Low boost on stock '86 930
 

The boost on my 930 goes up to 0.5 bar as measured by a separate VDO boost gauge -- the one that goes in the clock position. The boost pressure is tapped off of the brake booster vacuum line. This is with a completely stock engine and cat with 100K miles on it. The car was recently smogged on one of the dyno type machines here in Northern California and it passed with flying colors. When I replace the cat with a bypass pipe (actually a hollowed cat), the boost goes up to 0.7 bar. In both cases, I can hear the waste gate openning up.

My first thought is that the old cat is plugged up, and somehow affecting the boost limit. But the fact that the car passed smog makes me wonder if this is the case. Also, I can't really explain why a plugged up cat would cause the waste gate to open a a lower 0.5 bar pressure. I could see how a plugged cat could perhaps prevent the boost from going up all the way, but in that case, the waste gate would presumably not open. So I'm wondering if maybe there is another problem in the system.

Any help on this would be greatly appreciated!

-Juan

I am thinkin you need to measure it in the I.C.?or after that evvvven. or perhaps you will never know what the boost really is.............

I believe the brake booster vaccuum line is connected to the intake manifold, i.e. downstream of the IC. So that should be a pretty good place to measure the boost.

-Juan

Juan,
I have seen several80s US turbos with these symptoms that had melted/clogged cats. I think the problem is the exhaust doesn't let enough flow through that the turbo doesn't spool up. You can look into the inlet of the cat to check for damage. You should really get rid of the US muffler also and your car would run even better. The muffler sold by Fabspeed is a very nice piece
Dan J

Dan,

Your explanation is what I thought at first until I measured the boost. If the problem was that the turbo wasn't spooling up completely, then why would the waste gate trigger? I wouldn't think that the measured 0.5 bar boost would be enough to trigger the waste gate.

It doesn't make sense. Maybe there is a more complicated explanation of how it all works that would explain what is happening. I am curious to understand.

-Juan

Have you checked your wastegate to make sure its not open constantly?

Has the wastegate been rebuilt/replaced - I think 100K is pushing it

Even with a fully functional wastegate any exhaust restriction can cost you a few PSI of boost.

Without a boost controller the wastegate opens even before full boost is reached so hearing it open in either case doesn't mean much.

Wastegate open constantly... maybe that's why my smog numbers were so good -- it's all going out the wastegate instead of the tail pipe! :) Seriously, I'm pretty sure I can hear the wastegate open up, as it makes a very distinctive hiss when it does so. But maybe it is partially openning early? Is that a common failure mode? How would I check for good wastegate operation?

-Juan

The most common failure mode is a bad diaphram which will cause overboost. They sometimes stick too, but that will also cause overboost.

If your system is consistant then the problem at hand is just the effect of back pressure on a turbo's ability to spool.

Thats the reason why opening up the exhaust system after the turbo is a common turbo car tweak. All things being equal it will usually result in more boost on systems without boost controllers.

Hello!

There is probably nothing worng with your wastegate.

Plugged cat means that exhaust manifold pressure will be high. High pressure means that there will be lot's of force pushing on wastegate piston while spring force holding it back will remain constant -> lower opening boost.

I suggest you to replace cat as soon as possible. Low boost will be least of your problems. High backpressure means also high EGT and high EGT means overheated heads/valves -> problems later on.

Remember, wastegate is just a piston with spring pushing in one direction and diaphragm and exhaust pressure pushing in another. If gas pressure raises, it will open sooner.

I would not go as far as suggesting a new cat right off the bat since even a new cat might cause enough restriction to cause the changes he is seeing.

logician,

What type of mechanical boost gauge do you have? Is it a VDO with the tell tale needle? Do you notice a loss in power?

David

If the wastegate is functioning properly this is not true. It is another rumor like the G50 conversion add 600 lbs to your car.

If it were me I would test the wastegate to eliminate it as the problem.

To test- Put air pressure (less than 18 psi) into the side port of the wastegate with an air regulator and a gauge. And watch were it opens.

You are missing the fact that the pressure that cracks open the wastegate and the pressure that opens it a few millimeters can differ by a few PSI.

As the turbo reaches its sweet spot the wastegate often has to open several millimeters and that requires more than the base opening pressure of the wastegate.

For that reason everything from the flow dymics of the compressor and turbine to the wastegate spring rate and valve and diaphram geometry cause a complex control equation that causes the kinds of issues this thread is asking about.

its also the reason why 930s with functional wastegates will make more boost on cold days than hot ones.

I agree with BeepBeep and DanJ; it's probably a bad cat. As manifold pressure increases, it takes less pressure from the intercooler to open the waste gate (refer to diagram).

This further confirmed by the fact that you get normal boost when the cat is bypassed.

http://forums.pelicanparts.com/uploa...1107404956.gif

Thanks everyone for the insights! The explanation wastegate operation fascinating. Steve's diagram helps to understand Beepbeep's point. Does anyone know what pressure is in the exhaust manifold in a good system, and have any idea how high that might get in a plugged up system?

350HP930, nice thread on your rebuild! Thanks for the replies.

Shaka, the gauge is actually from NGK (Andial sells it). No tell tale. I don't quite understand your question about loss in power. The gauge certainly did not cause a loss in power when I installed it, if that is what you were asking.

-Juan

Well, concidering that the wastegate spring has a closing force of about 100 lbs, the control diaphram has a surface area of about 10 in^2 and the exhaust valve has a surface area of about 1.75 in^2 it would take an additional 15 psi of back pressure to cause the drop.

350HP930, your computation is exactly what I was getting at. Is 15psi a reasonable additional back pressure? If typical exhaust manifold pressures tens of psi, then that sounds plausable. I'd still like to know what typical exhaust manifold pressures are.

Also, you said previously that you would not go as far as installing a new cat off the bat. What would you suggest as the next step? I know that the test pipe feels really good, and gets the boost up to 0.7bar.

-Juan

If you have passed your test leave the test pipe on till its test time again.

I have no idea what back pressures are normal before a cat but I know its more than no cat and turbos love low pressure after the turbine.

The turbo will also increase back pressure to an amount close to the level of boost so my above pressure calculation would be in addition to those 'normal' restrictions.

If you are really curious you could tap a hose into the exhaust (there is an access port near the O2 sensor) and get some pressure readings.

Being in a non emission testing state I could never imagine paying good money for a catalyst and if I lived in california I would certainly try to cheat the system any way I could.