exhaust bottleneck debate

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exhaust bottleneck debate

Postby toayoztan » Wed Sep 22, 2004 5:49 pm

Okay, my friend has a wrx. he knows quite a bit about cars, but when it comes to something he doesn't know, he seems to act like an ass and TRIES to make you look bad. He's got a 3" exhaust, and he's putting a putting a 4" dp on...

To be blunt, what's bottlenecking? okay, and is 4" dp to a 3" exhaust bottlenecking? Would this be a good thing or a bad thing?

thank you
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Postby BraveUlysses » Wed Sep 22, 2004 6:07 pm

Bottlenecking is exactly what it sounds like, to generalize, you have two sections that are different diameters, and thus will have different flow charactaristics than if they both had the same inner diameter. In this case, the bottlenecking occurs because you are going from a 4" dp (freaking huge) to a smaller 3" ID uppipe (or whatever the wrx's have). I don't see this as giving huge gains on a two liter boxer engine, but whatever.

KO engineering posted dyno sheets showing the minimal difference between 2.5" and 3" dp.
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Postby RIalltrac » Wed Sep 22, 2004 6:07 pm

what does he need a 4" downpipe for? Is he running insane hp? Technically a bottleneck would be anywhere in a flowing system where there is change in the size of the route that results in a backup. So going from a 3" downpipe to a 2" midpipe. That would be considered a bottleneck, now depending on cats and how the pipes are bent and things like that would determine how much of a bottleneck. I doubt he would see too much of a bottleneck effect going from 4 to 3 inches, I just dont see why he would need a downpipe that big, unless he's flowing huge amounts of exhaust gasses. I know old school motor heads with big hp supercharged v8 engines that dont go that big on exhaust.
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Postby turbo4wd » Wed Sep 22, 2004 6:09 pm

bottlenecking is exactly like it sounds.. going from an open area to a smaller area.. In your friends case, putting on a 4inch dp will do no good because the rest of the system is 3inch.. Other than for bragging rights and maybe some "bling" its pointless unless the rest of his system was 4inches.. and even then unless hes producing a HUGE amount of HP, 4 inches is really pointless..

im sure someone here has the link talking about 3inch exhaust diameters.. either tim or maybe gary can link it.. haha
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Postby Gary » Wed Sep 22, 2004 6:33 pm

Sorry no link :D
A disadvantage of too large exhaust pipe is it allows exhaust to cool down faster and that's not good. Hot exhaust is lighter and easier to be pushed out by the exgine than cold exhaust (more dense). The heat will not go any way but the engine bay.

And 4 inch to 3 inch is definitely a bottleneck. That is no brainer. It won't do any good but adding more weight to the car and stress to the turbo
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Postby Griffin » Wed Sep 22, 2004 7:15 pm

I have seen numerous studies that prove that decreasing exhaust diameter as you get closer to the muffler as long as it is done gradually causes no loss in power. The reason is that the exhaust gas does cool as it travels back and the increase in density will allow the same mass of exhaust to travel through a smaller area at the same pressure.

In other words its okay to get smaller as you go further back in the exhaust.
Last edited by Griffin on Fri Sep 24, 2004 2:50 pm, edited 1 time in total.
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Postby Fat Trac » Thu Sep 23, 2004 2:00 am

4 inch downpipe... what a waste. especially pointless considering how the rest is 3
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Postby maroon_185 » Thu Sep 23, 2004 3:01 am

yeah
4" to 3" is pointless, 4" on a car that I am assuming not pushing out anythign over 350 crank is pointelss anyways.
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Postby alltracman78 » Thu Sep 23, 2004 3:37 am

Griffin wrote:I have seen numerous studies that prove that decreasing exhaust diameter as you get closer to the muffler as long as it is done gradually causes no loss in power. 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.

In other words its okay to get smaller as you go further back in the exhaust.


Yeah, that's pretty much how I feel about it.
However, 4" does seem like a bit too much for him.
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Postby busdriver » Thu Sep 23, 2004 5:13 am

I think unless his engine says Duramax or Cummins on it 4" exhaust is overkill. even if it is just the downpipe.
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Postby alltracman78 » Thu Sep 23, 2004 9:28 am

busdriver wrote:I think unless his engine says Duramax or Cummins on it 4" exhaust is overkill. even if it is just the downpipe.


:D
Or it's a 2JZ
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Postby Conrad_Turbo » Thu Sep 23, 2004 12:42 pm

Griffin wrote:I have seen numerous studies that prove that decreasing exhaust diameter as you get closer to the muffler as long as it is done gradually causes no loss in power. 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.

In other words its okay to get smaller as you go further back in the exhaust.


Bingo.

Bottlenecking doesn't necessarily mean the pipe goes from a larger one to a smaller one. A bottleneck is a restriction in flow, so technically the whole exhaust system is a bottleneck as compared to no exhaust. There are some sections of exhaust where it may be more bottleneck than other sections though.
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Postby toayoztan » Thu Sep 23, 2004 9:46 pm

Well, i just wanted to clear things up with my friend. All of you are right on what bottlenecking is, in my opinion. Any restriction in the exhaust, even going from small to big or big to small, or simply CATS and what not. He just doesn't believe that the 4" dp to a 3" exhaust would be bottlenecking. I don't really care what performance gains/loss he'll get out of it, but simply he's arguing it's not bottlenecking. He even showed me a beer bottle and said "see man, that's bottlenecking..small to big"...haha okay, whatever.

By the way, he's running STi turbo, STi tranny, 19psi. He's getting dynoed next week after his chip comes in. He should be running around low 13s, if not high 12s. as for the HP figures, no clue at all.

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Postby Gary » Thu Sep 23, 2004 10:21 pm

toayoztan wrote:He even showed me a beer bottle and said "see man, that's bottlenecking..small to big"...

In that case, http://dictionary.reference.com/search?q=bottleneck :D
From Fluid Dynamic class, bottleneck is a narrow passage and point of congestion where molecules of fluid get closer to each other and start colliding. Hence, the pressure is built up. 4 inch to 3 inch results in 44% reduction in crossectional area, you do the math :wink:
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Postby Darth Boost » Fri Sep 24, 2004 1:34 am

This thread reminds me of a thread on a Honda discussion board for two reasons...

1. Go look at a 4" DP. No, really. Go look. Because if you did, then you wouldn't be talking shit about how it's four inches and overkill and oh-so-bad, because it's not. It's four inches WIDE and about 2.5" TALL where it bolts to the turbo, meaning it's OVAL... then it tapers down to 3" where it bolts to the exhaust. Why's it oval? Because the TD04L is ineternally gated, and every internally gated turbo has an oval flange. You all make it sound like it's a 4" round pipe that butts up against a flange with a 3" hole in the middle.

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...
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