Our engines are air pumps!
Apr 8, 2009, 11:52 AM
#31
1. No
2. There are no exhaust products for the evo which are way bigger than needed.
3. Its not about size as much as it is about the transition from the turbine outlet. Basically you don't wan't a 5" pipe immediately after a 2" turbine outlet, but if you merged gradually from a 2" pipe to a 5" pipe, you wouldn't be hurting anything.
In terms of Evo's, you should be shifting focus from the DP and exhaust, to just the turbo elbow.
2. There are no exhaust products for the evo which are way bigger than needed.
3. Its not about size as much as it is about the transition from the turbine outlet. Basically you don't wan't a 5" pipe immediately after a 2" turbine outlet, but if you merged gradually from a 2" pipe to a 5" pipe, you wouldn't be hurting anything.
In terms of Evo's, you should be shifting focus from the DP and exhaust, to just the turbo elbow.
Agreed...
I should grab a picture of this O2 housing I have. I think the exhaust would be quite the setup for a RED...
In essence, the O2 housing transitions from 2.25" to 3.5" as it makes the bend before the wastegate is joined back in. While the wastegate port joins in the diameter goes from 3.5" to 4". It's not perfect, but the transistion is pretty smooth. Well, it's as smooth as I could manage as I took a 2" to 4" by 12" long transition and pie sectioned it into a 90 degree bend. Lots of welding, metal forming and frustration to get it to work.
The choice of 4" is for a reason other then flow. Turns out that the surface temperature of an aluminum tube is lower with 4" compared to 3".
Who wants a 15 pound 4" exhaust???
Last edited by 03whitegsr; Apr 8, 2009 at 02:26 PM.
Apr 8, 2009, 12:52 PM
#32
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1. I've run full 2.5" on my Talon and gone just as fast as many guys with similar setups running 3". We've also swapped from 2.5" to 3" on a friend's 20G DSM and noticed no gains at that level (approx. 400whp). Doesn't mean that 3" is "too big", but there is definitely a point of diminishing returns. The problem is most people don't want to buy exhaust twice and know they'll be making more power down the road, so most of the exhaust manufacturers tend to cater to that concept.
Apr 8, 2009, 03:42 PM
#33
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FWIW, my SRT-4 has a 3" O2 and downpipe into the stock 2.25" exhaust, but it has an electric cutout so I can open it to a full 3" or run 2.25" with the flip of a switch.
I can say from experience, from logging pulls both with the cutout open and closed, the 2.25" exhaust increases lag significantly, the turbo spools a few hundred RPMs sooner when the 3" cutout is open, not to mention that it simply makes more power with the bigger exhaust outlet.
Regardless of the obvious differences between 2.25" (too small) and 3" in terms of the amount of airflow, the added backpressure of the smaller pipes hurts spool dramatically. Backpressure = bad behind the turbo. At least from O2 down... turbine outlet size is another topic, but that's about the only place it would matter. You don't buy turbine outlets when you get a TBE...
I can say from experience, from logging pulls both with the cutout open and closed, the 2.25" exhaust increases lag significantly, the turbo spools a few hundred RPMs sooner when the 3" cutout is open, not to mention that it simply makes more power with the bigger exhaust outlet.
Regardless of the obvious differences between 2.25" (too small) and 3" in terms of the amount of airflow, the added backpressure of the smaller pipes hurts spool dramatically. Backpressure = bad behind the turbo. At least from O2 down... turbine outlet size is another topic, but that's about the only place it would matter. You don't buy turbine outlets when you get a TBE...
Apr 9, 2009, 09:14 AM
#36
That was based on the engine when it was making 650 whp. As the RPMS go up and the boost is increased, the engine simply flows more CFM. With increased CFM, the requirements also change concerning the exhaust's capacity to carry the increased volume of gas out of the engnie. Also, all of Davids dyno's on his car are in 3rd gear which will reflect slightly lower boost pressures than they would in a 4th gear pull. The pressure differential of incoming (pre-compressor wheel) and out going (after the turbine) is a primary concern when it comes to performance.
Apr 10, 2009, 07:22 AM
#38
Interesting posts you found there Zeus. But I think the OP of those posts has some serious contradictions with real world evidence. He said 3" is too small for 450 hp? Buschur tested going from 3" to 3.5" at 800 hp and saw no gains. So the OP was very wrong in that statement. Which is a pretty big issue!
Secondly, the OP said that if you don't taper the enlarging of the exhaust correctly, that it could have a detrimental effect on spooling the turbo. For example, someone bolting a 3" downpipe onto a stock turbo elbow (o2 housing) which probably has a diameter of less than 2.5". There is no taper there. None of us use any taper in our exhaust. So to take from the above quote that bigger is better after the turbo is not correct.
Secondly, the OP said that if you don't taper the enlarging of the exhaust correctly, that it could have a detrimental effect on spooling the turbo. For example, someone bolting a 3" downpipe onto a stock turbo elbow (o2 housing) which probably has a diameter of less than 2.5". There is no taper there. None of us use any taper in our exhaust. So to take from the above quote that bigger is better after the turbo is not correct.
The OP I quoted was correct in the context to which he was speaking. However, it's also correct in the general sense so long as you don't get wrapped around the axle about the details. In the example of one just pulling an exhaust system off a downpipe, would that not change the dynamics of the system? Although you may have stayed "safe" in your A/F by doing so, you HAVE effectively changed the dynamics of the equation. Basic science/real world testing dictates that you want the least pressure possible behind the turbine.
Last edited by Zeus; Apr 10, 2009 at 07:25 AM.
Apr 10, 2009, 08:06 AM
#39
I am starting to feel a bit dumb now 
In reading your guys posts, and thinking about it a bit more, I've realised what has happened. You see I had very strong evidence from myself and others that when you drop the pipe, the low end suffers. There was no doubt about it. This I was sure. And in randomly looking at the stock and aftermarket downpipe the other night, I got to thinking about velocity, and mistakenly thought it was the velocity post turbine outlet that was effecting spool when we dropped the pipe. But that wasn't the answer.
The answer was "load". I'm just realizing this now. Duh!
The tighter exhaust diameter with the pipe on creates more load on the motor, which therefore causes the motor to build boost sooner, which therefore gives you more power down low, and probably a greater torque peak. I feel so dumb because I used to think about this phenomenon often and would explain it by load but it had been so long since I thought about it I totally forgot.
Thanks everyone for contributing and even if I was confused and battled with you guys I did deffo learn a lot so thank you.
In reading your guys posts, and thinking about it a bit more, I've realised what has happened. You see I had very strong evidence from myself and others that when you drop the pipe, the low end suffers. There was no doubt about it. This I was sure. And in randomly looking at the stock and aftermarket downpipe the other night, I got to thinking about velocity, and mistakenly thought it was the velocity post turbine outlet that was effecting spool when we dropped the pipe. But that wasn't the answer.
The answer was "load". I'm just realizing this now. Duh!
The tighter exhaust diameter with the pipe on creates more load on the motor, which therefore causes the motor to build boost sooner, which therefore gives you more power down low, and probably a greater torque peak. I feel so dumb because I used to think about this phenomenon often and would explain it by load but it had been so long since I thought about it I totally forgot.
Thanks everyone for contributing and even if I was confused and battled with you guys I did deffo learn a lot so thank you.
Apr 10, 2009, 08:40 AM
#40
I would quit relying on your butt dyno as the center of this whole argument. Go log some data to validate that the low end actually suffers. it's much more likely that the low end is completely the same after you drop the exhaust but it just feels dead because the top end is so much better.
Apr 10, 2009, 09:26 AM
#41
Interesting posts you found there Zeus. But I think the OP of those posts has some serious contradictions with real world evidence. He said 3" is too small for 450 hp? Buschur tested going from 3" to 3.5" at 800 hp and saw no gains. So the OP was very wrong in that statement. Which is a pretty big issue!
Secondly, the OP said that if you don't taper the enlarging of the exhaust correctly, that it could have a detrimental effect on spooling the turbo. For example, someone bolting a 3" downpipe onto a stock turbo elbow (o2 housing) which probably has a diameter of less than 2.5". There is no taper there. None of us use any taper in our exhaust. So to take from the above quote that bigger is better after the turbo is not correct.
Secondly, the OP said that if you don't taper the enlarging of the exhaust correctly, that it could have a detrimental effect on spooling the turbo. For example, someone bolting a 3" downpipe onto a stock turbo elbow (o2 housing) which probably has a diameter of less than 2.5". There is no taper there. None of us use any taper in our exhaust. So to take from the above quote that bigger is better after the turbo is not correct.
there is some good info here
Apr 10, 2009, 09:30 AM
#42
no one know it all, and one can learn something everyday by listening.
dont fell bad bro
dont fell bad bro
I am starting to feel a bit dumb now
In reading your guys posts, and thinking about it a bit more, I've realised what has happened. You see I had very strong evidence from myself and others that when you drop the pipe, the low end suffers. There was no doubt about it. This I was sure. And in randomly looking at the stock and aftermarket downpipe the other night, I got to thinking about velocity, and mistakenly thought it was the velocity post turbine outlet that was effecting spool when we dropped the pipe. But that wasn't the answer.
The answer was "load". I'm just realizing this now. Duh!
The tighter exhaust diameter with the pipe on creates more load on the motor, which therefore causes the motor to build boost sooner, which therefore gives you more power down low, and probably a greater torque peak. I feel so dumb because I used to think about this phenomenon often and would explain it by load but it had been so long since I thought about it I totally forgot.
Thanks everyone for contributing and even if I was confused and battled with you guys I did deffo learn a lot so thank you.
In reading your guys posts, and thinking about it a bit more, I've realised what has happened. You see I had very strong evidence from myself and others that when you drop the pipe, the low end suffers. There was no doubt about it. This I was sure. And in randomly looking at the stock and aftermarket downpipe the other night, I got to thinking about velocity, and mistakenly thought it was the velocity post turbine outlet that was effecting spool when we dropped the pipe. But that wasn't the answer.
The answer was "load". I'm just realizing this now. Duh!
The tighter exhaust diameter with the pipe on creates more load on the motor, which therefore causes the motor to build boost sooner, which therefore gives you more power down low, and probably a greater torque peak. I feel so dumb because I used to think about this phenomenon often and would explain it by load but it had been so long since I thought about it I totally forgot.
Thanks everyone for contributing and even if I was confused and battled with you guys I did deffo learn a lot so thank you.
Apr 10, 2009, 09:32 AM
#43
It's interesting that no one has brought up the thermal side of in these exhaust theories. It is suggested that a gradual increase in diameter looks to carry some some weight here. A question I have is how the exhaust gases tumble as they move along the back half of the exhaust, where the tempature of the exhaust gases are actually cooling. Is it the inceased diameter helping to keep the flow of gases moving along so that tumble is minimized? If so could we just use something to help retain the heat in the back half of the exhaust to keep the velocity of the gases more constant?
Apr 10, 2009, 10:58 AM
#44
I think you'll find that the exhaust gas flow is very turbulent under boost and the idea of things like "turbulence" and "tumble" being trouble is kind of misleading. If my numbers are right, a 3" OD tube flowing 40 pounds/min of air at 1300F and 14.5 PSIA has a Reynolds number of 128,000. On a Moody charty though, it looks like you can treat a 3" tube as a smooth pipe, provided you have a decent tube.
At high reynolds numbers, about all that matters is flow seperation from the tubing walls. Make too much of a bend too quickly and the flow seperates from the inside wall and basically your effective flow area is reduced.
This idea of constantly increaseing corss section is "ideal" because it basically creates a diffuser out of the exhaust. If done in a perfect frictionless world, you would actually have a pressure at the turbine outlest that is LOWER then atmospheric pressure. In function, a constantly increasing diameter may help offset some of the frictional pressure losses.
But it's all acedemic, as I'd LOVE to see somebody make an exhaust that had a constant taper from turbo to tail pipe. If you were talking some drag car with a 2' long straight pipe off the turbo, you could pull it off but I don't think it's going to happen on a car with a full exhaust.
At high reynolds numbers, about all that matters is flow seperation from the tubing walls. Make too much of a bend too quickly and the flow seperates from the inside wall and basically your effective flow area is reduced.
This idea of constantly increaseing corss section is "ideal" because it basically creates a diffuser out of the exhaust. If done in a perfect frictionless world, you would actually have a pressure at the turbine outlest that is LOWER then atmospheric pressure. In function, a constantly increasing diameter may help offset some of the frictional pressure losses.
But it's all acedemic, as I'd LOVE to see somebody make an exhaust that had a constant taper from turbo to tail pipe. If you were talking some drag car with a 2' long straight pipe off the turbo, you could pull it off but I don't think it's going to happen on a car with a full exhaust.
Last edited by 03whitegsr; Apr 10, 2009 at 11:23 AM.
Apr 10, 2009, 12:26 PM
#45
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The flow at the turbine discharge can have a tangential swirl component, the intensity and direction of which depends on the operating point of the turbine. Basically, if you were able to look directly at the turbine discharge, sometimes the flow would be swirling clockwise and sometimes it would be counter-clockwise, or even have no swirl at all. It depends on the tip speed of the turbine inducer relative to the flow velocity there, which depends on the match point (exh flow, % wastegated, size of turbine etc).
Now picture a jet of higher-pressure wastegated flow impinging radially on this swirling flow field at the turbine discharge. It does turn things into kind of a mess--you have a region of undeveloped highly turbulent flow right at the turbine discharge. Yikes!
If you have a nice straight exhaust after this, then the flow has a chance to become fully developed and have an easier time managing the pressure drop(s).
Really you want to slow down the flow downstream of the turbine since the pressure drop depends on mach number. You want to be somewhere in the 0.3 mach number range to avoid having dynamic head loss getting out of control.
You can do this a couple of ways. The single best way to do this is to increase the diameter of the pipe--the relationship of pressure drop in a straight pipe to pipe diameter is an inverse relationship to something like the 4th power (!!). You could also slow the flow by increasing its density (by cooling it down), but this is a lot harder to do and a lot less effective.
Every bend and kink and step in the pipe adds an additional loss factor which ratchets the backpressure up further and further. Separation is bad. Diffusers are good.
Now picture a jet of higher-pressure wastegated flow impinging radially on this swirling flow field at the turbine discharge. It does turn things into kind of a mess--you have a region of undeveloped highly turbulent flow right at the turbine discharge. Yikes!
If you have a nice straight exhaust after this, then the flow has a chance to become fully developed and have an easier time managing the pressure drop(s).
Really you want to slow down the flow downstream of the turbine since the pressure drop depends on mach number. You want to be somewhere in the 0.3 mach number range to avoid having dynamic head loss getting out of control.
You can do this a couple of ways. The single best way to do this is to increase the diameter of the pipe--the relationship of pressure drop in a straight pipe to pipe diameter is an inverse relationship to something like the 4th power (!!). You could also slow the flow by increasing its density (by cooling it down), but this is a lot harder to do and a lot less effective.
Every bend and kink and step in the pipe adds an additional loss factor which ratchets the backpressure up further and further. Separation is bad. Diffusers are good.
Last edited by JKav; Apr 10, 2009 at 12:33 PM.