Is 3" all the way a good thing!!
 

Hello, I can't figure out if i should go with 3" downpipe all the way or 2.5".
What the difference besides more air flow and looks.

cost, exhaust volume, and fitting are the negatives of 3 inch vs 2.5.

Frank

You mean exhaust noise?

I'm the same boat.
at start I was tempted to do all 3" for future upgrades... but now I think that 2,5" is more than enough for what I want:gay:

didn't we JUST have a thread about this?

ohh sorry i didn't check to see if there was one

3" isn't really any louder with all things being equal. You can have a near silent but well flowing 3" system.

corky says that your downpipe MUST start at the size of your turbo outlet flange, and should only increase from there at no more than 15 degrees at a time. a sudden change in size, and therefore pressure, will result in a turbulent flow, and therefore more drag, making it less efficent than a smoothly changed diameter pipe.

after that, 3" would be better(no exhaust would be best) but a 2.5" system that is smoother with less restrictions would be better than a 3" system with shitty welds and lots of bends.

I don't think it matters. Hondas make 700+whp with 3" turbo back exhausts. I doubt "turbulent" flow is a concern in their minds when it comes to the exhaust.

Is there any dyno proof of corky's theory? I don't doubt the theory, but I do doubt that the turbulence has any noticeable negative effect on flow in a real-world application.

There is no dyno proof as far as I know. As far as I'm concerned there is no such thing as non-turbulent flow exiting a turbine.

I have a 3" vband welded to the turbine housing of my t3 60/63 and I can make 15psi just past 3500rpm on my honda 1.6l.

I'm all for 3" as soon as you can get it. To me its the best compromise of size/cost/fitment/sound etc...
On that note... I won't run any turbo car with less than 3" at this point.

Did I mention that the 3" off the turbo and all the way back... is on my beater?

Toyota Supras start at 4" downpipes, and they make 5" and even 6" downpipes. You want a 3" downpipe.

BEGI's 2.75" DP starts at 2.25" or so and steps up slowly like Corky talks about to the end size. You can have them make it into a 3" DP at the end for another $30 or so. Order up a 3" Enthuza setup, and the car will be nice and quiet and flow very nicely.

Avoid turbulence (easier said than done).
Avoid steps and abrupt transistions in size.
Mandrel-bent pipes.
Swage the pipes so pipe inlets fit "over" the previous pipe's outlet.
Avoid sharp turns or protruberances.
Use quality catalytic convertor and muffler.
Reduce backpressure as much as possible after the turbo.
Separate the wastegate flow from the main flow.
Don't forget you have to fit all of that under the car and into the engine compartment. The larger everything is the more likely the exhaust will bang on the frame under cornering or when the engine torques. Mazda Competition's racing motor mounts helped my car significantly.

My system that Corky custom-built for me?
Downpipe entry: 4 inches square area
Downpipe to cat: 4.9 inches square
Mid-pipe: 7.1 inches square
Muffler inlet: 8 inches square
Exhaust outlet: 9.6 inches square

And...he guaranteed me at least one horsepower increase over stock. Pretty good deal in my book....

Barry
PS. I'm still waiting to hear confirmation from Corky about the final details of the Bonneville Streamliner on which he modified the intercooling system. The fellow who ran that Streamliner and another one (the guy makes a good living) at Bonneville set a new class record and, first time at Bonneville, became the 8th fastest human on wheels. Corky does good work.

Now...time for me to get back to work.

yep sounds like my new setup:

turbo is 2.25" outlet
DP 2.5" steps to 2.75"
test pipe 2.75" steps to 3.0"
catback 3.0" to the tip.

Mines gonna be 3" turbo back.

Just wondering...How much did that one guaranteed horsepower cost? :nxsmile: for more than one guaranteed horsepower

you mean: 4" to 3" turbo back. :gay:

thats what i said :P

I used to make a turbo kit for the MKIV Supra, 3.1 inch ID on the DP, if I remember correctly. Which was an exact match for the turbine outlets we used. No difference on a car making 913 to the wheels when we switched to a 4 inch downpipe.

Thanks BEGITechRep and others for stepping in and helping me out. Maximum boost and the BEGI website is fresh in my mind, but since I dont have a turbo yet, I feel like I dont have any real rep points here. when someone starts spouting their HP PSI details at me, ive got nothing to go on, unless they have tried scientific studies with various sized pipes.

i also am aiming for a similar goal as corky's engineering goal, which is the most HP per PSI, not total PSI. once I get my system up and running, then I can start bouncing numbers back when people say "ZOMG, ive got 30 more HP than you running 15 more PSI!!!1"

Corky's engineering goal is obviously good, since it keeps heat down, but given equal IATs, a car making 220whp at 15psi is going to be more fun to drive than a car making 220whp at 6psi or 8psi.

These two statements seem to contradict themselves. "All things being equal" meaning 2.5 bare piping vs 3.0 piping into the same brand/length muffler, the 3 inch is substantially louder. Now if you add resonators, etc, you can certainly quite a 3 inch to 2.5 levels. But then you're adding cost.

Exhaust cools the further down stream it gets. A lot of Suby guys run 3 inch DP->midpipe into a 2.5 inch axleback. There isn't a significant difference (above the dyno error) in sub 350 whp cars when switching to a 3 inch axleback. ~5 whp difference which falls well within the dynos error.

Frank

Eh? How do yo figure?

ben, the blast of torque when the turbo spools most likely.

fmowry, Mark probably is saying. a 2.5" with one muffler will be about the same noise level as a 3" with one muffler. all things being equal as in the components on and before the exhaust. the variable being the size of the exhaust piping.

The only reason I dropped the hat on a 3" was because I've heard one that isn't any "louder" than my 2.25", just deeper. Granted it used one large muffler, but if need be I'll add a cylinder muffler between the cat and diff.

Ha, ok, I suppose some could see that as "fun." Personally, I think a more usable power band is more fun ;)

I want to see what the torque curves on a dyno look like with the same engine, but one setup at 220hp @ 15psi, and one at 220hp @ 8psi. then i'll figure out which I like best.

The way I imagine it right now is that the 15psi is like that super hot chick that is showing massive clevage and gets every guy to look at her, but if you dated her she would take all your money and be a total bitch and thinks she's amazing in bed but isn't, and just wants to party and hang out with her friends instead of you.
the 8psi sounds like a decent looking girl who dresses conservatively, but you can talk intellectually to her and she happens to be a nyphomaniac and bi-curious.

i just join the rest of the guys in checking out the hot chick with massive clevage, then turn to my girlfriend and she says "damn i'd like to see her naked"

Interesting comparison.. :nuts:

The 3-inch turboback on my car does pretty much what Corky describes -- only has slight bends, and keeps the wastegate pipe separate from the main pipe for as long as possible (in this case, almost all the way into a high-flow cat).

https://www.miataturbo.net/general-miata-chat-9/pics-my-3-inch-mazdaspeed-exhaust-divorced-downpipe-10096/

Do I really need this big of a pipe for 200 rwhp? No, but it's nice headroom for when I have more $$ and build a 300 rwhp beast someday. :)

Oh and it sounds awesome. Not obnoxious but a really nice rumble.

My setup is 2.5" DP and 3" from the DP back... I decided (For the looks) to go with dual exhaust, with 3" thus meaning a larger area for exhaust gases, but my design wasn't the best and I believe I have turbulence where the pipe splits... And I also added more weight, so my $0.02 would be to stay with a single pipe...
\
miatex

Am I off in thinking that a 15psi 220whp car will build boost sooner and produce a more useable powerband than a car that won't build boost as quickly, which will produce a peaky powerband? Or is it somehow the other way around?

15psi and 220whp makes me think late spool, and a huge, quick rise in torque near late in the rev band, whereas a 6-8psi setup would more than likely build boost much earlier in the rpm band, to perhaps a lower peak torque, but much more flat. I guess you could have a quick, early spooling 15psi setup only reach 220whp, but it's be horrible inefficient/poorly tuned!

Car A has a teensy turbo that needs to be pushed to 15psi in order to make 220whp, while Car B has a bigger turbo that only needs to run at 8psi to make the same 220whp.

Car A would likely make peak torque earlier in the rev range than Car B.

Talk about pooping out in the upper rev range ;) That's no fun at all!

It wouldn't necessarily poop out in the upper RPM range. It's got a turbocharger, so boost pressure is controlled. This means, as long as you're not out of the flow range of the 'charger, torque curve should remain relatively flat.

There are so many variables, though, it's silly to make sweeping statements like these.

I've got that :bigtu:....hope she doesn't read this though :nono:

Well there's one thing to be said though. If you can make 220 hp on 6psi you probably have a much safer engine. Besides, no one is going to think you are a tuning guru if you can only make more hp by blasting the shit out of psi. If you have over 1 bar of boost you better be making at least 350 hp and in my opinion..more than that.

Well, it goes without saying that if you and your buddy have the exact same turbocharger setup, but he's making 220rwhp out of 10-12psi and you're pushing 15psi to make the same 220rwhp, either you're engine is tired, you're tuning too conservatively or he's tuning to aggressively.

It obvious that you'd want the most amount of power per PSI without sacrificing spool.

hence the 3" exhaust :rofl:

Generally, hp aside, running a higher psi at your wg will cause your turbo to spool up faster because it will reach peak efficiency faster.

So whats the verdict? 2.5 inch or 3 inch when the target is 200 - 230 hp?

btw, whats the stock exhausts diameter?

Go big or go home, Oregon or bust!

for the mother fucking WIN

Stretch bent 1 7/8"

Whats the diameter of the drinking straws they hand out at the movies? :eek:

That's because the turbine exducer is 3", btw 7-11 degree tapers are thought to be optimum and yes there is proof that this is the case but you can get away with more taper than that. The idea is to keep the transition smooth and yes, it does make a difference on the dyno.

Mark

On the dyno I was not able to tell the difference between a really nice 2.5" system and a 3" dump pipe that exited under the car on the dyno. It didn't make a damn bit more power. This was with a GT2860R with an internal wastegate.

It didn't even spool any better to be honest. With that small of a turbine at 240 RWHP, the 2.5" system was plenty adequate and a lot more quiet than the 3" dump pipe. I subsequently made 300 RWHP on that same 2.5" system with a T3/T4 hybrid, but I am pretty sure that it is now an issue compared to a 3" system. 2.5" systems are fine up to about 250 RWHP. (I will concede that on a well designed 3" system with an external wastegate you can see benefits of a 3" system as low as 200 RWHP. YMMV)

Mark

Well then, I will have my BEGI DP go into a nice fatty 3" back pipe with the external waste gate dumping to the atmosphere.. By 2008 that is..

We've done 250 whp on a stock exhaust before. (2" or 2.25" I think?)
My car has done 243 on a 2 1/2" exhaust.

Now that the 3" dual is on, I think that the power band is much smoother. There is not the sudden sit you back in the seat. But it has not been back to the dyno. The 3" sound is deeper. I wish I had put the 3" on sooner. :)
Stephanie

The best exhaust for a turbo is no exhaust at all. Period.

Probably the pinnacle of turbocharged piston engine technology was WWII era airplanes. They ran no exhaust, turbine housing straight to atmosphere, along with lots of other fun stuff (nitrous, alky injection, cockpit boost controllers/overboost). No exhaust is unrealistic on a car but I'd never suggest smaller is better. I may suggest that smaller is sufficient but you'll always have gains with a bigger exhaust, no matter how minute they may be. Period.

If you have the ability to go 3" all the way from the start, do it. If not, 2.5" will probably be fine for anything under 300hp.

^

The pinnacle of turbocharged piston engine technology was the F1 turbo era.Where cars were making 1300+hp in qualifying trim on 1.5 liter 4 cylinder engines.

You got me there.

What did they run for exhaust?

^ I'm pretty sure it was open header/flame thrower.

Ahhh skeet skeet skeet
Man i love F1.