i has an sc300 w/1JZ..which size exhaust pipes from back to down pipe should i run to acheive the most hp??3'' or 4''?? thks-

3" otherwise you lose lowend torque and it'll seem like you've increased turbo-lag...if you're going to go with aftermarket turbo(s) then go 4" but only if you plan on exceeding 600rwhp .

i have a 3 1/2" on mine......its nice, deep

Since you're turboed (1jz-gte), the less restrictive the exhaust, the better. Only NAs have the problem with losing low end power because of a bigger pipe.