bill mcdonald

I know in theory with a turbo, bigger is better as far as max power.

I have 2.5 inch down pipes on this twin set up, and I wanted to go 3". It is pretty tight in there and trying to go 3" would mean rebuilding and such...

So, how much power can a 2.5 downpipe support?

Is this one of those things were it could make 800 hp, and changing the down pipes to 3" would wake it up a bit?

By the way, this is a T3 turbine. so the exit size is something like 2.5".

Could someone explain how a 2.5 exit going into a 3" pipe would make more power? does it even matter?

Baldturbofreak

well, do a simple test. Drill a hole in your exhaust and weld in a nipple. Hook that nipple up to a pressure gauge and find out if there is any left behind the turbine. the idea is, the greater the delta across the turbine, the greater the impetus. The 3 pipe might give you a quiker spoolup. HP is the final say. If your making 800rwhp you should definitely have dual 3in.

mn_vette

Alot of it depends on what the rest of your exhaust is like. If your catback is 2.5" then you won't see much if anything. I've you've got a 3" low restriction exhaust it will help. You still have to push all your hp through 2 2.5" pipes. It will spool the turbo up faster and the pressure drop across the turbo will also be greater which should causing less back pressure in the headers and more HP from the engine.

The pressure gauge idea is a good way to find out how much pressure you've got, the more pressure, the more it'll help.

bill mcdonald

I guess the point of this question was to get a whole engine combo layed out. I am trying to decide what it will become. an 18 degree head 1000 hp engine. or would the power be hung up because of a design issue and I would just waste money on high end parts because I am choking it.

I picked this turbo system up off another forum member. I am trying to retro-fit it to my engine and some things are not working out. I was aiming to improve things as well since it looks like it will be getting rebuilt.
I am shooting for as much HP as possible.
I am going for a 406 short block.
debating on 18 degree heads and converting a carb intake to EFI.
If I go to 18 degree heads, I want to go all out.

Currently the down pipes look like a huge problem. I am starting to wonder if there is enough room to do this 3" pipe.
the more I think about it, the more I think I should just do what meaney did. but I insist that the exhaust has to run to the rear of the car. Maybe something like monty's old set up would work.

as far as the exhaust, I will follow the down pipes all the way out, dual exhaust, to power effects mufflers, which is pretty much a 3" straight through set up when in the open position.

what are cats?

bill mcdonald

Another things I was wondering about. manifold design vs downpipe.

currently I have block hugger headers going to a 2.5 inch J pipe. With all this crap in there, it looks like a 2.5 inch downpipe is all I can do.

I have been thinking, if I went to a log manifold, I might fit a 3" downpipe if the log hugged the head tight.

which is the lesser evil? tubular exhuast, or larger downpipe?

mn_vette

More exhast is better, but if you look at some of the supra guys they are flowing some pretty big numbers out of a single pipe.

I think they use a 3.5" pipe which has a slightly more cross sectional area than two 2.5" pipes. just something to think about

Another option is to get some oval piping.

StealDads67

SUpra guys go for the Single 4" pipe back. That supports plenty of flow.

mn_vette

Most of them still run 3" to 3.5" 4" is reserveded for the really big dogs. But in any case the cross sectional area is still eqivilant to two 2.82" pipes. So even if you could go with 2.75" it might be worth it.

The problem might also be the T3 flanges. Those aren't able to support more too much more than 300hp a piece. If you are using internal wastegates then those may be your down fall.

Another little tip that may help is if you are using the internal wastegate build a seperate downpipe for the wastegate outlet. If you can get a 2" wastegate dump with a 2.5" downpipe then you'd be better off then just a 3" downpipe.

AKS Racing

I wish I could find the online pics of my engine combo with the 18° heads.

Unfortunately, it is not just about the addition of heads. On my "large motor", it involved the replacement of the heads, intake, pistons, offset lifters, custom pushrods, different headers, and valvecovers. When the single four is converted, it does make for a very nice intake.

As a side bonus, the heads flowed in the high 370 cfm on the intake side, which is more than enough to support a blown 447 ci sbc.

Aaron