Since I am not one to just listen to the mainstream and I always wondered about the need of 3″ exhaust on our 4 cyl engines I decided to do a little scientific research. The main measure of exhaust system effectiveness is Back-pressure. Too much back-pressure hurts turbine efficiency and affects the power output and Knock sensitivity. Noise is also a factor but not my main concern. Read on to find my suprising results…
My test setup was on my stage 4+ Viggen. I used a Fluke digital pressure gauge and my Fluke meter to record maximum pressure readings on the same test loop driven at full throttle both at peak toque and horsepower. I started backwards and tested the already installed JT3″ complete system with JT racecat and 1 muffler. The maximum readings were very high in front of the cat 21.3psi or 1.5bar Way too high! Next to locate the restriction I moved the pressure pickup to behind the catalyst. The problem was immediately apparent, peak readings of 4.4psi or 0.3 bar. Now to find out why. First step to take the system off the car. Even off the car you cannot look in and see the catalyst core so I had to cut it apart. Here are a couple images.
Expecting it to be visibly damage. It appears fine! I plan to cut it further to inspect. Now with no exhaust system on the car it was time to dig into my collection and get the Original Saab accessory remus catback system with straight through center pipe. After adding a few extra sensor fittings to one of my off the shelf stainless downpipes with larger metal core catalyst. The downpipe is 3 inch tubing and reduces to stock 2-3/8 (or so) to mate to the original flex pipe. Knowing the books say this is fine for my ~340 hp but “they” say I need 3inch… In the same test conditions the peak pressure before the catalyst was now only 6.9 lbs or 0.47bar I added the wideband and pressure ports but forgot the pressure port after the cat.. The sound is nicer too.
This system stays on the car for now. The difference in the power and drivability is huge! So this is the extent for now. Off to the Dyno tomorrow. My plan is to collect more data from stock exhaust and a few other combinations as time permits. Hope this is as interesting to you as it is to me. Gallery of related Images