Cold pipe size, 3" or 4"?

Building a set of s300/s400 twins in my 89. I've got the hot pipe built out of schedule 40 3" black pipe. I originally bought 4" before asking, that was a mistake, it was huge, took it back and got the 3".

So now I'm asking before having to make any extra trips. Okay I lied, I've already got some nice mandrel bent 4" elbows and what not from the shop, but can take them back no problem.

What is the word on cold pipe size with twins? My s400 is a plain jane 171702 with the v-band compressor outlet. I've got two nice 90* cast aluminum elbows, one is a short short radius and the other is a longer radius, both are nice junkyard pieces and are to be used with a 4" boot. If I use these, obviously I'll use 4" exhaust tubing for the cold pipe. If I have to use 3" or 3.5" it will cost be a weld flange ~$70. I've heard it both ways, 3" and 4". Comments are appreciated.

I've always used 4" on mine.

I've got 4" cold pipe for mine also. I did the same thing with the hotpipe :doh:
3" schedule 40 on the way.

I never asked and I haven't seen it in person yet...but I'm pretty sure Hellmann used 4" on mine.

I have used 3'', 3 1/2 and 4''. The only difference i did see is the amount of room and the boost response time. The 3'' is quicker to move the gauge between the chargers but i saw the cool down time and top end heat up. Down low in the rpm range it seemed better than the 3 1/2 and 4''. Between the 3 1/2 and 4'' in all instances i couldnt see a difference.

I used 3" on mine and basically copied a couple of other twin setups I looked into. They all had 3" .

Mine is 3.5 and Nathan Wright usually use the same to my knowledge......Unbroken yours looks to be 3.5 to cause the top compressor is a 4" and It has a reducer going to it.

Brett

It very well may be....like I said I've only seen pics. It just loooked pretty good size in the pics.
Can't wait to see it in person next week.

Thanks guys. I believe I'm gonna try 4".

Another question, might start a new thread over though. Any problem building the support for the primary coming up off the motor and welding the bent/fabbed support to the hot pipe? As opposed to fabbing a support to thread into the bottom of the center section on the primary/oil drain etc.?

The way we've built the hotpipe it positions the primary forward and directly below the s300 and almost right next to the oil filter. It just looks easier to build a support up to the hotpipe. Any ideas on this?

use 3.5 "

ATP has an aluminum adapter that clamps directly ro rhe S400

Used 4" aluminum on my '93.

You could make your mount like i did for Gus Farmers twins. On the backside of the bracket that bolts to the block there is a plate that will not allow the charger to drop below what you see there. The plate only uses 1 bolt in the block and 2 in the hot pipe flange. Hope that helps.