They are one off turbos which is making identification difficult. So I can't answer your question. I have tubular manifolds.

And you're right. The smallish exhaust flange could be causing part of the problem. The downpipe is 3".

 

That sucks to hear. I hope I don't have that problem.

 

sorry to hear of the bad news , hope you get it sorted also i think soon you will be welcomed to the 900whp+ club

 

The Exhaust is plenty for a 346, the cam is large for a 346 turbo car. The triple intercooler is a real bottleneck.

Before you go too far, I would disconnect/remove and inspect the intercoolers, i think they may have debri or jus not flow well.
Good luck, we are all just throwing out ideas because we are not there to systematically eliminate the possibilities and get to the root of the problem. If your exhaust is not plugged, I would not worry about the size, it is not your problem.

Steve

 

also check your blowof valve it might be seeping boost , also do a boost leak check.

 

Listen to Dr. Phil or you will end up broke and upset. Many have put down over 900rwhp with 2.5 inch exhaust with turbos comparable to yours..

 

What valve springs and lifters are you running?

 

Free up everything after the turbos. I run 3 inch pipes after my 182 CI 6 cylinder! I know folks running 4" from turbo back and seeing significant increases.
May even want to consider porting or clipping turbos to eliminate back pressure the engine sees... Turbos must breathe AND NOT CONSTRICT! I admit I'm only pulling around 510 HP, but again, its only 182CI, and of course going to AWD!!

 

Yes, going with larger diameter piping will help but there is something other than the downpipe and catback causing the issues. I still think its the housings if these are T25 based turbos because nothing else really explains why the power doesnt increase with boost. A restriction on the cold side doesnt really explain it because power will still increase with boost and even with a boost leak power will increase with boost.

corvettesforfun: Can you confirm if these turbos have the t2 flange with a picture or by contacting the builder or previous owner? If the turbos are T25 based like i suspect you will be very limited on the turbine side. I am not sure if there is a option for a larger ar ratio so unless you are willing to change the flange on the manifold to a T3 flange, replace the housings and modify the downpipe there may not be any other options go get you to or near 800hp on the same setup.

I would be willing to bet this is why tti and lingenfelter stopped using the T25 based turbos with their setups.

Carl

 

dont change the turbos , i did 664whp on a 347ci with mildly ported heads stock cam and a 2.5inch exhaust, start working outside on the piping on your blow off valve and manifold gaskets and then after you have eliminated all those then think about changing major components.

 

Most of you were right. Dropped the exhaust at the downpipes and car picked up some HP but not substantial and still fell on its face at 6000 at 10psi.

Next step remove the wet to dry intercooler and back to the dyno.

I have gotten over the hurdle of having to drop the engine again.

 

I'm with Arun. I think its in the tune. I've seen a couple cars do the same thing, even more so when their boosted. I would make sure your tuner knows what they are doing in regards to the tables he mentioned before chasing these hard parts. That and put two good intercoolers in it and ditch the 3rd.

 

You said backpressure is 4.4:1. Are you refering to the turbine expansion ratio? So you are measuring exhaust pressure before and after the turbo? If so what are the measurements?

You also said they hogged out the inside of the turbo. This scares me. What did they do? The way you worded this makes it sound like they increased the internal volume of the housings. I hope this is not the case. The volute geometry is critical for aero performance and should never be modified.

One more question: what kind of coupler do you have running between the filters and the compressor inlet? If they are relatively flimsy silcone type you may want to try going with a more rigid pipe as the ones you have now may be collapsing due to the high negative pressure at the compressor inlet. I work for Garrett on the OE side and have seen this before on some production applications.

Are you running internal or external wastegates? Some other things to check: BOV, WG spring stiffness, intake manifold temp (would give you an indication of inefficiency of your cooler setup), intercooler delta P, and ignition retard (are you sure it's not pulling timing?). It would also be good to know the exact turbo you are running (i.e. wheel size, trim, etc.).

Good Luck,

 

Backpressure measured on the hot side at the turbo. Couplers not collapsing. Int wastegates have been fixed. Waste gate springs about 10lbs and equal, IATs good on meth, not pulling timing. Turbos are GT35R but can't answer other questions.

Wet to dry intercooler has been pulled. Dyno again Friday.

 

Pulled wet to dry intercooler and made a trip back to the dyno. Only modest gains though. At 10lbs on meth at 19 degrees timing at 6300 RPM the car made 700HP. The car again was very strong up to the 'wall' then abruptly fell off power wise. I have a video and graph coming I'll try to post. At 15lbs the power fell off at 5300rpm.

I'm convinced the flanges are too small and perhaps even a T2 instead of a T3. We'll have the turbos out of the car next week. And we'll see.

They are Garrett. Who do you recommend rebuild them and make the recommendations for changes?

 

Why would you rebuild the turbos? It's hard to recommend any changes without knowing exactly what you have.

What is the backpressure measurement?

 

I should have used language like inspect the turbos and rebuild as necessary. The main purpose would be to find out what these 'one off' turbos are in terms of measurements. If they are hybrid turbos instead of stock GT35s then maybe that's my problem.

I'm still confused. It seems that normally GT35s come with a T3 flange. I still don't know what to make of things. I'm getting a lot of conflicting advice still.

Backpressure is 4.4 to 1.

 

Precision Turbo and Engineering and be done with it.

 

The T3 flange is not the restriction. Imports can make more power than you are making with only 1 turbo on a motor with 1/3 the displacement.

I would suspect that those intercoolers you have are causing restriction.

Also where are the dump tubes from your waste gates going?

 

I am still not sure what you mean by this. If you are talking about the expansion ratio (which is turbine inlet pressure divided by turbine outlet pressure - we measure this as a total / static ratio) then that is very high and could be an indication that your turbine stage is too small. You could be choking the **** out of your engine. Increase turbine trim (or most likely increase A/R) and that could help it breath easier. It doesn't seem like twin GT35s should be choked at your power level but if the 4.4:1 you are talking about is the expansion ratio, then I would be suspicious of what you really have. Measure the turbine wheel inducer (larger diameter) and exducer (smaller diameter) and let me know what it is.