Does bigger exhaust pipes really mean more flow?
02-20-2011, 11:41 AM
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Does bigger exhaust pipes really mean more flow? Help me understand guys...
Does bigger exhaust really increase flow significantly (especially for the 1000 bucks some of these go for)?
Where has my math/science gone wrong?
Exhaust diameter = dual 2.5" (stock C5)
Exhaust area = 4.91 * 2 = 9.8 in^2 = 0.068 ft^2
Speed of sound = 65,000 ft/min
Exhaust flow rate with dual 2.5" pipes = 0.068 ft^2 * 65,000 ft/min = 4400 cfm (exhaust will choke at the speed of sound, and probably a little faster because exhaust isn't as dense as cold air)
I know there is more friction in a smaller pipe. But fact is, you will not choke the stock exhaust even if you make 7000 hp (4400cfm * 1.6 = hp). I also figure the practical limit is a bit lower, but who knows exactly.
So what's the deal? Is better exhaust worth the money? Can i still make 600-700 whp with the stock exhaust?
Does bigger exhaust really increase flow significantly (especially for the 1000 bucks some of these go for)?
Where has my math/science gone wrong?
Exhaust diameter = dual 2.5" (stock C5)
Exhaust area = 4.91 * 2 = 9.8 in^2 = 0.068 ft^2
Speed of sound = 65,000 ft/min
Exhaust flow rate with dual 2.5" pipes = 0.068 ft^2 * 65,000 ft/min = 4400 cfm (exhaust will choke at the speed of sound, and probably a little faster because exhaust isn't as dense as cold air)
I know there is more friction in a smaller pipe. But fact is, you will not choke the stock exhaust even if you make 7000 hp (4400cfm * 1.6 = hp). I also figure the practical limit is a bit lower, but who knows exactly.
So what's the deal? Is better exhaust worth the money? Can i still make 600-700 whp with the stock exhaust?
02-20-2011, 11:46 AM
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Oops, forgot to include the effects of pipe length. Longer pipes of course mean more friction and flow loss. So a larger pipe will essentially "shorten" the pipe length, or in other words, the length/diameter ratio would be smaller, meaning less flow loss.
I get that. Therefore, I can dump right behind the front wheels and make big gains in flow, just like the expensive pipes.
Correct?
I get that. Therefore, I can dump right behind the front wheels and make big gains in flow, just like the expensive pipes.
Correct?
02-20-2011, 12:23 PM
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sounds like you definitly did some thinking on this .
bump for someone else who knows more then me to chime in
from my understanding, a short pipe just dumped wont generate much backpressure at all , leaving not great low end , and nice top end.
Full exuast will help make back pressure helping you pick up in the low end to get going.
I am sure it might be capable to make massive ammounts of horse with the stock set up, but who would want to hear that ?
bump for someone else who knows more then me to chime in
from my understanding, a short pipe just dumped wont generate much backpressure at all , leaving not great low end , and nice top end.
Full exuast will help make back pressure helping you pick up in the low end to get going.
I am sure it might be capable to make massive ammounts of horse with the stock set up, but who would want to hear that ?
02-20-2011, 02:10 PM
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About the number crunching part im not sure, but theoreticaly yeah for our cars you do want the best flow, and that means bigger diameter exhaust. When you think of the stock exhaust theres more than just the 2.5" pipe, theres the length of it all, the bends and flat part in the y pipe, the muffler, tips, cats, etc. There are restrictions in it, now, a 2.5" straight back pipe compared to a 3" straight back pipe, im sure your numbers would be closer to and yeah it would definatly be fine im sure. When you look at dyno numbers you can always see a gain from cutouts over any other exhaust, that right there tells you that the exhaust can be more free flowing and the best numbers would probably be from open headers (mind you this is not on a tuned application such as a race car that have more engineering behind backpressure, etc.) When the stock exhaust gets upgraded all of this stuff adds up. Just pipe size 2.5 vs 3" i think is mostly for sound.
02-20-2011, 02:53 PM
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I'm doing 1 7/8 - stepped to 2" LT's....3" pipes off my collectors into a Flowmaster merge, that has a 4" outlet that runs back to the Magnaflow muffler. It will flow GREAT for a 441ci and be relatively quiet when driving it casually and normally. But it'll sound like it does now under WOT...pure evil.
But like Mighty Texan said.....a cut out is great. I'll have a 4" electric cutout immediately after my merge right on the 4" outlet......it'll add some power and be SICK *** loud..........
I think thats the perfect system without getting all expensive and crazy about it.......
.
But like Mighty Texan said.....a cut out is great. I'll have a 4" electric cutout immediately after my merge right on the 4" outlet......it'll add some power and be SICK *** loud..........
I think thats the perfect system without getting all expensive and crazy about it.......
.
Last edited by LS6427; 02-20-2011 at 03:13 PM.
02-22-2011, 01:03 PM
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Yes, bigger pipe "flows more". That's how you get sold
bigger pipe. Question is, how much is exhaust restriction
costing you power-wise?
If you had your EGT and figure exhaust mass airflow is
close enough to input mass airflow, you could probably
figure volume airflow and a pipe based head pressure
for length. Pressure*volume_airflow is a power number
that requires only units conversion. What does that
tell you?
My gut says that the problem is elsewhere, than 3" vs
4" pipe. Because if there were a meaningful quantitative
difference, vendors would be quantitative instead of
"selling you sizzle when you ask for steak".
bigger pipe. Question is, how much is exhaust restriction
costing you power-wise?
If you had your EGT and figure exhaust mass airflow is
close enough to input mass airflow, you could probably
figure volume airflow and a pipe based head pressure
for length. Pressure*volume_airflow is a power number
that requires only units conversion. What does that
tell you?
My gut says that the problem is elsewhere, than 3" vs
4" pipe. Because if there were a meaningful quantitative
difference, vendors would be quantitative instead of
"selling you sizzle when you ask for steak".
