Judging from this screenshot of my data log, what would be the things to check to reduce pre-turbinePSI backpressure? At 22psi of boost I'm seeing about 31psi of turbine backpressure. At 26psi of boost I'm seeing 40-43lbs and at 30psi of boost I'm seeing 53-55psi of backpressure.

The turbo is a Precision PT70-GTQ DBB (7080RH) and it's rated at 900HP. It has a single inlet 3 bolt .85 AR exhaust housing. It's physically good with no bent fins and it's not rubbing the walls of the housing.

I know turbos and exhaust housings have a lot to play in the equation. Am I out of turbo even though it's rated for 900HP? If it supports that much, something in my combination is the issue. Here's where I'm asking for suggestions. Cam too big? Cam too small? Opinions? Educated guesses.

Engine Details:
My engine is a built Buick V6. It's stroked to a 249cid with Champion brand aluminum Race heads, Champion GN1 intake. The compression ratio figures out to be 9.3. Camshaft is a 212-212 with .570 lift at the valve with 113* lobe separation. Springs check at 200 seat pressure & 500 on the nose of the cam. Cam is set up straight up without advance or retard. Intercooler is doing its job with minimal to no differential backpressure. Equal tube length headers with 3.5" downpipe into single 3" exhaust with about 1.5lbs of backpressure taken at the bottom of the downpipe. The air filter is clean too. The last time I put the car on the dyno it made 600 rear wheel horsepower with 3.42 gears through a 200-4R transmission.


#ad

 

What is the exhaust system post-turbo?

 

I would hope that the 600hp was at 22psi or less, otherwise something is up....
When I ran a V6, I was running a billet 71, 239ci, home ported aluminum heads etc, not high compression. It would run 135mph at 3800lbs at 21psi for comparison. You still aren't 2:1 backpressure, you would think there is more in it?

 

It's a 3.5" downpipe about 4 foot long leading into a single 3" exhaust system with two mufflers that are straight through with the last muffler having two tail pipes that exit right behind the rear tires. The last time I checked the backpressure it was about 1.5lbs and was taken at the bottom of the 3.5" downpipe. I should check it again too confirm. My car is in the deep 10's. 1/8th mile is 6.48 at 108mph with a 1.49 sixty foot. This was with the boost starting at 23 and creepng up to 27psi. I've since gotten the boost creep to a minimum. Car weighs 3580lbs.

 

I'm trying to figure out what part is causing the issue if any. Seems like for what I've got, it should be at least mid 9 second car.

 

The 900 hp rating really means nothing without a compressor map, which Precision chooses not to share. Your back pressure isn't totally out of line, so some hp numbers or trap speeds at various boost levels would help a great deal. Have you tried open exhaust, and no air filter? definitely worth a try. Also, what fuel, and have you read plugs for ign timing? So many possibilities, but I'm guessing your na hp is enough to be pushing that turbo near it's limits with those back pressure numbers on a bb turbo. A turbo upgrade wouldn't be a bad thing.

 

What mph are you running in the quarter? Rpm? Maybe you are blowing through the converter? I was running a PTC 9.5" non locker which was great, very efficient up top.

 

Going to make some assumptions since you didn't give any detail, egts (1500), mass flow (65lbs/min). Also assuming precision basically cloned a Garrett t70 with a 62mm turbine, I'll use their new gtx35 turbine flow map since it's the closest in size (62mm), but will surely flow more than the older t series. At 30psi with 1psi pressure drop through the air filter your right at 3.2 pressure ratio, 66lbs/min, your right in the sweet spot for that compressor.
Problem is the turbine side. Assuming 1500° egts your exhaust corrected flow is 40lbs, turbine will require a 2.2 expansion ratio to power the compressors 3.2 pressure rise. Looking at the gtx35 turbine flow map, 2.2 and .83 a/r housing puts you at 26lbs/min of flow. Your at 40, 153% over max flow for that turbine stage, and that's assuming the t series turbine flowing what the gtx does, which it doesn't. So your probably 160%.

Remember this is me assuming some numbers, the egts will change the outcome alot.

As of right now with these numbers, you need something around the 1.06a/r housing. A turbine rework would help things as well.

 

His turbo has a Q trim turbine wheel, which is a 68mm exducer iirc.

 

Fuel type is everything in this situation. It sounds like maybe are using typical race fuels, instead of alcohol.

I would recommend switching to Ethanol (E55 to E85 is fine) or spraying water, 100% distilled or 60/40 meth is fine.
Either would reduce EGT and improve the exhaust gas backpressure, water would pull power out of it but alcohol would add some I bet.

EGT is useful to a turbine but it contributes to exhaust gas volume which causes excess pressure. The real solution is improve the turbine but I assume we are looking for things to try that don't involve surgery.

This is all Mnlx's idea anyways. He thought it up first I just stole it to try and sound smart by saying it before he can

 

I haven't tried no air cleaner or open exhaust. I'm going to test my exhaust system back pressure soon.

I have an 8th mile run. 1.49, 6.48@108mph weighing 3580lbs. RPM is 5700-6000 at shift points with a 200-4R transmission with a PTC non-locker 17 blade converter. The RPM drop is about 800rpm on the shifts.

I am not shy about changing the cam to something a little bigger but nothing that is going to sacrifice drivability. I was thinking going up a step in duration...something like a 214 to 218.

I am using Q16 Race fuel. My injection system is set up with 12 injectors. 6 low boost/cruising injectors and 6 fuel injectors for race gas. It automatically switches over from the low octane injectors to the high octane injectors at 12psi of boost and then automatically switch back when the boost falls back below 12. There isn't E85 stations around my area for me to convert over to it.

I have nothing to monitor exh temp but I have thought about it. This car is used on the street 98% of the time. I rarely go to the track.

600HP was at 27psi but that was many years ago. What was your 8th mile on that run?

This is all I have to show a quarter mile time. The tracks in my area are 1/8th mile only. I used a Dragy to get this and I hit the rev limiter on 2-3 upshift due to a shift flare. This was 30psi of boost.

#ad

 

I wouldn't worry about the cam, silly to mess with the cam or engine internals at this point. Turbo will ultimately control the power, not the cam, for this situation anyways.

The fuel you are using is 'hot' like gasoline. It kind of relies on forged pistons which work well at high temp. The high temp however is an issue if the exhaust gas pressure builds up a bottleneck at the turbine, which is where power bottlenecks with exhaust gas, go figure.

If the compressor has headroom you could cool the EGT using water then raise the boost to compensate, resulting with higher flow, and similar EGT with higher boost pressure. But that is a bandaid and assumes the compressor is still in the same efficiency island range with increased boost which is anybody's guess and your boost is already so high I'm not sure thats even a band-aid at that point.

Imo bottom line,
Seems like there is a need to cool the thing down with alcohol fuels, or switch to a larger turbine.
Anything else is going to nickle and dime you to death, and barely worth any progress. I mean, to me progress is like +100hp or so. Some people are happy with 20 or 40hp though, its up to you what you think is worth. I can a gain 100 or 200hp from a turbine swap if done right with plenty of compressor to back it up.

 

Really need to know egts, I don't think turbine is the issue, I think it's the housing. Doesn't help there isn't much out there for specs for that turbo.

 

When you say turbine, or housing, whats the difference in this context? When I say turbine i literally mean the entire back part of the turbo, the wheel, the hous... oh I get it now. So I've been saying turbine and to you that means literally the wheel. Did not even realize

I was thinking of mainly the volume with which to occupy exhaust gas at some temp T, since equation of state PV=nRT , we can decrease T and V or T and P, smaller turbine = smaller V (the housing?) which means at some P, T will be higher or lower. Or visa versa, specific T will give a specific P based on V.
e.g. A small V means a larger P at the same T.


Words like differential, and atomization are the worst. Is it rate of change, or gradient pressure, or difference? Are molecules of water in a gas state 'atomized' if there are not any individual atoms?

Turbine "the back part of the turbo" could include the housing IMO on the internet, people use that words interchangeably I've found.

Me: "whats a centrosome, anyways?"

"Just make exhaust flow better by making the numbers larger on the back part of the turbo."

 

Yep, there's 2 points to choke and limit max flow in the hot side of the turbo, the "nozzle" in the housing, and the exducer of the turbine wheel. If the housing chokes, the bottleneck is before the turbine, so no matter what size turbine is put into that housing it won't make a difference. It's like if your intercooler is too small and limits flow, so you install a larger throttle body.

It's the most mis understood and under appreciated part of the turbo, while also being the most important.

 

Before posting my question on the board, I was thinking the turbo was probably the cause of high BP.

 

Numbers are indicating you aren't using a wastegate? When I correct the flow to 26lb/min turbine flow (.83a/r housing on gtx35, 65lb/min @ 2.2 expansion ratio, i realize you have a larger turbine but may also have a more restrictive exhaust system) I get exactly 56.73psi of drive pressure.

 

Those 3 bolt .85 a/r housings have always been a bottle neck in my eyes.
But thats from being around peeps that wanted to grunt these motors down low with boost. Which just about always wipes out high rpm flow thru the turbine.
I realize peeps have made good power with them. But what was the engine combo details to do that?

If you decide to make cam changes to work with the back pressure, I suggest you get with someone to build you one. The timing events are key to avoiding exhaust gas reversion back into the cylinder. Longer duration alone on exhaust could make the reversion worse depending on timing events.

 

It could be just that simple. Due to your good flowing heads/intake, and more cubic inches your turbo could just be out of breath. Everything in your combo sounds good and very comparable to my old setup, my turbo probably flowed more, and had less cubic inches to keep back pressure at bay. Even with my ported stock heads and single pump w meth injection, it ran 102mph 1/8 (off idle) and 133mph in the quarter at less boost.

 

I think folks have goe fast with them, but with 231 cubes and way less head. It would seem as though the hot side is choking this combo. Having 3" exhaust isn't helping anything either. But even with an open 3.5" the housing will choke it due to the engine airflow. Would be neat to see him get a T4 housing with a .96 or so A/R. I bet it would open it up nicely.