SR-71 SS

iTrader: (2)

I am just about ready to order all my parts for my opel and had a question on down pipe size.

I know the consensus on down pipes is generally the bigger the better but where is the point of diminishing returns? This is a tube chassis car so I have room to work with up to 4" maybe a tad more per side. The minimum planned as of now is dual 3", but since space is available how big should I go?

This is essentially a street car and will have exhaust running at least to the rear axle.

Pics of car here https://ls1tech.com/forums/conversio...9-opel-gt.html

Combo is
402 cube LS2 9:5 compression
TU2 cam PRC 237 heads
Twin 6266 turbos
4l80 trans 9" rear

thanks in advance

c0ncEpT

iTrader: (4)

I have no 1st hand experience but maybe bump it to 3.5 to go out the back. If the exhaust side is a 3" v-band you would think that would because the restriction before a 3.5" pipe. Again just speculating here.

ls1_chevelle

iTrader: (9)

Im running 3.5" all the way back and would never do it again. 750rwhp. After seeing so many successful 2.5-3" turbo to tailpipe cars making 1000rwhp...

With that said, Id run 3".

LS1NOVA

iTrader: (12)

I think full 3.5 is best in your case.

jridenour31

3.5, more than enough with those turbos and you won't need to redo them if you step up to 7675's or something down the road.

LilJohn

125% of the turbines exducer is the minimum rule of thumb.

3 inch has supported 700 @ the wheel on certain combos.

With the length of run you are talking... id personally do 3.5.

I would also reconsider your cam choice if it hasnt been purchased yet.

qqwqeqwrqwqtq

We've built several single 7675 cars at 900+rwhp with a single 3in downpipe.

That being said, dual 3 inch is not going to restrict you at all. (Not ideal but we've seen 1000++rwhp with dual 2.5 systems).

If you can easily fit dual 3.5 it certainly wouldn't hurt but I don't think it would pick up any power.

Looks like a great combo (including the cam!) in my opinion.

jridenour31

That's pretty impressive. How much could you expect to pick up when going from dual 2.5" to dual 3" or 3" to 4"?

SR-71 SS

iTrader: (2)

looks like 3.5 duals it is. I prefer to overbuild now and not have to redo later should I add more cubes or more turbo down the road. I appreciate the input from everyone who posted. I will update the build thread once parts arrive and fabrication begins. Since the knickname for the opel gt is the mini vette I think it would be kind of neat if I could run the downpipes as sidepipe exhaust like the old stringrays did.

LilJohn

That would sound pretty wicked... i like it.