Building a 120hp Duke

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:

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:

Model Year	Distributor	Ignition Coil	Camshaft	Throttle Body

1984	Standard	External	Standard	Old design
1985	Standard	External	Standard	Old design
1986	Standard	External	Roller	Old design
1987	DIS	Coil Packs	Roller	New design
1988	DIS	Coil Packs	Roller	New design

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.

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.

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.

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.

(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.

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.

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.

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.