What is the right header size for my 700bhp build
 

Hi. I have a 2.5cdb with 99.75 je 9.2 compression custom pistons , pauter rods, ported and polished 2.5sti avcs heads, supertec +1 valves , springs etc. Power division s2 cams. Brian crower adjustable cam sprockets. Proccess west intake manifold. 75mm boomba racing throttle body, aps725 intercooler and gtx3582r Garrett turbo. I also have a killer b sump that can accommodate el headers.
What I want to know is what is the best internal diameter size on each header to have the best spool and performance.
My car has managed 677bhp and 741nm torque with a 39mm internal diameter nel headers.
I don't know if I should go el or nel. And what size should i go on headers and the uppipe?
thanx

With that build spec your obviously not shy of spending a bit of cash,have yòu thought about the killer b holy header?
I'm not sure about internal dimensions but they get good results with them.

I thought about it. But the killer b is 1.5" (39mm) in diameter on its primaries. My ports are 46mm. I thought it would be too small.

Seen some of the folk running big power remove those killer bee headers

To replace with a set of perrin big tube ones

Made gains will try find he link :)

Thats interesting...at what sort of power were they becoming restrictive?

:lol1:.SJ.

I was wrong :brickwall

After digging around i found it,

The guy removed his Killer Bee to allow his friends race car to get going,

Had nothing to do with restriction

Apologies for the misinformation :(

He did say that swapping from KB to the Perrin big tube improved spool tho think it was on a Gt30 but after the mince i says earlier believe me at your peril hahah

Today i contacted www.goodfabs.com. tbey used to do the exhausts for the prodrive wrc cars and they also did the gobstoppers. Told them my spec over email. Now waiting to see what they will say

Bigger is not better in this case ;)

forgive my tech knowledge, but wouldnt fatter headers mean the turbo will struggle to spool/make boost?

I've started reading up on things like compressor surge and up pipe diameters and its fascinating how everything has to be in sync to perform at its best.

Exactly. I measured my heads each port is about 36.5mm so I think I will stick to 39mm headers and go equal length. I will also get it ceramic coated and fit a turbo blanket on the hot side to keep the temps inside so the gasses flow faster. It will also keep the engine bay temps low.

The Holy header is specially designed to make more power than any other header on the market. With true equal length primaries and smoother bend radiuses, the exhaust gases can maintain a much higher velocity. The cast collector at the merge is CNC designed and flow tested giving optimal flow into the secondary. This provides for a much fast spool time and a much larger power curve, sustaining boost higher into the rpm range.

The diameter of a large tube header will rob speed and heat, taking away efficiency from the turbo and increasing spool time. Also the sharp bends in the header and rough merge of the collectors cause turbulence, also robbing form your end power goal.

The Killer B Motorsports Holy Header is also built of a much higher quality 321 stainless steel alloy, making them much more durable and lighter than that of the competitors. Backed by a lifetime warranty as well. The Holy Header sets the bar for Subaru Performance Headers.

Can you get Katy to check emails please.

Best headers we've ever tested :thumb:

Did you get a response? Check your spam folder, we did have an e-mail server issue recently that may have caused the e-mail to be redirected. Let us know either way :)

No,nothing yet...I've checked junk which is where her emails normally end up.

Does heat wrapping effect the warranty on the headers?
If so can they be ceramic coated by someone like Zircotec here on the UK?

OK. If you can, shoot me a PM and I will get back to you. I'll follow up with katy, but suspect she's still got unresolved issues here :/

We do not warranty headers that are wrapped because it dramatically accelerates surface oxidation when used in daily driving conditions.

We love coatings because not only does it improve performance (engine bay temps and header performance), but it also insulates the metal from oxygen drastically reducing the oxidation of the metal; prolonging the metal's lifespan.