is 3" exhaust worth it?

[jonnygsx]

I'm trying to decide whether to upgrade my exhaust or not. When i was first starting out modifying my car I got a really good deal on a 2.5' catback (less than $100). So I just stayed with the 2.5' theme and got a 2.5' test pipe, and 2.5' downpipe. Now I'm solid into my mod's... I'm looking to hit high 12's on a small 16g. But, I'm trying to decide whether it is worth upgrading to a 3" exhaust or not. I've heard that the o2 housing exit is only 2.5" big and that an exhaust much bigger than that doesn't really help.... What are your guys thoughts?

Jon

 

[Nappy94]

Stick with the 2.5" exhaust. If you are not aiming for 400+ BHP then a 3" exhaust is not necessary.

 

[HyperGSX]

Stick with 2.5". People have already run the times you want and beyond with a 2.5" exhaust.

 

[jdmawd]

Get the 3" exhaust, 2.5 sounds like ass and the cost difference is very small. Plan for the future.

 

[Vegas Smith]

Originally posted by jdmawd
Get the 3" exhaust, 2.5 sounds like ass and the cost difference is very small. Plan for the future.

Sounds like ass???? I have 2.5 all through and i have yet to hear an import that sounds better than mine. If anything, the 3 inches sound like behind.

 

[MNGSX]

I've heard that the o2 housing exit is only 2.5" big and that an exhaust much bigger than that doesn't really help.

It does help to have a bigger exhaust than the o2 housing exit. Just use a long transition cone instead of a steped adapter. You want gradual changes in diameter. As long as the slope of the change is'nt too steep the tranisiton to larger diameter will increase flow and not hurt it, Towards the front you can have it narrower. It is still plenty hot and the velocity is high despite the diameter. As it travels farther down the line the gasses slow and cool and you need more ID to move the exhaust gases at the same rate.

~300hp 2.5"
~350?-400hp 3"

over 400 I would actually start to look bigger than 3"

http://www.vishnutuning.com/exhaust101.htm

 

[jdmawd]

my o2 housing is 3" and its not flared

 

[MNGSX]

These are better than what you find at say pep boys. No ledge just a taper but short.

http://www.burnsstainless.com/Transitions/transitions.html

The best way
http://www.burnsstainless.com/Megaphones/megaphones.html

Another good source
http://www.coneeng.com/exhbld.htm


I'm building an O2 housing based on the Garret engineers advice.

The wg tube will be smoothed into the main pipe about 12" down line.
I'll use a long transition from the turbine outlet diameter to 3". then a long 3" to 3.5" megaphone after the DP to transition to the cat back.

 

[DSM Stang-Eater]

wait, wouldnt u still gain more hp from a 3' regardless? its not like u would loose power from a 3' inch rite? or would a 2.5' with external dump be close to a 3'?

 

[MNGSX]

Originally posted by DSM Stang-Eater
wait, wouldnt u still gain more hp from a 3' regardless? its not like u would loose power from a 3' inch rite? or would a 2.5' with external dump be close to a 3'?

You could theoretically make more power. Since the wg'd exhaust flow does'nt flow thru the same pipe anywhere. It does'nt distrupt the velocity of the main flow when the gate opens by re-entering the system.

 

[doug]

Originally posted by MNGSX
~300hp 2.5"
~350?-400hp 3"

over 400 I would actually start to look bigger than 3"

http://www.vishnutuning.com/exhaust101.htm

Interesting read. Although it seems to contradict some of the things Corky Bell says in Maximum Boost. Seems to make sense, though. Looks like more research is in order on my end....

 

[diamondstar36]

If you're not flowing the exhaust gases to support it, adding a 3in will do about nothing. Anything under 300hp and I couldn't imagine your hp increase with a half inch bigger exhaust to be anything over 1-5%, if that. But on the other hand, locally we have 2 cars making well over 500hp on 3in no problem.

 

[GoFastTSI]

Don't get me started on Vishnu. I wouldn't ever, ever, everrrr follow their instructions even if it were to save my life. Them and their "magic dyno".

 

[Lord Khan]

I don't see what the big deal is of NOT getting a 3" exhaust if you have the money for it. It really isn't THAT much more expensive than an 2.5 inch, and if you have the money for it, why not, if it doesn't give you power loss. It really depends on what you want to do with your car.

From what I've been told about turbo cars, the bigger the exhaust, the better. Its really best if they don't have an exhaust at all, but thats not going to happen with street laws.

Personally, I got the 3" Thermal catback, and plan to go full 3" when I get some more money. I'm not sure what turbo I want to get just yet, but I do know that the 2nd gen. Eclipse has been my domestic dream car since they first came out, so I plan to keep mine til it falls apart, so I'm trying to get the best quality upgrades I can for it, within a reasonable price. If some are overkill, then so be it, as long as it doesn't impeed performance, and it always leaves me the option of upgradability in the future.

 

[doug]

Originally posted by Lord Khan
From what I've been told about turbo cars, the bigger the exhaust, the better. Its really best if they don't have an exhaust at all, but thats not going to happen with street laws.

You often hear "the best exhaust for a turbo car is no exhaust," which is true. The problem, as I see it, is that many misinterpret that as "a bigger exhaust is always better." And this may not necessarily be true, in my opinion. Corky Bell explains it in his book, although to me, not very well. He says that the bigger the exhaust piping, the faster the exhaust velocity (which I have questions about actually, but that's another subject), and that at a point, the exhaust velocity becomes too high and creates turbulence. Now, if you have literally NO exhaust, then that's not a problem. But even if your exhaust is 3" or slightly more, it's still in a confined space and that turbulence will in itself create unwanted backpressure, he says, and decrease performance.

He recommends determining what your HP goals are and choosing an exhaust piping size based on that and the chart he provides in his book. That's why you sometimes see people say that 2.5" piping is good for up to around 450-500HP-- because that's what his chart indicates, although it's a little ambiguous since it's only a line graph with some rough numbers along the axes. That's my interpretation anyway-- I'd still love to see some before-and-after comparison tests of 2.5" vs 3" on the same car.

 

[MNGSX]

Originally posted by doug

You often hear "the best exhaust for a turbo car is no exhaust," which is true. The problem, as I see it, is that many misinterpret that as "a bigger exhaust is always better." And this may not necessarily be true, in my opinion. Corky Bell explains it in his book, although to me, not very well. He says that the bigger the exhaust piping, the faster the exhaust velocity (which I have questions about actually, but that's another subject), and that at a point, the exhaust velocity becomes too high and creates turbulence. .

That is worded goofy. A narrow pipe has more velocity. Think of a river during flood stage. The areas with canyon walls will be a raging torrent with alot of pressure at the canyon entrance. Where down stream hmm say with patures on either side it will be a big sluggish mud puddle. The atmosphere is the ocean.

Its like head port sizing. Striking the right balance between velocity and volumetric flow.

Velocity is'nt volume or lbs of air. It's just the wind speed if you will in that pipe. It has no problem getting thru the tapered 02/DP section. It is hot and leaving higher pressure for low. Power comes from fuel. More power.. more fuel and the extra air needed to burn the extra fuel.

Originally posted by doug

Now, if you have literally NO exhaust, then that's not a problem. But even if your exhaust is 3" or slightly more, it's still in a confined space and that turbulence will in itself create unwanted backpressure, he says, and decrease performance.

I agree with the no exhaust. But flow will increase with the tapered difusser attached to the turbine exducer as opossed to just a big dump pipe or nothing.

A gradual increase in diameter to a realistic diamerter will decrease backpressure. Going directly from say 2.5" to 6" in a step change will have alot of turbulance at the step and very slow flow in the pipe. A gradual taper... like a 7 degree or less included angle will have next to nothing for turbulance at the diameter change. If the new diameter is just right both velocity and the amount of air will be just fine.

Originally posted by doug


He recommends determining what your HP goals are and choosing an exhaust piping size based on that and the chart he provides in his book. That's why you sometimes see people say that 2.5" piping is good for up to around 450-500HP-- because that's what his chart indicates, although it's a little ambiguous since it's only a line graph with some rough numbers along the axes. That's my interpretation anyway-- I'd still love to see some before-and-after comparison tests of 2.5" vs 3" on the same car.

For that comparison I'd also like to see 2.5" and 3" with say a 16g then with say a 20g. If an exhaust for a 450hp setup could hamper a 350hp setup. Also how bad exhaust more suited for 350 chokes 450 down.

A test of o2 and dp sizing would be nice too. This would illustrate the difference from gradually tapering to full exhaust diameter vs the turbulance caused by jumping right to it.

Maybe his chart is right for the first section of an exhaust system and for short unmuffled race systems.

 

[MNGSX]

Originally posted by GoFastTSI
Don't get me started on Vishnu. I wouldn't ever, ever, everrrr follow their instructions even if it were to save my life. Them and their "magic dyno".

If I could find the link to the actual post they quoted from the Garrte engineer on the impreza board I'd have put up that link.

I don't care what vishnu thinks either... but a Garret engineer I'll listen to and soak it all in.

 

[doug]

Originally posted by MNGSX
A narrow pipe has more velocity.

Yeah, that's what I thought too. I need to go back and read that section of the book again to try and figure out why he said that, or if I'm just remembering it wrong.

 

[THEDUKE]

go 3

 

[800_Speed]

stick with 2.5", save your money for something else... like say a prostitute, or better yet other parts for your car

 

[RnR Racing]

Sorry guys, but you got to go with 3" turbo back even if you don't think you need it. It sounds better and performs better. I can assure you that even on a slightly modded car you will pick up some power and then when you mod it more you can have more room to work without needing to upgrade later. This is just my persoanl opinion. Also on a turbo car the 3" isn't that loud and we use it on all our street cars. It also isn't much more money.

 

[Cirus_93TSI]

although porting ur manifold and o2 housing will make a good ammount of power when combined wih a 3" exhaust, the 3" alone be a huge difference from the 2.5", i cant see y so many people waste money buying 2.5" exhausts, u should have thought about this in the begining and bought the 3" instead of wasting money on a 2.5" that isnt going give u much more power over the stock exhaust
on any turbo car the bigger the exhaust the more power, the o2 and mani do restrict air flow with a 3" but u will get more power then a 2.5, and when u do get the o2 ported u can get even more power, take ur exhaust completly off and run an open mani at the track and see how much faster u run, it will supprise u how much of a difference it makes
and u do realize that having a ported manifold gives u almost no power increase unless u have the o2 ported to match it? not that it would give u any power if u ported it anyway cause then the small 2.5" would restrict the airflow instead
u should be able to hit high 12s on stock turbo with a 3" and a few more mods like a fuel pump, with ur 16g u should be runnig mid 12s, but u need the 3" if u want to go that fast
i can tell u this, my best friend has a 96 tsi FWD with a 3", an MBC, intake, a front mount, and IC piping with a 1g BOV and he is running mid 13s on the stock unported turbo, ur awd, u should be able to go 12s easily, u need the 3"
and even the cheapest 3" exhausts like the ones custom made at exhaust shops sound much better on a turbo car then the best name brand 2.5" out there

 

[doug]

Originally posted by Cirus_93TSI
although porting ur manifold and o2 housing will make a good ammount of power when combined wih a 3" exhaust, the 3" alone be a huge difference from the 2.5", i cant see y so many people waste money buying 2.5" exhausts, u should have thought about this in the begining and bought the 3" instead of wasting money on a 2.5" that isnt going give u much more power over the stock exhaust

Well, you can keep saying this as much as you want (and quite poorly, I might add), but I am not going to be convinced unless I see some actual proof.

Maybe some day I can dyno my car with the 2.5" and switch over to a 3" and see for sure if I make more power.... Hey Ryan, wanna send me a 3" system to test?