I have to build a new turbo manifold for my Opel Gt/ Turbo II project car (the turbo won't fit in the engine bay without its being moved). I am using the stock turbo, stock ABV, stock intercooler (front mounted) and my boss and I are fabricating all new plumbing. Anyhow, when we measured it today, it seemed that I won't be able to use the 2" ID bends and pipes, as they are too large for two to fit. Its a lot more probable that I can use 1.5"-1.75" ID bends and pipes, but is this large enough? The bends I am talking about are weld-els, a type of cast, schedule 40 steel. My original plans involved a 3-3.5" downpipe, but that surely will not fit, the maximum now will be a 2.5"-(hopefully) 3" downpipe.

To be a little more specific, the turbo will be mounted in the front right of the engine (in front of and to the side of the pulleys) at a 45 deg. angle to the engine, to decrease the number of bends the exhaust has to make on its way out of the car.

But to make things more interesting, I was reading Maximum Boost by Corky Bell and according to that, smaller runners are, to some extent, better. I'm not sure the maximum diameter I will be able to use on my turbo manifold, but I am sure that 2" (each) will be a stretch, if possible at all. Is there a size that would be (dare I say more?) appropriate or even just the best of the worst? I'll have to go and remeasure later, but now I'm thinking the max. diameter will be roughly 1.75" each. Will this cause too much backpressure, or be decent in regards of speeding up exhaust gas velocities? I do plan on porting the wastegate. Any help would be appreciated.
Thanks,
-Rob

 

Your gonna find that anything smaller the 2" primaries will make it spool extremely fast (so fast that you might be in boost during cruising), but at a cost of horrible boost control and creep. You could use a smaller restrictive down pipe to control boost, but now your introducing more heat into your turbine, engine etc and will make less power and spool slower. 2" OD mani runners, and a HEAVILY ported gate with either 2.5", 3", or 3.5" will work. The larger the exhaust after the turbo the quicker it will spool and the more power it will make per psi verses a smaller one. Keep in mind the larger the exhaust the harder it will be to control boost too. Its all a trade off and ultimately your set up should be determined by what you want from your car and if you think you might want more later.

~Mike..............

 

Thanks Mike,
Thats about what I thought it would be, but backup info is always nice. Ultimately everything will be dictated by the size constraints of the engine bay, but when its up and running I'll post back some results.
-Rob

 

Ah, another thing I thought of, (despite the fact that I cant see how much room I have now) is that it might be possible to run two different sized downpipes (one from the wastegate and one from the turbine). Of course, within the given size constraints, the optimum size would be roughly 2" at the wastegate and 2.25" at the turbine. These would need to be welded together to make an area to decrease turbulence between the two (an area where the tubes come together at an angle and have a hyperbola cut from them). The area of these tubes would be roughly the same as that of a 3" dp and, have less turbulence, and possibly conserve space. As there will be no cat and probably no silencers or mufflers, backpressure should not be much of an issue. Also, porting to the wastegate might become unnecessary if I can devise a system that would allow it to open fully instead of the ~7mm it opens now. If anyone can find a flaw with this (I'm sure there is one), I'd like to know, and I'll do my measuring tomorrow.
Thanks,
-Rob

 

I don't think that's entirely correct. Two pipes of the same internal area, would be more restrictive than one large pipe with said area. The smaller pipes would have more surface area of contact with the flow, and thus slower ie more restrictive airflow. You are on the right track that there are proper ways to join the WG back into the DP to reduce turbulence.

Also, back pressure is key for spooling the turbo. Speaking strictly to spooling, this has everything to do with the size of the exhaust on the turbo, and nothing to do with the size of the WG dump tube. Once the Wg is open you are effectively spooled, and then you are simply working to maximine flow so as not to choke top end. So it does play a role, but not in helping to spool.

Now, you can use an overly large WG and dump tube, to help reduce the backpressure and the "limited" top end associated with using a conventionally smaller turbine housing.

-Chris

 

Thanks Chris, I hadn't thought of that. However, the lack of top end I was looking at was in respect to the small diam. runners I will have to use on the turbo manifold, not so much the turbine housing.
Aso, I got the measurements of the box I'm trying to route both my turbo manifold and the downpipe(s?) through. It is 8"(h)x~4-6"(w). The downpipe (by itself) has a box 5"(h)x2"-4"(w) to go through.
I realize this idea might not be the most efficient plan, but the space constraints almost dictate this, or something way more restrictive.
Thanks again,
-Rob

 

Edit-
After looking at the flanges and whatnot yesterday, it might be more beneficial to add two downpipes, but instead of one directly off of the turbine and one directly off of the WG, have one directly off of the turbine and another between the turbine and the WG to pick up flow from both locations. Oh, and just to clear up any confusion, when I referenced the 3" DP, I meant it only as a reference, I didn't actually want the flow of a 3", but more like a 2.5-2.75"
Sorry, I meant to add this last night, but thanks again,
-Rob

 

Ah, I had to go back and read your original post, which I think I must have overlooked the first time. I couldn't figure out why you were having so much trouble fitting this in an Rx7....DUH! Sounds like a fun project. Depending on how much power you are looking to make, the smallermanifold runners would work fine I suspect. Boost control would certain come into play though with a small turbo like the TII's, and small runners. Might want to search for Blue TII's project car. He's running a hybrid stock turbo, and it took a 60mm gate to keep things undercontrol!

Good luck.

-Chris

 

I would just go for it.
Turbo exhaust manifold pipe sizing is always safer to go smaller.


-Ted

 

Thanks guys,
I have to get the engine mounted, and then I'll be able to tell more accurately what I'm looking at. Anyhow, I found a really cool metal and part surplus place today, complete with a dimly lit warehouse and most any metal part I can think of. So I'll probably be getting my stuff from there. Anyhow, thanks again.
-Rob