Outer Limits 4x4 Board

my question is,

is it bad for the engine to run an exhuast with no back presure?

On an N/A engine you will find it will use more fuel and have less power and torque. On a turbo motor, post turbo, less back pressure is better.

coxy321 wrote:On an N/A engine you will find it will use more fuel and have less power and torque. On a turbo motor, post turbo, less back pressure is better.

I have to disagree here in the first part of the answer..... no back pressure is good .. however having said that, I doubt you could have an exhaust system (going the length of the vehicle) that has no b/p. While dyno results do tend to show if you have the minimum exhaust system, there is a drop in h/p and/or torque (not always the case). This is generally because normal cam/ign timing is more suited to some scavenging/back pressure..... however, if you have the same motor with cam/ign timing optimised for open exhaust it WILL make more power/torque.
With turbo...no b/p is best.

My understanding is that you are after the least amount of back pressure possible while still having a high enough velocity to create scavenging in the desired rev range. That is why some people feel they have lost torque with a really big system. It is not designed to be it's most efficient at low revs. It may make more power over all but less where you want it.

On a turbo engine you are after a pressure drop behind the turbine and have no interest in back pressure.

yeah its for an non turbo 3RZ.

i'v been told no back presure is bad for ur engine as it loses compersion in the valves and farks them after a while. i donuo realy

but what you all have said sounds right to me

RED60 wrote:

coxy321 wrote:On an N/A engine you will find it will use more fuel and have less power and torque. On a turbo motor, post turbo, less back pressure is better.

I have to disagree here in the first part of the answer..... no back pressure is good .. however having said that, I doubt you could have an exhaust system (going the length of the vehicle) that has no b/p. While dyno results do tend to show if you have the minimum exhaust system, there is a drop in h/p and/or torque (not always the case). This is generally because normal cam/ign timing is more suited to some scavenging/back pressure..... however, if you have the same motor with cam/ign timing optimised for open exhaust it WILL make more power/torque.
With turbo...no b/p is best.

Makes enough sense to me! You don't see hi-po N/A engined cars worrying about exhaust backpressure.

RED60 wrote:

coxy321 wrote:On an N/A engine you will find it will use more fuel and have less power and torque. On a turbo motor, post turbo, less back pressure is better.

I have to disagree here in the first part of the answer..... no back pressure is good .. however having said that, I doubt you could have an exhaust system (going the length of the vehicle) that has no b/p. While dyno results do tend to show if you have the minimum exhaust system, there is a drop in h/p and/or torque (not always the case). This is generally because normal cam/ign timing is more suited to some scavenging/back pressure..... however, if you have the same motor with cam/ign timing optimised for open exhaust it WILL make more power/torque.
With turbo...no b/p is best.

YES, somebody gets it. Try explaining that to an exhaust shop. Once the exhaust passes the extractors (if theyare a good set) the lower the backpressure the more the power, torque and economy once the mixture and timing has been set. Tends to be a tad noisy though.

Joel

PGS 4WD wrote:

RED60 wrote:

coxy321 wrote:On an N/A engine you will find it will use more fuel and have less power and torque. On a turbo motor, post turbo, less back pressure is better.

I have to disagree here in the first part of the answer..... no back pressure is good .. however having said that, I doubt you could have an exhaust system (going the length of the vehicle) that has no b/p. While dyno results do tend to show if you have the minimum exhaust system, there is a drop in h/p and/or torque (not always the case). This is generally because normal cam/ign timing is more suited to some scavenging/back pressure..... however, if you have the same motor with cam/ign timing optimised for open exhaust it WILL make more power/torque.
With turbo...no b/p is best.

YES, somebody gets it. Try explaining that to an exhaust shop. Once the exhaust passes the extractors (if theyare a good set) the lower the backpressure the more the power, torque and economy once the mixture and timing has been set. Tends to be a tad noisy though.

Joel

I will agree with this - once the mixture and timing has been set. Throw a larger exhaust on a n/a engine without altering fuel delivery and I believe you'll lose bottom end torque. You may make more power at the high end, but you'll lose performance off idle.

If people want to discuss "exhaust velocity" and "scavenging effects" make sure you distinguish between replacing headers/extractors (from the head down) and simply replacing the exhaust (e.g. cat back, manifolds back etc.) The headers will have the most significant effect on gas velocity out of the cylinder, & hence exhaust scavenging from the cylinder. A larger exhaust behind stock headers won't slow down the gas flow out of the exhaust port.

coxy321 wrote:

RED60 wrote:

coxy321 wrote:On an N/A engine you will find it will use more fuel and have less power and torque. On a turbo motor, post turbo, less back pressure is better.

I have to disagree here in the first part of the answer..... no back pressure is good .. however having said that, I doubt you could have an exhaust system (going the length of the vehicle) that has no b/p. While dyno results do tend to show if you have the minimum exhaust system, there is a drop in h/p and/or torque (not always the case). This is generally because normal cam/ign timing is more suited to some scavenging/back pressure..... however, if you have the same motor with cam/ign timing optimised for open exhaust it WILL make more power/torque.
With turbo...no b/p is best.

Makes enough sense to me! You don't see hi-po N/A engined cars worrying about exhaust backpressure.

But you will find they have spent a lot of time developing their headers to create optimum scavenging in the desired rev range.

-Scott- wrote: A larger exhaust behind stock headers won't slow down the gas flow out of the exhaust port.

It may also not increase the flow either. I think a lot of the power people think they have gained by their "seat of the pants" test of their cat back system on a N/A engine when it is just an illusion becaus it sounds faster.

On an N/A engine the biggest gain will be in the headers.

hilux79 wrote:

coxy321 wrote:

RED60 wrote:

coxy321 wrote:On an N/A engine you will find it will use more fuel and have less power and torque. On a turbo motor, post turbo, less back pressure is better.

I have to disagree here in the first part of the answer..... no back pressure is good .. however having said that, I doubt you could have an exhaust system (going the length of the vehicle) that has no b/p. While dyno results do tend to show if you have the minimum exhaust system, there is a drop in h/p and/or torque (not always the case). This is generally because normal cam/ign timing is more suited to some scavenging/back pressure..... however, if you have the same motor with cam/ign timing optimised for open exhaust it WILL make more power/torque.
With turbo...no b/p is best.

Makes enough sense to me! You don't see hi-po N/A engined cars worrying about exhaust backpressure.

But you will find they have spent a lot of time developing their headers to create optimum scavenging in the desired rev range.

Yes. And they don't care about off-idle torque, nor low-rpm driveability.

he lower the backpressure the more the power, torque and economy once the mixture and timing has been set

cam timing in particular.

one thing we had many years ago was sometimes the scavenging was a little to good especially in the low rpm range. it sucked out a lot of the air (and fuel) before the exhaust valve shut. either redo the cams or re tuned the extractors to be poorer in the low rpm range. the lack of scavenging keeps the air/fuel in and you get more torque and better economy while still keeping the power in the top end.
what you have to watch is some manufactures are lazy and do that as per standard. (especially on parts bin cars ie car is mix and match various bits from other models).
change the exhaust and it effectively throws the cam timing out. adjust the cam timing to suit the new exhaust and it will run sweet.
roughly put, not many are that bad.

backpressure, lower is better but you still have to have everything tuned right.