dholling13

Fellow Porschefiles,

I am new to this forum, but have owned a c2 Turbo for about 2 years. I recently deleted the cat from my exhaust system, and I have been experiencing problems with the over-boost circuit. The car has a 1.05 bar spring in the wastegate, but when the cat was still on the car, the manual gauge never showed above 12 psi's. Now that the cat is off, the gauge shows between 14-15 psi's and hits over boost at 5k rpm's. Any experience or help would be great. Happy Holidays to you all.

Drew

PorschePhD

The overboost switch on the intercooler is weak. You need to replace it if you are really only at 1.05 bar. It shuts the fuel pumps off in a over boost situation. That should be 1.3 bar, not 1.05.

JBH

After removing the cat, it sounds like exhaust flow is now less restricted so you are hitting the 14-15 psi anticipated with your wastegate spring. How far are you overboosting? What turbo do you have?

There are previous threads that describe overboost situations where flow from the wastegate is too restricted, but I thought this situation was associated with the GHL header design and a high flow turbo.

dholling13

Guys thank you for responding,

I have a stock k27 turbo... Is this Stephen Kaspar from the PCA tech section? If so, I like most PCA members are very thankful for your time and energy you put into the club tech section.

So your message states the overboost switch is weak. Can the motor handle 1.05 bar? Where do you get a better or more tolerant switch for the intercooler?

Drew

911addict

Can someone explain this overboost to me?
My car is a standard 3.3 but with a 1 bar spring (probably!) and 3.2 exchangers with cat bypass and muffler bypass. Will this affect the boost, and or mixture. It ran fine before the cat delete, and still runs fine but pops a whole lot more on lift-off :-)

nathanUK '81 930 G50

Mark, there is a pressure switch screwed into the intercooler, when boost pressure exceeds say 1.3 bar then the fuel pumps are cut and the engine dies when the fuel is burnt. The boost pressure then drops and then the fuel pumps will come back on and it will go through this cycle again until you lift off.

JBH

Yes your engine can take 1 bar boost if the air/fuel ratio is properly adjusted.

I think the point is, you should not be overboosting. The pressure limit switch is there to protect your engine - it is not intended to be part of normal operation.

The wastegate is designed to open when a preset pressure is reach - determined by the spring - overboost is caused by the inability to relieve pressure fast enough when the wastegate opens. This can be caused by a constriction before or after the wastegate. The problem is exacerbated by high flow turbos that can build pressure very quickly. I don't think this is your case since you are using the stock k27

911addict

Thanks guys.
Out of interest, what is the sign of overboosting?

38D

The power will cut. You will know it when it happens.

dholling13

Has anyone ever had success curing this problem by solely replacing the overboost switch?

Staffan

Yes, very common fix to this problem.
If you still have this problem after getting a new one (which I doubt) you could try having the switch read manifold pressure so that you avoid the spikes acting inside the intercooler

JBH

If the switch is faulty and cuts out at or below your wastegate setting, then yes changing the switch is all you have to do, but that is not overboost - just a faulty switch.

Just for clarity, overboost occurs when the boost pressure exceeds the wastegate setting.

dholling13

In our generation turbo cars with CSI, then the wastegate is controlled by the manual spring inside. If the car is overboosting, would the wastegate be at fault? Tonight, on the cars manual gauge, when I accelerate in fourth, the psi's are between the 16-18 range. If there is a 1.05 bar spring inside the waste gate, shouldn't the waste gate open and dump the excess boost? I didn't hit the overboost tonight when I was on this test run, I let off when I saw these readings.

Guys, thanks for your input...

Kevin

You have to realize the relationship between the turbine section of the exhaust driving the hot side turbine wheel and the compressor stage of the turbocharger. It sounds like you have a 1 bar boost spring. like you mentioned. Your wastegate valve and guide could be sticky/sluggish. But the high pressure you are seeing after you slam the throttle-body shut is caused by a restriction from your bypass valve circuit. Installing a Forge or Bailey bypass valve with correct spring will cure the after throttle/shut spikes.

Watch your manual gauge in 3rd and 4th gear under load. This will let you see what the compressor wheel is doing.

dholling13

Kevin,

Thanks for the info. I had a 996 diverter valve put in last year... Can we use manual boost controllers in our turbo system? If the car has a 1 bar spring, could we install one of these and keep the pressure under 16 psi's?