Jacko

2nd May 2002, 13:21

Hello Techies,

in light of the numerous headers threads that are bouncing about, I thought I would ask two questions on a proper BBS. Here goes.....

1/ Is there any performance benefit from running equal length runners/headers?

To my knowledge, very short runners/headers etc will consume less exhaust energy than the long equal length runners (not good for turbine efficiency). Equal length runners are best suited to naturally aspirated engines where you utilise the runner lengths and inner diameters to induce ramming/scavenging, so therefore tuned to give torque benefits at a desired rpm and cam phasing. The general rule is, long runners for low rpm, short runners for high rpm, therefore there must be minimal benefit on the Impreza using equal length runners, may be even bad due the extra energy consumed??

2/ Is a restriction/backpressure post turbo good for torque?

As far as backpressure is concerned, once the gases have met the turbocharger, the remaining exhaust energy is consumed and the gases from the turbine outlet wish to find the quickest exit. Surely any backpressure will hinder this!

If this theory is correct then surely a 3" exhaust system will be better than a 2.5" system.

I always assumed that a 2.5" system would generate higher gas velocities Vs a 3.0" system, this would give better torque but not be able to flow as much gases at higher rpm. However on reflection, once again...this probably only applies to naturally aspirated engines not on Turbocharged.

Your thoughts would be appreciated.....

Jacko.

[This message has been edited by Jacko (edited 02 May 2002).]

in light of the numerous headers threads that are bouncing about, I thought I would ask two questions on a proper BBS. Here goes.....

1/ Is there any performance benefit from running equal length runners/headers?

To my knowledge, very short runners/headers etc will consume less exhaust energy than the long equal length runners (not good for turbine efficiency). Equal length runners are best suited to naturally aspirated engines where you utilise the runner lengths and inner diameters to induce ramming/scavenging, so therefore tuned to give torque benefits at a desired rpm and cam phasing. The general rule is, long runners for low rpm, short runners for high rpm, therefore there must be minimal benefit on the Impreza using equal length runners, may be even bad due the extra energy consumed??

2/ Is a restriction/backpressure post turbo good for torque?

As far as backpressure is concerned, once the gases have met the turbocharger, the remaining exhaust energy is consumed and the gases from the turbine outlet wish to find the quickest exit. Surely any backpressure will hinder this!

If this theory is correct then surely a 3" exhaust system will be better than a 2.5" system.

I always assumed that a 2.5" system would generate higher gas velocities Vs a 3.0" system, this would give better torque but not be able to flow as much gases at higher rpm. However on reflection, once again...this probably only applies to naturally aspirated engines not on Turbocharged.

Your thoughts would be appreciated.....

Jacko.

[This message has been edited by Jacko (edited 02 May 2002).]