exhaust bottleneck debate

Q&A regarding engines, turbos, and intercoolers and power upgrades

Postby busdriver » Fri Sep 24, 2004 2:14 am

"The reason is that the exhaust gas does cool as it travels back and the reduction in density will allow the same mass of exhaust to travel through a smaller area at the same pressure."

He might have said reduction but if you read what else he said it's right, cooler (more dense) air, takes up less volume per unit mass, so the same volume can flow through a smaller area at the same pressure.

Just out of curiousity, you said with the 4" downpipe that it is 3" diameter where it bolts to the exhaust? Wouldn't that be a 3" downpipe then? I was under the impression that a downpipe, where it bolts to the turbo, must be a certain size, the size of the turbo, and that size had no influence on whether it was a "3 inch" or "4 inch" downpipe? I am asking here so please don't take this the wrong way.
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Postby alltracman78 » Fri Sep 24, 2004 2:15 am

I think he meant it takes up less volume.
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Postby busdriver » Fri Sep 24, 2004 2:30 am

Typos happen, nothing to rant about. :smokes:
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Postby Darth Boost » Fri Sep 24, 2004 4:17 am

Typos do happen, but an inaccuracy in a technical discussion isn't a typo.

Jake, the Subee guys call it a 4" downpipe, so for the sake of arguement, I guess we're all calling it the same thing. In person, it looks HUGE, because you can only view the major dimention of the oval, not the minor, but upon further inspection, you can see it tapering as it goes down the firewall and under the car. You are correct in your impression, and I agree. It's one of those things I never gave much thought to, really. There's more pressing issues that need my attention. Like typos. ;) :P
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Postby busdriver » Fri Sep 24, 2004 5:45 am

It's all good. A couple more Subaru questions though.

Do they even make a "3 inch" downpipe for the subaru? Or is the 4" by 2.5" that tapers to 3" what they get to buy? Do they make one for them like the Aussie DP for ours?
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Postby Darth Boost » Fri Sep 24, 2004 10:50 am

busdriver wrote:Do they even make a "3 inch" downpipe for the subaru? Or is the 4" by 2.5" that tapers to 3" what they get to buy?


I'm not sure. I don't own one, so I don't really know what's available for 'em.

Do they make one for them like the Aussie DP for ours?


You mean, "Do they make a downpipe for them that has the worst wastegate outlet/ turbine outlet merge possible?" Again, I don't know. But I can tell you this- the Aussie DP leaves a LOT to be desired in the way that collector is designed.
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Postby Griffin » Fri Sep 24, 2004 3:45 pm

Darth Boost wrote:2. Someone mentioned exhaust gas cooling and becoming less dense. Uh, you sure about that? So hot gas is more dense than cool gas? Air is a gas. So by your reasoning, a hot air intake is a swell idea, because it's ingesting nice, hot, dense air, right? Might wanna rethink that one, bucko...


Hmmm - this is my new favorite person. :wink:

Reduction was a typo and the point of my post was obvious. I post on here when I get a few minutes here and there at work or at home and I appologize, but because I am not getting paid to post I don't necessarily have the luxury of taking the time to proofread, spellcheck and re-edit all my posts. I appologize for the inconvenience that may have caused.

That being said, if the gas was expanding or increasing in density when cooling (which of course violates Boyle's law) then a reduction in pipe diameter would obviously NOT be ok for continued flow at the same pressure. Everyone besides you understood exactly what I was saying, however you were apparently unable to extrapolate that information from the context of my post. So who is the moron - the 99% of the people who understood the obvious - or you - the small minded individual that didn't?

On another subject, standard industry terminology refers to exhaust pipes by diameter. If the pipe is not round the appropriate method to calculate the stated or advertized size is to still find the inside circumference of the pipe and calculate for size as though it was in fact a circle. The reason for this is obvious in that as long as the crosssection of pipe has the same area it pretty much flows the same amount of gas. Your argument is that the WRX has what is referred to as a 4" downpipe by the seller or the Subaru community, but that its actually an oval and the 4" section is the wider section of the oval. If is the case then you can not accurately use the terminology 4". I think most people here (like myself) don't really care about the Subaru community enough to familiarize ourselves with their little nuances. So if people come here and use incorrect terminology which gives the impression that someone is doing somethign silly, I think its appropriate to expect folks to be skeptical about the value of their actions.

I think its pretty clear that despite your efforts to correct and ridicule the world at large you simply succed in proving yourself a misguided and childish person.
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Postby tim » Fri Sep 24, 2004 4:11 pm

But I can tell you this- the Aussie DP leaves a LOT to be desired in the way that collector is designed.


1. Find me the space. I have yet to see any downpipe - aside from one-offs - for the CT26 or CT20b that runs a divorced wastegate setup. Space is very limited in the real estate around the turbo.

2. Help me make economic sense of it. People already flinch at the price of most DPs on the market ("you want me to pay HOW MUCH for a 2 foot long piece of metal?!"). The extra fab time=extra costs to the consumer (which they have no interest in paying).

3. Most importantly, find me the need. Cars equipped with Aussie DPs have started hitting the 280-300hp mark with startling regularity. Given that 300hp is pretty much the upper limits of the CT26 and CT20, I'm not sure what I'm going to gain through all the extra work. I don't see turbulent exhaust flow costing me a whole lot of HP here.

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Postby Darth Boost » Fri Sep 24, 2004 11:25 pm

Griffin wrote:So who is the moron - the 99% of the people who understood the obvious - or you - the small minded individual that didn't?

I think its pretty clear that despite your efforts to correct and ridicule the world at large you simply succed in proving yourself a misguided and childish person.


The fact that you make it personal, as I've outlined above, prevents me from further discussion with you. It's a shame, because I would have rather enjoyed a technical debate. But since you stoop to this childish level you reference, I feel quite compelled to stop posting in this thread and distance myself from any association with you at all. Please don't reply to me in the future, and I'll do the same. That is, unless you feel the need to continue being... "childish."

Tim- I agree with all your points. I'm not saying Aussie is crap. I think TIG welds would be nice, if nothing more than for the estetics (sic), but what do I know, huh? All I was saying was that the collector is far from optimal.
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Postby gearhead313 » Fri Sep 24, 2004 11:50 pm

...i can't believe no one has mentioned turbulance in the exhaust. a 2" exhaust opening next to the wastegate flapper all dumping into a massive 4" pipe..... :crazy:
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Postby MrWOT » Sat Sep 25, 2004 2:33 am

gearhead313 wrote:...i can't believe no one has mentioned turbulance in the exhaust. a 2" exhaust opening next to the wastegate flapper all dumping into a massive 4" pipe..... :crazy:


Funny, I was thinking I couldn't believe noone named the real reason the 4" downpipe would be bad. The answer is volume. When you increase volume in a given area, potential flow may increase, BUT, as gases expand they lose velocity, the 4" dp may give a tiny increase in top end if any, but the loss of velocity will be more harmful then any increase because after it expands and slows down, he's then trying to cram it back into the 3" pipe with lower energy available (since the gas cools as it expands), expansion is a free action except in regards to velocity, however compression costs energy in this case, so he's losing on 2 ends, loss of velocity and recompression. If he went for a full 4" system back, meh, but pretty useless for the kind of gain over 3" you would see on a streetable system, since he would only flow that much around 9k+ rpm :wink:

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Postby furpo » Sat Sep 25, 2004 4:56 am

i have often woundered what the point of having an exhaust bigger than the turbine outlet side would be? obviously the section would need to be made a little bigger where the wastegate goes in.

this link has a picture of the ct26 and the size of the outlet. http://www.mjmturbos.com/CT26OEMvsUPDRADE.Htm

so from that about 2 1/2 inches would be about right.
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Postby Griffin » Sat Sep 25, 2004 10:31 am

Darth Boost wrote:
The fact that you make it personal, as I've outlined above, prevents me from further discussion with you. It's a shame, because I would have rather enjoyed a technical debate. But since you stoop to this childish level you reference, I feel quite compelled to stop posting in this thread and distance myself from any association with you at all. Please don't reply to me in the future, and I'll do the same.


:cry: I'm crushed...

right.... so anyways - you saying "So hot gas is more dense than cool gas? Air is a gas. So by your reasoning, a hot air intake is a swell idea, because it's ingesting nice, hot, dense air, right? Might wanna rethink that one, bucko..." and "Typos do happen, but an inaccuracy in a technical discussion isn't a typo." in response to an obvious typo is Ok and perfectly acceptable - but me taking exception to the aforementioned smart-assedness is overreactionary? Thats hardly fair. No offense bro - but if you're going to dish it out you best be prepared to take some in return.
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Postby gearhead313 » Sat Sep 25, 2004 2:13 pm

MrWOT wrote:
gearhead313 wrote:...i can't believe no one has mentioned turbulance in the exhaust. a 2" exhaust opening next to the wastegate flapper all dumping into a massive 4" pipe..... :crazy:


Funny, I was thinking I couldn't believe noone named the real reason the 4" downpipe would be bad. The answer is volume. When you increase volume in a given area, potential flow may increase, BUT, as gases expand they lose velocity, the 4" dp may give a tiny increase in top end if any, but the loss of velocity will be more harmful then any increase because after it expands and slows down, he's then trying to cram it back into the 3" pipe with lower energy available (since the gas cools as it expands), expansion is a free action except in regards to velocity, however compression costs energy in this case, so he's losing on 2 ends, loss of velocity and recompression. If he went for a full 4" system back, meh, but pretty useless for the kind of gain over 3" you would see on a streetable system, since he would only flow that much around 9k+ rpm :wink:

I've said my bit, and I'm off to bed.



of the very little i know about thermodynamics, this is excellently said.


I think another point is dumping the wastegate right next to the turbo outlet. I think if you kept the same size piping and simply separated the 2, ran a 1.5" wastegate pipe a foot down, it would improve flow more than the stupid 4" piece.
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Postby toayoztan » Sun Sep 26, 2004 4:31 am

My WRX friend said something about the 4" dp being a bell curve (bell housing) that goes into a 3" flange for the rest of the exhaust system. He said this improves airflow to make the air flow alot smoother...and he said the dp was good at 400hp (proven)...well, whatever. I was just trying to tell him that it was a bottleneck lol.

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