Tjabo

Hey guys, the search function doesn't like the number 3 and just returns every thread with the word downpipe or dp in it, so I figured I'd just ask what you all think, and risk being abused for not searching. Haha...

I'm getting close to starting a turbo project on a small displacement V8, that I just can't see how to fit bigger than a 3" downpipe. Once I get down past the firewall I can run 3.5" or maybe even 4" the rest of the way back. Currently I'm planning 3.5".

I'm also planning to recirculate the wastegate dump, and I'm not sure yet if I'll be able to run that tube separately back to where the diameter goes up to 3.5" or not.

So with all that in mind, what do you all feel are the limitations of 3" downpipe, both with and without the WG dump recirced.

Thanks!

yenkomike

iTrader: (2)

3" is not a real issue the std exaust housing on a tc 78 or on3 78/75 is 3" so making enuff power with on to go into the 9s is not going to be to hard. I have seen some posts on here at near 800 hp using a 3" downpipe.

Tjabo

Thanks yenkomike, I appreciate the input! Puts my mind at ease a bit.

willizm

iTrader: (10)

With a pt7675 and 3" downpipe I was still able to go 9.75.

Jetn3

What did you dyno at and at how much boost?

NemeSS

iTrader: (127)

Have seen a consistent 660rw thru 3in dp with a c/o before the muffler box and with a older model t76 turbo w older compressor and turbine tech
Pretty much out of turbo at that point. On 370ci

Tjabo

Thanks guys!

Can I assume all of these experiences were with open WG dumps to the atmosphere, or were any of them recirc'd back into the 3" downpipe? I'm guessing that is where the problem might really become apparent...

willizm

iTrader: (10)

Last time I was on a dyno it made 780whp but that was a while ago and i've been tweaking the tune since. I was pushing 22psi but I wasn't much faster over 18psi so I suspect I was choking on backpressure. Went with a bigger turbo and 4" downpipe but it was a bitch to get the pipe to fit.

LJMSJohn

typically you can see 700-750 hp before the downpipe starts creating post turbine pressure.... which is a really really bad thing to have happen.

Tjabo

Is this definitely assuming an external dump (or separate tube to a larger part of the exhaust) for the wastegate, or am I mistaken about that?

Shownomercy

iTrader: (1)

Are talking a 3 inch downpipe thats dumped right under the car, or a full three inch exhaust out back?

And or, wheel hp, or flywheel?

LJMSJohn

Short downpipe....

not full exhaust...That would just make it that much worse.... Length of run can hurt worse than diameter on a short downpipe.

Tjabo

What about the effect of recirculating the WG dump? Big problem with a 3" downpipe, or not a big deal?

LJMSJohn

Thats just going to lower the point at which it chokes... it'd be better to dump it to atmosphere.

NicD

iTrader: (7)

Yup, dump it to atmosphere with that small of a downpipe/exhaust as a 3" downpipe will be a restriction. I currently have a 3" downpipe on my Camaro and really really wish I could make it bigger but with the kmember being in the way with my setup I can't. Either way with that limitation my car has still run a 9.17 @ 152 mph on a stock cube LS1 with an older 76GTS which is really similar to the 7675. Heck at some point I may try to switch out k-members to a more turbo friendly one and redo the downpipe but that's just more money I don't want to spend.

Shownomercy

iTrader: (1)

You have a dumped downpipe I assume?

Tjabo

Thanks guys!

NicD, just making sure I understand 100%, you're WG is being dumped, right? That's already amazing that you can run that kind of trap with a single 3". Great info!

So since I REALLY don't want an atmo dump on this car, I will look into running a parallel 1.75" or 2" tube run for the WG back to where I can jump up to 3.5" exhaust. Should be just behind the bottom of the firewall. I think at the power level I will be at (probably never reach 600wheel), it should be okay to recirc the wastegate gasses into the 3.5."

Thoughts on that?

NicD

iTrader: (7)

Kind of but not really, it still has a straight through muffler and ends right before the rear axle so I've at least eliminated some bends.

NicD

iTrader: (7)

Yes my WG is dumped to atmosphere.

Honestly for under 600 rwhp it's really not going to make much of a difference IMO. I think running a parallel 2" tube running from the WG to the back where it can flare to a 3.5" pipe would work well at 600 rwhp.

Tjabo

Thank you!