exhaust diameter sizes

[S4evr]

So this summer i was going to redo my exhaust but am unsure of what i want to do. currently, i have piggies w/testpipes into a single 3" all the way back. my first thought was to make a dual 2.5 all the way back, but that would require alot of work and welding as well as tubing (I am making this myself). so then i thought, why not just go single 3.5" all the way back? i guess derek (deb38) is running something similar to this and said clearance was good enough for it to fit with ease. today however i compared cross sectional flow area between dual 2.5's and a single 3.5", and the dual 2.5" setup would yield me something like .2" more area of flow over the single 3.5" so with that being the case i wouldnt want that.. what are the possibilities of running a 4" exhaust? does it fit under the car, and would it be beneficial over a single 3.5" even if i still had piggies? i plan on welding in a y pipe as far up as possible on the dp's to improve flow, so almost right after the o2's. so from there back i plan on running 4".. can it be done?

dual 2.5" flow area = 9.82"
single 3.5" flow area = 9.62"
single 4" flow area = 12.56"

[JaredVL]

Why in the world do you want this? A single 3" will out flow what a K04 car is capable of making. A 3.5 would be more than ample for even a GT car.

[Jung]

a few weeks, perhaps a month ago there was an exhaust thread like this, and i think someone said that someone ran a single 4 and that it was slightly difficult to fit, but doable. give her a shot. lets see some action!

[S4evr]

Why in the world do you want this? A single 3" will out flow what a K04 car is capable of making. A 3.5 would be more than amble for even a GT car.

because i like to be creative, and having too large of an exhaust wont hurt. better than too small.

[Luca12]

Yes having too large of an exhaust CAN hurt.

[Jung]

this is true. why do you think we actually have an exhaust and not just a straight dump from the turbos? you need to have a little back pressure, a tad. you never see a b18 honda, no matter what the power level, running more than a 3.5" exhaust (unless it's a gayrod ricer). why? without some backpressure, you get some pretty substantial torque losses

[dc878mmm]

You mean it will hurt for the low end or for the clearance? Jason Harbinson at JHM has done a dual 3" to a single 4" for a RS6. He says, "the bigger the better, but thats as big as the car will fit".

[JaredVL]

Too little back pressure is a myth on a turbo car, the turbo creates any and all that you want. You want it as free as possible post turbo. cars dont come with open exhaust post turbo due to noise, emissions and customer tolerance restrictions.

But I will reiterate that 4" is way overkill and pointless for anything under 600 wheel, even at that its still excessive. The hurdles for fabrication, $$ and day to day usability due to how low it hangs make it pretty hard to justify once you have actually done one unless you really need the flow. As the gases travel down the exhaust and cool they loose volume, you can actually slowly taper down as you go out and not restrict anything. There are also gas velocity arguments that can be made against excessive sizes.

[BB2001S4:Tim]

Pics are from USP GT 2871R S4






4" Is JUST too much!!!! Especially w/o a muffler/resonator. But hey if you wanna give it a shot...go for it....GL

And Dereks car is effin LOUD...It does sound good though!

[dc878mmm]

Damn. I want that! ....but with dual tips.

[sCeRaXn]

you never see a b18 honda, no matter what the power level, running more than a 3.5" exhaust (unless it's a gayrod ricer).

You obviously never go into the FI forum on HT lol Everybody with a gt40, and alot of people with 35's have a 4" exhaust. I almost got an AFI 4" Al exhaust for a civic i was building, but i bought the s4 so all that went out the window.

For this application...i dont think 4" would be necessary. In fact, itll be a gross overkill...especially with piggies. Our turbos will never flow enough volume to need an exhaust of that size. The only thing youll do is slow the velocity...which is one of the only good things about having tiny turbos with high egt's lol

[sCeRaXn]

As the gases travel down the exhaust and cool they loose volume, you can actually slowly taper down as you go out and not restrict anything. There are also gas velocity arguments that can be made against excessive sizes.

The gases will lose velocity while cooling, not volume. Its theoretically impossible for them to lose volume. The volume can and will be displaced, which is what i think you meant. The affect that cooling has on velocity is best described in the Joules-Thompson Theory...which is far too long and drawn out for me to really get into. But if you get the time you should look up this theory and the ten thousand others that are related to it. It comes in really handy with all sorta of shit. Ive used it to help model everything from exhausts and int/exh manifolds to port shape designs.

A properly designed aft-tapered exhaust would outflow basically anything other than an open dp. The gains that the old school Group-B Rally RS200 Evo got from a tapered exhaust were astronmical. I cant remember the exact numbers, but i know it made like 40hp more than a straight 3.5" pipe...and the tapered exhaust ended at 2".

Theres a product idea for you Jared. A Stasis aft-tapered exhaust for the S4! Youd spend 100's of thousands of dollars in proper design and manufacturing...and you probably wouldnt come close to making it all back...but damn that would be fucking awesome! lol

[dc878mmm]

So Jared....I'm getting open dumps/exh bypass valves soon. Do you think I would have a better run if I clicked the switch to open the valves at 3500 rpm and have them closed before that or should I just have them open from the start?

[S4evr]

that 4" does look like a squeeze. i wouldnt run straight pipes either, i have done it before and it is far too loud for my liking. i would most likely add in a resonator to reduce the noise a little bit. i will pull off my old exhaust, and take a look at it to see what i can work with. will most likely go with a 3.5" tho.

[The_Jerbel]

Too little back pressure is a myth on a turbo car, the turbo creates any and all that you want. You want it as free as possible post turbo. cars dont come with open exhaust post turbo due to noise, emissions and customer tolerance restrictions.

But I will reiterate that 4" is way overkill and pointless for anything under 600 wheel, even at that its still excessive. The hurdles for fabrication, $$ and day to day usability due to how low it hangs make it pretty hard to justify once you have actually done one unless you really need the flow. As the gases travel down the exhaust and cool they loose volume, you can actually slowly taper down as you go out and not restrict anything. There are also gas velocity arguments that can be made against excessive sizes.

2nd.

Jarrett,
Gases do contract a little when they cool down due to the loss of energy in the gas molecules, and the resulting decrease in intra-molecular vibration and collision. They ALSO lose velocity.

[Crash6]

I've always read that the theoretical ideal exhaust for turbo applications begins with a collector (down pipe) that has the same diameter as the turbo outlet, then gradually tapers out for the length of the pipe(s).

I've been doing this kind of thing for a while, albeit with other makes/models of cars, and the downpipes have always proven to be the biggest power adders. The torque feels reduced down low, but it is typically gained across a larger power band that results in more power over all. This was especially true of the Ko3 equipped GTI's and A4's. Even when bottle necked (a big no-no) in a system that went from a 3" DP to a 2.5" cat back, there were significant gains over a straight 2.5" TB.

The gains for these cars are in the downpipes. ASP (and maybe APR/B&B) are the only 3" DP's that I am aware of that are true 3". Too bad they are both $1500 +.

For the money, the TT 2.5" TB, with true 2.5" dual cat backs seems like the best all around solution.

JMHO

[lil' is 300]

Supras with 800+hp run 4'' exhausts on inline-6. I agree with Jared 4'' for K04 is overkill - not that bigger is bad, but you won't see any gains over a 3.5'' and it's way easier to fit.

[Jung]

You obviously never go into the FI forum on HT lol Everybody with a gt40, and alot of people with 35's have a 4" exhaust. I almost got an AFI 4" Al exhaust for a civic i was building, but i bought the s4 so all that went out the window.

For this application...i dont think 4" would be necessary. In fact, itll be a gross overkill...especially with piggies. Our turbos will never flow enough volume to need an exhaust of that size. The only thing youll do is slow the velocity...which is one of the only good things about having tiny turbos with high egt's lol

im just saying, because my cousin has a t67 on his b18c '00 Si pushing 480 fwhp, and he's running a 3". he says he never saw bigger than 3.5 on a car like that, and he's been a JDM guy his whole life. but yah i guess that there are some people out there that go with the whole "bigger is better" mentality

[saucys4]

The gases will lose velocity while cooling, not volume. Its theoretically impossible for them to lose volume. The volume can and will be displaced, which is what i think you meant. The affect that cooling has on velocity is best described in the Joules-Thompson Theory...which is far too long and drawn out for me to really get into.

Google "Charles Law". Maybe your saying something that I am not following but gases do change their volume as temperature changes. In simple terms, D=M/V and the density of a gas changes with T and M is constant. Therefore the volume would have to change.

And the whole thing about gases having less volume is true BUT the effect of back pressure will be negligible compared to the reduction of back pressure from the cross sectional area.

[JaredVL]

The gases will lose velocity while cooling, not volume. Its theoretically impossible for them to lose volume. The volume can and will be displaced, which is what i think you meant. The affect that cooling has on velocity is best described in the Joules-Thompson Theory...which is far too long and drawn out for me to really get into. But if you get the time you should look up this theory and the ten thousand others that are related to it. It comes in really handy with all sorta of shit. Ive used it to help model everything from exhausts and int/exh manifolds to port shape designs.

A properly designed aft-tapered exhaust would outflow basically anything other than an open dp. The gains that the old school Group-B Rally RS200 Evo got from a tapered exhaust were astronmical. I cant remember the exact numbers, but i know it made like 40hp more than a straight 3.5" pipe...and the tapered exhaust ended at 2".

Theres a product idea for you Jared. A Stasis aft-tapered exhaust for the S4! Youd spend 100's of thousands of dollars in proper design and manufacturing...and you probably wouldnt come close to making it all back...but damn that would be fucking awesome! lol

Good job on finding google.com...

While you are at it how about you find me a theory that talks about exhaust (spent fuel, gas, air) and or anything similar not expanding when hot and contracting down in volume/mass/size whatever you want to call it as it cools

Sorry if that sounded rude, but c'mon man lets get real.

And yes I agree on the tapered system, thats what i was talking about before. I just chose to keep it short and sweet.

[BITRBO]

There was a very good discussion on this subject (as there have been many), that involved the late-great DoubleNutz to name a few... It was more about piggies vs. DP's, but still relevant.

There is obvisouly a limit to everything, including exhaust sizes. As Jared mentioned, turbo'd cars don't usually need any sort of exhaust (post-turbo) since the turbo provides enough backpressure already. That is why your WSC, Prototype, and LM cars (Porsche, Audi, etc.) will dump their exhaust post turbo ASAP, usually in 12" or less... In domestic applications, exhaust is required to control sound, emmissions, and heat (to name the big ones). The performance of a t/c'd engine relies heavily on the pressures before & after the turbine.

Now N/A cars are totally different, and can actually benefit somewhat in the torque-department from an "un-opened" exhaust. Here the effects of tapering actually help more, since you are relying more upon gas velocities than pressures. A properly designed exhaust system for a N/A car will provide sufficient taper to maintain velocity for scavanging, but being careful not to include too much backpressure to be harmful.

But getting back to our cars...

In t/c'd engines bigger is usually better, but not ALWAYS; the law of diminisioning returns does apply. If the exhaust size is too large (for a given length), the exhaust gases will cool too rapidly, slow, CONTRACT (i.e. densify), and block those exiting gases coming from the engine. As such, the exhaust post turbo should taper UP to the optimal size (to aid spool up) and mantain that diameter for it's [calculated] efficient length. But that's not to say tapering up to a 4" and carrying that to the bumper will work. Again, diminishing returns may apply here...

Anyway, here's some literature from an engineer at Garrett for those looking for further enlightenment on the subject. I would probably consider him to be an expert on the subject, although some of his information may not be relevant:


http://www.tantrumwerks.com/html/Spl...t%20theory.pdf

[TalipseAWD]

Damn
Nice read bitrbo

[dc878mmm]

Very credible source. Some seriously detailed info.

[sCeRaXn]

Good job on finding google.com...

While you are at it how about you find me a theory that talks about exhaust (spent fuel, gas, air) and or anything similar not expanding when hot and contracting down in volume/mass/size whatever you want to call it as it cools

Sorry if that sounded rude, but c'mon man lets get real.

And yes I agree on the tapered system, thats what i was talking about before. I just chose to keep it short and sweet.

Yes, Google was one a hell of resource when i was researching this over 5yrs ago. So was my Fluid Dynamics:Theoretical and Computational Approaches textook...but thats neither here nor there.

Give me a day or so and ill find you everything you want on "exhaust or anything similar" lol. However, im curious as to what youre trying to find out those things...as the theory ive already quoted will answer many of your questions. Not to mention that "contracting" down in volume would basically mean an aft-tapered exhaust, which has already been proven to work many times over. I wont be able to find a single theory that directly relates to your question because one probably doesnt exist...but im sure i can find many of them that will answer your questions when combined. This is why you have to actually do "research" to find answers...not everything is posted online and in one spot for you to steal your knowledge from.

I can start you in the right direction in case you wanted to actually do the research on your own. Look at the properties of the individual gases/substances created by the combustion process and how they act under different situations(such as large piping/low pressure, small piping/high pressure etc). If you want to get deep with it you can find the specific gravity of each subsequent gas, which will tell you the difference in speed between the individual gases and how they affect eachother. I can go on and on, but this is honestly way to deep to go when talking exhaust design.

Edited for the sake of peace!

[sCeRaXn]

In t/c'd engines bigger is usually better, but not ALWAYS; the law of diminisioning returns does apply. If the exhaust size is too large (for a given length), the exhaust gases will cool too rapidly, slow, CONTRACT (i.e. densify), and block those exiting gases coming from the engine. As such, the exhaust post turbo should taper UP to the optimal size (to aid spool up) and mantain that diameter for it's [calculated] efficient length. But that's not to say tapering up to a 4" and carrying that to the bumper will work. Again, diminishing returns may apply here...

This man knows his shit. This is basically what i said, except its formatted much better for an explanation. I wonder if he learned all of this on Google too? lol

[lil' is 300]

Google makes everyone a mechanical engineer...

[S4evr]

cant wait to take some more engineering orientated classes next year :-)

[BITRBO]

Google makes everyone a mechanical engineer...

...so does a masters and a P.E.

[sCeRaXn]

Google makes everyone a mechanical engineer...

Was this directed at me? Im not an Engineer...but my previous job basically required me to know as much about airflow as possible. The only reason i said any of that is to help those who like to know the details behind the usual responses.