As posted on page 2.

 

They are posted

 

You guys have to realize that the turbo is an On-Center exhaust housing. And Has shown to be signigicantly restrictive and reduces the Power capability of the turbo.

Tang Housing>>>>>On-Center Housing...

BTW, I am at the limit of my system on a 370 at 15 psi and 653rwhp (SAE). So yes its a limitation of the turbo kit in using the on-center housing.

Ryan K.

 

A log manifold and on-center housing.......the odds are stacked againist your system indeed.

 

The only real options I see are pulling the engine and decreasing the CI or pulling the turbo kit off and going with a tubular header setup and at least a 76GTS. You could maybe add a 76GTS to your current setup and add a 3.5 inch dp but it probably wouldn't be enough to make the bp reasonable again.

JMHO of course...

 

I think you could add a PT88 and still have problems. The log manifold I suspect is where the problem lies. I don't think any turbo will get those #'s down. The turbo is compounding the problem, but not solely creating it.

 

Based on experience with my own large cid log manifold setup, you can get backpressure down with the current engine size and log manifold. I have 388 cid and a 3" log manifold, but my backpressure is under 36 psi at 18 psi boost. With my current turbo system, I've run 363, 383, and 388 cid with no significant change in backpressure. Also, I built a similar combo with 377 cid and a T76GTS with tubular headers and still didn't see a significant difference in backpressure.

I believe that the problem is a) small Q on-center exhaust housing and possibly b) too much pressure drop through the cold side (restrictive intercooler, leaking BOV, restrictive piping, restrictive air filter). If it is a Precision Turbo, you can probably just purchase the exhaust wheel/shaft and exhaust snail and upgrade it to the GTS .96 Tang and see some huge reductions in backpressure and power gains.

I think that's what's limiting this system and RyanK's.

 

Its gonna require some hotside refabbing, but it would be worth it.

 

And cold side, since your shifting the unit to the side. I hope their is room. Last LS1 log setup I saw had the turbo less then 1/4" from the frame rail and right next to the accessory. I would imagine he would need to stick with an on-center housing unless he plans to start cutting.

 

How much room would I need? (tang vs on center do you all think?

 

To put in a Tang, you would need to do a bit of cutting/welding on the chassis right next to the turbo, Channeling the frame). And the Down Pipe would have to be redone. As far as the cold side, that would be very minor...

Ryan K.

 

What is the big benfit of tang mount?

 

Hey, sorry about the dropped call earlier. I hate Cingular.

The tang ex housing flows more than the on-center. However, I think that the size of the wheel (GTQ) and A/R are more of a problem.

I found on Precision's website where they offer a T67GTS on-center. I'm not sure if it's a typo because all other GTS's are tang. I wonder if you could mix and match the GTS on-center to your T76 compressor. . . You would still be stuck with the .85 A/R and the on-center housing, but at least the wheel would flow more and spin the compressor faster. Probably not the most effective way out, but definitely the cheapest and easiest.

Mike

 

Or you could go with a T-netics 76 if you want a direct boltin and then have an option of using the on-center housings that go all the way up to a .96A/R or even a 1.30. Something you might want to think about. I believe the MPS turbine can be had as well.

 

Even for a stock inch LS1 I dont know why you guys are going with a Q Trim wheel. JMO.

I've been talking to Shane quite a bit (locally). The Q Trim wheel doesnt help nor does the "small" on center turbine housing with that size of a motor.

I had a 76GTS with a log design on a smaller motor (granted more stroke/less bore) and I was running into the same issues (lack of increased power beyond about 16-17psi). I think you guys need to be STARTING with a S Trim wheel. I'm no expert though, just a self-taught redneck! LOL

 

I have a GTS .96 tang housing from my old T74GTS unit and since the rest of it is pretty much scrapped, the turbine housing is the only thing in working condition.. lemme know if you wanna upgrade and I will sell ya my housing. may help both of us out some!

I will say this, I had no problems spiking my T74GTS to 27psi on a stock cammed 355 LT1.. and I had NO backpressure issues on a 3" semi log style manifold.. although the only part that was like a log was the very last primary that fed into the 3" turbine feed pipe.

I just ordered the turbonetics T76MPS from Jose last friday and am hoping on my lil old 355 with a cam this go around and some head work she will make it into the 800rwhp range.. the cam I will be using is a 224/224 .570 .570 115lsa. wish me luck!

and yes looking at your graph.. I would try grabbing a PT88, it has both larger turbine and comp wheel and will much better meet the flow requirements of your engine and is the smallest big little frame turbo you will be able to put your hands on. fab work is in your future my friend.. and that backpressure figure is frightening... fix it before your shopping for more then just a turbo!

Chris

 

Yeah in the works. Need to sell this turbo. If anyone needs one let me know I will make pretty good deal on this one.

 

Well guys I have the specs on the log setup. 3 inch downpipe, 2.25 crossover pipe, 3 inch intercooler pipe inlet and outlet, 1 5/8 primaries, 2.50 log. I ran some tests and found that 35 psi on the outlet side of turbo with only 15 psi in the manifold. I am assumming that the intercooler can't flow enough. What do you guys think?

 

That setup is tiny, thats why others have had better success (like engineermike). Q trim vs S trim aside, Mike runs an SLP header on the pass. side and I believe he has ~2.5" crossover, 3" log with 1.75" primaries and a 3.5" DP.

I wonder how much power you would pickup (and drop the bakpressure) if the kit you have was upgaded to Mike's specs.

 

My spec's are 3" downpipe, 2.5" crossover, 3" intercooler pipe, 1 3/4" primaries, and 3" log. The only one on your setup I would have concerns with is the 2.5" log, but it may work anyway. See my comments below. . .

Are you saying that the compressor has 35 psi at its outlet and you're only getting 15 psi in the plenum??? If so, then "Houston, we found the problem." That is WAY WAY WAY too much pressure drop through the cold-side piping!!! Of course, less is better, but the rule of thumb is that your cold-side pressure drop should be <10% of boost. So, if you're running 15 psi boost, you would expect to lose 1.5 psi through the intercooler piping, intercooler, MAF, and throttle body. You're losing 20 psi. That's just ridiculous. This is where all your backpressure problems are coming from. Cold-side pressure drop makes the backpressure look bad for 2 reasons: 1. Reduces boost in the plenum. 2. Makes the turbine work extra hard (more pressure) to spin the compressor faster to reach the desired plenum boost level. Forget about the turbo for now and fix the cold side pressure drop. 3" cold-side piping should be big enough. With that much pressure drop, I would look for debris clogging the intercooler or piping, leaking BOV, or TB not opening all the way. If you don't find problems with that, then I'd look real hard at the intercooler.

Once you fix this problem, you will have either a) 35 psi boost and 65 psi backpressure or b) turn the boost down and have 20 psi boost and 50 psi backpressure. 50/20 still ain't the greatest, but it's worlds better than 65/14. In order to improve further beyond this, you will need a GTS exhaust side and maybe .96 Tang housing.

By the way, to all that think that a T76 won't perform on large displacement, I just went 146 mph at full weight with a 388/T76, LT1, log header, and blower cam.

Mike