Engine Vacuum lbs??
03-19-2009 | 10:50 PM
#17
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From: Stockton, CA
David
Did you tap into vacuum after the blue/black check valve? That is a one way valve....so the vaccum should go up pretty high, but it will come down FAR slower...in theory not at all? However the valves fail and leak....
Typically I get 22-25 inches of vacuum at idle at various HVAC vac pots.....but when attached directly to the port on the brake booster (no check valve) it goes up and down FAST....it should go to ZERO at WOT....
Did you tap into vacuum after the blue/black check valve? That is a one way valve....so the vaccum should go up pretty high, but it will come down FAR slower...in theory not at all? However the valves fail and leak....
Typically I get 22-25 inches of vacuum at idle at various HVAC vac pots.....but when attached directly to the port on the brake booster (no check valve) it goes up and down FAST....it should go to ZERO at WOT....
03-19-2009 | 10:54 PM
#18
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From: Acworth, GA
David,
First, you are not measuring the true manifold vacuum of the engine - you are measuring the boosted vacuum used to power the brake booster and the HVAC. The venturi next to the intake increases the vacuum at the fitting that you used. Go to the back of the engine to the "tree" fitting to get intake manifold vacuum.
My guess on your engine is that you have the common GTS oil consumption problem, caused by poor oil control at the piston rings. Lots of trouble to fix - is it worthwhile for you?
BTW - Malcomb's air pressure trick to change the valve stem seals works, but there is a safer way. Get about three meters of soft braided nylon rope, perhaps 10 mm diameter. Tie a good solid knot in one end. Feed the other end down into the spark plug hole until you have all of the rope in the cylinder - you may need to rotate the engine slightly to get it all in. Slowly and carefully turn the engine in the correct direction until the engine stops and you can't turn it any more. The rope will now hold all the valves in that cylinder closed. The only problem with the method on the 928 is that you do have to turn the engine backward just a little to release the pressure on the rope so that you can pull it out.
My personal opinion is that you can put a lot of oil thru the engine before you ever pay for the gaskets and parts to fix the oil consumption.
First, you are not measuring the true manifold vacuum of the engine - you are measuring the boosted vacuum used to power the brake booster and the HVAC. The venturi next to the intake increases the vacuum at the fitting that you used. Go to the back of the engine to the "tree" fitting to get intake manifold vacuum.
My guess on your engine is that you have the common GTS oil consumption problem, caused by poor oil control at the piston rings. Lots of trouble to fix - is it worthwhile for you?
BTW - Malcomb's air pressure trick to change the valve stem seals works, but there is a safer way. Get about three meters of soft braided nylon rope, perhaps 10 mm diameter. Tie a good solid knot in one end. Feed the other end down into the spark plug hole until you have all of the rope in the cylinder - you may need to rotate the engine slightly to get it all in. Slowly and carefully turn the engine in the correct direction until the engine stops and you can't turn it any more. The rope will now hold all the valves in that cylinder closed. The only problem with the method on the 928 is that you do have to turn the engine backward just a little to release the pressure on the rope so that you can pull it out.
My personal opinion is that you can put a lot of oil thru the engine before you ever pay for the gaskets and parts to fix the oil consumption.
03-19-2009 | 11:26 PM
#19
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From: Bend, Oregon
David--
If you are searching for a place to test manifold vacuum, try the connection to the fuel pressure damper at the front of the motor. T in there with a stub to the damper.
If you are searching for a place to test manifold vacuum, try the connection to the fuel pressure damper at the front of the motor. T in there with a stub to the damper.
03-20-2009 | 12:16 AM
#21
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From: Lillington, NC
Yeah, the engine just runs too good as is to worry about replacing valve guides or rings.
The smoke is not that bad since I added the SharkVent from 928 Specialists. You would be surprised at how much oil that catches, that used to dump into the intake.
If money was no isssue, I would get the car to Greg Brown and have him rebuild it and regrind the cams and Sharktune it. No need for stroking it, 5.4L is enough for me. With the right cams, head/intake work, and tuning the 5.4L could make real close to 400 rwhp, in my limited opinion.
The smoke is not that bad since I added the SharkVent from 928 Specialists. You would be surprised at how much oil that catches, that used to dump into the intake.
If money was no isssue, I would get the car to Greg Brown and have him rebuild it and regrind the cams and Sharktune it. No need for stroking it, 5.4L is enough for me. With the right cams, head/intake work, and tuning the 5.4L could make real close to 400 rwhp, in my limited opinion.
03-20-2009 | 12:18 AM
#22
Race Car
Joined: Dec 2003
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From: Atlanta, GA
Engine vacuum should be around 16-18inHg at idle and exactly 0 at full throttle. On full deceleration with fuel cut off from above 3000RPM I have seen mine go as high as 23inHg or so.
Dan
'91 928GT S/C
475hp/460lb.ft
Dan
'91 928GT S/C
475hp/460lb.ft
03-20-2009 | 12:41 AM
#23
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From: Manchester,NH
Normal engine vacuum at idle should be 16-20 or 18-22 inHg, depending on the source. A vacuum gauge is a useful tool for engine diagnosis. Any time the throttle is opened vacuum drops due to atmospheric pressure entering the engine. Wide open throttle should go close to 0. The highest is on decel with throttle closed, can range from 21-27 inHG. A good gauge at good souce is crucial. Constantly changing vacuum readings at low engine speeds may indicate a problem with worn valve stems, guides, or seals. Higher engine speeds tend to temporarily cancel out this problem.
Have also replaced seals on numerous engines with compressed air method and this tool.
http://www.lmctools.com/product_info...ducts_id=20004
Have also replaced seals on numerous engines with compressed air method and this tool.
http://www.lmctools.com/product_info...ducts_id=20004
