Backpressure stuff
Registered User
iTrader: (12)
Joined: Nov 2002
Posts: 18,369
From: Reno, NV
Car Info: 1993/2000/2001 GF4 mostly red
The intake is not helping either. The MAF sensor is sensitive to changes and I would be willing to bet the car is running quite rich at the stumbling points.
Thread Starter
Registered User
iTrader: (2)
Joined: Jul 2004
Posts: 1,659
From: USA
Car Info: 1990 thing
Originally Posted by BAN SUVS
No, he was saying that a certain amount of backpressure is good. But if your car still has stock headers and the stock cats, then the catback doesn't do anything but make noise really. What year is your car and what intake do you have? That's the more likely culprit, barring a mechanical issue like bad wires or coils.
Registered User
Joined: Aug 2004
Posts: 69
yes you are very right when it starts to stumble or even when it doesnt at that point i can see teh black cloud behind me telling me that it is a very rich burn. what suggestions if any do you guys have that could fix my problem thanx
ride4life32
ride4life32
VIP Member
Joined: Mar 2003
Posts: 2,023
From: Phoenix, AZ
Car Info: Subaru Impreza WRX 2002
Originally Posted by jdepould
that's exactly the type of thinking I'm trying to dispell. Backpressure is bad, period. When you open up the exhaust too much, you lose exhaust velocity, and lose power, which is entirely separate from backpressure.
Registered User
iTrader: (12)
Joined: Nov 2002
Posts: 18,369
From: Reno, NV
Car Info: 1993/2000/2001 GF4 mostly red
In a normally aspirated car, I believe that's correct meileres, although I've never studied the thermodynamics to find out for sure. In a turbo car, however, exhaust gas velocity is what drives the turbine, so any loss of that through larger manifold runners and uppipe results in lower boost.
Thread Starter
Registered User
iTrader: (2)
Joined: Jul 2004
Posts: 1,659
From: USA
Car Info: 1990 thing
Originally Posted by meilers
What you are talking about is technically called "scavenging," correct? The idea is to have enough pressure to create pockets of lower pressure in the exhaust pipe that help to pull the gases through the pipe and out the end. Too large of a diameter of pipe allows for turbulence, which is exhaust gases flowing over themselves and not straight out the pipe. Is this the idea?
Registered User
iTrader: (6)
Joined: Mar 2004
Posts: 1,139
From: San Jose, CA
Car Info: '06 RBP Impreza 2.5i
Originally Posted by BAN SUVS
I agree with your post, although I would say that the ideal size of an NA 4 cylinder exhaust varies with the displacement of the motor. 2.25" works GREAT on EJ25s, and 2.5" is good too. On a ~2.0 liter motor, 2" is probably best.
Registered User
Joined: Nov 2002
Posts: 56
Originally Posted by BAN SUVS
I agree with your post, although I would say that the ideal size of an NA 4 cylinder exhaust varies with the displacement of the motor. 2.25" works GREAT on EJ25s, and 2.5" is good too. On a ~2.0 liter motor, 2" is probably best.
We can't make the generalization of thinking that certain engine sizes go with certain exhaust piping size. It all depends on the air flow capacity of that engine. A Honda S2000 probably wouldn't do best with a 2" exhaust.
Piping size doesn't matter on a turbocharged car since exhaust velocity isn't an issue since the header runs into the turbo itself (Uppipe). The only piping size you'd be concerned with here is the uppipe.