Back Pressure: Facts, Fiction, Myths, Rumors, and Lies

[Vettezuki]

That's it. I can't take it anymore. Everyone state your case if you think engine back pressure IS or IS NOT good (i.e., desirable). Qualify your assertion with details, context, quantification, etc. Links to actual studies and engineering docs would be nice. There seem to be roughly two schools on this:

A: It's all crap. You don't ever want engine back pressure. Headers exist to create a scavenging effect, not back pressure. You want as little as possible to keep the "air pump" (a.k.a., engine) flowing as efficiently as possible.

B: Some back pressure is necessary. Not sure why, something to due with cylinder pressures, and therefore torque/power. The most typical case sited is small import engines that LOSE power when big exhaust systems are put on them.

[BRUTAL64]

That's it. I can't take it anymore. Everyone state your case if you think engine back pressure IS or IS NOT good (i.e., desirable). Qualify your assertion with details, context, quantification, etc. Links to actual studies and engineering docs would be nice. There seem to be roughly two schools on this:

A: It's all crap. You don't ever want engine back pressure. Headers exist to create a scavenging effect, not back pressure. You want as little as possible to keep the "air pump" (a.k.a., engine) flowing as efficiently as possible.

B: Some back pressure is necessary. Not sure why, something to due with cylinder pressures, and therefore torque/power. The most typical case sited is small import engines that LOSE power when big exhaust systems are put on them.


A) that is it--no back pressure more power. End of story.:drink::pot_stir:

B) over scavenging can effect power--but no back pressure makes more power than any amount of back pressure.:lhdevil:

[94cobra69ss396]

My answer is B and I found this http://www.custom-car.us/exhaust/default.aspx to back it up. I don't believe you need back pressure as it is a sign of a restriction but it's my opinion that you need a small enough pipe to keep up the velocity without restricting it. If you go too big then you lose velocity and therefore the scavenging effect.

[Vettezuki]

My answer is B and I found this http://www.custom-car.us/exhaust/default.aspx to back it up. I don't believe you need back pressure as it is a sign of a restriction but it's my opinion that you need a small enough pipe to keep up the velocity without restricting it. If you go too big then you lose velocity and therefore the scavenging effect.

Haven't read the article yet, but is this part of the reason for the stepped headers (smaller off the heads stepping up to a slightly larger size) on dragsters and other mega-power vehicles?

[94cobra69ss396]

Haven't read the article yet, but is this part of the reason for the stepped headers (smaller off the heads stepping up to a slightly larger size) on dragsters and other mega-power vehicles?

That would be my guess. It's also why guys that run open headers add an extention pipe that's 18-24 inches long after the collector.

[BRUTAL64]

Increasing exhaust gas velocity reduces back pressure. The faster the spent gases move away from the exhaust port the more the intake change can fill the cylinder. Yes a smaller to larger pipe (stepped) can increase velocity. Increased velocity makes less back pressure.:drink:

[enkeivette]

I'm with Glenn, before ordering parts for my stroker build I read a "how to build high performance engines" book. It states, that if an engine is tuned properly, it will always make more hp. But having back pressure can help tune the torque curve, so you might opt for manifolds in a motorhome engine over headers, get it? So for this reason I vote B.

The only exception I would say, is that heads seem to flow slightly better with short primary tubes attached. Then again, I wouldn't consider a 4" primary tube a backpressure creating restriction. It probably just acts like an extension of the port and somehow smooths the air flow to increase velocity? :huh:

So once again, talking horsepower, and not developing torque at a certain rpm, the engine will always make more hp with less back pressure. Assuming of course the engine will be tuned to run correctly in that state.

[Leedom]

I remember reading an article in MMFF (http://www.musclemustangfastfords.com/tech/mmfp_0807_2005_mustang_gt_american_racing_headers/index.html) a while back about exhaust and how bigger is not always better, but that article talked about the exhaust pulse and how headers can play a role in them (so this article does not support me thinking but does talk about the effect of the exhaust as a whole). The exhaust was more complicated than I once thought and based on the article what pretty much everyone thought. I think that eliminating back presure has a negative affect on the velocity and as a result possibly the scavenging. i think like everything pretty much in your car it is a delicate balance of the entire set-up of the eninge as to what ammount of back presure would be ideal. If straight pipes were the best then everyone would do it. I had a buddy years ago who put a stright pipe on his 4.0L ranger and said the car did not seem to have as much power and went back to a mufler.

[enkeivette]

I remember reading an article in MMFF (http://www.musclemustangfastfords.com/tech/mmfp_0807_2005_mustang_gt_american_racing_headers/index.html) a while back about exhaust and how bigger is not always better, but that article talked about the exhaust pulse and how headers can play a role in them (so this article does not support me thinking but does talk about the effect of the exhaust as a whole). The exhaust was more complicated than I once thought and based on the article what pretty much everyone thought. I think that eliminating back presure has a negative affect on the velocity and as a result possibly the scavenging. i think like everything pretty much in your car it is a delicate balance of the entire set-up of the eninge as to what ammount of back presure would be ideal. If straight pipes were the best then everyone would do it. I had a buddy years ago who put a stright pipe on his 4.0L ranger and said the car did not seem to have as much power and went back to a mufler.

What he was missing was the low end torque, especially because his intake manifold/ cam were probably tuned for low end also. But I'll bet that he made more hp at 5500 rpm.

[BRUTAL64]

It appears that the term "back pressure" is not being understood here.

What are you guys thinking back pressure is??

[enkeivette]

When the little Asian girl stands on your back on the massage table and digs in with her toes.

[94cobra69ss396]

When the little Asian girl stands on your back on the massage table and digs in with her toes.

:rolling:

[BRUTAL64]

When the little Asian girl stands on your back on the massage table and digs in with her toes.

Yep, that works.:thumbs_up:

[SeanPlunk]

When the little Asian girl stands on your back on the massage table and digs in with her toes.

Winner :laugh:

[BRUTAL64]

Ok, what do you guys think Back Pressure is??


Hint, have you ever watched the Weather Channel?:smack:

Ok, so you know we are working with 14.7 pounds of air pressure--sea level.


I could address this Back Pressure crap in the head porting class. A picture is worth 10 million words here.

[Vettezuki]

Ok, what do you guys think Back Pressure is??


Hint, have you ever watched the Weather Channel?:smack:

Ok, so you know we are working with 14.7 pounds of air pressure--sea level.


I could address this Back Pressure crap in the head porting class. A picture is worth 10 million words here.

Without looking up the actual definition I'm going to think about what I think it means and answer as follows:

If at the end of the combustion stroke, when the exhuast valve opens, and the pressure in the exhaust manifold/header is greater than in the cylinder, that's back pressure and will exhibit as a tendency to:

- create resistance against the piston as it evacuates the cylinder, clearly robbing power from the engine
- if there's overlap, and both intake and exhaust valves are open simultaneously, it could interfere with filling the cylinder with the A/F charge

The part that I struggle with is that it could also lead to somewhat higher cylinder pressures after intake (if carefully designed as OEM engines should be) but would depend on the interplay of several complicated variables (int.exh duration, overlap, valve and port size, . . ). That is, the pressure inside the cylinder at the end of intake before compression could be greater than atmospheric pressure. This would equal higher combustion pressure and therefore higher torque under some conditions. However as engine speed increases the negative effects of this "back pressure" become greater than the positive effects at lower speed. In support of this hypothesis consider that given an engine like the stock LS1, if you remove the stock manifolds and put on a nice set of equal length tuned LT headers that flow vastly better, you will LOSE some low end torque, but the increase in breathing efficiency at high rpm results in considerably higher peak power. :huh:

[BRUTAL64]

Without looking up the actual definition I'm going to think about what I think it means and answer as follows:

If at the end of the combustion stroke, when the exhuast valve opens, and the pressure in the exhaust manifold/header is greater than in the cylinder, that's back pressure and will exhibit as a tendency to:

- create resistance against the piston as it evacuates the cylinder, clearly robbing power from the engine
- if there's overlap, and both intake and exhaust valves are open simultaneously, it could interfere with filling the cylinder with the A/F charge

The part that I struggle with is that it could also lead to somewhat higher cylinder pressures after intake (if carefully designed as OEM engines should be) but would depend on the interplay of several complicated variables (int.exh duration, overlap, valve and port size, . . ). That is, the pressure inside the cylinder at the end of intake before compression could be greater than atmospheric pressure. This would equal higher combustion pressure and therefore higher torque under some conditions. However as engine speed increases the negative effects of this "back pressure" become greater than the positive effects at lower speed. In support of this hypothesis consider that given an engine like the stock LS1, if you remove the stock manifolds and put on a nice set of equal length tuned LT headers that flow vastly better, you will LOSE some low end torque, but the increase in breathing efficiency at high rpm results in considerably higher peak power. :huh:

So, all this "I don't know" crap from you and it turns out you already knew. :drink:Where you testing us??:smack:

But still your use of the term Back Pressure is a little off but for this issue, you pretty much have a grasp of the goings on in the cylinder and ports.

Like I've always said, you can keep up. That's why I enjoy talking to you.:thumbs_up::thumbs_up:

[Leedom]

Gotta love Wikipedia! (http://en.wikipedia.org/wiki/Back_pressure)

[BRUTAL64]

Gotta love Wikipedia! (http://en.wikipedia.org/wiki/Back_pressure)

Yea, they, pretty much, have a grasp of what Back Pressure is.:thumbs_up:

[BADDASSC6]

I voted b only because A is an absolute and therefore incorrect.

But since this is a performance car forums I would go with A.

[enkeivette]

I like Ben's definition, except for where he said the pressure in the exhaust is greater than that in the cylinder. If this were true, your exhaust wouldn't flow out.

I would say, as the exhaust gas flows into the exhaust chamber, and the chamber is unable to accomodate the full amount of gas without offering resistance (as the gas is being compressed in the exhaust) that is back pressure.

In other words, a 4 year old kid doesn't exert more force than Stone Cold Steve Austin... but he does exert some force on him.

[Vettezuki]

I like Ben's definition, except for where he said the pressure in the exhaust is greater than that in the cylinder. If this were true, your exhaust wouldn't flow out. . . .

Sure it would. It flows out to the atmosphere where the pressure is lower and some back into the cylinder where the pressure is lower. Imagine pressurizing a tube of air. Now open a vent on both ends. Pressure "normalizes" from both ends.

[BRUTAL64]

I like Ben's definition, except for where he said the pressure in the exhaust is greater than that in the cylinder. If this were true, your exhaust wouldn't flow out.

I would say, as the exhaust gas flows into the exhaust chamber, and the chamber is unable to accomodate the full amount of gas without offering resistance (as the gas is being compressed in the exhaust) that is back pressure.

In other words, a 4 year old kid doesn't exert more force than Stone Cold Steve Austin... but he does exert some force on him.

Very good Adam!!:drink:
You have it. People are mistaking Back Pressure with resistance to flow.

Scavenging is something that has to be addressed- that is the final factor.


A balancing act is what is needed with scavenging. Too much and you lose low and mid range. Not enough and the top end suffers. :drink:

[BRUTAL64]

Sure it would. It flows out to the atmosphere where the pressure is lower and some back into the cylinder where the pressure is lower. Imagine pressurizing a tube of air. Now open a vent on both ends. Pressure "normalizes" from both ends.

What you have is hot expanding gas and a piston trying to push the spent gases out a little hole- resistance to flow. So you use scavenging to PULL the exhaust out of the cylinder. Very simple.:drink:

[BRUTAL64]

Ben, do you have your answer now?

[Throttle Crazy]

I am going to input my $.02 on this one. Any guy who has ever been BLUE BALLED knows exactly what back pressure is!:nuts:
On a more serious note. Back pressure is created when the volume of exhaust gas leaving the combustion chambers,exceeds the capability of the exhaust system to release exhaust gas. This lack of ability to release exhaust gas can take place at any point in the exhaust system (normally cat.). If the exhaust system is not capable of releasing that volume of air before the next exhaust valve opening, then the exhaust system will continue to bulid "back pressure". Most production exhaust systems will maintain around 2 lbs to 3 lbs of pressure on acceleration. Anything over about 4 lbs is excessive. If there is excessive restriction in the exhaust system then the "back pressure" will continue to bulid until it equalizes with the pressure in the cylinder putting back pressure against the piston. When the intake valve opens this pressure will release against the volume of fresh air trying to be drawn (normally aspirated) or forced (super charger, turbo) into the cylinder, reducing the engines ability to pull fresh air into the cylinder and affecting engine vaccum. This is why a restricted exhaust can cause a mass air flow sensor code to set. The computer sees throttle angle and engine load but not enough air being drawn into the engine so it thinks the maf is not working properly.

Scavenging takes place when an exhaust system is tuned to create a negative pressure back pulse in the exhaust system. this negetive pressure creates a vaccum to draw the inert gas from the combustion chamber,while at the same time drawing fresh air in through the intake valve on overlap for a few deg. before top dead center. I believe that a very slight exhaust back pressure in the exhaust system (tuned exhaust) between exhaust valve openings creates a vaccum in the exhaust sytem which helps draw exhaust out of the combustion chamber. As the exhaust pressure is leaving the exhaust system it creates a venturi vaccum effect which draws exhaust gas from the combustion chamber.


Thats my $.02.

[Vettezuki]

I am going to input my $.02 on this one. Any guy who has ever been BLUE BALLED knows exactly what back pressure is!:nuts:
. . .

Most production exhaust systems will maintain around 2 lbs to 3 lbs of pressure on acceleration.

A ha! That's why I get blue balls while trying to accelerate in a production car.

This has been instructive. Is there a way to measure back pressure for the common man or is that only a big fancy OEM task?

With a ported TB, LS6 intake moderate cam, high flowing heads, and stock manifolds and cats, I must have quite a bit of BP. :huh:

[Throttle Crazy]

Testing exhaust back pressure is simple. remove the pre cat. oxygen sensor from the pipe or exhaust manifold and adapt in a vaccum/pressure guage. Start the engine and give it a couple of good revs while watching the guage. Look to see if the guage goes over 4psi. I have seen a plugged cat go as high as 30psi. normally a restricted cat. ends up around 10 to 15 psi. Twice in the 17 years as a tech I have seen an exhaust so
plugged the car would not start. Most tool trucks sell an exhaust back pressure tester kit with all the adapters needed to perform the test. Your car may build some back pressure but if it was excessive it would be low on power. Of course with a properly tuned exhaust I am sure there is a lot of power to be gained.

[Vettezuki]

Testing exhaust back pressure is simple. remove the pre cat. oxygen sensor from the pipe or exhaust manifold and adapt in a vaccum/pressure guage. Start the engine and give it a couple of good revs while watching the guage. Look to see if the guage goes over 4psi. I have seen a plugged cat go as high as 30psi. normally a restricted cat. ends up around 10 to 15 psi. Twice in the 17 years as a tech I have seen an exhaust so
plugged the car would not start. Most tool trucks sell an exhaust back pressure tester kit with all the adapters needed to perform the test. Your car may build some back pressure but if it was excessive it would be low on power. Of course with a properly tuned exhaust I am sure there is a lot of power to be gained.

Thanks for the advice. I'm galdurned curious.

[BRUTAL64]

People are mistaking Back Pressure with resistance to flow.
If you want to use that term, not a problem. I guess it is just easier to say "Back Pressure" than resistance to flow.



Back pressure (resistance to flow) is bad. Yery simple.

Maybe the "Back Pressure" people are talking about is the 14.7 lbs at sea level.

[Throttle Crazy]

Actually "resistance to flow" would be considered the cause and back pressure would
be the result. Take for example a garden hose with no nozzle on the end, when it is turned on it will flow at line pressure. When you put the nozzle on the end of the hose you have just caused a resistance to flow and there for created a back pressure in the hose. Make sense? Your exhaust works the same way. If you have a resistance to flow exhaust then you will cause back pressure in the system prior to the point of that resistiction. After the restriction you will have a low pressure. If you think about how an a/c system works this principal also applies. The compressor outputs a high pressure gas which travels to the condensor. The condensor condenses the gas to a high pressure liquid. Then that high pressure liquid flows through a restriction known as the orfice tube where it is converted to a low pressure liquid on the other side of the resitriction. From there it travels through the evaporator where it is converted to a low pressure gas and sucked back into the the compressor again. The restriction is what creates that high and low pressure with the help of the compressor.

[Vettezuki]

Actually "resistance to flow" would be considered the cause and back pressure would
be the result. Take for example a garden hose with no nozzle on the end, when it is turned on it will flow at line pressure. When you put the nozzle on the end of the hose you have just caused a resistance to flow and there for created a back pressure in the hose. Make sense? Your exhaust works the same way. If you have a resistance to flow exhaust then you will cause back pressure in the system prior to the point of that resistiction. After the restriction you will have a low pressure. If you think about how an a/c system works this principal also applies. The compressor outputs a high pressure gas which travels to the condensor. The condensor condenses the gas to a high pressure liquid. Then that high pressure liquid flows through a restriction known as the orfice tube where it is converted to a low pressure liquid on the other side of the resitriction. From there it travels through the evaporator where it is converted to a low pressure gas and sucked back into the the compressor again. The restriction is what creates that high and low pressure with the help of the compressor.

This is definitely how I was thinking about it.

[BRUTAL64]

This is definitely how I was thinking about it.

So do you still think any "back pressure" is good for exhaust?
Yes, it's good for AC, but we are talking exhaust.

I still believe BACK PRESSURE is the wrong term -- Kinda like Harmonic Balancer.
It's really Harmonic Dampener. But for the sake of discussion I'll use the term Back Pressure.

So, again, wasn't your question something about "Back Pressure" being good for some engines?

[Throttle Crazy]

Here is my final answer. Back pressure should remain as close to zero as possible while maintaining the highest velosity of exhaust flow. It depends on where your engine will benifit from the power the most. If your engine is a low rpm engine then you need smaller exhaust diameter to maintain high velocity in the mid range. If you put to large of exhaust you slow down the velocity in that range. If you have a motor that makes big power at 7,500 then you need large exhaust to maintain that close to zero back pressure and maintain higher velocity of air flow in the exhaust. The "bigger the better" is not the key to a tuned exhaust. SO THE ANSWER SIMPLIFIED. YES, BUT AS CLOSE TO ZERO AS POSSIBLE IN THE RPM RANGE YOUR ENGINE WILL BENIFIT THE MOST!!!

[Vettezuki]

Here is my final answer. Back pressure should remain as close to zero as possible while maintaining the highest velosity of exhaust flow. It depends on where your engine will benifit from the power the most. If your engine is a low rpm engine then you need smaller exhaust diameter to maintain high velocity in the mid range. If you put to large of exhaust you slow down the velocity in that range. If you have a motor that makes big power at 7,500 then you need large exhaust to maintain that close to zero back pressure and maintain higher velocity of air flow in the exhaust. The "bigger the better" is not the key to a tuned exhaust. SO THE ANSWER SIMPLIFIED. YES, BUT AS CLOSE TO ZERO AS POSSIBLE IN THE RPM RANGE YOUR ENGINE WILL BENIFIT THE MOST!!!

This I understand.

BRUTAL,
It seems back pressure is non desirable in any case. The big diameter exhaust problem typical on small import engines (when guys just put on big exhaust) is not because of reduced back pressure, but because of reduced exhaust velocity. It think you said more or less the same thing referring to scavenging.

TC,
If the intersection of low BP (close to zero as possible) and high exhuast velocity in typical operating range is most desirable, then why do most OEM engines have that low amount of BP at cruise/off idle? I guess that's just the most optimum position they can engineer for the total system, but it has nothing to do with "utilizing" back pressure.

My guess that back pressure in small amounts may be desirable because it may increase cylinder pressure and therefore torque seems to be wrong. There is never a benefit to back pressure (talking just about the efficiency of a NA engine). BP is just a consequence of other design elements in the system, it is not intentionally designed into the system.

:huh:

[BRUTAL64]

Here is my final answer. Back pressure should remain as close to zero as possible while maintaining the highest velosity of exhaust flow. It depends on where your engine will benifit from the power the most. If your engine is a low rpm engine then you need smaller exhaust diameter to maintain high velocity in the mid range. If you put to large of exhaust you slow down the velocity in that range. If you have a motor that makes big power at 7,500 then you need large exhaust to maintain that close to zero back pressure and maintain higher velocity of air flow in the exhaust. The "bigger the better" is not the key to a tuned exhaust. SO THE ANSWER SIMPLIFIED. YES, BUT AS CLOSE TO ZERO AS POSSIBLE IN THE RPM RANGE YOUR ENGINE WILL BENIFIT THE MOST!!!

You don't have to convince me, I already know the answer.:p


It's Vettezuki that needs to know.:leaving:

[BRUTAL64]

This I understand.

BRUTAL,
It seems back pressure is non desirable in any case. The big diameter exhaust problem typical on small import engines (when guys just put on big exhaust) is not because of reduced back pressure, but because of reduced exhaust velocity. It think you said more or less the same thing referring to scavenging.

TC,
If the intersection of low BP (close to zero as possible) and high exhuast velocity in typical operating range is most desirable, then why do most OEM engines have that low amount of BP at cruise/off idle? I guess that's just the most optimum position they can engineer for the total system, but it has nothing to do with "utilizing" back pressure.

My guess that back pressure in small amounts may be desirable because it may increase cylinder pressure and therefore torque seems to be wrong. There is never a benefit to back pressure (talking just about the efficiency of a NA engine). BP is just a consequence of other design elements in the system, it is not intentionally designed into the system.

:huh:

Scavenging is the adjustable part of the issue. You want no Back Pressure and you use scavenging to pull the exhaust gas out of the cylinder. The amount of scavenging is adjusted from a little to a lot depending on the flow of the intake charge and exhaust port. You adjust the scavenging ( like adding a 30 inch pipe to lenghten the collector to give more mid range torque) to fit the engine.

I don't like the term Back Pressure.

Throttle Crazy--"Actually "resistance to flow" would be considered the cause and back pressure would be the result"---This is as close to correct as we are going to get.:judge:


You want no "Back Pressure".

[Throttle Crazy]

This I understand.

BRUTAL,
It seems back pressure is non desirable in any case. The big diameter exhaust problem typical on small import engines (when guys just put on big exhaust) is not because of reduced back pressure, but because of reduced exhaust velocity. It think you said more or less the same thing referring to scavenging.

TC,
If the intersection of low BP (close to zero as possible) and high exhuast velocity in typical operating range is most desirable, then why do most OEM engines have that low amount of BP at cruise/off idle? I guess that's just the most optimum position they can engineer for the total system, but it has nothing to do with "utilizing" back pressure.

My guess that back pressure in small amounts may be desirable because it may increase cylinder pressure and therefore torque seems to be wrong. There is never a benefit to back pressure (talking just about the efficiency of a NA engine). BP is just a consequence of other design elements in the system, it is not intentionally designed into the system.

:huh:
Bingo!! Back pressure is used to determine if there is a restriction in the exhaust system. Also bp is a result of emissions equipment and exhaust tuning. If you have an oem engine that makes good torque numbers at 2,200 and good hp up to 5,800 the exhaust needs to be tuned to create a balance. If the exhaust is to large then you loose the velocity at low rpm. If the exhaust is to small you have a "resistance to flow" which creates back pressure in the high rpm.

[Vettezuki]

Scavenging is the adjustable part of the issue. You want no Back Pressure and you use scavenging to pull the exhaust gas out of the cylinder. The amount of scavenging is adjusted from a little to a lot depending on the flow of the intake charge and exhaust port. You adjust the scavenging ( like adding a 30 inch pipe to lenghten the collector to give more mid range torque) to fit the engine.

I don't like the term Back Pressure.

Throttle Crazy--"Actually "resistance to flow" would be considered the cause and back pressure would be the result"---This is as close to correct as we are going to get.:judge:


You want no "Back Pressure".

Crazy ass tangent.

We know that some (super fancy race) engines utilize variable length intake runners. Most modern super bikes use this technology. Has there ever been an engine with variable length exhaust?

[BRUTAL64]

Crazy ass tangent.

We know that some (super fancy race) engines utilize variable length intake runners. Most modern super bikes use this technology. Has there ever been an engine with variable length exhaust?

The only ones I've ever seen is the 180 degree exhaust systems. :D But I think that was to due to having to run header tubes all over and under the engine.:rolleyes2::drink:

[Vettezuki]

The only ones I've ever seen is the 180 degree exhaust systems. :D But I think that was to due to having to run header tubes all over and under the engine.:rolleyes2::drink:


As in they actually mechanically change length over the rpm range. For instance longer to shorter on intake.

[Throttle Crazy]

Crazy ass tangent.

We know that some (super fancy race) engines utilize variable length intake runners. Most modern super bikes use this technology. Has there ever been an engine with variable length exhaust?

I do not know about variable length, but I do know of late model ferrari and the corvette z06 have computer controled exhaust flaps to tune the velosity
at different rpms and load conditions.

[BRUTAL64]

As in they actually mechanically change length over the rpm range. For instance longer to shorter on intake.

No, haven't seen anything like that. There might be something to that. Instead of variable timing, variable exhaust length.:thumbs_up:

[BRUTAL64]

I do not know about variable length, but I do know of late model ferrari and the corvette z06 have computer controled exhaust flaps to tune the velosity
at different rpms and load conditions.

That makes sense. Actually "resistance to flow" could be adjusted that way.:drink:

[94cobra69ss396]

I do not know about variable length, but I do know of late model ferrari and the corvette z06 have computer controled exhaust flaps to tune the velosity
at different rpms and load conditions.

Yah, on the Z06 it's worth 6HP. :laugh: On a side note I don't think that OEM's take into account performance when they design an exhaust for new cars. I think it is more a factor of what will fit and emissions. That's why you can gain so much HP by upgrading the exhaust on some cars such as the 03/04 Cobras.

[Vettezuki]

Yah, on the Z06 it's worth 6HP. :laugh: On a side note I don't think that OEM's take into account performance when they design an exhaust for new cars. I think it is more a factor of what will fit and emissions. That's why you can gain so much HP by upgrading the exhaust on some cars such as the 03/04 Cobras.

I'm goddmaned curious what my "BP" is now. . .

I know my exact motor with headers, but with a much less high flowing intake (LS1, I have an LS6) was making 432WHP in a C5 on an aggressive tune. I'm making 391 with stock headers and a very conservative tune (18 degrees total as I recall) in my C3. :huh:

[Throttle Crazy]

I'm goddmaned curious what my "BP" is now. . .

I know my exact motor with headers, but with a much less high flowing intake (LS1, I have an LS6) was making 432WHP in a C5 on an aggressive tune. I'm making 391 with stock headers and a very conservative tune (18 degrees total as I recall) in my C3. :huh:I have a back pressure tester if you would like to borrow it.

[Vettezuki]

I have a back pressure tester if you would like to borrow it.

Please and thank you. Do you live close to your brother? Maybe I can just come out and use it on the spot?

[Throttle Crazy]

Please and thank you. Do you live close to your brother? Maybe I can just come out and use it on the spot?

Yes, within 5 miles. I can just leave it with my brother.