Exhaust Flow Question.
12-28-2003, 08:32 AM
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Exhaust Flow Question.
I have a 3" Thermal R&D Turbo catback...taht will be going on my SOHC turbo and i was planning on getting a 3" DP...but then i found out i'd have to ditch my AC...i was told to get a 2.5-3 flared downpipe...but wouldn't that be pointless? i was just thinking why would having a full 2.5" be any worse than a 2.5 flared to a full 3" catback...since the air has to go through the 2.5" section anyways....
i am tired and couldn't figure this out, post your thoughts.
i am tired and couldn't figure this out, post your thoughts.
12-28-2003, 09:04 AM
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Re: Exhaust Flow Question. (meams)
Even though it is somewhat being restricted at the 2.5" dp, it will still flow a lot better once its past that point. I have a custom 2.5" dp on my car with a 3" catback exhaust, and I didn't have to remove the a/c or p/s.
12-28-2003, 10:20 AM
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Re: Exhaust Flow Question. (meams)
idk if this is exactly right but think of it like this: onces it gets into the 3" section the pressure will drop. a lower pressure on the exit side of the 2.5" section will help it go through there faster as well. making the system overall have much less pressure.
now listen to this. someone told me that on a car making under 300hp a 2.5" system would actually have LESS pressure because of higher exhaust velocity and less drag due to surface area. now that I know exhaust velocity would be higher and drag would be lower, but wouldn't overall volume compensate for this?
Also, I'd like to here real answers, not 3" is bigger so its better....
now listen to this. someone told me that on a car making under 300hp a 2.5" system would actually have LESS pressure because of higher exhaust velocity and less drag due to surface area. now that I know exhaust velocity would be higher and drag would be lower, but wouldn't overall volume compensate for this?
Also, I'd like to here real answers, not 3" is bigger so its better....
12-29-2003, 01:17 PM
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Re: Exhaust Flow Question. (meams)
"the best turbo exhaust is no exhaust" - Corky Bell, Maximum Boost.
The main thing you are trying to do is eliminate back pressure after the turbine wheel. I am not sure how much velocity is an issue but I feel these sizes/HP are the best compromise of power/money/sound/fitment. Don't be scared to go big.
-250hp 2.5"
-350hp 3.0"
-500hp 3.5"
-700hp 4"
The main thing you are trying to do is eliminate back pressure after the turbine wheel. I am not sure how much velocity is an issue but I feel these sizes/HP are the best compromise of power/money/sound/fitment. Don't be scared to go big.
-250hp 2.5"
-350hp 3.0"
-500hp 3.5"
-700hp 4"
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12-29-2003, 04:11 PM
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Re: Exhaust Flow Question. (DIRep972)
I've gone 3" and would again, but its just something I was thinking about.
No exhaust is the best, but its different than bigger is closer to no exhaust.
You want the least amount of back pressure. This guy was stating that you would have MORE back pressure w/ 3" at around 300hp.
1) there would be more back pressure
2) you would loose too much heat causing the gas to densen up and slow down.
Who knows...
No exhaust is the best, but its different than bigger is closer to no exhaust.
You want the least amount of back pressure. This guy was stating that you would have MORE back pressure w/ 3" at around 300hp.
1) there would be more back pressure
2) you would loose too much heat causing the gas to densen up and slow down.
Who knows...
12-29-2003, 05:09 PM
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Re: Exhaust Flow Question. (00SilverLS)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 00SilverLS »</TD></TR><TR><TD CLASS="quote">I've gone 3" and would again, but its just something I was thinking about.
No exhaust is the best, but its different than bigger is closer to no exhaust.
You want the least amount of back pressure. This guy was stating that you would have MORE back pressure w/ 3" at around 300hp.
1) there would be more back pressure
2) you would loose too much heat causing the gas to densen up and slow down.
Who knows... </TD></TR></TABLE>
I dont see how that guy can state that for a turbo car. Thats basically the theory for picking an exhaust for a Naturally Aspirated car. NA cars use exhaust gas velocity to make power while turbo cars use a pressure ratio. You want velocity, but you want it in the turbo manifold. The less pressure after the turbine discharge and the less energy it takes to drive the the turbine. This results in even less pressure and increased velocity in the manifold. The main reason "bigger is better" (to a point) is because the main goal of a turbo exhaust is to releave pressure at the turbine discharge. Whether the gas escapes from the exhaust itself fast or slow it does not matter, as long as it reduces the pressure after the turbine.
Heres a pretty good article on exhausts written in part by a Garrett turbocharger engineer. http://www.vishnutuning.com/exhaust101.htm
No exhaust is the best, but its different than bigger is closer to no exhaust.
You want the least amount of back pressure. This guy was stating that you would have MORE back pressure w/ 3" at around 300hp.
1) there would be more back pressure
2) you would loose too much heat causing the gas to densen up and slow down.
Who knows...
</TD></TR></TABLE>I dont see how that guy can state that for a turbo car. Thats basically the theory for picking an exhaust for a Naturally Aspirated car. NA cars use exhaust gas velocity to make power while turbo cars use a pressure ratio. You want velocity, but you want it in the turbo manifold. The less pressure after the turbine discharge and the less energy it takes to drive the the turbine. This results in even less pressure and increased velocity in the manifold. The main reason "bigger is better" (to a point) is because the main goal of a turbo exhaust is to releave pressure at the turbine discharge. Whether the gas escapes from the exhaust itself fast or slow it does not matter, as long as it reduces the pressure after the turbine.
Heres a pretty good article on exhausts written in part by a Garrett turbocharger engineer. http://www.vishnutuning.com/exhaust101.htm
12-30-2003, 07:07 AM
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Re: Exhaust Flow Question. (DIRep972)
for the record, all i was asking is that which applies to the weakest link theorum...
If your exhaust has to go to through a 2.5" will there be any point in having a 3" catback? since the exhaust has gone through a 2.5" section?
If your exhaust has to go to through a 2.5" will there be any point in having a 3" catback? since the exhaust has gone through a 2.5" section?
12-30-2003, 10:03 AM
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Re: Exhaust Flow Question. (00SilverLS)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 00SilverLS »</TD></TR><TR><TD CLASS="quote">now listen to this. someone told me that on a car making under 300hp a 2.5" system would actually have LESS pressure because of higher exhaust velocity and less drag due to surface area. now that I know exhaust velocity would be higher and drag would be lower, but wouldn't overall volume compensate for this?</TD></TR></TABLE>
Drag (gas flow friction) is basically a function of two things, VELOCITY and SURFACE AREA. For most normal flow rates, i.e. not supersonic, drag is approximately proportional to the SQUARE of the VELOCITY, and is linearly proportional to the SURFACE AREA. As a simple thought experiment, compare a 1.5" exhaust to a 3" exhaust. For the same volume flow, the 1.5" exhaust will have 4 TIMES the exhaust velocity and therefore 16 TIMES the drag per unit area. However, since the 1.5" exhaust has 1/2 the surface area of the 3" exhaust, it will only have about 8 TIMES the drag of the 3" exhaust. So yes Dorothy, there's a real answer for ya, on a turbo engine, BIGGER...IS...ALWAYS...BETTER, and it'll be really difficult to convince me (or Corky Bell) that it isn't! Unless it means no A/C, and then it sucks.
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 00SilverLS »</TD></TR><TR><TD CLASS="quote">2) you would loose too much heat causing the gas to densen up and slow down.
Who knows... </TD></TR></TABLE>
That is a good thing. When the exhaust gas cools and gets denser and slows down, its momentum and deceleration causes a partial vacuum behind it and helps to pull the exhaust flow along. This is just like using a flared diffuser at the end of subsonic flow in a pipe, except that instead of the cross-sectional area getting larger, the flow volume gets smaller. Grab someones sweater when they are running by you and you will slow them down, but they will pull you along. The only time that heat loss from the exhaust is bad is when it either sets something on fire or it leaves too much water condensation in the exhaust.
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by meams »</TD></TR><TR><TD CLASS="quote">for the record, all i was asking is that which applies to the weakest link theorum...
If your exhaust has to go to through a 2.5" will there be any point in having a 3" catback? since the exhaust has gone through a 2.5" section? </TD></TR></TABLE>
The most critical things to exhaust flow are 1) smooth transitions from flange to flange 2) smooth transitions from 2.5" to 3" and 3) smooth radii that aren't too sharp, i.e. Rbend >= 3xRpipe where possible. And yes, there are good benefits to having a 3" catback after a 2.5" section.
Drag (gas flow friction) is basically a function of two things, VELOCITY and SURFACE AREA. For most normal flow rates, i.e. not supersonic, drag is approximately proportional to the SQUARE of the VELOCITY, and is linearly proportional to the SURFACE AREA. As a simple thought experiment, compare a 1.5" exhaust to a 3" exhaust. For the same volume flow, the 1.5" exhaust will have 4 TIMES the exhaust velocity and therefore 16 TIMES the drag per unit area. However, since the 1.5" exhaust has 1/2 the surface area of the 3" exhaust, it will only have about 8 TIMES the drag of the 3" exhaust. So yes Dorothy, there's a real answer for ya, on a turbo engine, BIGGER...IS...ALWAYS...BETTER, and it'll be really difficult to convince me (or Corky Bell) that it isn't! Unless it means no A/C, and then it sucks.
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 00SilverLS »</TD></TR><TR><TD CLASS="quote">2) you would loose too much heat causing the gas to densen up and slow down.
Who knows...
</TD></TR></TABLE>That is a good thing. When the exhaust gas cools and gets denser and slows down, its momentum and deceleration causes a partial vacuum behind it and helps to pull the exhaust flow along. This is just like using a flared diffuser at the end of subsonic flow in a pipe, except that instead of the cross-sectional area getting larger, the flow volume gets smaller. Grab someones sweater when they are running by you and you will slow them down, but they will pull you along. The only time that heat loss from the exhaust is bad is when it either sets something on fire or it leaves too much water condensation in the exhaust.
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by meams »</TD></TR><TR><TD CLASS="quote">for the record, all i was asking is that which applies to the weakest link theorum...
If your exhaust has to go to through a 2.5" will there be any point in having a 3" catback? since the exhaust has gone through a 2.5" section? </TD></TR></TABLE>
The most critical things to exhaust flow are 1) smooth transitions from flange to flange 2) smooth transitions from 2.5" to 3" and 3) smooth radii that aren't too sharp, i.e. Rbend >= 3xRpipe where possible. And yes, there are good benefits to having a 3" catback after a 2.5" section.
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