exhaust bottleneck debate

toayoztan

Moderator
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
Bryan
 

BraveUlysses

New member
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.
 

RIalltrac

Active member
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.
 

turbo4wd

Moderator
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
 

Gary

Moderator
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
 

Griffin

New member
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.
 

maroon_185

New member
yeah
4" to 3" is pointless, 4" on a car that I am assuming not pushing out anythign over 350 crank is pointelss anyways.
 

alltracman78

Active member
Griffin":jchxubop said:
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.
 

Conrad_Turbo

New member
Griffin":1hcbx6vx said:
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.
 

toayoztan

Moderator
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.

Bryan
 

Gary

Moderator
toayoztan":1v8y0xxp said:
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:
 

Darth Boost

New member
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...
 

busdriver

New member
"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.
 

Darth Boost

New member
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
 

busdriver

New member
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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