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Discussion Starter · #1 ·
I am beginning to wonder if bigger is really better when it comes to exhaust. I bought Corky Bell's Maximum Boost and his chart shows that a 2.5" diameter exhaust is good for up to 500 hp (which is farther than I will ever go). Exhaust engineers say that too large of an exhaust will cause the exhaust to cool quicker and just makes it more difficult to push the (increased density) air through the exhaust. Does anyone have any data showing that a 3" diameter exhaust system is better than a 2.5" system? (No opinions please) :) .
 

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KEITH said:
I am beginning to wonder if bigger is really better when it comes to exhaust. I bought Corky Bell's Maximum Boost and his chart shows that a 2.5" diameter exhaust is good for up to 500 hp (which is farther than I will ever go). Exhaust engineers say that too large of an exhaust will cause the exhaust to cool quicker and just makes it more difficult to push the (increased density) air through the exhaust. Does anyone have any data showing that a 3" diameter exhaust system is better than a 2.5" system? (No opinions please) :) .
data??

sorry man you are not going to find it except what the dyno shows. Reason is to capture data requires money and lots of it.
 

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Are you talking about an NA car or turbo?
 

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Discussion Starter · #5 ·
Turbo. I feel pretty confident that the information above did come from data. Since I already have a 2.5" high flow cat and 2.5" muffler, I don't want to scrap it thinking I am going to get more hp from a 3" system.
 

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boost freak
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KEITH said:
Turbo. I feel pretty confident that the information above did come from data. Since I already have a 2.5" high flow cat and 2.5" muffler, I don't want to scrap it thinking I am going to get more hp from a 3" system.
heh. if you don't believe it, do a search here on SF and take a look @ all the MKIV guys that pick up 20-50 HP swapping from a 3" to a 4" exhaust....you can't put too large an exhaust on a turbocharged car.

as for corky bell, he is the same guy that touts the wonders of the aerocharger or whatever in his book....look where that went!
 

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RoastBeefCurtain Commando
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drjonez said:
heh. if you don't believe it, do a search here on SF and take a look @ all the MKIV guys that pick up 20-50 HP swapping from a 3" to a 4" exhaust....you can't put too large an exhaust on a turbocharged car.

I agree with Dr J to an extent. There is alot more in it than people think though. but thats engineering shit.
Diameter, length, back pressure, velocity and all that crap plays a way one way or another.
Bigger for turbo helps, but not needed in most cases. Depends on the application really.
 

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I started a thread on here about backpressure/exhaust power. Here is the thread, alot of contradiction by members here, but there is some science to it. Contradiction for NAs anyway, but here ya go. Hope it helps you. I guess the jist for GTEs is bigger is better.

http://supraforums.com/forum/showthread.php?t=355874
 

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the reason for turbo charged vehicle (regardless of motor) to have the biggest exhuast is that the turbo is already a HUGE restriction itself. You want those gases to get out as fast as possible from the turbo and to do that you need the biggest pressure differential.
 
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drjonez said:
heh. if you don't believe it, do a search here on SF and take a look @ all the MKIV guys that pick up 20-50 HP swapping from a 3" to a 4" exhaust....you can't put too large an exhaust on a turbocharged car.

as for corky bell, he is the same guy that touts the wonders of the aerocharger or whatever in his book....look where that went!
max boost is the best turbocharing book ive ever read. and porsche just adapted a version of an aerocharger (aero charger is just a variable vein inlet turbo) on the new 911 and the torque curve is almost dead flat from 1,000 rpm to redline.

then again...im still a newbie
 

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Agreed w/ everyone else in here: No exhaust is too big for a turbo car. This is why you get even more power when you run open dp: less restrictions.

Exhaust sizing comes into play on n/a motors because if you don't have enough backpressure, you'll burn up your exhaust valves among other things...
 

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239RWHP @ 9PSI
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drjonez said:
doesn't matter. a larger exhaust will ALWAYS make more peak power.
but isnt it bad to have a 3" for a NA? or do you still get power but lower torque?
 

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driftedmk3 said:
but isnt it bad to have a 3" for a NA? or do you still get power but lower torque?

it depends on what the application for said N/A is

If you want to street drive it. More than likely you will fall flat on your face down low. Now if it is all out road race then you want those gasses out of the engine as fast as possible without no care for velocity.
 

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The 3" DP will be nothing but a bottleneck for a 3.5-4" catback. In otherwords... it makes no sense at all to go bigger then the DP... or the turbo elbow.
 

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driftedmk3 said:
but isnt it bad to have a 3" for a NA? or do you still get power but lower torque?
AGlobalThreat used a 3inch catback on his NA, and he had one of the most powerful, and fastest NAs here. MK3 that is. But he also had other mods like headers, intake etc etc. He said that he saw a noticeable gain in power from upgrading from a 2.5 to a 3 on his NA. So experience speaks the best to me.
 

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AlexSmith said:
AGlobalThreat used a 3inch catback on his NA, and he had one of the most powerful, and fastest NAs here. MK3 that is. But he also had other mods like headers, intake etc etc. He said that he saw a noticeable gain in power from upgrading from a 2.5 to a 3 on his NA. So experience speaks the best to me.
Power is not all about peak ;)

Area under the curve is more critical.
 

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AlexSmith said:
AGlobalThreat used a 3inch catback on his NA, and he had one of the most powerful, and fastest NAs here. MK3 that is. But he also had other mods like headers, intake etc etc. He said that he saw a noticeable gain in power from upgrading from a 2.5 to a 3 on his NA. So experience speaks the best to me.
hes not the fastest na here.. there is a na with forged piston. he is the na god but craig is around there. haha
 
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