Iron Duke Performance FAQ:

Here are some interesting incites into the Iron Duke’s performance potential.

by: Gary Ohst

>> I was hoping anyone could tell me what the difference is between the SD valve cover and the stock 2.5. Specifically is the bolt pattern the same and is there room for needle bearing rockers?<<

The bolt pattern is the same for all years. There is plenty of room for valve train mods. That cover is too tall to fit under the stock air cleaner intake housing. There are no provisions for a PVC valve port. It would require some mods to fit on a production motor with all emission control hooked up.

>> Well I finally found the supplier of the Super Duty Block online. Check it out at:<<

Doug Thompson took over production of the SD block when GM dropped the ball. You should know that the cost is prohibitive at around $2,500 each. They are actually stronger than the original SD block and can accept a variety of cylinder heads. Buying a complete used SD race motor and de-tuning it for street use would be far more economical. Aries pistons makes an aluminum SD block for around $4,000 if you think $2,500 is bad. By comparison, the last new SD iron block I bought from GM cost $850.

>>I wonder if the block, crank, cam and connecting rods will work with stock type pistons and a stock cylinder head?<<

Most parts interchange, but… The SD4 cranks have 2.10 rod journals (SBC). The stock crank is 2.00. If you run a SD crank you need to change the rods. The rod length and crank stroke influence the compression height spec of the piston. If you increase stroke or change rod length, new pistons are required. All SD heads use 1/2 head bolts. All production heads use 11mm. You need to drill and re-tap production blocks if you run a SD head. SD blocks already have 1/2 head bolt holes. SD blocks also have all the stock mounting bosses, so you can bolt up all the stock accessories if you want.

>>I know this sounds like a “halfway” job but my reasoning is simple, use these components with about 9:1 or 10:1 compression, at about 2.7-3.0L, do a mild porting job on the stock head, install lighter valves and stronger valve springs with better rocker arms (rollers maybe).<<

The production “swirl-port” intake ports are restrictive. The first thing you should do is hog out the stock intake ports or use a SD head. The next weak link is the crank. The S10 truck block and 1988 crank with center oil pump drive gear are the strongest stock parts. If you add displacement with a larger stroke SD crank, you are into a SD head to match increased flow requirements. An SD head on a production block is a far better match than a production head on an SD block.

>>I have looked into various options for my engine rebuild. A local machinist I visited, who builds racing motors said that the problem with the Duke is not necessarily the bottom end but the head. It is restricted, thin, and prone to cracking.<<

Yes. They are OK for performance street use if you open up the intake port and check for cracks.

>> So, maybe the way to go for a half and half is to leave the bottom end basically as it is and go for the upper…….<<

The bottom end is good for 150+- HP. Maybe 180 with the right components (88 crank/S10 block). The duty cycle goes down as power output goes up. Don’t expected it to last like a 100 HP production motor.

>>I need facts here, not rumours or speculation. What is the problem with the Iron Duke regarding the 5000rpm ABSOLUTE rev limit I have heard mentioned.<<

It was a low-cost, entry-level power plant for a lot of GM cars, not just Pontiac Fieros. The crank was made as light as possible for throttle response and fuel economy. It was designed for low performance cars and low performance driving. The stock crank is cast and weighs 30-35 lbs. The Super Duty counterpart is forged 4340 and weighs 50 lbs. The SD motors started to blow up at about 9,200 RPM according to Vanderly. These were under controlled dyno room conditions. Don’t know of hard data on production crank limits, but sometimes they flex and cause rod bearing failure before they break. If GM “rated” it at 5,000, it can go higher. Just like the Getrag 5-speed is “rated” at 200 ft lbs torque, but handles more than that. You are cutting into life expectancy when these guidelines are exceeded. GM has warranty concerns in mind when they “rate” parts.

>>Is there an inherent problem with the crank and bearings? The block appears to have five well supported main bearings (even if the main caps are not exactly “beefy”). The connecting rods are reasonably long so they should not suffer from angularity problems. With a good forged crank and good quality connecting rods (tested so as not to be flawed as many of the early ones were) will this engine hold together.<<

There is nothing wrong with the design. It was just optimized for economy and not performance. Of the bazillion 2.5 liter motors made, how many saw performance driving?. If you were GM, how strong would you have made the production crank? A forged crank supports over 200 HP. The problem is that while they are out there, they are near impossible to find these days. Only option is a billet crank from SCAT for $1,800.

>>Are people making to big a deal about this engine being weak because Pontiac ran the SD program to build a racing engine and made the stocker look inadequate or is there real cause for concern.<<

Because it was designed as light as possible on purpose, it runs into performance limits sooner than other motors. The same rules apply to other motors, more power and higher rpm equals stronger components. The SD program came about because GM knew they could not get where they wanted to go modifying production hardware. 150 hp is not a bad goal for four cylinders. How many eight cylinder SBCs do you know of making 300+ HP without upgraded internal components? The difference is there is a sliding scale of performance hardware for the SBC. The Iron Duke is black or white, SD or not.

>> Thank you very much, you have cleared up many of the unanswered questions on the 2.5. If I am reading you correctly, a good 2.5 block with no cracks, properly cleaned up but not overbored, combined with a good stock crank and good quality connecting rods. (I guess an 88 crank will not fit the earlier block, an 84)<<

Yes, the 88 crank will fit. The extra mass of the oil pump drive gear adds strength to the unit, but fits in all blocks. The major dimensions are all the same. Just be sure to use a 84-87 cam with the oil pump drive gear. The 1988 cam has no distributor or oil pump drive gears.

>> A cam designed for better mid range power (2000 to 4000 rpm) a porting job on the stock head (check for cracks first, don’t remove too much material), an exhaust header and a holley intake (if you can find one) or a stock intake with port matching. Combine with a free flow exhaust system and the stock electronics would I be reaching 120hp (with a 5000rpm redline)?<<

Porting the head is most important. In the area under the intake valve in particular. Holly TBI & manifold will not add much. Stock intake manifold/TBI flows enough. You may need a higher pressure fuel pump and adjustable regulator. Use more fuel pressure to adjust for VE improvements in the engine. That saves messing with the ECM fuel maps.

>>Would I have a strong running motor capable of hauling the tall 3.32 ratio four speed at a relaxed highway speed with some power left for passing, and decent reliability?<<

It is harder to get more torque at the same rpm than it is to get more hp at higher rpms. If you want a lugger, then a stroked SD crank may be what you want. 125 HP will work on stock bottom end components in rebuilt condition, but peak HP will be at higher rpm than stock.

>>I was on the MSD ignition website today and found that they have a new super duty billet aluminum distributor. The description mixes Iron Duke with SD. Are distributors interchageable?<<

Nope. It’s a “dumb” system that can’t interface with the variable timing feature of the stock ECM. It’s just has a mag pick-up that matches the standard MSD modules. Nice hardware, but only necessary for higher rpm use. Just upgrade the coil on the stock ignition as the first step. Unless you increase compression ratio or spin higher rpm, the stock system design is fine. Money is better spent elsewhere.

If they were to say it is “compatible”, you should be suspect. Since they say it only works with their module, you pretty much know for sure there is no way it will interface direct.

>>This distributor is designed to be used with their ignition controller (such as the #6). I have seen other cars with aftermarket ignitions. I understand the principle of multiple spark and cylinder effects. Could not get a straight answer about emissions control/ECM/TBI etc…… How does installing one of these affect the computer control of the car?<<

It does not interface directly. See above comments.

I am not a big fan of MSD, but their marine grade stuff may be Ok. My street driven beater car uses the stock system w/upgraded coil. No reliability problems, plus it’s emission legal that way.

I do run a simple Adrenaline CD system on another car rated at 180mJ (mil joules) per spark. If you look into it, that’s a lot of spark energy for a single spark system. The mJ spec is most important, but often never published by ignition peddlers… I wonder why?? Racer Wholesale used to sell these systems a few years ago. The Adrenaline brand was nice stuff with mil-spec components, but I don’t think they sell direct under their own name anymore. The trend is to get rid of the distributor, get rid of the spark plug cables, and now to focus on a better spark event. For a peek into the future of automotive ignition systems, check out the Adrenaline web page. Probably a few more years before these new systems become cost effective and a mass marketer picks up the design.

A 2.5 liter, 3 inch stroke 1988 production crank (left) next to a 3.0 liter, 3.625 stroke SD crank (right) so you can see the significant differences.

Variations of the 2.5L ohv GM four cylinder engine.

There are several variations on the 2.5L engine used in GM models. Before using a non-Fiero block as an engine replacement check out the following list.

The Information in this table is also courtesy of Gary Ohst. The comment which prompted this response on the Fiero Mailing list was from Sketch and an excerpt of his letter is reprinted below:

“…I recall a URL posted on one of the lists quite some time ago that was a technical article about the Duke, on a site oriented towards professional mechanics. They mentioned one problem with the stock FWD blocks was that two of the cylinders at one end were actually closer together. Instead of having round cylinder liners, they cut a slice off of one edge on two of the cylinders, and moved them closer together to make the engine shorter….”


Stay away from the oddball FWD only VIN U motor, too many parts are different compared to all other 2.5 motors. It was used only in FWD N-cars, Grand Am, Calais, etc. Crank thrust bearing and cylinder head bolt pattern are a few (see below). The VIN R (Fiero included) motors were used in both larger FWD cars (X and A body cars) and RWD applications with no special mods.

The S-10 truck block is good because it has solid main webs like the SD block. It is cast from average grade iron though, where as, the SD block uses 40,000 psi high nickel iron. The SD blocks come drilled for a starter mount on both sides.

The Vin E 2.5 liter truck motor was used in both Astro/Safari vans and the S-10/S-15 trucks. I think the later model 1991+ VIN A was only used in the pick-up trucks. The pick-up trucks have the starter on the right and the vans have it on the left (fiero side). Starter location can be changed

Here are some general VIN notes regarding the various 2.5 liter motors:

Vin V 1977-1980 Carb non cross-flow head (intake & exhaust manifold on same side)
Vin 1 1978-1979 Carb non cross-flow head
Vin 9 1979 Carb non cross-flow head
Vin F 1980-1983 Carb export only
Vin 5 1982-1983 Carb cross-flow heads from 1982 on
Vin 2 1982-1986 TBI Thrust brg length 1.009, Head bolt spacing 3.75
Vin R 1982-1992 TBI FWD/RWD/Fiero Thrust brg length 1.009, Head bolt spacing 3.75
Vin U 1985-1991 TBI N-car FWD Only Thrust brg length 0.940, Head bolt spacing 3.67
Vin E 1985-1991 TBI S-10, Astro Van Thrust brg length 1.009, Head bolt spacing 3.75
Vin A 1991-1993 TBI S-10 Thrust brg length 1.009, Head bolt spacing 3.75

The VIN E and A truck motors were also used in postal vehicles. That’s right, the mailman has a better block than you do ;)



by S. J. Wynman

Most Fiero owners have 4 cylinder engines to power their cars. It appears no one has ever cared to have any real performance enhancement for these Fieros – well, your time has come! Now you can read about some methods to improve the overall engine performance of the stock Fiero 4 cylinder motor. This article is divided up into several topics:

  1. Throttle body enhancements
  2. Port Matching
  3. Octane Tuning
  4. Decarbonizing
  5. Valve Guide Sealing
  6. Increasing the Redline
  7. Harmonic Balancer usage
  8. Performance Build Up Parts List

Not all of the recommendations will apply to every year 4 cylinder Fiero. The first thing you must realize are the differences between the various 4 cylinder motors GM installed in the Fiero. The Table below summarizes the differences as they pertain to the article:


Back in the mid-1980′s HOLLEY felt they had a quick answer to the lack luster performance offered in all GM 2.5 Liter motors. They actually hit on something good, but it appears the idea was not an original one. GM used a performance Throttle Body Injection (TBI) unit on one of the original 1984 INDY Pace cars. GM later provided HOLLEY one of the prototype units and HOLLEY mass-produced them. Not only was the throttle body bore increased from 1.68″ to 2.00″, but the single injector output was increased to make use of the increased air flow. Although the result was impressive, HOLLEY wanted even more. They developed their own intake manifold, which had a larger intake runners, and TBI mounting bore to accommodate the larger throttle body bore. As a result, the combination proved to be excellent. This TBI can even bolt onto the stock 1984 through 1986 intake manifold. HOLLEY provides an adaptor gasket to allow the new TBI to bolt onto the old manifold. A few years ago HOLLEY discontinued their intake manifold, but you can still realize many of the HOLLEY intake manifold benefits. What you should do is machine out the opening in your present intake manifold. Do not open it up to the full two inches as this will probably interfere with the existing EGR passages. Use your judgement as to how much material can be removed. You can use the 2″ HOLLEY gasket as a guide. Scribe this diameter onto the intake manifold. Stuff the intake hole with moist rags to help catch the aluminum which is being removed from the intake manifold.

If you own a 1987/88 Fiero, you will need to fabricate an adaptor plate. GM totally changed the design of the TBI and intake manifold. You cannot use an earlier design intake manifold as the head on the 1987/88 has also changed. The earlier intakes will not bolt up to the newer head. All this means that a simple adaptor plate will have to be fabricated allowing the old bolt pattern of the HOLLEY TBI to be mounted onto the new GM designed intake manifold. Use a 1/4″ aluminum plate along with a new 1987/88 TBI intake gasket. Angled screws must be used to maintain a low adaptor plate height. Remember, you must still open up the TBI bore on your intake manifold. If you are lucky enough to find one, use a HOLLEY intake manifold (except on 1987-88).

As a simplistc example, an engine can be seen as an air pump. The more air the engine pumps. the more power it develops. In order to increase air flow, any obstructions must be removed the air flow path. The first and most obvious obstruction in the 4 cylinder Fiero is in the exhaust system. When the muffler and catalytic converter are replaced with high flow units, and perhaps headers are installed (although they are not really a big advantage here), it is time to look at the intake system. The easiest improvement is to replace the air filter with a K&N Air Filter. The next step was previously discussed in the topic entitled Throttle Body Enhancements.

Once this has been completed, it is time to go inside the engine. This brings us to another 2.5 Liter engine improvement which is to perform port matching. The only safe way to perform this improvement is to remove the intake manifold, exhaust manifolds, and the head from the Fiero. Otherwise, dirt and metal particles will enter the cylinders and drastically shorten the life of the engine.

This is a process where the gasket, between the intake manifold and the head, are used as a template for both the intake manifold and the intake ports on the head. There is enough metal on the desired work surfaces so the port matching can be accomplished safely. The head should be off the engine and all of the valve train should be removed. After the work is completed, the heads should be thoroughly rinsed and cleaned. Follow the GM or Haynes shop manual for the removal of these components.

HEADS. The intake ports are first painted with a blue dye. The intake to the head gasket is then laid on the heads intake side using a manifold bolt as an alignment device. Use an Exacto knife to scribe the intake passages of the gasket onto the head. You will notice the port openings of the gasket are larger than the intake port opening of the head. Now use a 3/8″ flame shaped carbide rotary file installed in a die grinder (capable of 20,000+ RPM speeds) to expand the intake passages of the head. Only enter the head by 1″. Going beyond this point may result in head, valve, or valve guide damage unless you have previous experience in the porting and polishing process. Use your fingers as a guide to determine how the work is progressing. The newly improved openings should have a smooth transition into the head.

The exhaust side of the heads can use the same improvement. The exhaust side will require substantially less work because the ports are closer in size to the exhaust manifold gasket ports. Use the technique above to port match the exhaust ports.

INTAKE MANIFOLD. The same gaskets used to improve the intake ports on the head are now used to open up the intake manifold to head ports. This is the most restrictive area in the intake tract. Make sure the holes are open and clean. Use brake cleaner and Gum Out to clean the passages.

Later, when installing the intake manifold, use FELPRO BLUE as a gasket sealant on both sides of the gaskets and follow the shop manual torque sequence for tightening the intake to head bolts.

The intake manifold was factory painted with a silver paint on most 4 cylinder Fieros.

EXHAUST MANIFOLDS. The exhaust manifold is made of tubular stainless steel. The manifold is basically a four into one header. Believe it or not, this manifold is relatively free flowing. Concentrate your efforts in the area where the “header” connects to the rest of the exhaust. Do not remove more than the obstructions, as the manifolds are relatively thin walled. Also, do not trim or port match the mounting flange of the ports themselves, as this is where the welds are, which hold the mounting flanges onto the stainless steel pipe. You will end up cutting through the welds which hold the stainless steel tubing to the mounting flanges. Clean out all of the metal particles, otherwise they will end up either in your engine or in the catalytic converter.

The exhaust manifold was not factory painted.

One of the gasolines octane rating characteristics is its resistance to pre-ignition. The higher the octane, the higher the resistance to pre-ignition. As an example, 93 octane gasoline is more resistant to pre-ignition than 87 octane gasoline.

To a limit and as a basic rule of thumb, the more your engine timing is advanced, the more power it will make. The two draw backs to this theory are that more advanced ignition timing settings will tend to promote pre-ignition and also increase exhaust emissions.

The secret here is to use high octane gasoline at all times. Once you have purged the lower octane gas from your tank, by constantly diluting it with higher octane gasoline, you can reset the timing of your Fiero. This trick is limited to 1984 through 1986 2.5 Liter motors as the timing is totally ECM controlled on the 1987/88 4 cylinder Fieros. You should follow the Fiero shop manual for the details of this procedure, however the following are abbreviated steps on how to perform this task. This is done with a timing light and 13 mm socket. Run the engine warm and then loosen the distributor. Move the distributor to increase initial base timing. The normal base timing is 8 degrees BTDC. Now advance the timing to 10 or 11 degrees BTDC and tighten up the distributor. By doing this you are resetting the initial timing of the engine. The ECM cannot determine the base timing and assumes you have set the initial timing to 8 degrees BTDC The ECM now adds its own calculated advance curve to this new base timing. This means the 10 or 11 degree advance you have set will remain in place throughout the entire RPM range.

Now you have must test the new timing settings. In 95% of the cases the new timing will not be a problem. For the other 5%, the problem is with the motor. High carbon deposits and poor quality gasolines are just two examples. With the side windows open, accelerate and listen for a rattling or knocking noise. Hopefully you will not hear either sound, meaning everything is alright. However, if you hear these sounds, back off the timing by 1 degree and repeat the test. Repeat this step until no strange noises are heard.

The advantages you will experience by completing this task are more power and better fuel economy. The down side are increased exhaust emissions. This is the reason why GM limited the Fieros 2.5 Liter timing to B degrees BTDC. It is for this reason you should advance the timing only if your Fiero is to be used in off road applications.

Carbon deposits have always been a fact of life with the internal combustion engine. Unfortunately, the introduction of fuel injected engines during the mid 1960′s had introduced a new set of problems. The problems included new areas of carbon deposits. Carbon deposits accumulate everywhere in your engine. The only way to slow down this process is to use premium name brand gasoline. According to a spokesman from MOBIL, a detergent with anti-carbon properties is placed in all grades of gasoline sold by MOBIL. The only difference is the concentration of this detergent varies with the grade of gasoline. The premium grades contain the highest levels of the detergent additive.

Carbon deposits form in quantity in two areas of the 2.5 Liter Fiero engine, the intake tract and around the intake valves.

The intake tract can be completely clean only through disassembly and the use of a wire brush. Aside from doing this you can clean another area, which immediately affects performance, and is accessible with the engine fully assembled and also still being in the Fiero. The others are in the throttle body. Excessive carbon can prevent the throttle plates from closing completely or even blocking some of the vacuum passages. The throttle plate can become stuck (open or closed) and even end up setting some ECM codes. The air cleaner side is always nice and clean. As soon as you pull on the throttle cable to open the throttle plate everything is black. It is this black carbon which you must clean. This is accomplished with a brass tooth brush and several cans of Gum Out Carburetor Cleaner. First place a small clean damp rag just past the throttle plates. This will prevent debris and excessive amounts of Gum Out from entering the engine. Soak the blackened area with Gum Out and then use the brass brush to loosen it. Remove the debris with a clean rag. Spend extra time cleaning behind the throttle plate, the throttle plate pivots, and the throttle body bore area where the throttle plate touches.

Carbon around the intake valves is a much more severe and difficult problem to correct. The problem arises when the cool gasoline leaves the fuel injector and hits the heated valve causing some microscopic deposits to be left behind. This problem is further compounded when the intake valve guide seals (two per intake valve, an Umbrella type and the standard “0″ Ring type) deteriorates which allows oil to build up on the valves, which can amount to an incredible build up. This can actually block most of the air flow into the cylinders. This results in a choked engine which refuses to perform. Some companies sell products which claim to remove this carbon build up, but I have not experienced ant to perform as claimed. The only efficient way to remove this intake valve carbon build up is to remove the intake valves from the heads and either replace them, or clean them with a sharp chisel and a wire wheel. This procedure requires removing the heads from the Fiero.

Virtually all engines contain valve guide seals. The purpose of the seals are to prevent oil from entering the valve bowl area and either getting burned (intake) or leaving as a cloud of smoke (exhaust). Heat and time take their toll on the seals. The result is carbon depositing on the intake valves developing quicker (see above), resulting in the Fiero smoking after sitting still for sometime.

The valve guide seals can be replaced without removing the heads from the engine. As a quick overview the following must be performed. Follow the shop manual for detailed procedures on each of the following over viewed steps:

  1. Remove the valve covers
  2. Remove all four spark plugs
  3. Remove all eight rocker arms
  4. Install a compressed air adaptor into the spark plug hole and apply compressed air. This will hold up both intake and exhaust valves for the cylinder being worked on.
  5. Using one of the rocker arm retaining nuts, and a lever type on-car spring compressor, depress the top of the valve retainer, With needle nose pliers remove the two valve stem keys (keepers). Release pressure on the valve spring, then remove the retainer and valve spring. The compressed air will hold up the valve, Remove the old “0″ Ring seal (both the intake and exhaust valve have this seal), and wiper seal if you are working on the intake valve, Install the new seal(s) and reverse the process to replace the valve keys.
  6. Repeat steps (4) and (5) above for the other 7 valves.
  7. Re-assemble the remaining components of the motor per the shop manual.

The redline, or maximum operating RPM of the 2.5 Liter engine is 5000 RPM. Most stock four cylinder engines have a hard time finding the high side of 4000 RPM. Many engine parts can limit the maximum RPM of an engine.

A large step can be realized in achieving that 5000 RPM redline in the 1984 through 1986 Fieros. This is done by replacing the ignition coil. GM makes some nice ignition parts, but fell short with the coil. The answer: Bolt on an ACCEL, or other brand of high performance coil onto the rear trunk sheet metal, near the original stock coil location. With a coil change, a free flowing catalytic convertor, and a new TBI alone, you will really wake up the little motor.

A harmonic balancer absorbs the torsion vibrations which occur within the crank shaft. These vibrations are caused as each cylinder fires and part of that “power pulse” is sent to the crank shaft, in a direction which does not contribute to the power being output to the flywheel. Having a harmonic balancer does not eliminate the pulses, but it does give them a place to go. This will contribute to longer crank shaft and timing gear life. GM introduced a harmonic balancer for the 1987/88 Fiero 2.5 Liter motor. The part number is 10101369. Unfortunately, GM did not think about the earlier Fieros models.

GM developed a nice 2.5 Liter engine program. This program is called the Super Duty Four Cylinder. Once this motor is built up correctly, it will give a 1996 Corvette a run for its money. The bad news is that just about every piece of this motor is race ready, and very expensive. A completely assembled motor will run about $7000.00. As you can see, this is a little on the high side. Do not give up hope or even your stock four cylinder. if you follow all of the previous suggestions, your stock 2.5 Liter will be good for about 110 to 115 HP. Considering the stock motor puts out about 94 HP this is not too bad. If you want more, then a complete rebuild is necessary. Standard Blueprinting practices are recommended, along with the following parts:

  1. Pistons – TRW piston part number 248SF-SO, TRW ring part number is T8370-30. This is a forged 0.030 oversized piston. I do not recommend an over bore larger then this as the block is not to strong or thick in the cylinder area. Make sure a deck plate is used when boring and honing the cylinders. The compression ration will increase to about 9.1:1 where stock is approximately 8.4:1. I strongly recommend the use of the previously described improvements if you decide on these pistons. These pistons will fit in the 1984 through 1988 motors.
  2. CAM Shaft – TRW TP-209, Intake Lift = 0.409″, Exhaust lift = 0.516, Duration lIE = 204~/2 140. This is a nice improvement over stock and which will not require head work for the spring perches. The stock ECM will also work with this cam shaft. I strongly recommend the use of the previously described improvements if you decide on this cam shaft. This cam is a direct bolt in for the 1984/85 motor. You can use it in the 1986 to 1988 motors if you use conventional lifters in place of the roller lifters. You will also have to use the 1984/85 push rods in the later motors if you use this cam shaft.
  3. Timing Gears – Cloyes makes a nice all steel set. GM has improved their own timing gears which can be ordered under part number 10101790. This gear set will fit the 1984 through 1988 motors.
  4. Valve Springs – TRW part number VS1 100, using retainers SR363. These parts can be used only in the 1984/85 motor. Use the stock springs with the 1986/88 motor.
  5. Oil Pump – TRW High Volume part number 50132. This oil pump can only be used only in the 1984 to 1986 motors. The 1987/88 motors use a gyrator type pump which is part of the balancer assembly.
  6. Use brand new GM Head, Connecting Rod, and Main Bearing Cap Bolts. Follow the latest Torque-to-Yield Bolt specifications.
  7. Replace the catalytic convertor with a high flow ACCEL unit.
  8. If the oxygen sensor is 2 years old or older, it should be replaced. The accuracy decreases the range of forcing the engine to run lean.

Balance the reciprocating crank shaft mass to within + 0.5 gram. This includes the crank shaft, pistons, rods, rings, bearings, front hub, and flywheel. This will remove much of the engine vibration characteristics and provide a slight increase in power.

Increase the fuel pressure on the HOLLEY TBI unit (it is adjustable) to 15 pounds.

After the stock or aftermarket header, replace the stock exhaust with a 2 1/4″ stainless steel unit such as the one sold by the Fiero Store. This removes the stock restrictive muffler and replaces it with resonator tips. The sound is a little louder, but it does sound nice.

This is about as good as you can expect with the stock block, crank, and head. You can expect the engine to produce 120 to 130 HP if all of these modifications are made.

We can debate this until the cows come home….. but unless you go the SD route you ain’t gonna make a significant difference in the overall performance of street driven Dukes with add-ons (been there – done that). If you are successful in cranking out significantly more HP, the lower end will not last much longer. Experience has taught me (and others) that when you start consistently pushing a Duke over 5,000 rpm it is going to self destruct. It ain’t a matter of “If”, it is when. Do you ever wonder why the early 2.5 turbo kits died off so quickly? It wasn’t the introduction of the V6 – they were a prescription for a broken crank or pitched rod.

Still, there are some things you can do to a Duke to improve performance that are simple, cheap, don’t compromise reliability and should be done anyway. Sadly, they are often overlooked or ignored. It has been at least 5-6 years since I put this on the list, but it is still valid. If you have previously done these, GREAT! If not, try it – you’ll like it. (You will also get better gas mileage)

  1. Replace the fuel filter. Do this every other oil change or 6,000 miles. Restricted fuel filters lower fuel pressure. Lower fuel pressure translates to a drop in performance. This is often overlooked and is true for the 2.8 as well.
  2. If you still have the stock converter, replace it with a new high flow unit. Gains will be noticeable as the original L4 brick catalyst is very restrictive.
  3. Drop in a K&N air filter – or at least a new paper one.
  4. Pop in a new oxygen sensor
  5. Change to synthetic oil
  6. On 84-86 Dukes, bolt on a performance coil and a new set of lower resistance plug wires.
  7. Install a fresh set of spark plugs – NEVER put Bosch Platinum plugs in a DIS car (87 & 88) If you have not added a higher voltage coil and wires, close the gap .005 less than specified. “Trick” plugs (Splitfire, etc.) are OK, but a waste of money as far as performance gains. A set of new steel electrode AC plugs will do just fine.
  8. Bolt up the dual outlet muffler and pipes from a 85-87 V6. There are tons in the junk yard for little or noting. Don’t expect a great performance improvement, even if there is one less chamber in the V6 muffler, but it looks and sounds better.
  9. Run maximum allowable air pressure in your tires and avoid those tall fat ones – especially on the back. They can change your effective gearing and reduce HP at the point where the rubber meets the road.
  10. Try a 160 or 180 degree thermostat. It may take longer for the car to go into closed loop but Dukes seem to run better in the 180-200 degree water temp range. Add a manual fan switch so you can cut it on at will, don’t wait for the fan temp switch to do it or the water temp will soar even with a low temp thermostat.
  11. Be sure no calipers are dragging – especially the rear EM brake.
  12. Run a batch of carbon removal and top engine cleaner through (Ed Parks sells this and the stuff really works!).
  13. For quicker off-on throttle response, disconnect and plug the vacuum hose to the EGR valve. May have to move to mid grade 89 octane gas for this. Also increases emissions. Will not give more overall HP – only quicker throttle response. On a 2.5 the ECM doesn’t know this has been done. (Note this may cause emmissions problems. I.C.)
  14. Give the car a wash and wax, clean cars always run better – you know, less wind drag over a slick surface thing.

If you have not done these low cost items, you will be amazed in how much better your Duke will run. Then, if you gotta learn the hard way (like I did) go ahead and face the wind and pull those bills out of your pocket for your visit to the johnny house…… or you could just donate them to me.

No way (short of the SD-4 – see and click on “Blast From the Past” for SD 4 articles) will you get the HP gains you want with the other higher cost add on mods for the L4. Port matching and larger TB’s won’t hurt – but they won’t help much either unless you go the whole nine yards (heads, headers, cam, etc.).

One of the most important things you have to be aware of when trying to improve the performance of the Iron Duke is the well-known fact that it has a weak block. Remember that this engine was designed from the ground up to be as fuel efficient as possible. High performance was never a major design goal of the engine. Therefore, strength and the ability to accommodate higher performance was sacrificed in the name of weight reduction. Low cost was another design goal, so more aggressive steps to increase efficiency such as porting and polishing were overlooked. It is in areas such as these that I believe the performance can be gained; not by pulling more power out of the engine, but rather removing obstructions or restrictions that are consuming the power that the engine already has. I believe that if you are going to be successful, you have to accept the concept of ‘power through efficiency’. Any other approach is going to result in a blown engine. I can almost personally guarantee it.

That said, the next most important thought you should keep in mind is that of properly matching all the parts during the rebuild. An engine with a well-matched set of conservative parts will usually provide better results than an engine full of high performance parts that are not matched. Take a good long look at your driving habits and your expectations for this vehicle. If the car has a manual transmission, what RPM ranges do you normally shift at? What gear do you prefer to use when passing people? What speed are you normally driving at when you decide it’s time to ‘get on it’? Is this a daily commuter or a weekend sportster? Take these thoughts into account before, during and after you start to rebuild the engine.

You also need to be realistic about your expectations. The Iron Duke is a great engine for efficiency and moderate to respectable performance. If you are looking for a truly significant jump in performance, say more that a 25% increase over factory rated HP; this is not your engine. By the time you pull that much HP out of the engine, you will have spent a small fortune and in all likelihood the engine will fail very quickly. This article is for those that have more time and patience than money and just want to have a reliable, efficient, smooth running little 4-banger that gives them reasonable performance when it is occasionally needed. If you are expecting more, go for a V6 or V8 conversion. The bang to buck ratio will be much better with a conversion approach and they are not as expensive as you might think.

If you are an experienced mechanic and are comfortable with choosing your parts, by all means do so. If you are a little foggy in this area, like myself, seek out the advice of an expert in your area. Most of these people are more than happy to share their knowledge with you, especially when they see that you have already given some thought to how you want the engine to perform and considered how your driving habits have played into your decision. If you don’t trust your mechanical skills at all, you can have a local outfit build the engine for you. Just make sure you pick a quality shop to do the work. If you have no fear of spinning wrenches, feel free to ask an expert for guidance on how to best match and assemble the right parts for your needs. They usually don’t mind being used as a resource because they know that you will have to have certain machine work (boring, milling, turning, etc.) done somewhere, and if they are helpful, they just might get your business.

The first step to building an engine is to select a camshaft that matches your performance expectations. This one part more than any other will determine the true nature of your new engine. Again, be realistic. If you try to put a full race cam in, you will have to take extreme measures to take advantage of it, by which time your will likely have exceeded the block’s capabilities. Once you have chosen the camshaft, you should build the rest of the engine around that. This will affect what type and compression of pistons you use; how you have the head milled and what type of valve springs you will use. It can affect what kind of exhaust system you install and what type of ignition products you buy. Based on the camshaft, you might choose to port and polish the head and intake manifold. You may opt for a three or five angle valve job. Of course, some of this may be a bit overboard, and if you choose a cam that HAS to have all of this, you’re probably being a bit aggressive. Again, consider a conversion instead of going overboard.

I was fortunate enough to have an engine designed for roller cams, which made having a new profile ground onto the original cam a viable possibility. Roller cams have far more success than standard tappet cams with longevity after being reground. This was certainly a good thing because there were not many aftermarket options for a new cam at the time. Kams, Inc. in Oklahoma City put a mild RV profile on the cam that adequately matched my driving style. It was certainly streetable; started easily even in winter; had a good mid-range power curve and respectable fuel economy at highway speeds. From there, I chose 9.5:1 compression pistons that would pull a little more power out of the fuel without inducing too much stress on the weak cylinder walls. I had a great local outfit named Buddy Rice do all of the machine work on the engine. They were the ones that helped me choose parts that would perform well with the camshaft profile I had selected. The block and head were both cleaned. Then, I had the centerline bore reestablished on both the camshaft and crankshaft bores. They also trued up the bores on the connecting rods. They milled the head and performed a 5-angle valve job, and then shimmed the valve springs to match them with the camshaft. The crankshaft journals were also turned, along with the mains that were ‘recentered’ to match the work done on the block. They looked at the idea of balancing the crankshaft, but ultimately decided against it because of its light weight and the fact that it was already fairly well balanced. While the engine was still at the shop, we carefully looked over all the parts for any signs of wear. Anything that showed any significant wear was replaced. Better to catch the problem now that to find it later after it fails.

With the block and head prepped, I took all the parts home and started putting the engine together. I didn’t do anything special while assembling the engine. I just made sure I was liberal with the engine assembly lube. I use this stuff on everything, not just the camshaft. I also made sure that I followed the torque specifications for everything; using several increments up to the final torque for anything over 25 ft/lbs. When working on the engine or suspension, I never tighten anything with an ordinary wrench. I always use a torque wrench to make sure I am not overtorquing something. I also do this to make sure that matching bolts on a part are torqued uniformly to prevent warping.

One note of caution; whenever you are not working on the engine, make certain to but a thick plastic trash bag over the engine and seal it airtight with tape. When I first started the engine, I had no oil pressure at all. After taking it back apart, I found that a wasp had built a mud nest in the oil pump that completely blocked the passageway to the oil filter. Fortunately, the liberal use of the assembly lube saved the engine from damage. The bag will also help protect the engine from dust and humidity.

At the time of this writing, this was the extent of the work done to the engine. Notice that the only higher than stock performance parts are a mild camshaft and slightly higher compression pistons. Everything else was just removing any possible sources of resistance, and making sure that each part could do its job in the best possible environment. The engine already performs extremely well. As time permits, I will port and polish a spare head and intake manifold from another engine. I will install them along with a new exhaust, headers, Holley’s Big Bore TBI and a good aftermarket coil. Some additional notes would be that I removed the restrictor plate underneath the TBI in the intake manifold; I use a cooler thermostat and coolant fan switch; and that I disconnected and capped the vacuum line to the EGR valve. If you choose to do the same, make sure you reconnect the EGR for one day every month and take the car for a long drive. That way, it will work when it is time to get your inspection. Something else to keep in mind is that I personally would probably never go this far with another Iron Duke. When I started, I did not know about the ease and tiny cost of certain V6 conversions. Also, I was fortunate enough to have a rare engine. This is a 2.5R engine, not the normal 2.5L. GM ran the numbers on it and describes my engine as an ’84 Fiero Second Design’ engine. They swear it is definitely for the ’84 Fiero even though it has roller lifters, which the ’84s are not supposed to have. I’m certain this plays a part in the better performance, and in this case, makes the extra effort worth it. I have a 4 banger that performs very close to many of the V6 Fieros, yet retained the efficiency of the 4 cylinder and did not sacrifice reliability or life in the process. I paid a whopping $200 for the car and spent around $1,000 rebuilding the engine. As long as I take care of it, this little baby will last forever.

The Pontiac Fiero has been available with three differently geared Muncie 4 speeds, an Isuzu 5 speed and the Muncie Getrag 5 speed. The 3 speed model 125 is the automatic transmission (with lock up torque converter). Here is a brief run down of what was available and what it means.

Transmission: Simple working definition, a device for transmitting engine power to the the final drive (and then on to the wheels).

Transmission lore is a mixture of fact and myth. Let us explore some of these points with an eye to the Fiero. (Note: emphasis on issues of importance to potential engine swappers)

Automatics, in the case of the Fiero a TH125 three speed with torque converter lockup. A reliable transmission, if not exactly an exciting one. The automatic is a good choice with V6 models, or sufficiently beefed up for a 3800 or V8 engine swap.

4 Speeds, again the four speeds are an excellent choice for the high torque, low rpm powerplants. A 1984 economy 4 speed (option MY8) would make a very good high speed cruiser with a Chevy V8, allowing decent fuel economy, relaxed cruising (about 2000rpm at 60mph) and plenty of performance because of the flat torque curve of the typical 305 or 350. The 3.65 ratio 4 speed used in the V6 (M17) would also prove a good choice giving better acceleration but a rather busy highway cruise and lower fuel economy, this transmission would be a better choice for a 3800 V6. The 4.10 4 speed (M19) is best left to the drag racers (or as an acceptable but less than ideal unit for the high RPM twin cam engines.). There is no real difference between the 4 and 5 speeds other than the fact that the 5 speed has more intermediate ratios which allows you to keep the engine in its powerband.

That’s it! As a matter of fact when Porsche built their first Turbo 911s they actually replaced their normal 5 speed with a beefed up 4 speed since they could build a very strong 4 speed which would be lighter than an equal 5 speed (Their normal 5 speed was not strong enough). They reasoned correctly that the increased torque of the turbo made the extra ratios unnecessary. Marketing however showed that people saw a 4 speed as being “low tech” so the 5 speed soon returned BUT ONLY BECAUSE PEOPLE WHO KNEW LITTLE ABOUT CARS DEMANDED IT. Just as a 5 speed V8 combination is unnecessary in a Fiero unless you plan on competitive road racing. This does not mean that a 4 speed is preferred over a 5 speed, just that if you have a perfectly good 4 speed you may not NEED a 5 speed unless you are racing.

5 Speeds, The Isuzu 5 speed attached to 85-88 4 cyl Fieros is not a good choice for a high torque swap, it is not strong enough. This transmission may cause problems because of weak cast alloy (pot metal) shift forks, which could break if abused especially at high rpms. Careful shifting can make these transmissions survive since it is the shift forks, and not the gears, which causes the problems. The gearing of the Isuzu transmissions seems to be a better compromise than the Getrags, but the reliability issue can cause some concern. This leaves the Getrag, although not perfect it is probably the best transmission used in the Fiero. It is strong enough for the high torque V8s and V6s but with the better ratio spread necessary for the cammier engines such as the 3.4 twin cam V6, the Quad Four and some V8s tuned for top end power. If you have one this is the transmission to use for any swap.

Now you may ask what is the point of this little discussion. Simple, many of you may be contemplating a V8 swap but are concerned about the added cost and complexity of finding and installing a Getrag. If you have a V6 with 4 speed, or an 84 with the economy ratios, you don’t need the Getrag. Also decent 4 speeds are available for much less than the typical Getrag ( All Citations with manual transmissions used this 4 speed, but with different combinations of ratios, such as the 2.5 with 3.32 final drive but 0.81 4th gear). In an extreme case, the 6 speeds Chevy sells in the Camaro and Corvette are mainly meant to allow tall gearing for good fuel economy and as a marketing gimick (no flame please, I would love to own one I just want to point out that a 9000rpm Honda VTEC engine would need that transmission, a 6000rpm 5.7 litre V8 does not). With big torquey engines 4 speeds are fine. Now the Iron duke 2.5 like mine, it could use all the help it can get, but since I drive an 84 I have the four speed.

The moral of the story is, if your perfect Fiero must have a V8 than a 4 speed is fine, but if you long for the high pitched scream of an engine on the far side of 6000rpm the 5 speed is the logical choice. The wide ratios of the economy box in my 1984 suits my slow revving iron duke just fine.

Note: All indications are that NO V6 ever left the factory with the 4.10 ratio 4 speed, it was a 1984 only option.

Automatic Transmission Final Drive Ratios:

84-86 L4 had 3.18 final drive.
85-86 V6 had 3.06 (86, at least the GT haven’t seen any in SE had 3.33 optional)
87-88 L4 had 2.84
87-88 V6 had 3.33 (2.84 optional)

All will interchange


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