Backpressure-

[texan]

Originally posted by rybred
hey! so by what you explained there, do you think it's only wise to go with 3 inch exhaust piping only if my engine was fully blown pushin about 10psi boost or more??
I'm going to get some custom piping done and i was thinkin to go with 2.5 inch tubing, with my 92 lude si, and i don't have a turbo in yet, but would it be wise to go with a smaller 2" or so until i have a turbo in? Also do you know what an effect a 3 angle valve job would have on backpressure if any? I got that done recently all in part of the prep for the turbo install! Any feed back would be greatly appriciated! Thanks!

When it comes to a turbocharged motor, it's virtually impossible to go too big on the exhaust diameter. 3" is definitely a great size, and anything larger would be very difficult to fit under the car without serious scraping problems.

A 3 angle valve job only helps to efficiently flow air into and out-of the motor, it has no direct effect on exhaust backpressure. I don't know about the H23A, but the 5th gen Prelude H22A4 comes from the factory with a 3 angle valve job.

[Someguy]

As Texan said, its possible to go too big with the exhaust with a turbo (especially the down pipe), but not easily. Too large of a pipe is effectly like not having a pipe at all, and as a result you can cause some turbulance and reversion issues, but that's not going to be a problem here. 3 inch is fine for the turbo'd motor.

It is big for the motor in NA though. If you are planning to add a turbo why not save yourself some time and money and just do it all at once?

[Prelussion]

Originally posted by Someguy
As Texan said, its possible to go too big with the exhaust with a turbo (especially the down pipe), but not easily. Too large of a pipe is effectly like not having a pipe at all, and as a result you can cause some turbulance and reversion issues, but that's not going to be a problem here. 3 inch is fine for the turbo'd motor.

It is big for the motor in NA though. If you are planning to add a turbo why not save yourself some time and money and just do it all at once?

W/ a large down pipe you will likely lose your A/C and if you have a short shifter and a 3" exhaust i bet it'll rub. plus your exhaust will rub other places as well.

[Marasmus]

Originally posted by Someguy
As Texan said, its possible to go too big with the exhaust with a turbo (especially the down pipe), but not easily. Too large of a pipe is effectly like not having a pipe at all, and as a result you can cause some turbulance and reversion issues, but that's not going to be a problem here. 3 inch is fine for the turbo'd motor.

It is big for the motor in NA though. If you are planning to add a turbo why not save yourself some time and money and just do it all at once?

Correct me if i'm wrong here - i'm totally speculating, as I've never personally worked with turbos - just superchargers.

A turbo functions by allowing exhaust gases to flow by a spool, which gathers rotational velocity from the exhaust gases. The higher the rotational velocity (and the geometry of the spool), the higher the boost.

Preludes come with 2 or 2.25" piping if I remember correctly. If you stick a fat 3" pipe on something like a prelude si, you're significantly slowing down the velocity of the air which turns your spool. What this will do is raise the bottom-end RPM at which your turbo is useful, and lower the PSI boost of the turbo at most RPMs (excluding high RPM, at which you can reach a terminal rotational velocity due to the geometry of the spool). If I'm thinking about this correctly, this can seriously weaken the power of your turbo.

I did some number-crunching on my car (2000 integra GS-R), taking displacement (1800cc), top RPM (8000 rpm) and pipe diameter(2.25in) (and thus cross-sectional area) into account. I came up with a maximum velocity of 219 ft/s from the car, stock. At top RPM, it's still below the 240 ft/s "magic number". In other words, I'd have to do some max RPM changes or forced-induction before my exhaust velocity reached an alarming level.

Adding a turbo to my car, pushing 8 psi (beyond that would probably trigger detonation due to the 10.0:1 compression ratio), would not increase my exhaust pressure by 8 psi because turbo sucks gas from the exhaust to feed the intake. Let's take a random guess, for the sake of example, that 50% (4 psi) of that psi boost gets recycled to the intake. That means that the default ambient pressure, 14.696 psi (1 atmosphere at sea level) would be increased to 18.696 psi. That's a 27.2% pressure increase. In order to keep the overall maximum exhaust pressure the same, I would need to increase the cross-sectional area of my exhaust piping by 27.2%.

The math:
exhaust dia^2 * 1.272 = ideal dia^2
2.25in ^2 * 1.272 = ideal dia^2
6.4395 sq. inches = ideal dia^2
2.538 in = ideal diameter

So.. if I bolted on an 8psi turbo, my ideal piping diameter, to keep my exact max velocity (which is a little bit on the low side, actually), would be 2.5 inches. I could just scam Type-R headers and cat, and grab some 2.5" aftermarket cat-back exhaust. However, a 3" pipe would completely drop my exhaust velocity, slow down the turbo, and really hurt my low-to-mid range.

Unless you're planning something like a 15psi boost on a 'lude, I don't think there's ever a need to go to 3" exhaust on a street car.

If anyone sees anything I screwed up here, please shout! I'm just making some guesses here based on my very rusty math.

[texan]

That all makes perfect sense to me... if pressure concerns at low airflow levels are the most important constiuent to turbocharger exhaust design AFTER the turbine section. Which to my knowledge is not the case, and here's my reasoning:

The turbine section of any turbocharger represents the primary restriction to flow in the exhaust path, and is always responsible for a great deal of pressure gain in the manifold itself. Basically any portion of the exhaust system which is ahead of the turbine inlet has high pressure under non-idle and cruise engine running, which means everywhere we are concerned about performance. Additionally, what makes a turbo spool up quickly is, as exhaust system design from one side of the turbine to the other is concerned, is pressure differential. At lower flow levels the large (sometimes ridiculously large) piping aft of the turbine presents the largest possible difference in pressure from one side to the other, hence also aids in spooling the turbo better than any other setup. To make the piping smaller will have no positive effect on lowering gas pressure, since the dynamics of exhaust flow change so extremely after flowing through a turbine to begin with. Think of the turbine as the blender, the exhaust gasses as the veggies, and I think you'll get the general idea. What it will have an effect on is overall flow ability vs. pressure, and that effect will be negative concerning any remotely workable piping size (surely if you went to 7" wide you might find problems, but you couldn't fit such a large pipe under there in the first place). So in essence, a turbo exhaust is a completely different beast than NA or supercharged engine setups and therefore requires a different approach to maximizing performance through piping size choice.

Basically, what we're after is a piping size that will not present any significant pressure gains at the airflow levels we will be seeing, especially at higher RPM and boost levels. The low RPM spool caracteristics will take care of themselves well with this line of thinking, and ensure that not only are we allowing spool to commence unimpeded by pressure gain aft of the turbine, but also that overal flow will not be restricted at high RPM's and high boost.

[Someguy]

My reasoning is along the lines of Texan's. The intake side of a turbo is very similiar to that of an oddly placed centrifical supercharger. The exhaust side is completely different then that of a supercharged or NA engine though.

On a NA engine the exhaulst gasses act as a series of pulses. In that situation exhaulst tuning becomes very critical since the response of a linear system to a pulse is typically a series of oscilations. To make the best effiency in your system you have to hit the lows and miss the highs of that response. That's pretty tricky. A similiar situation occurs with a supercharger, besides you have possitive intake manifold pressure, and is therefore easier to overdo it (a large portion of your intake charge goes right back out your exhaulst ports during valve overlap).

With a turbo the input of the system is much more steady. This is because, as Texan mentioned, the turbo acts as a major restriction. The exhaulst manifolds for turbos are much different then for NA applications. A good one won't have much of anything in the way of tubes. Basically its where pressure builds. The turbo does "blend" the flow also. A good demonstration of that is if you take the muffler(s) off a turbo'd motor and off a NA one. The NA will be loud as hell, while the turbo'd motor will be much quieter. Its the difference between gasses "pouring" out and "punching" (for lack of better terms) out.

Well, time for bed.

[Someguy]

Originally posted by Prelussion


W/ a large down pipe you will likely lose your A/C and if you have a short shifter and a 3" exhaust i bet it'll rub. plus your exhaust will rub other places as well.

Finding the balance between ideal and practical is the difference between science and engineering; IE art.

[rybred]

thanks everyone! that pretty much cleared everything up in terms of backpressure...at least i have a much better understanding of the difference between superchargers and turbos!

[HACO_ID_85]

I couldn't read the whole thing i got bored so i'm just going to ask. I went and baught a muffler then i went to a muffler shop and got 2.5" piping running from the cat back. I was talking to the guy and he said that if i were to gut or replace my cat and add headers i would lose my low end torque. is this correct. or can i add headers and gut the cat .I drive a 90 accord

[93vtecer]

Yup he sure is right you need a certain amount of back pressure on low end. Ask anyone who has had an exhaust leak, your car may even idol funny. You will see the gain at higher rpm's, You can just look at your tach and you will see the rpm's raise quicker as you approach red line.

[zippeay]

Ok I just put a 2 1/2 inch exhaust on my 92 civic "with a b16a" full header back exhaust with a high flow cat. Its so DAMN LOUD!!! I don't like it, plus I was racing my friend "he has a 2001 VW golf 1.8T" and I was doing fine until I hit 100 then he passed me by like I was standing still. Is this because my exhaust is too big? Should I put a stock cat back in? The only thing else I have done right now is cold air intake. Please help

[eckoman_pdx]

I have a 92 Civic with a LS in it, also, no header, only a Cold Airi intake It's currently an LS auto). When I bought a universal muffler and had someone build me a catback system, he said that anything above 2 1/4 and I'd lose pressure. It would hurt performance. Since I have an LS, he told me to go with a 2 if its an AT, since the AT needs more backpresuure at the low end. He said that a 2 1/4 would give me a little more top end, not quite as much low (whihc isnt as much a problem for MT). Anyways, he said dont go above 2 1/4, so I didnt, and thats where I sit. I think 2 1/2 would be too much for a b16a. I could be wrong, but thats what I heard. Also, I put a resonator on my exhaust system, really quited it down. Thats what DC sports uses, a 15" or 18" resonator, I think. Hope this helps. If I'm wrong on any of this, feel free to correct me

[Hatchie Boy]

you guys are smart
ooooh ahhhh smart

[97gsr]

NICE WRITEUP....THANKS!!


here... i'll throw up my setup...
and let me know what you guys think...

'97 GS-R...
I have AEM intake...
just bought the rs-r ex mag exhaust(2.36 inch)
and will mate it with the jdm oem 4-1 header(2.5 inch)
modified free flow cat(not stock cat)

after that, i might do cams, and wrap it up with fuel pressure regulator, apexi v-afc and some tuning!

thanks guys!!
/hijack

[Ricochet]

Wow, spend $600+ on a great exhaust, $400+ on a tuned header, $200+ on an intake, and you gain a whopping 15-20 horsepower. Thanks for all the tech info, but it's such a large headache to think about with minimal payback in real life.