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Dyno Cell: Is Bigger Really Better?

Thermal Research & Development 3-inch versus 2-1/4-inch Honda/Acura Exhaust

Robert Choo
Aug 8, 2002

From the title, you'd think we are talking about human anatomy, but since our company recently went through sensitivity training, that subject is off-limits. The real question we are asking is does a larger diameter exhaust piping make a difference in horsepower? We are constantly surrounded by overzealous enthusiasts trying to stuff the largest diameter wheel under a fenderwell or an oversized turbo in the engine compartment, but when does it become too big. Can exhaust piping diameter affect performance? This is the question we seek to answer in this Dyno Cell article.

Aftermarket turbo kits are hot commodities. With the promise of V8 power and I-4 economy, it is easy to see why. Factor in the downfall of factory turbocharged offerings and the bloodlust of power-craving enthusiasts, and the popularity of pressure is even more clear to see. Today, turbo boosts for Civic, Integra, Prelude, Eclipse, Miata, 240SX, Accord, CRX and many more can be readily bolted on.

Many have made the mistake of adding a forced induction kit without upgrading the exhaust system. Even though most enthusiasts will already have installed an exhaust system prior to the turbo kit, the exhaust system that was installed was probably not developed to handle the exhaust flow characteristics of a turbo engine, which means useable power is being left on the table.

It is an obvious fact that the exhaust system on a 1.5-liter Civic is considerably smaller in piping size and muffler section than a Supra twin-turbo. The most obvious reasons are displacement and the power output of the two engines. One produces an anemic 80-plus hp, while the other pumps out a staggering 320 hp. It's simple science; as horsepower output increases, more air and fuel are burned, which means more exhaust gases will be produced. This same theory can be applied to vehicles utilizing a bolt-on turbo kit. For example, a vehicle that has been modified with a turbo kit but still utilizing a normally aspirated exhaust can benefit from a purpose-built turbo exhaust system with larger diameter piping, which decreases exhaust backpressure and eliminates pumping losses. A properly sized exhaust system can produce more horsepower and increase turbo response.

To illustrate this point, we called on Thermal Research & Development out of Canoga Park, Calif. The company has realized a need for performance turbo exhaust systems for Hondas and Acuras, and has been working closely with top import racers to design and develop the perfect exhaust system for both the street and the strip. Extensive R&D efforts have resulted in the development of a complete line of 3-inch mandrel-bent, stainless-steel exhaust systems specifically for turbocharged Honda powerplants. By effectively sizing the piping diameter and muffler flow characteristics, the Thermal system allows the engine to produce its full potential. Thermal offers both 2 1/4-inch and 3-inch exhaust systems for both normally aspirated and turbo engines. The 2 1/4-inch system is a bolt-on affair and does not require any modifications, but the 3-inch system requires the mounting of an extra 3-inch flange to the catalytic converter.

Our tester was a newly rebuilt 1990 Acura Integra LS utilizing a custom XS Engineering turbocharger and front-mount intercooler. The engine has been fortified with 9.0:1 JE pistons and Saenz rods. The engine still utilizes the stock head and cams, and currently runs an upgraded in-tank fuel pump and boost-dependent regulator for fuel enrichment. Our baseline run at 7.0 psi of boost running the normally aspirated Thermal 2 1/4-inch exhaust system registered 189.6 hp and 165.9 lbs-ft of torque. By just bolting the Thermal 3-inch exhaust system in place, we were able to increase peak horsepower power output by 2.2 and peak torque by 2.4 lbs-ft without touching the boost controller. Peak power now checked in at 191.8 hp and 168.3 lbs-ft of torque. We were also able to see an increase in power output throughout the entire powerband with jumps of 4-5 hp. Overall, we were happy with the increase generated by simply swapping exhaust systems. We are positive that when boost pressure is turned up, there will be a more significant increase in power with the larger diameter exhaust system. We are planning to re-test the exhaust systems when we upgrade the head and cams and increase boost pressure.

Having a plan of attack is the best way to prevent excess spending. By determining which performance route you plan to take, whether it is normally aspirated or forced induction, it is wise to purchase the right exhaust for your needs the first time. And thanks to Thermal Research & Development, you can plan to boost to stratospheric levels and still have a system that muffles and allows the realization of maximum power.

By Robert Choo
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