im looking at a couple down pipes. a 3 inch and a 4 inch. i was curious to know if there is such a thing as to big of a down pipe? ill be running a fresh streetported S4 with a HKS "LOG" manifold with a turbonetics 62-1 with .96 hotside and remaing exhaust will be 3 inch high flow cat and a 3 inch HKS HI_POWER old school exhaust.

here are the downpipes:

4 inch:
http://www.himni-racing.com/index.ph...oducts_id=1025

or 3 inch:
http://www.himni-racing.com/index.ph...roducts_id=839

 

im running a 80mm downpipe (little bigger then 3 inches) its fine , the bigger the better!

 

Turbos like to breathe!

 

The faster that the exhaust gets out of the turbo, the faster it will spool.... so yes, bigger=better.

 

If it is a turbo car the short answer is no, the bigger the better.

The long answer involves issues of the stock turbo, wastegate, and boostcreep. The less exhaust back pressure the greater the propensity for boostcreep.

 

If there were no such thing as too big, all of the performance downpipes would be 18" diameter or larger.

 

you need some level of back pressure .. but at the same poiint you want it to be free flowing as possible. if the diameter is larger then the outlet on the exhaust side then the point is moot.

 

it has a 3inch outlet on the turbo.
i wasnt sure. i figured bigger DP = faster spooling.
plus the 4 inch is ONLY about 10 bucks more.

 

If it's only a 3" outlet, and you're going to be running 3" parts on down the exhaust line there, then there is no point to run 4" at the downpipe only to have it restricted before and after.

 

You don't need any backpressure. Turbos need as large a pressure differential between the turbine inlet and outlet as possible. The only real limitation is physical space. After about 4", it becomes difficult to package things in the engine bay (you still need room to remove and install the pipe, and for wastegate plumbing).

 

that was another issue i wondered about was packing a wastegate and piping for that. id like to have a 4 inch but i am curious as to actually how much room there would be?

 

Also, at some point it is a game of diminishing returns. When you are putting out power in the 700-800 HP range, the difference between 3" and 4" is quite a bit different than in the 300-400 HP range.

Depending on the turbo and fuel system you are running going from 3" to 4" may do very little for you other than cause space issues as previously mentioned.

 

My pipe is just under 4" OD and it fit without too many issues. Which is pretty good, considering how far the NA intake sticks out compared to the TII.

 

ok, thanks. i think ill go with the 4 inch.