Very scary compression numbers on my turbo car
10-05-2011, 12:04 AM
#81
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Do I want these or something hopefully cheaper since I want no increase in RPM?
http://thmotorsports.com/i-69199.asp...ingSeanVehSpec
If I must do headwork, we're doing it right.
http://thmotorsports.com/i-69199.asp...ingSeanVehSpec
If I must do headwork, we're doing it right.
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10-05-2011, 12:36 AM
#82
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FWIW, when I've done leakdown tests, I often see the valves leaking slightly. I think it's due to bits of carbon or something getting stuck between the valve and the seat and preventing a complete seal. If you crack the exhaust valve open with pressure in the cylinder, that'll usually blow it loose, then go back and it will seal.
--Ian
--Ian
I was wondering this myself. What happens if you apply pressure and try moving the crank slightly by hand. Could a timing belt stretch enough to allow the valves to open slightly? Was the lash on all exhaust valves the same (primarily interested in #4 with respect to the others)?
You may just have some worn valve springs that aren't pulling the valves shut firmly enough.
EDIT: I recall a story similar to this from my auto mechanics class in high school. The car would drive normal, but the compression test or whatever would show a leak. It turned out that one of the valve springs actually cracked/broke so that it had enough spring to pull the valve shut, but not hold it shut very tightly.
You may just have some worn valve springs that aren't pulling the valves shut firmly enough.
EDIT: I recall a story similar to this from my auto mechanics class in high school. The car would drive normal, but the compression test or whatever would show a leak. It turned out that one of the valve springs actually cracked/broke so that it had enough spring to pull the valve shut, but not hold it shut very tightly.
By looking at the crank, using a long screwdriver to verify TDC, and looking at the cam gears to verify the compression stroke.
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10-05-2011, 01:56 AM
#84
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Trey, did you let it die off the seafoam or just sip a liitle? I always let my cars die off the seafoam, wait 30min, then start up for the smoke show. I'd like to see you do it again and recheck the compression.
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10-05-2011, 05:33 AM
#86
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Nah, if the particles are loose then they won't stick around when there's pressure in the cylinder. It only happens when doing a leakdown test because you're moving the crank slowly, by hand.
At least, that's the theory I have to account for the fact that I get exhaust valve leakage during a leakdown test, but if I crack the valve open and then close it again the cylinder jumps from around 80% sealing to 99%. This was really apparent when I was doing a leakdown test once with the valve cover and timing belt removed, so I could spin the cams with a wrench.
--Ian
At least, that's the theory I have to account for the fact that I get exhaust valve leakage during a leakdown test, but if I crack the valve open and then close it again the cylinder jumps from around 80% sealing to 99%. This was really apparent when I was doing a leakdown test once with the valve cover and timing belt removed, so I could spin the cams with a wrench.
--Ian
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10-05-2011, 10:07 AM
#87
For valve springs I would probably go with a good aftermarket replacement for a stock ground cam. Overly stiff springs aren't necessarily a good thing if you aren't running higher lift cams. My vote is still on springs, but carbon on the valve seats sounds good too.
EDIT: my engine building knowledge comes form pushrod V8s so take that into account with my spring info.
EDIT: my engine building knowledge comes form pushrod V8s so take that into account with my spring info.
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10-05-2011, 11:26 AM
#88
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You should pull the head, last leak down test I did I didn't have anything leaking by the head, it was the rings. You are lucky if its just the head like it sounds.
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10-05-2011, 11:06 PM
#92
I use to check for valve leakage at a engine shop. Pull the head, find a flat steal plate the size or bigger than the head surface, drill holes in plate in the middle of each cylinder and tap air fittings in, bolt the plate to the head with a rubber gasket (I just cut a sheet of rubber about the same size as plate), with valves closed pressurize the cylinder head, if leaks bad valves or cracked combustion chamber. I filled the port I was looking for leaks in with water cause it bubbles when it leaks. The fist plate is a pain to make but it's a tool you use forever and never need more than a few times. Plus it works on other cyl heads of similar sizes.
Edit: a new idea just hit me. If you place the air fitting in the right spot you can use it to pressurize the ports with the valves closed to ensure they seal under boost/exhaust back pressure. Or to see how much pressure it takes to crack the valves open.................. I need to make a plate for my Miata now
Edit: a new idea just hit me. If you place the air fitting in the right spot you can use it to pressurize the ports with the valves closed to ensure they seal under boost/exhaust back pressure. Or to see how much pressure it takes to crack the valves open.................. I need to make a plate for my Miata now
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10-06-2011, 12:11 PM
#93
Read up on using a vacuum gauge to help diagnose engine issues. Lots of good info to help pinpoint what your problem may be. This link is just an example...
www.myclassiccar.com/pdf/howto-usingacompressiongauge.pdf
www.myclassiccar.com/pdf/howto-usingacompressiongauge.pdf
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10-06-2011, 02:14 PM
#94
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Read up on using a vacuum gauge to help diagnose engine issues. Lots of good info to help pinpoint what your problem may be. This link is just an example...
www.myclassiccar.com/pdf/howto-usingacompressiongauge.pdf
www.myclassiccar.com/pdf/howto-usingacompressiongauge.pdf
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Stock motor idled at 25"
10-07-2011, 12:02 PM
#100
Have you seriously overheated lately? Warpage happens from high heat. And it would be the aluminum head, not the iron block. Is there a continuous stream of bubbles coming up under radiator cap? Oil in the coolant? Coolant in the oil? White smoke from tailpipe? One or more plugs look like they were in the dishwasher? Probably not it. Might not be anything. At this point have you tried a snap-on or craftsman compression tester? No? Then stop worrying until you do.
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