Back pressure and exhaust discussion

I started this thread because I recently upgraded my exhaust to a full turboback 3" SS system including a 3" high flow cat and a 3" resonator. My seat of the pants feel after install was that I lost a bit of low end torque. This may just be in my head though because of the louder and different exhaust note. I've heard it can play tricks on the "seat of the pants dyno". My intention is not to create a cross posting but I didn't want discussions about torque to take away from the experience or workmanship of the system which were both great. I love this exhaust system!!!

I have done much research on exhaust systems and have talked to a couple of "muscle car guru's" at work. Every conclusion that I come up with suggests that a large pipe diameter will hamper low end torque BUT only on N/A cars. All pulsation and savenging effects and theories that come into play with N/A cars are thrown out the window with turbo cars because the back pressure is built before the pre-turbo and for the most part the flow after the turbo is fairly smooth therefore there is no "negative pressure behind an exhaust pulse that can be utilized by smaller pipe (scavenging)". I can state a couple of sources here for my research and I was going to post a new thread to discuss this in detail and I still may, but if anyone has any input on the subject I'd be excited to read others points of view.

Here are a couple of documents that I have mined from the Net that have relevance to the topic.

The first document is a posting from Jay Kavanaugh on a Subaru forum username JKav back in 2003. He claims to be a turbo systems engineer at Garrett. The Doc is in PDF format and is located HERE.

The second document is just from a random search on Google but I thought it also had relevance so I copied it as well. I don't have the author for this one but it was found on a DSM Mitsubishi forum if I'm not mistaken. The Doc is in PDF format and is located HERE.

Both of these documents explain the theories and misnomers of back pressure, exhaust pulsations, exhaust velocity and scavenging effect. Also both documents claim that with a turbo car, conventional N/A exhaust theories are thrown out the window and basically "bigger is better".

Now, all that being said, I do plan on upgrading the turbo and doing a doing a full tubing upgrade (intake, TIP, MAF, MAP, IC Pipe). All of which will benefit from the open exhaust.

Comments and theories are welcome!

I agree it does seem with FI, the bigger and straighter the exhaust, the better (for power and EGT's).

I'd also like to put the myth to bed that taking out the cat on a FI car will cause a measurable (seat of pants) loss of torque.

My car has completely straight exhaust and it didn't lose any low end.

I think we should maybe specify Turbo instead of FI (forced induction) here as a supercharger is classified as a forced induction system but would be effected by the absence of exhaust velocity and theoretically could be negatively effected by having an exhaust that is "too big" because of the loss of scavenging.

Oh... good post!
I do a test last week-end, pressure made by my cat because i think is dead.
Exhaust sytem on my car is 2" tube with original cat on my 478,000km car.
The mesure of pressure is take at the downpipe with 0-30psi gauge at full throttle, pressure number are rounded.
3psi at 2000rpm
6psi at 3000rpm
12 psi at 4000rpm!

I found this document for pressure in exhaust system.
http://www.articlearchives.com/1645176-1.html

My cat is not dead but is too restrictive for performance setup!
I order a high flow cat for remplacement.


Dieseleux

I have said this before and others have argued. Going with a larger exhaust (3" in your case) can cause your powerband to move up (EVEN ON A FORCED INDUCTION CAR!!!) It won't move near as much as a NA motor but it can still cause a shift. It usually is not a big deal because you are gaining torque and power, not a lot but I bet if you dyno before and after you will see a gain.

With that said, the largest exhaust you can fit will be the best exhaust. Of course a 5" exhaust probably will not fit and will not see any gains over a 3" exhaust. I would be surprised if you can get more than 3" in there. YES I KNOW someone on here got a larger exhaust to fit, but mass production will never work).

I plan on putting a 3" Turbo back on as well. The parts are here, just have to build it