Hi everyone I’m doing a turbo kit for naturally aspirated holden commodore 3.8 v6 with 9.4 compression (will probably raise it abit more) and will having supporting mods (valve springs double row timing chain...), stage 2 cam with a 2500rpm/3000rpm to 6500rpm power band, with stock stall converter and was wondering what size pipes I need to make it support both low setting and high settings? the turbo im going to go with a 62mm or 67mm wheel gt35 with a 1.06 a/r housing with 4” dump pipe 3” single cat and 3” catback and a 38mm wastegate, now this is where I’m lost on what size I should do on a mild steel steam pipe log manifold setup? The factory size ports on the heads are 1 1/2” (same size as the factory exhaust manifolds, I’m planning on making 230rwkw on low street setting and 350rwkw on the high street/strip setting with a turbosmart dual stage boost controller, my rev limiter will be 6500rpm with a redline of 7000rpm, I don’t want it to spool at 4800rpm but I don’t wanna choke the top end ether!!

 

Leave the compression where it is, make power with boost. The 1.5" primary will be plenty. 2" crossover or log is fine for your power goals.

 

Ok cos I’ve got 2” tee pieces that the primary will feed into so it’ll be 1 1/2” off the exhaust flange stretched to fit into the 2” tee piece then continue 2” for the crossover and same for other side 1 1/2” stretched for primary into 2” secondary then put a cone piece that goes from 2” to 2 1/2” onto the turbo flange into the turbo, but what about if I was to make 420+rwkw later would 2” primary into 2 1/2” secondary be fine for that power goal

 

Yes, you will be fine. 2" is still going to flow better than the turbo itself and not be a restriction.

 

So pretty much 1 1/2” primary and 2” secondary is plenty for even 400+rwkw?

 

Yes, If you really want to go bigger you can, but for that power goal on a 6 cylinder I don't think it will matter. Lot of guys run stock manifolds that are smaller cross section than that with 2" crossovers and do fine.