z28adiction

iTrader: (1)

Will be building my own set of turbo headers but unsure on what diameter I should use. With me 400 cu in 6 bolt head engine with max psi of 25 too guess. School me guys. Turbos will be around 70 mm thx.

DEFYANT

1 3/4" to 2 1/4", 2 1/4" to T4 or whatever flanges you have.

LASTLS1

iTrader: (7)

1 7/8

Forcefed86

iTrader: (4)

Ask 10 engine builders get 10 different answers. Either can make plenty of power. I'm with the smaller is better train of thought. How much power are you planning to make? RPM? Heads used?

This is more on hotside sizing in general but it has manifold opinions as well....

https://ls1tech.com/forums/forced-in...er-pipe-6.html

Longer thread with even more discussion about hotside sizing.

http://www.yellowbullet.com/forum/sh...d.php?t=306399

ezstriper

considering stock truck manifolds are good for 2000hp you do the math...

SwampWS6

iTrader: (7)

Since you are 400ci I would use 1 3/4. You really dont gain anything by going larger size like a NA motor. You would want to gain as much velocity as possible (which means smaller). Think of it like blowing through a straw and blowing through a toilet tissue roll, which has more velocity? (Straw) 1 5/8 is a bit of a gutsy call for your ci, but imagine the spool response and velocity you would be pushing through a smaller header. 1 7/8 is a no no I think unless your an all out race build with a larger turbo. JMO......

Forcefed86

iTrader: (4)

Regardless of the power people have made on a stock manifold, a properly built performance manifold with a merge type collector will always perform better than a factory manifolds. I think the problem is most go nutty on the runner diameter when building a manifold.


Looking at you’re signature I’d guess a stepped 1 3/4 to 1 5/8 manifold would be more than enough for your needs.

The smaller piping really shines when dealing with single turbo setups suffering from high drive pressure ratios. "Smaller" piping allows you to run a much larger exh. housing. This often results in less overall drive pressure and improved "spool" time over "larger" piping. The big debate is at what point smaller piping becomes a restriction. Everyone has their own theory on this, but all agree you don't want the exh gasses to exceed the speed of sound. (1100fps or so)

Going with large twins, drive pressure probably won't be an issue for your specific setup. This means you can build it more like a max effort NA combo. I don't think larger diameter manifold runners will make or break a combo with 1:1 or better drive pressure ratios. I also don't think "oversized" runners will help any as the turbo scroll exit is usually the bottle neck in the system.

LASTLS1

iTrader: (7)

I've personally run both . I started as a 920 hp street car to now over 2000 hp drag car. There's a good chance your turbos will get bigger over time, I would really go the 7/8. I'm on my 5th set of headers and turbos, lol.

Just my .02

Good luck with the build

z28adiction

iTrader: (1)

Thx guys happy to see this thread got some attention. Was slow at first. The car will always be a stick car. I don't think ill put bigger turbos on this car as it prolly will never hook it's power potential anyway.
If I ever want to build a true race car I'd prolly do a very large single but there's a slim chance ill be able to find a true race car lol. The next car I want to do is a ground up build n/a iroc z for road racing. But lets not get ahead of ourselves. Lol
I think ill roll with a heavy duty thick 1 3/4 headers just for space savings and velocity being a 6 speed. I'm sure it will meet my needs but still open to other comments.