Good debate I was involved in: 2.5" or 3"
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Good debate I was involved in: 2.5" or 3"
There was a novice "know-it-all" who was regurgitating some info his tuner had relayed to him about a 3" exhaust. The mentality of a 3" exhaust is better than a 2.5" exhaust is heavily debatable on both ends. You can make almost equal arguments to both sides, however the "bigger is better" mentality is always in the back of everyone's minds. Let's just nail down some stuff prior to opening this can of worms, which I am sure is going to create a stir, however all of the data I am going to relay is 100% factual and can be verified at any time.
Let me first state that a turbo system does not like back pressure at all. I will quote the turbo God, Corky Bell "the best turbo exhaust is no turbo exhaust". With that being said, there is no debate there, that any larger exhaust diameter is better than stock. The stock rinky dink 2" just is an utter killer of power no matter what. So, let's lead into some basics of a gasoline combustion engine first. The simple premises of "Suck, Squeeze, Bang, Blow". No this is not porno talk, but laymans terms for the combustion cycle. I want to specifically focus on the blow section considering this is what is debated: exhaust.
Assuming that the exhaust diameter is sufficient, because 2.5" has good flow properties and can keep the air moving VERY fast until about 500HP at the crank. Then the air velocity speeds up too much and a result is HUGE back pressure due to exhaust version that occurs in the pipe diameter due to drag of moving past .4 MACH.
With all exhaust diameter being the same for arguments sake, I am going to play devil's advocate here.....So assuming a 2.5" exhaust. Assuming the same boost pressure and same turbocharger, only difference being a larger turbine wheel and a larger A/R, the exhaust gas in the larger A/R/larger turbine wheel will take longer to spool, but move more air effectively. I think we can all agree there correct? We are not speaking about a stock exhaust, which I think we can both agree that it will make some serious loss in power. Anyway, my point is that he will have that wall first.
Lastly, let's recall the premise of a gasoline combustion engine: Suck, Squeeze, Bang, Blow. Let's focus on the blow. Where does the exhaust gas go? First after the valve opens, it enters the manifold, assuming no reversion, it then enters the turbine housing. From there it begins to spin the turbine wheel. Let's pause right there. We haven't even gotten to the downpipe yet in the equation, which is why I say what I said and quote "think about it"....
"# After the fuel is burned in the cylinder it is exhausted during the cylinder’s exhaust stroke in to the exhaust manifold (5)
# The high temperature gas then continues on to the turbine (6). The turbine creates backpressure on the engine which means engine exhaust pressure is higher than atmospheric pressure
# A pressure and temperature drop occurs (expansion) across the turbine (7), which harnesses the exhaust gas’ energy to provide the power necessary to drive the compressor
--Taken from Garrett's website:
Then more....
"Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing, as it is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel. This provides increased turbine power at lower engine speeds, resulting in a quicker boost rise. However, a small A/R also causes the flow to enter the wheel more tangentially, which reduces the ultimate flow capacity of the turbine wheel. This will tend to increase exhaust backpressure and hence reduce the engine's ability to "breathe" effectively at high RPM, adversely affecting peak engine power.
Conversely, using a larger A/R will lower exhaust gas velocity, and delay boost rise. The flow in a larger A/R housing enters the wheel in a more radial fashion, increasing the wheel's effective flow capacity, resulting in lower backpressure and better power at higher engine speeds. "
The overall argument was on a Honda. The premises of turbocharging holds true almost all the time regardless of application so long as it's a gasoline combustion engine. Diesel is an entirely animal when it comes to the combustion and aftermath, which is not what we are concerned with.
The cliffs notes more or less are:
*Turbine A/R and stage turbine wheel restriction will be met before exhaust restriction
*A larger turbine A/R and larger staged turbine wheel will make smaller exhaust systems more efficient at higher RPMS due to flow properties and linear fashion of boost delivery
*Bigger isn't always better
*You will meet cam, valve, cylinder head, exhaust manifold and turbine restrictions before you meet almost any exhaust system restriction.
*Smaller A/Rs will require larger exhaust systems to keep flow up on higher RPMS.
*3" may be required for some due to turbine arrangement and power made.
*2.5" can be used to make some VERY good power assuming your ability to switch to a larger A/R turbine arrangement
*Everything has a trade off in life. Pick your poison.
Learn something?
Let me first state that a turbo system does not like back pressure at all. I will quote the turbo God, Corky Bell "the best turbo exhaust is no turbo exhaust". With that being said, there is no debate there, that any larger exhaust diameter is better than stock. The stock rinky dink 2" just is an utter killer of power no matter what. So, let's lead into some basics of a gasoline combustion engine first. The simple premises of "Suck, Squeeze, Bang, Blow". No this is not porno talk, but laymans terms for the combustion cycle. I want to specifically focus on the blow section considering this is what is debated: exhaust.
Assuming that the exhaust diameter is sufficient, because 2.5" has good flow properties and can keep the air moving VERY fast until about 500HP at the crank. Then the air velocity speeds up too much and a result is HUGE back pressure due to exhaust version that occurs in the pipe diameter due to drag of moving past .4 MACH.
With all exhaust diameter being the same for arguments sake, I am going to play devil's advocate here.....So assuming a 2.5" exhaust. Assuming the same boost pressure and same turbocharger, only difference being a larger turbine wheel and a larger A/R, the exhaust gas in the larger A/R/larger turbine wheel will take longer to spool, but move more air effectively. I think we can all agree there correct? We are not speaking about a stock exhaust, which I think we can both agree that it will make some serious loss in power. Anyway, my point is that he will have that wall first.
Lastly, let's recall the premise of a gasoline combustion engine: Suck, Squeeze, Bang, Blow. Let's focus on the blow. Where does the exhaust gas go? First after the valve opens, it enters the manifold, assuming no reversion, it then enters the turbine housing. From there it begins to spin the turbine wheel. Let's pause right there. We haven't even gotten to the downpipe yet in the equation, which is why I say what I said and quote "think about it"....
"# After the fuel is burned in the cylinder it is exhausted during the cylinder’s exhaust stroke in to the exhaust manifold (5)
# The high temperature gas then continues on to the turbine (6). The turbine creates backpressure on the engine which means engine exhaust pressure is higher than atmospheric pressure
# A pressure and temperature drop occurs (expansion) across the turbine (7), which harnesses the exhaust gas’ energy to provide the power necessary to drive the compressor
--Taken from Garrett's website:
Then more....
"Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing, as it is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel. This provides increased turbine power at lower engine speeds, resulting in a quicker boost rise. However, a small A/R also causes the flow to enter the wheel more tangentially, which reduces the ultimate flow capacity of the turbine wheel. This will tend to increase exhaust backpressure and hence reduce the engine's ability to "breathe" effectively at high RPM, adversely affecting peak engine power.
Conversely, using a larger A/R will lower exhaust gas velocity, and delay boost rise. The flow in a larger A/R housing enters the wheel in a more radial fashion, increasing the wheel's effective flow capacity, resulting in lower backpressure and better power at higher engine speeds. "
The overall argument was on a Honda. The premises of turbocharging holds true almost all the time regardless of application so long as it's a gasoline combustion engine. Diesel is an entirely animal when it comes to the combustion and aftermath, which is not what we are concerned with.
The cliffs notes more or less are:
*Turbine A/R and stage turbine wheel restriction will be met before exhaust restriction
*A larger turbine A/R and larger staged turbine wheel will make smaller exhaust systems more efficient at higher RPMS due to flow properties and linear fashion of boost delivery
*Bigger isn't always better
*You will meet cam, valve, cylinder head, exhaust manifold and turbine restrictions before you meet almost any exhaust system restriction.
*Smaller A/Rs will require larger exhaust systems to keep flow up on higher RPMS.
*3" may be required for some due to turbine arrangement and power made.
*2.5" can be used to make some VERY good power assuming your ability to switch to a larger A/R turbine arrangement
*Everything has a trade off in life. Pick your poison.
Learn something?
Originally Posted by highvoltage1
So what would be the ideal exhaust for a 16g when i will be running around 5-7 psi and am looking to get around 250whp??
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SL Member
Joined: Nov 2006
Posts: 1,480
From: Arizona
So what would be the ideal exhaust for a 16g when i will be running around 5-7 psi and am looking to get around 250whp??
2.5".
But if you want to get down to specifics...we would need allot of info. But lets just say it would be smaller than 2.5". ;)
NICE WRITE UP PAUL!!!
Regards-
Todd
all you have to do is quiet it down and they wouldnt know you are overkilling your setup lol.. but i dont think having a 3" on s/c isnt that really bad, not just getting the most of the advantage of the 3"
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Joined: Aug 2006
Posts: 4,888
From: Miami
Originally Posted by draxcaliber
great, so now i have epically loud 3 inch exhaust for nothing? how am i supposed to justify it now?
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Joined: Sep 2006
Posts: 11,141
From: Maryland
Originally Posted by m6ar2cel6oTC
Originally Posted by draxcaliber
great, so now i have epically loud 3 inch exhaust for nothing? how am i supposed to justify it now?
