1 5/8th, 1 3/4 or stepped headers?

[rfiresnake]

Made my mind up on kook headers and im trying to figure out the perfect size. My car was dynoed at 680rwhp and 630ish tq. I'm thinking 1 5/8 may be a bit small but i could be wrong. Would it benefit me more to go with stepped headers 1 5/8 to 1 3/4 or strait 1 3/4 headers?

thanks

 

[va boy]

My car dynoed a bit less than yours. I ended up going with the Kooks steeped headers and am very happy with them. My tuner had already suggested them, but I was a bit hesitant due to the price difference. Then, I witnessed a car with similar mods to mine. He already had solid numbers then had a set of the steeped Kooks installed, no other changes, same temp, humidity, etc. It made me want some too! The claim by Kooks is that the steeped design increases RWTRQ, I studied on them a lot becasue of the price, Kooks did their homework on these, they work! I'd be happy to share my tuners name and number with you if you have more technical questions that I may not be able to answer that I'm sure he could. PM me if you're interested. Good luck with whatever route you go, because they are pricey, but man do they fit and sound great!

 

[GodStang]

Straight 1-3/4".

 

[LSX_Terminator]

My car dynoed a bit less than yours. I ended up going with the Kooks steeped headers and am very happy with them. My tuner had already suggested them, but I was a bit hesitant due to the price difference. Then, I witnessed a car with similar mods to mine. He already had solid numbers then had a set of the steeped Kooks installed, no other changes, same temp, humidity, etc. It made me want some too! The claim by Kooks is that the steeped design increases RWTRQ, I studied on them a lot becasue of the price, Kooks did their homework on these, they work! I'd be happy to share my tuners name and number with you if you have more technical questions that I may not be able to answer that I'm sure he could. PM me if you're interested. Good luck with whatever route you go, because they are pricey, but man do they fit and sound great!

+1 for stepped
You do not want to go to big or the headers will hurt your power more than help

I know coming from the GM LSX side, we used a 1-3/4" or 1-7/8". Not sure if these motors need the 1-7/8" though?

 

[evolve]

Flowing that much air, I agree with GodStang. You want 1 3/4 with collector back at 3". Ive seen multiple FI setups that pick up 20+whp at the higher end of the rpm range going from 2.5 to 3". All of them making over 650whp.