That's all good info if we were naturally aspirated T25... but with turbos I'm afraid bigger really is better.

 

Turbochargers change the equation slightly, but not much. On a stock 1.6 I would laugh at anything bigger than 2.25". On a turbo 1.6, a well sorted full 2.5 is going to be a screamer, and be cheaper than a custom (probably not well thought out and built) 3".

A well built 3" DP MIGHT be worthwhile but NOT, and I repeat, NOT a full 3". And it seems that because it is a waste of time, there isn't a well built one available.

 

Here I was thinking this whole discussion was entirely about downpipe size. I've been listening because the DP sitting in my garage is 3" with a step to 2.5" right at the cat.

 

The cat will cool and therefore compress the gasses, so 2.5 after is all you need.

 

Turbochargers change the equation a bit more than slightly...first off, you have a huge restriction in the exhaust directly after the primaries...a naturally aspirated car doesn't have that. This basically means exhuast gas velocity and pulse tuning are a moot point, unlike in a naturally-aspirated car. You no longer have defined pulses because they've all be chopped up and thrown into one big gas plume by the turbine.

Why do you think a turbocharged car spools better when it's running merely a downpipe and no cat-back? Funny thing is, after the primaries are properly collected on an N/A car's header, you actually want the pipe to open up as much as possible there as well (assuming smooth transition). Say your N/A car's header primaries collected into 2.5"...a smooth, short transition from 2.5" - 4" into open air would make more horsepower at no expense in torque compared with a full 2.5" cat-back. Same goes for the exhaust dia. post turbocharger.

Here's something for you to chew on since, basically, the burden of proof rests on you, as you're going against all modern thought on the topic: since a turbocharger relies on a pressure differential across the turbine to spool and produce boost, explain to us why you wouldn't want the largest diameter possible exiting the turbine?

 

You would be right here if you weren't so wrong

 

man, all those dragsters and race cars running 4" diameter dump pipes are sure gonna be mad they forgot to run an exhaust!

 

t25... you really are off base. A turbo is happiest with NO piping at all, just something to smooth exit gases and the open air make it happiest.

I run a full 3" DP on my 1.6L civic... and will be running a full 3" on my 1.6L miata. Its an easy size to get piping in and it has proven gains over smaller piping.

If I could fit 4"+... I would use it.

 

I don't think the question should be: Is a 3" DP and exhaust better than 2.5?

I think the question should be: Is a 3" DP and exahaust worth the trouble and expense?

 

I don't know this trouble everyone speaks of. I have a 3" exhaust, and it wasn't difficult at all. the downpipe is the hard part. there's pics in my garage.

 

Some would say that having a custom exahaust made is more hassle than buying an off the shelf 2.5" exhaust. Is there anyone making a bolt on 3" exhaust?

 

oh, i see. but they'd still be wrong. it's easier than intercooler piping.

 

I'm sure if you have access to the tools and skills to use them you would be correct.

 

I dont know what you guys are saying... I have a bunch of friends that have had evo's,sti's,GTI's all with big downpipes. You lose lowend but top end pays off for it. One of my buddies was running a GTI 1.8T with a gt30R and a 3 inch turboback. Car ran amazingly. I remember he made my 350z look like it was stopped on the freeway.

 

you dont lose low end from a big exhaust. if you run a GT30R on a 1.8 you lose low end.

large downpipe improves spool which improves low-end.

 

You would certainly lose low-end if you want from a log- or short-runner tubular manifold to a super long-runner manifold (like this: http://www.turbotechnologyinc.com/pr.../e&e/e&e02.jpg ), on the same engine, with the same turbo with the same downpipe size.

Post turbine, however, you want the larger MF'n diameter you can get.

 

If it's so easy and so little hassle you should make some up, there's at least one guy in this thread who'd buy it.

 

All i'm sayin is it took a 6ft straight pipe, 1 u-bend, 1 muffler, 1 or 2 sets of flanges, and four or five cuts to make. i then took the whole lot to a muffler shop and had it welded. It really isn't too bad. IT even clears a 7" diff without modifying the rear subframe brace.

 

Have you seen how far back a greddy is mounted on the manifold?

 

http://www.bellengineering.net/Pages...peupgrade.html

Read it. I am happy to stand corrected on several of your points. Dragsters are not road cars, don't use them to back you up cause they are so different they aren't comparable. And if they use 4", what makes you think that a pissy little 1.6 needs 3" :gay:

Like I said, I stand corrected on some things and have learned a lot in this thread, and also laughed at a lot of comments. Don't put me down because of **** you have read on forums. ********* like us post on forums, what makes what we say right? I'd trust Begi though, they research.