i measured the output of the turbo where the downpipe goes and it is only 2.25 inches....why does it make sense to put a 3 inch pipe on there if its pumping the exhaust out of a 2.25? i dont see how it would restrict the car to run a straight 2.25 inch pipe....im building a kit car and the pipe has to me made custom locally so i was just wondering what would be the disadvantages of using a two and a quarter inch pipe instead of a nice three inch pipe.

 

Back pressure, from my experiance Rotarys love to breath if your giving them power cause the stock is like 2.5 from the cat beyond and 2.0 at the exhaust on the N/A's and I went 3 after the cat and then resignator it got loud as hell with the header but could feel the difference. Then when I did the turbo swap with a 2.5 down pipe and cat and the 3 it was great. Didnt get a chance to put the resignator on before I popped a Apex seal. But gettin rebuild goin with in the next 2 months to be back on the road. But 3 inch is better for power cause Turbos and Rotary's both need to breath freely for performance.

 

It's better to have a free flowing exhaust then it would to have a small restriction up from. 2.25" in very small for turbo to breath. The motor is like your lungs. Easy to breath in easy to breath out.

I mean you CAN do 2.25" but why not just do a 2.5" all the way back if it's a kit car. I'd prefer 3" for the fact it's going to help the turbo with spool.

 

depends on the power your want out of it. higher boost works better with better flow. if your're just going to put around at stock levels then there is no point of bigger exhaust, just mandrel bends.

 

you also have to consider that the turbo can be thought of as a torch tip, after the tip the flame expands. being that the outlet of the turbo is only 2.25" doesn't mean that the exhaust cannot be tuned by running larger diameter piping beyond that point, generally speaking the exhaust into and just out of the turbo are smaller than what you need post turbo anyways for optimum results.

if the ports inside the turbo were too large the turbo would lack in response, in the reverse scenario, turbos need decently sized piping beyond the turbo to breathe for upper end power generation. in non turbo applications this is also somewhat true but for low and mid range response, the diameter of the piping becomes crucial.

 

going from 2.5 to a 3" downpipe makes such a huge difference in response its amazing. peak power seems close, but the turbo really responds to having the big pipe volume after it. you don't need to run 3" all the way, but you should for the first few feet.

 

alright guys....thing is the kit car is a vw beetle so the engine is in the back and the muffler will only be a few feet anywaz and have alot of bends in it...ill see if the local muffler shop on the island can do it in 2.5 from the turbo to the muffler.....any ideas on a muffler canister?

 

I had a freshly rebuilt s4 engine, around 4K miles on it, with a s5 turbo. ran the old RB 2.5" full exhaust then got a deal on the REVII. seriously I didn't feel a bit of difference. before I swapped that engine in I had a old jspec s5. the new s4 had a new SRS lightweight flywheel. the difference between the two engines with the old RB exhaust? notta one IMO. maybe its the 4000 feet elevation that made it seem that way. this is just my experience. the only difference with exhaust was with an FD. GReddy catback swapped for an unbaffled muffler resulted in a 6 PSI gain.