Iam getting my exhaust done tomorrow what size should I do?

 

2 1/4 get them to piece together with madrel bends...

 

3" If you are going to run Forced Induction......?

 

I did 2.25 with two resonators

 

Nice and quiet....Sleeper

 

3" even going FI on the L15 is a bit much. 50mm is about perfect staying NA, 54mm I still feel would be beneficial up high, but the L15 in stock form doesn't really run up high.

 

I thought that at first also but I have had great hp gains from my 3" with my T25 Turbo setup.

 

You can go as ruthlessly large as you want to with FI with no loss of low end power. A correctly sized turbo will always have enough back pressure to maintain reasonable exhaust velocity. Open down pipe via cut out is a realistic option too. For NA, I wouldn't waste my time with anything cat back until you delete the cats, unless its just for sound. Very restrictive cats in this ulev vehicle. Reference my WR cat delete from when I was NA for proof.

 

A 3" pipe may be great for a car equipped with with a turbo but It might be a little to much for a supercharged engine at least on the low end.. I have the Megan header, resonated down pipe and B pipe with an Apexi exhaust control valve into an 18" long Vibrant resonator followed by an Aero Tubine 2540i 4" inside diameter straight through muffler with 4" outlet... Everything is 2.5" inside diameter to the muffler and without the control valve there was a slight loss of low end power... It wasn't a problem except for the noise of having to rev a little higher as slowly letting off of the clutch to get underway.. I installed the control valve to lower the volume of the exhaust but found that by using it 1/3 to 1/2 closed in stop and go driving using very little throttle that the throttle response and ability to pull at very low RPM was dramatically improved... I figure if a 2 1/2" pipe hurt the bottom end on a KWSC high boost kit equipped Fit and just an axle back pipe with a 12" long glass pack muffler had the same effect on the same car it it's stock form, then a 2 1/2" axle back pipe on a N/A engine will have a negative effect even more and further up into the power band... The larger diameter pipe doesn't allow the flow of exhaust from the exhaust ports to create the velocity needed to draw all of the air/fuel mixture past the intake valves and into the combustion chamber during cam overlap when intake and exhaust valves are partially opened at the end of the exhaust and beginning of the intake stroke.

 

Ah good point tc, with a SC you can have too large a diameter exhaust.

 

Once a turbo is spinning it acts as an exhaust scavenging pump so in addition to force feeding the intake it is also sucking out the exhaust and pulling the air/fuel mixture into the the combustion chamber so exhaust flow velocity isn't even an issue..