How to Determine Necessary Exhaust Pipe Size

[todd383]

well 2 years ago when i had my 08 gt500 mustang, making 670 at the wheels with factory exhaust but with hi flo cats, 2.50" pipes. i switched to 3" straight thru mufflers. everybody on the mustang sites said you will get another 30 + hp! no way! had it dyno tested. lost a couple of hp and tq. and the noise level went thru the roof! so of it came and put on stock exhaust. My challenger with  a 383  with 500 hp had 3 in. and went to 2.50 in. and it gained almost 20 ft.lb. of tq. lost no power. my new motor im doin (489) im putting on factory manifolds and original exaust. (resonators and mufflers out back) i hate noise! but anyway my freind put original exhaust back on his car (470 cu.in.) and there was no difference in power. it felt better, and smoother. large exhaust is a waste on the street!

[cudabob496]

well 2 years ago when i had my 08 gt500 mustang, making 670 at the wheels with factory exhaust but with hi flo cats, 2.50" pipes. i switched to 3" straight thru mufflers. everybody on the mustang sites said you will get another 30 + hp! no way! had it dyno tested. lost a couple of hp and tq. and the noise level went thru the roof! so of it came and put on stock exhaust. My challenger with  a 383  with 500 hp had 3 in. and went to 2.50 in. and it gained almost 20 ft.lb. of tq. lost no power. my new motor im doin (489) im putting on factory manifolds and original exaust. (resonators and mufflers out back) i hate noise! but anyway my freind put original exhaust back on his car (470 cu.in.) and there was no difference in power. it felt better, and smoother. large exhaust is a waste on the street!

Makes no sense. 670 rwhp, and you put on a less restrictive exhaust, and you lose hp and tq. Maybe the dyno was out of calibration. I think everybody on the sites that said you would gain 30+hp were probably right.  Then you have a friend with a 470 cu in that installs the original exhaust (instead of 3 inch, or something) and there is no diff in power output. Just find it hard to believe. Its like defying the laws of physics. If what you are saying is true, then everyone at the drag strip can put on smaller or stock exhausts, and either gain power, or not lose any!? I guarantee you the times will slow down, if they restrict their exhaust size/flow.

The chart clearly says that going from a 3 inch to 2.5 inch exhaust will remove 200 hp capability from your engine.  Power is all about moving
air,fuel, and exhaust, and if the exhaust is restrictive, you cannot make as much power. Kinda like trying to blow through a straw.

Large exhaust is not a waste on the street, if your engine will rev to 6000 rpm.

[Aussie Challenger]

Makes no sense. 670 rwhp, and you put on a less restictive exhaust, and you lose hp and tq. Maybe the dyno was out of calibration. I think everybody on the sites that said you would gain 30+hp were probably right.  Then you have a friend with a 470 cu in that installs the original exhaust (instead of 3 inch, or something) and there is no diff in power output. Just find it hard to believe. Its like defying the laws of physics. If what you are saying is true, then everyone at the drag strip can put on smaller or stock exhausts, and either gain power, or not lose any!?

A lot of modifications we do to our vehicles don't make sense, computor programs are just a bunch of numbers fed in, crunched and fed out and people tend to believe only those numbers. There are too many variables which computors do not have a hope of understanding, they are can not programed in. A bit like Grandma's cake recipe, when it is put down in writing and copied by someone else it doesn't turn out the same. Computor programs do not have the ability to take advantage of the velocity or heat of the exhaust, different mufflers, bends and so on. High velocity of the out going exhaust can be impeded if the exhaust is too restrictive, but as we are trying to get across on a street vehicle using 2 1/2" over 3" the overall numbers don't vary much if at all. Going on bigger is better then you need to ask why do we not now all have 5" or even 6" diameter exhaust pipes?
Crossover pipe or "H" section at or near the collectors, X pipes also can not be fed into computor programs, these modifications came about because somebody looked outside the "square", fitted, tested and retested until the performance gains could be duplicated with success.
Resonance is another factor which can not be fed into a computor program but can play havoc with the wrong headers, mufflers or resonators.
This list can go on and on, this why different race teams do things differently but can get the same winning result.   

[MizzouRT]

The 679 is the max hp a 3 inch system can handle, before you need larger pipe.
The max a 2.5 inch system can handle is about 460 hp.
For me to have a 600 hp, 500 cu in motor, the chart says I need at least a 3 inch system.
The chart says a 2.75 inch dual exhaust would limit my engine power to 566 hp.

No, the chart is saying that until you exceed 566 hp, the biggest size exhaust you need is 2.75".  If you have a 900+ hp engine, going from a 3.5" exhaust to a 2.5" pipe won't magically cut your power from 935 hp to 463 hp.  But at 900+ hp you are making enough power and flowing enough air that the 2.5" exhaust is a choke point.  Instead of 935 hp you will get something less, probably something in the 800-900 hp range but certainly not the 463 hp listed as the "max" for 2.5" pipes.  The "max horsepower" just says that once you go over that number, you start to lose a little peak power to the exhaust.  Go over it slightly, and the loss will be a small percentage.  Go over it a lot, and the percentage will climb but always be far below 100%.

Note the current GT500 makes 662 hp with a 2.75" factory exhaust.  Ford was running a 2.5" exhaust on the 2010 GT500 that made "only"  540 hp.  If there had been a lot of cheap power with a 3" exhaust over the 2.75" at 662 hp, Ford's engineers certainly would've gone with the bigger pipes.

[cudabob496]

No, the chart is saying that until you exceed 566 hp, the biggest size exhaust you need is 2.75".  If you have a 900+ hp engine, going from a 3.5" exhaust to a 2.5" pipe won't magically cut your power from 935 hp to 463 hp.  But at 900+ hp you are making enough power and flowing enough air that the 2.5" exhaust is a choke point.  Instead of 935 hp you will get something less, probably something in the 800-900 hp range but certainly not the 463 hp listed as the "max" for 2.5" pipes.  The "max horsepower" just says that once you go over that number, you start to lose a little peak power to the exhaust.  Go over it slightly, and the loss will be a small percentage.  Go over it a lot, and the percentage will climb but always be far below 100%.

Note the current GT500 makes 662 hp with a 2.75" factory exhaust.  Ford was running a 2.5" exhaust on the 2010 GT500 that made "only"  540 hp.  If there had been a lot of cheap power with a 3" exhaust over the 2.75" at 662 hp, Ford's engineers certainly would've gone with the bigger pipes.

They could increase their max power with larger pipe, but they probably wanted to keep the torque levels flatter in the lower rpm range,
to make the car more streetable.

[cudabob496]

A lot of modifications we do to our vehicles don't make sense, computor programs are just a bunch of numbers fed in, crunched and fed out and people tend to believe only those numbers. There are too many variables which computors do not have a hope of understanding, they are can not programed in. A bit like Grandma's cake recipe, when it is put down in writing and copied by someone else it doesn't turn out the same. Computor programs do not have the ability to take advantage of the velocity or heat of the exhaust, different mufflers, bends and so on. High velocity of the out going exhaust can be impeded if the exhaust is too restrictive, but as we are trying to get across on a street vehicle using 2 1/2" over 3" the overall numbers don't vary much if at all. Going on bigger is better then you need to ask why do we not now all have 5" or even 6" diameter exhaust pipes?
Crossover pipe or "H" section at or near the collectors, X pipes also can not be fed into computor programs, these modifications came about because somebody looked outside the "square", fitted, tested and retested until the performance gains could be duplicated with success.
Resonance is another factor which can not be fed into a computor program but can play havoc with the wrong headers, mufflers or resonators.
This list can go on and on, this why different race teams do things differently but can get the same winning result.   

would have to disagree. Computers, calculations, thermodynamic analysis, fluid dynamic analysis, are very precise, and allows for all variables to be taken into account. Otherwise, how did we land men on the moon, and bring them back, in 1969?  What happened on the rocket launch pad had to be precisely supported by the calculations.  And an auto engine is much simpler than a Saturn Rocket.  If 463 is not the max hp a 2.5 inch system can support, then there must be an error in the chart, because there is a max hp a 2.5 inch system can support. It should easily be able to be calculated. Again, there is only a fixed amount of exhaust that can pass through a 2.5 inch pipe, before it can't pass anymore. The ol' blow through a straw analogy, or choked flow, as discussed below:

http://en.wikipedia.org/wiki/Choked_flow

[cudabob496]

No, the chart is saying that until you exceed 566 hp, the biggest size exhaust you need is 2.75".  If you have a 900+ hp engine, going from a 3.5" exhaust to a 2.5" pipe won't magically cut your power from 935 hp to 463 hp.  But at 900+ hp you are making enough power and flowing enough air that the 2.5" exhaust is a choke point.  Instead of 935 hp you will get something less, probably something in the 800-900 hp range but certainly not the 463 hp listed as the "max" for 2.5" pipes.  The "max horsepower" just says that once you go over that number, you start to lose a little peak power to the exhaust.  Go over it slightly, and the loss will be a small percentage.  Go over it a lot, and the percentage will climb but always be far below 100%.

Note the current GT500 makes 662 hp with a 2.75" factory exhaust.  Ford was running a 2.5" exhaust on the 2010 GT500 that made "only"  540 hp.  If there had been a lot of cheap power with a 3" exhaust over the 2.75" at 662 hp, Ford's engineers certainly would've gone with the bigger pipes.

Interesting. the Mustang went from 540 to 662 hp, a 122 hp gain, when it went from 2.5 inch exhaust, to 2.75 inch exhaust.  The chart says going from 2.5 inch to 2.75 inch exhaust allows a 103 hp gain. But we don't know what other changes Mustang made (upped the boost?), so its hard to compare. At least the chart is in the ball park on this one.

Wait a minute. Is the Mustang supercharged?? This chart is for naturally aspirated, I would think. Having more pressure up stream from a turbo or supercharger should change the whole equation.

[todd383]

Interesting. the Mustang went from 540 to 662 hp, a 122 hp gain, when it went from 2.5 inch exhaust, to 2.75 inch exhaust.  The chart says going from 2.5 inch to 2.75 inch exhaust allows a 103 hp gain. But we don't know what other changes Mustang made (upped the boost?), so its hard to compare. At least the chart is in the ball park on this one.

Wait a minute. Is the Mustang supercharged?? This chart is for naturally asperated, I would think. Having more pressure up stream from a turbo or supercharger should change the whole equation.

The biggest change that mustang did was installing the TVS 2.3 supercharger and upping the cu.in. from 5.4 to 5.8 and the boost level also, from 8.5 to 15 lbs. i beleive.

[todd383]

Makes no sense. 670 rwhp, and you put on a less restrictive exhaust, and you lose hp and tq. Maybe the dyno was out of calibration. I think everybody on the sites that said you would gain 30+hp were probably right.  Then you have a friend with a 470 cu in that installs the original exhaust (instead of 3 inch, or something) and there is no diff in power output. Just find it hard to believe. Its like defying the laws of physics. If what you are saying is true, then everyone at the drag strip can put on smaller or stock exhausts, and either gain power, or not lose any!? I guarantee you the times will slow down, if they restrict their exhaust size/flow.

The chart clearly says that going from a 3 inch to 2.5 inch exhaust will remove 200 hp capability from your engine.  Power is all about moving
air,fuel, and exhaust, and if the exhaust is restrictive, you cannot make as much power. Kinda like trying to blow through a straw.

Large exhaust is not a waste on the street, if your engine will rev to 6000 rpm.

no, the dyno was not off, my mustang and challenger ran like crap with 3" pipes! as far as my friends 470 goes, he lost around 8 hp at 5800 rpms and tq. stayed the same. He does have exhaust manifolds on it. he had 3" pipes also but going back to original  hardly made a difference. besides being quieter! There are a few people i know that has been experimenting exhaust, and going to larger pipe hardly makes a difference. I rather give up some hp. and be quieter, just my

[72cudamaan]

Has anybody even verified any credentials of the owner of the article? Way too much emphasis is put on "articles" that have
 no scientific value or basis involved. That's why I can't stand getting into arguments with people that don't have any clue
what they are talking about other than reading a silly misinformed article. Might as well look this stuff up on wiki.   

[HP_Cuda]

Anyone use the square pipe that is now available so you can get even more clearance under the car?

[1970 RT Challenger 1970]

Interesting stuff.

[cudabob496]

Found another chart, from Magnaflow.

http://www.magnaflow.com/07techtips/faq/question10.asp


And for you strong but silent types:


http://www.pontiacstreetperformance.com/psp/exhaust.html

[cudabob496]

interesting: Below are a few articles I came across, but I did not come across what I was looking for: A test of a big block where you have a dual exhaust, and you vary the exhaust pipe size from say 2 inchs, to 2.25, 2.5, 2.75,3, 3.5 or 4 inches, and see how the dyno numbers change.  That would be a good way to validate the chart at the beginning of this thread, and definitely help a person with a high hp engine put the most power to the rear wheels.  I have, however, seen countless articles that show open headers give the best 1/4 mile ETs and top speeds.


http://www.carcraft.com/techarticles/116_0312_exhaust_pipes_test/viewall.html

http://www.moparmusclemagazine.com/techarticles/engine/mopp_0212_exhaust_system_and_headers_upgrade_and_dyno_test/viewall.html

Flowmaster Muffler Comparison

[cudabob496]

Anyone use the square pipe that is now available so you can get even more clearance under the car?

or you could get 4 inch pipe and hammer one side flat! haha