i have been through ALOT of threads about exhaust but i have not seen a question on sizing. I am looking to get an Alta exhaust or possibly magnaflow but i have seen that the exhausts are offered in a 2.5 inch and a 3 inch diameter. obviously the 3 inch will flow better but does a 2003 Cooper S really need a 3 inch exhaust or is this overkill? does the R53 need a certain amount of backpressure to run properly? Im definately no expert but i dont think it would due to the forced induction. I have a few mods and am always in the market to get more. at what point ( if any ) do you NEED a 3 inch? If the choice exhaust is indeed a 3 inch then why even sell a 2.5??
i appreciate your help in advance.

 

sorry i just realized i posted on the wrong forum. my bad

 

Even though this is in the wrong spot. The 2.5" is the idea choice on the MCS 02-06. UNLESS you plan to twin charge, or single turbo the car. Other wise you need a bit more backpressure to make the addl. TRQ.

Let me know if I can help with this or any other choices in the future!

 

While this started for the R53, I questioned whether 2.5" or 3" was the way to go for the R56. After reading an article from a Garret Tech, I feel much better about the idea of running 3" without any losses in torque. The general premise of his article is that the flow must be accelerated with a properly sized exhaust manifold using small but non-restrictive runners. The turbo will generate enough back pressure on its own. The down pipe should be large enough to allow the least restrictive flow possible to allow the turbo to spool effortlessly. The old rule of diminishing returns applies and eventually a larger pipe will no longer provide gains.

I just thought it was a great article that answered some of the criticism I have seen of the 3" on a turbo Mini.

 

58driver: EXCELLENT point!

 

ugh. backpressure talk
you never need backpressure, it's one of those horrible myths.. plenty of knowledgeable people/articles on the subject all over the internet. google "backpressure myth"
eg.
http://www.uucmotorwerks.com/html_pr...torquemyth.htm

you go with a smaller diameter pipe to increase the velocity & help with the scavenging/extraction from the exhaust, too large and the pulses don't make it cleanly out the tailpipe..

Turbo's are a bit different beast with the turbine eating up the exhaust pulses , you can go too large, but it's pretty hard. Larger equals quicker spooling and ultimately more power & more potential for power.

plenty of math out there, eg. http://www.autolounge.net/calculator...ustsizing.html

Of course I'm sure your tech's have it all figured out, 3" for the alta Turbo-back is a great compromise for increased size & still making it fit.
I used to build & tune systems for the Porsche club guys in Redmond, NA & Turbo, we also used a Dynopack as well, best dyno & tuning system ever.

 

The article I read also said that all scavenging is complete at the turbo and there is none needed after. It said that you want higher velocity before to spool the turbo and free flowing after to prevent any restriction to the spool up. The bigger pipe helps it spool much faster.

 

Its always a balance. For NA engines, too big and bye bye low end power but gains up top are found. For a Turbo car the same type of mentality applies before the turbo. Bigger is freer flowing, but will cause some low end loss do to the turbo spooling later. But after the turbo, as big as you can go is best! If we relate this to the R56, there isn't really an opion for Pre turbo piping only post turbo piping. And 3" is the best for performance and sound. While a 4" exhaust might be a little better, its gonna be alot louder and be a little tight fitting in places. In my opinon, the added cost, weight, and noise from anything bigger than 3" isn't gonna net much more HP.

When people say "A turbo system needs no back pressure", they should follow that up with, "After the turbo". The turbo creates back pressure in the exhaust before the turbo and if this was super free flowing you have a dog of a car as the turbo would be very lazy to spool.