Can somone explain to me the theory of Exhaust Back Pressure. 2.5 vs. 3in? Does a suppercharged car want more or less back pressure? Does a N/A car want more or less back pressure? How does it affects Tourqe and preformance in general?

Please don't give oppinians, I just want to know the strait facts from somone who knows a lot about this toppic

Thanks a lot for the help

 

I'm not an expert by any means, but whoever wrote this article seems to know their stuff.

 

Thanks,
So what I got from that is that bigger exhaust is better for a S/C engine... Correct?...

 

What I gathered is that a f/i car should run a larger exhaust than an n/a for similar engine sizes, but in the end, bigger is not always best.

 

It's not just backpressure but also exhaust velocity. Boosted vehicles use bigger exhaust systems because the exhaust velocity is much greater and back pressure is less needed.

 

LSJ = 134ci

 

bigger is better to a point, you need a certain amount to keep your torque constant. a lot of people putting a 3" on a stock car are seeing their torque curve fall off. it has something to do with the exhaust not getting hot enough to scavenge the gases like it should or something, so the gases start to cavitate and don't move like they should. a 2.5" exhaust seems to be good up to about 320hp or so.

turbos seem to react better to completely free-flowing exhausts, IMO because they don't seem to rely so much on the exhaust as the turbine in the exhaust half of the turbo itself for resistance.

just my $0.02 FWIW.

 

Ummmmmmmmmmmmm... no, the LSJ is not 134ci. The 2.2l in the base Bolt is 134ci. The LSJ is 122ci.

 

so in the end... would you use a 2.5 or 3 on the LSJ(stock)?, LSJ STG2?

 

I think it is a Flip of a coin really.... I am Contemplating the same thing you are but I am getting stage 4 but I don't know if I want a 3" exhaust around here just because the cops are DICK'S

 

2 1/2" is fine.

 

2.5"

 

Thanks

 

Like the guys said, 2.5 is adequate. If you plan on going up over 350 HP then go 3".

 

Not sure about that HP # but I think the whole 2.5" vs. 3" comes from looking at turbocharged vehicles.

The reason guys with turbochargers get such big exhaust systems is because the less backpressure they have, the faster and easier their turbines spin inside their turbochargers. Other ways of achieving this besides getting an overall big exhaust system is by doing an exhaust cut out (basically makes you have a huge exhaust leak) or running open downpiped (like I'm doing currently ).