Engine - Exhaust System

The factory exhaust in the Geo Storm is very good quality, made of 409 alloy stainless steel. After 20+ years, most of the rear mufflers have been replaced (multiple times if aftermarket replacement rear muffler assemblies were used), but the mid pipe and down pipes typically remain original, due to their high durability and quality.
But the original exhaust system is undersized for optimum flow and has restrictive squeeze bends. It was designed for quiet vehicle operation. There is a lot of performance to be gained with the right exhaust upgrades.


Cat Back Exhaust
The original pipe diameter is 2 inch for the DOHC engine, and 1.75 inch for the SOHC engine. After many years of disagreement over 2 ¼ vs. 2 ½ inch pipe and squeeze bend vs. mandrel bent pipe, Isuzuperformance published the only exhaust system dyno comparison test ever done for Geo Storm and its Isuzu sister cars, which proved that 2.5 inch diameter, mandrel bent pipe, with a Magnaflow straight-through style muffler, yielded the highest power improvement for the Isuzu engine. The test further showed that as long as the mufflers are straight-through type, using multiple mufflers in series provided further noise reduction with no drop in performance. The test compared the Isuzuperformance exhaust system to the Pacesetter exhaust system, and found the Pacesetter product to be undersized, made of thin, untreated pipe, poorly fitting, and making very little power compared to the baseline level of the stock system.
Isuzuperformance is the only company to have conducted extensive dyno testing of exhaust systems for Geo Storm and Isuzu cars, and published those findings on their website. Those results remain the most cited documents in discussions concerning engine power and appropriate exhaust pipe size, for not only Isuzu and Geo Storm, but for all small and compact car groups and forums.

Isuzuperformance
Isuzuperformance is the only company ever to have offered a true, quality, dyno proven, performance exhaust system for the Geo Storm, built for maximum power delivery. The system featured 2 ½ inch, heavy wall, aluminized, mandrel bent pipe, with choice of single or two muffler configuration, and excellent fit with quick and easy installation by the vehicle owner in an afternoon of work in their own driveway.

Exhaust Header
The original forward exhaust system of a Geo Storm is a cast iron manifold and down pipe. Both were designed for maximum durability. Actually, the manifold is rather extravagant for an economy car, pairing four exhaust ports to two, which then merge again into a single pipe in the down pipe. This is called Tri-Y, because there are three Y connections, and it provides a wider power band with better torque across a wider range of engine speed. A lot of much more expensive and much more popular cars in the same market segment do not incorporate such an expensive design element in their exhaust systems, showing that Isuzu made the effort to get the most performance out of the stock vehicle. But the manifold and down pipe were designed for quiet operation, and are undersized for maximum performance.
Isuzuperformance published a second comparison, this time a header dyno comparison test covering Tri-Y vs. 4-into-1 and pipe diameter for the collector pipe. The comparison included the Isuzuperformance header and the Pacesetter header and showed Tri-Y design and 2 ½ inch diameter provided not only a wider torque curve, but also provided higher peak power. The Pacesetter header actually lost power compared to the factory manifold and down pipe, and the quality and fit were so low that it leaked, immediately burned off the black painted finish, and rusted.
Isuzuperformance is the only company to have conducted extensive dyno testing of headers for Geo Storm and Isuzu cars, and published those findings on their website. Those results remain the most cited documents in discussions concerning engine power and appropriate design and pipe size, for not only Isuzu and Geo Storm, but for all small and compact car groups and forums.

Isuzuperformance
Isuzuperformance is the only company ever to offer a true, high quality exhaust header for Geo Strom, built for maximum power delivery. The header featured the same heavy wall, mandrel bend pipe, waterjet cut flanges, full emissions compliant with fittings, a high quality, spiral wound flex pipe, and Tri-Coat multi layer, high temperature, ceramic finish. Fit and finish were again top quality, and installation easy for the owner, in an afternoon of work in their driveway.

EGR Valve Delete or Block Off – A Bad Idea
The Exhaust Gas Recirculation Valve diverts a small amount of spent exhaust gasses, back into the intake system, via the EGR pipe. The EGR valve and its control system, prevent these spent gasses from entering the intake system at idle speed, and allow the spent gasses to enter the intake system only above idle speed and when the engine is under load. The purpose is to reduce combustion chamber temperatures by about 500 degrees, which reduces the formation of nitrogen oxide (NOx).
The punch line in this description is the phrase “reduce combustion chamber temperatures”. Combustion chamber temperature determines engine operating temperature, and a device that reduces the combustion chamber temperature also reduces the engine operating temperature and the demand on the cooling system, and the need for a larger, heavier cooling system. This is one of those rare cases where the environmental protection requirement actually benefits the function of the vehicle. The vehicle manufacturer sized the cooling system based on the demands of an engine with a properly functioning EGR valve. A failed or disabled EGR valve will result in higher engine temperatures, typically above the redundancy built into the cooling system. The two things to keep in mind when considering this are:
• No one has ever measured a power improvement in the Geo Storm or its Isuzu sister cars by blocking off or disabling the EGR valve.
• Those who have blocked off or disabled the EGR valve in a Geo Storm or its Isuzu sister cars, also complain about engine overheating problems.
For those who insist upon this, usually based primarily upon hatred and mistrust of any and all emissions control devices, the following are suggested:
• Upgrade to a larger radiator to provide additional cooling.
• Consider any minute, measured performance gain on the dyno (if any is found), against the performance lost by adding 10-20 pounds of water and radiator to the weight of the vehicle.

High Flow Cat
The catalytic converter is often overlooked, and becomes the bottleneck in the system when this is the case. The original catalytic converter is good quality and flows well, but the inlet and outlet are matched to the factory exhaust pipe size which is 2 inch. Put a 2 ½ inch collector pipe in front of it, and a 2 ½ inch cat back exhaust behind it, and the catalytic converter is literally the bottleneck in the path of flow.
A good quality converter is made with flow passages through the catalyst brick that match the pipe size, in order to minimize turbulence.

Isuzuperformance
The Isuzuperformance high flow cat uses a Magnaflow/Carsound brand converter with Isuzu pattern flanges for bolt on installation. The improve flow compared to the stock cat is due to the larger size, and there really is no measurable power improvement when comparing the high flow car to a straight pipe or test pipe. So in this case, complying with the law involves no sacrifice in performance.


Hollowing Out the Stock Cat - A Bad Idea
One thing to avoid is hollowing out the original converter. The large, open cavity within the converter will lead to turbulence in the exhaust flow. This usually causes harmonic disturbances, or hiccups as the engine passes specific speeds, and looses power for a split second as the wave reflects back into the engine. And the inlet and outlet of a hollowed out factory converter will still be undersized to match the larger header collector and performance cat back on each side of it. If a high flow converter is not an option, then a straight pipe, matching the size of the collector and cat back pipes, is really the only alternative.

Click here to return to the Geo Storm Owners Page Tuning Index.