Backpressure topic

[smurfbus]

Not much in the 930 forum archice so lets try a new thread with example to open up discussion.

I swapped a holset hx40 super out for a spare mismached hx40 with 8 blade compressor which should be 58mm and if I searched correctly the turbine wheel is about 60/73 or 64/76 and the spare hothousing is big 18cm^2. That makes a small cold side and a large hotside which is a odd ball and IMO should have good low backpressure.

Im also testing how high My current dual ultragates with 7psi springs can go so I have manifold pressure at the top niple.

I just rigged up a sensor for backpressure. EDIT: 2nd gear max 1.85bar BP with tapering boost at 0.45 bar @5800rpm. Third gear BP 1.5bar with 0.86bar @4200rpm

What is going on and is the anything to make it lower? Already 2 wgs and split flange and zork. I did drop a 11mm wrench in the up pipe but no sounds.

I can make some test runs and even swap back the hx40 super but first Iwould like to test this turbo with higherboost. 4 port macvalve is on it way and looking for 10 psi springs.



These pics are from internet.

[Mixed76]

Where are you measuring backpressure?

The turbine works on differential pressure; first place to look at lowering pressure is your outlet pipe. Yours looks pretty good, maybe some obstacles there. Consider checking backpressure with the pipe removed.

Next place should be diameter and smoothness of plumbing feeding the turbine.

If both those are good then look at turbine specifics.

I agree that your ratio of backpressure to boost is high.

[smurfbus]

Just before turbo flange. I use the same VPV 3bar MAP for BP as for MAP and same settings on analog 2.


BP is low under 180kpa being under or same as MAP, over that game over. I hope that spare MAN truck turbo just wont work with 3.3 liter engine.



Here the same WOT run but a bit later BP is already 250kpa

[smurfbus]

Turbo cold side pic





MAP and BP sensors are those 3bar units.



There really should not be much backpressure in that Zork tube with that flap horisontal.

[smurfbus]

Looking at the above comp map (if its correct) and assuming my engine flow @1bar comp pressure as follows (got these from my hxsuper map I had done earlier these may be flawed with wrong VE or cooling assumptions):
40kg/s 4500rpms
50 5500
60 6500
70 7500 or 1.2bar and 7000rpms

This above hx40 MAP is out-flowed already at 5500rpms.

What happens when engine outflows compressor? heat and choking? Well I get BP but my MAT did not raise much on the third gear pull above. I think I need to reinstall my superhx40 and see if it still smokes after I reinstalled my scavenge pump.

[Mixed76]

Are your pressures absolute or gauge?

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[smurfbus]

Tunerstudio has absolute and my calcs above for flow gauge pressures.

[Mixed76]

...

[smurfbus]

I have my both turbosmart 38 ultragates setup so that engine gets max boost so I can later try to control it with mac valve. They have 7 psi springs and wg hose on the top port to act against backpressure. This is not permanent setup but a test to see the potential of the turbos before limiting them. They work fine when hose is in the lower port and als owhen backpressure goes above spring+MAP.
Today I swapped in my original K27-7200R turbos just to test them with EFI. They dont have twinscroll but they are much smaller and spooled earlier but also gave even more backpressure and I could hear the wastegates open on second gear when BP went to 300kpa.

[Mixed76]

I went back and looked at the timestamps and rpm range, changed my mind.

You've got gradual boost increase with hardly any backpressure, which could be open wastegate or big turbo. Looking at the timestamps and the rpm range I think the turbo is big and the gradual boost rise is lag. Is the compressor side too small, so you run out of compressor flow capacity once the turbine gets into its operating range (as seen by the knee in the backpressure curve)? That might give you excess backpressure as the wastegates force all flow through the turbine trying to hit a boost target at a flow rate that the compressor is too small to achieve. Not convinced though, backpressure comes from doing work, and if the compressor can't move the air it isn't doing the work to create backpressure.

If the compressor is big or correctly sized, boost should rise together with the backpressure, which I don't see from the plots. If the wastegate is limiting boost it should also be limiting backpressure.

Maybe Mr turbocraft can unravel this one.

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[Mixed76]

In your first post the units are mixed.

1.9 bar(a) is not the compressor dp; inlet to the compressor should be close to 1bar(a).

2.9 bar(a) is not the turbine dp; turbine discharges to atmosphere which is at 1 bar(a).

So you're looking at about 1.9 bar exhaust to make 0.9 bar intake, which is about right. Or, if you have a little loss on both sides you would be using 1.8 bar exhaust to make 1 bar intake, again pretty usual.

I think there's nothing to see here.

[smurfbus]

Yup, messed the first posts backpressure numbers should have been 1bar lower so
"2nd gear max backpressure 1.85bar with tapering boost at 0.45 bar @5800rpm. Third gear BP 1.5bar with 0.86bar @4200rpm"
and now with smaller 7200R
2nd gear 2bar BP at 1bar boost @4600rpm and third gear 1.2bar Bp at 1bar boost @3800rpm.

So these old engines with two valve heads with pancake IM just cant get a nice backpressure/boost ratio?

[Mixed76]

A turbo with compressor and turbine sized to match one another better can do better, has nothing to do with #of valves or the intake shape at this low rpm.

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[smurfbus]

Follow up and new question: So I got my BP readings a bit lower by ditching the flapper on the zork. The springs I had on it did not hold it all the way open when on boost.

Q: How does cam timing affect backpressure if at all? I've had my cam timing max advanced for mid range but now that I've sealed the cylinders again I might try 1.26mm lift for max top end with 964 cams. I have two turbosmart WGs so there is enough dumping volume after req boost is reached.

Retarding the cam would cause the intake valve to close later, bleeding off some cylinder pressure through the intake port but at the same time it should cause more exhaust volume at higher rpms where the BP is worst?