Turbine Back Pressure

[TwinTurboM3]

on my 3.2L inline six BMW engine, back pressure on the exhaust manifold is very high.

twin turbocharged. GT40 x 2

76 mm x 2 downpipes.

no muffler, no midpipe. straight to road discharging.

back pressure is taken between the turbo and cylinder head.

3:1 exhaust pressure to intake manifold pressure ratio

i have changed to bigger A/R turbine housings and the ratio dropeed to 2,6: 1.

still at the high side.

power does not go up when i add boost. same power @ 16 psi. same power @ 21 psi.

intake manifold temperatures are close to ambient temps, so the temps dont increase, stay close to ambient.

Cams are high overlap, a total of 162 degree cam overlap.

Would you please comment on the high back pressures?

[r33_racer]

Exhaust manifold design perhaps? Possibly the size of the runners? Perhaps they are too restrictive???

[Fastrotor]

Where are you measuring the BP ? At the turbo flange or at the head ? I'd check at both ends to gague the drop across the manifold runners to determine if there undersize. Having large BP with a big overlap cam is not great... you'd probably find putting in a smaller cam without changing anything else would gain power.

But thats not what your after obviously. Have you also measure pressure drop across intercooler and piping ?

Does the Head flow enough for the required power output ?

Are the airfilters on the turbos big enough ..... have you measured the intake vacuum ? For best results you really dont want more than 10" of water vacuum between filter and turbo. (assuming you run them)

Did the power increase a decent amount when you changed A/R ratios and the BP dropped to 2.6 ? (assuming same boost) ? If so , maybe the A/R's are still just to small and at the limit of the power your making currently ?

Anyway some food for though. Sounds like a monster either way, do you have some photos to share with us ?

Gary

[Lithium]

The cam profile thing comes up again - a couple of us answered this on GTRUK forum and there was no answer, hard to feel motivated to add more if you don't know if the person is reading or considering the suggestions.

[TwinTurboM3]

First of all, thank you for your comments.

Back pressure dropped after decreasing the cam overlap.

I have made the following PRESSURE measurements:

Measurement 1: Pressure at the discharge of each turbo is the same. I have put 2 fittings to the pipes,these pipes connect the turbos with the intercooler.

Both turbochargers flow at the same pressure. Identical pressure ratios.

Measurement 2: Pressure before and after intercooler

Before intercooler, measured @ intercooler entrance end tank 2,5 bar

After intercooler, measured @ intercooler exit end tank 2,45 bar.

Pressure drop accross the cooler is not much.

Measurement 3: Boost pressure after the 6 individual throttle bodies is 1,4 bar. The engine has 6 individual throttle bodies.

This means there is a 1 Bar Boost Loss between the intercooler and cylinder head.

The 1.4 bar boost pressure is measured AFTER the throttle bodies, boost is measured between the 6 throttle bodies and the cylinder head.

Tomorrow I will weld a fitting to the pipe which connects the intercooler with the intake manifold. and see if the loss is due to this pipe.

Moreover, I will weld a fitting to the intake manifold and see if the pressure is being lost because of the intake manifold.

I suspect the 2 following points:

Idle valve may leak under boost.

Or the throttle bodies may leak. As there is a shaft which moved the throttle plates, we may be leaking through the holes of this shaft.

Would you please comment? Many thanks

[chrissso]

You have 2 GT40's? They're pretty big.. At whet rev's do you get 21psi? What's the AR of the exhaust housings?

Overlapped cams are more for N/A applications. The the inlet cams open before the exhaust cams are shut, which is good for na cars, but you get exhaust manifold pressure flowing back into the intake manifold on a turbo car if both exahust and inlet valves are open at the same time.

So unless I've got my terminology wrong, you maybe using naturally aspirated cams in a turbo car.

[TwinTurboM3]

yes twin Gt40s.

i have a dyno dynamics in my garden, and have 21 psi @ 5400 rpm if i load the dynamics.

during a power run, in Shoot out mode, 21 psi comes @ 8800 rpm.

as i have a 1 bar leakage, the boost should come on earlier when fixed.

i switched back to less overlap cams and back pressure is now 1,8:1. so i am happier with the back pressure.

but the boost loss of 1 bar is making me sad.

idle valve or throttle body shaft or some leak?

[Lithium]

yes twin Gt40s.

i have a dyno dynamics in my garden, and have 21 psi @ 5400 rpm if i load the dynamics.

during a power run, in Shoot out mode, 21 psi comes @ 8800 rpm.

as i have a 1 bar leakage, the boost should come on earlier when fixed.

i switched back to less overlap cams and back pressure is now 1,8:1. so i am happier with the back pressure.

but the boost loss of 1 bar is making me sad.

idle valve or throttle body shaft or some leak?

I hate boost leaks A tool which could pump "smoke" through the system post intercooler would be very handy at this stage to see if or where it came out. One then we did when I had a leak with mine was squirt compressed air at different points of the outside of the piping etc while the car was idling and at least in my case you could hear the engine react to the higher available airflow. I assume you have ruled out any chance of the blow off valve or a join in the piping leaking?

Good luck with this, this sounds like it will end up completely insane when done - how far did you go with the twin GT30R setup?

Edited June 19, 2008 by Lithium

[TwinTurboM3]

I hate boost leaks A tool which could pump "smoke" through the system post intercooler would be very handy at this stage to see if or where it came out. One then we did when I had a leak with mine was squirt compressed air at different points of the outside of the piping etc while the car was idling and at least in my case you could hear the engine react to the higher available airflow. I assume you have ruled out any chance of the blow off valve or a join in the piping leaking?

Good luck with this, this sounds like it will end up completely insane when done - how far did you go with the twin GT30R setup?

Thanks a lot, but we dont have the smoke machine here.

With GT3076 i went maximum to 18-19 psi and then w/o maxing them out, i installed the GT4088.

[Dale FZ1]

Exhaust manifold design perhaps? Possibly the size of the runners? Perhaps they are too restrictive???

If high turbine inlet pressure aka back pressure is an issue, manifold design would be well worth revisiting. Looking at the overall spec and rpm being used, is this thing a dyno queen or drag monster? What sort of power target is there? You'd have to be aiming for fairly big numbers, so pipe diameter / length / merge designs would be critical in keeping TIP down and power production up.

The cam profile thing comes up again

Back pressure dropped after decreasing the cam overlap.

This means there is a 1 Bar Boost Loss between the intercooler and cylinder head.

I will weld a fitting to the intake manifold and see if the pressure is being lost because of the intake manifold.

Or the throttle bodies may leak. As there is a shaft which moved the throttle plates, we may be leaking through the holes of this shaft.

i switched back to less overlap cams and back pressure is now 1,8:1. so i am happier with the back pressure.

but the boost loss of 1 bar is making me sad.

idle valve or throttle body shaft or some leak?

How did you decide on camshaft spec? While reduced overlap has helped in some respects, shouldn't cam lift + duration + valve size + port flow capacity be geared to match the turbo size (and vice versa)? As described, it seems that things just aren't working in harmony.

Can issues with leaks in the throttle bodies be sorted with new O rings? Using the smoke method would be a good test (if available ) to check for leaks before pulling things apart.

Can you remove / blank off the idle valve purely to remove it from the potential source of leaks? Might have to run it at a fast idle while testing, but at least you'd get an answer.

Best of luck with things - sounds like a wild ride.

[Lithium]

If high turbine inlet pressure aka back pressure is an issue, manifold design would be well worth revisiting. Looking at the overall spec and rpm being used, is this thing a dyno queen or drag monster? What sort of power target is there? You'd have to be aiming for fairly big numbers, so pipe diameter / length / merge designs would be critical in keeping TIP down and power production up.

Can you remove / blank off the idle valve purely to remove it from the potential source of leaks? Might have to run it at a fast idle while testing, but at least you'd get an answer.

The high turbine pressure versus intake pressure at this stage could possibly be largely due to the fact that the inlet pressure is disappearing after the intercooler?? Going by his pressure measurements as far as the turbo is concerned its running 2.4bar of boost - it just so happens the boost is not making it all the way into the engine.

For it to have pumped itself up to 2.4bar it'd have to have developed sufficient turbine pressure for it to reach the shaft speeds necessary to do that, which would be why you are still running high exhaust manifold pressures. I would probably try and drop the boost level while trying to resolve where the boost is going so you don't blow a turbo trying, as whereever the pressure is going is not doing anything any good! Though how far did you drop the cam duration??

Does the car idle high or do you have any lean miss condition going on? I suppose if its still running an aggressive cam it would be hard to identify this kind of thing.

Edited June 19, 2008 by Lithium

[nitestalkr]

yes twin Gt40s.

i have a dyno dynamics in my garden, and have 21 psi @ 5400 rpm if i load the dynamics.

lol.. your sh!tting us right?

[TwinTurboM3]

the main problem can be the 6 individual throttle bodies.

drive by wire system operates the 6 throttle bodies and under 100 % TPS the 6 throttle bodies may not be opening. Thus restricting air flow into the cylinder head.

i will check if the drive by wire opens 100%.

[Dale FZ1]

Have you searched for answers from this forum:

http://forums.bimmerforums.com/forum/index.php

Running the same machines, they would surely have sorted through generic type issues, and there are some guys with big output BMW engines. If electronic throttling is a problem, then someone there would have encountered and resolved it.

[TwinTurboM3]

Latest measurements

2.0 bar at the exit of each turbocharger

2.5 bar at the entrance of intercooler

2.45 bar at the exit of intercooler

2.2 bar at the intake manifold. A fitting is welded to the intake plenum.

1.2 bar AFTER the SIX individual throttle bodies.

Am I flowing 2.2 bar into the engine according to the Intake Plenum readings or flowing 1.2 bar according to After 6 Throttle Bodies readings?

The `after throttle bodies` measurement is made, where there are 6 runners after the throttle bodies, and each runner has a pipe which is connected to a main TUBE. I take the readings at the end of this TUBE, which gets air from all six runners.

AM I feeding 2,2 bar or 1,2 bar into my engine?

I checked the Drive By Wire and it opens 6 throttle bodies 100 %. So the throttle bodies work properly.

Ambient temps were 30 deg celcius and intake manifold temps were 32 degrees. So the turbochargers are not being overrevved. At 2,2 bar, the intake manifold temps should be higher than 32 degrees.

I cant solve this pressure difference across the 6 throttle bodies.

One way would be to remove drive by wire, put a single throttle body. But the VANOS, cam control does not work without the drive by wire.

[HIJAKD]

valves are most likey too small,as is the ports,i wouldnt imagine the manifold runners are the problem.