2.5 inch vs. 3.0 inch exhaust
 

So is the verdict out on if the 2.5 in is sufficient for the R56 turbo?? I like the sound of the Invidia (even though I am a little leary of the droning some have reported) but don't want to lose any HP's if the 3.0 is better. Is bigger actually better in this situation?:confused: I want to get this ordered! Thanks

I think you'll find 2 distinct schools of thought on this one.
1 - 3" is too big, 2.5" is fine.
2 - You can never go too big with a turbo.

I really don't think there is much of a "hp gain" difference between any of them particularly if you are going Catback.

I went with what my install shop could easily get, install, support. I also took into consideration that I'll be adding the Downpipe next year.

It may be quite a bit dated and not devoted to our cars but the knowledge is sound and still applicable.
http://www.nsxprime.com/FAQ/Miscella...austtheory.htm

Here's an excerpt dealing with the question specifically:

We've seen quiet a few "experienced" racers tell people that a bigger exhaust is a better exhaust. Hahaha… NOT.
As discussed earlier, exhaust gas is hot. And we'd like to keep it hot throughout the exhaust system. Why? The answer is simple. Cold air is dense air, and dense air is heavy air. We don't want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large exhaust pipe will cause a slow exhaust flow, which will in turn give the gas plenty of time to cool off en route. Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our header tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe. Coating the entire exhaust system with an insulative material, such as header wrap or a ceramic thermal barrier coating reduces this effect somewhat, but unless you have lots of cash burning a hole in your pocket, is probably not worth the expense on a street driven car.
Unfortunately, we know of no accurate way to calculate optimal exhaust pipe diameter. This is mainly due to the random nature of an exhaust system -- things like bends or kinks in the piping, temperature fluctuations, differences in muffler design, and the lot, make selecting a pipe diameter little more than a guessing game. For engines making 250 to 350 horsepower, the generally accepted pipe diameter is 3 to 3 � inches. Over that amount, you'd be best off going to 4 inches. If you have an engine making over 400 to 500 horsepower, you'd better be happy capping off the fun with a 4 inch exhaust. Ah, the drawbacks of horsepower. The best alternative here would probably be to just run open
exhaust!

I went with a T304 SS 2.5" exhaust system last year when I got my car and I did so because I knew I wasn't upgrading the turbo anytime soon, so with a 1.6L engine its more than enough, especially when you consider the downpipe is 2.6", then it goes into a 2.4" in the middle and then a 2.2" towards the rear of the car. My car gained power right away, although it was seat of the pants it really pulls well. I elininated the rear cat as well as went with NO resonater, but I may put it back as after hearing the JCW car we have in our shop (I'm not saying my car is going to sound like that after adding a resonater, but afterall the resonater is just for toning down the sound) Speaking of that, I think the factory JCW resonater is about 2' long:eek: , so I'm not sure if I'm going that large, but I think I'm doing something though. I've lived with the sound for this long and it doesn't bother me, but some I know it does, so its a personal thing for sure. My suggestion is to try and hear some of the locals cars and go from there:thumbsup: Good luck to you!

*kickin back with the popcorn*

This is probably one of the most hotly debated subjects on the board after what gas to use and how to break-in the car...

:lol: so true.

It's been quiet for a while.....

There's plenty to read on this subject that has already been posted.

It's not like anyone here is making enough power on an R56 for it to really matter :sad:

...Still waiting for a big-turbo kit...

:( poor slow mini coopers

is that article about NA engines or turbo engines... there is a difference.

must be N/A they are talking about header tuning :popcorn:



To add to this: to a point you want the greatest delta in pressure between manifold and exhaust but the tiny exhaust housing and wheel will only flow so much so after that the gains would be negligible.

If running on stock dowpipe meaning stock cats and don't plan to upgrade later go 2.5" without giving it much thought.

They talk about both in the article, mostly it's just general exhaust theory.

2.5" is plenty for 99.9% of the cars here. Ify ou have a stock turbo, then 3" is ONLY for bragging rights. An ideal exhaust is shapped like a funnel. As the exhaust comes out the turbo, it comes out with alot of velocity. that velocity starts to slow down as the gases cool off. In order to keep the gases flowing a funnel would be ideal but its not easy to make.

Im sure you have been at a place that has ducting overhead. If you notice the a/c duct is large when it starts and it tappers down. This is to keep the velocity up because it slows down teh farther back it goes. The same thing with your exhaust.

I can make both so i can sell you what ever you want but personally I would go with a 2.5". 3" will not hurt you but you dont need it, trust me on this one.

Here is a crazy duct setup. :)

http://www.infrontsports.com/fileadm...verview_02.jpg

Notice how the pipes tapper down.

Happy boosting!

Hector

so how do you make this happen if I just am doing the cat back?

Wish more people would believe this around here!

:thumbsup:

so when are you coming out with this 3in -> 2.5 downpipe :wink:

2.5 for all of our slow minis will be fine....if you plan on going with a bigger turbo down the line get a 3" if you dont plan on making 300+hp out of your mini stick with 2.5 Hector knows what me is doing tapering your exhaust helps with the flow. FYI my brothers subie is 320whp that about 440 crank hp and he has a 2.5" exhaust. so to sum it up 2.5 is just fine, but 3" if you plan on making a beast later down the line

My experience on is similar to your brothers before the mini I had a WRX, and STI and then an EVO all were modded and judging by those cars 2.5 may verge over kill on our little motors/turbos. There was virtually no top end power difference on the EVO or WRX between 2.75 and 3.0 DP's and the smaller diameter pipes allowed the turbo to spool more quickly and increased the RPM range in which useable TQ was availible. Looking to those cars your dealing with larger motors, significantly larger turbos and on the EVO 2X the boost of the MCS.

First of all, I'd like to say "Hi, I'm new- long time listener, first time caller"

To help put this topic into perspective, my '87 Grand National 3.8L came stock with a 2-1/4" DP, coupled with a 2-1/4" single exhaust, in order to make high 13's-low 14's out of the box. Commonly, Grand Nationals reach 10's in the quarter with 3" DP's and 2-1/2" duals (among other upgrades, of course). My point is that an engine with more than double the MINI's displacement is proven to adequately breathe with 3" behind the turbo. I am sure that a 2.5" turbo-back setup should not significantly effect power output for even the most potent MINIs. IMHO.

Also, I would like to add that the ventilation systems are set up "funnel" style in order to maintain an air velocity within the system so that the vent will "blow". As the radius of a pipe is reduced, a fluid will increase velocity in order to move the same volume. This works fantastic for a system designed to maintain pressure and velocity; however, a turbo will breathe best with the absolute smallest amount of backpressure possible. Ideally the least amount of backpressure would mean a slow exhaust velocity, so call me crazy, but I don't really want any exhaust gasses leaving my tailpipes with much of a velocity -otherwise one would infer that there is a significant amount of resistance in the system. :nod: