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Discussion Starter · #1 ·
ZmokE said:
Well..i finally bought one via Ebay (280 euro's, not too much i guess)...
Think i'll receive it next week...
I hope it's not to difficult to set up....
First do some other checks as, ignition, fuel transport, compression of the cilinders etc.
Well...today i checked the compression. Each cylinder gives a value of 90 PSI.
Is this a good value? It sounds a bit low to me.
 

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Compression pressure....

ZmokE - Rini: Compression pressure depends on several things and it is more important that the pressure is similar in all four cylinders than the actual value.
See what happens with the carburettor wide open and a squirt of oil put in through each spark plug hole before taking a reading.
The later, low compression, Opel motor has a compression ratio rated at 7.6:1 so its compression pressures are nothing to write home about! Its "real" compression ratio is probably nearer 7:1
So 14.7 psi X 7 = 102.9 psi is the theoretical best you can expect!

BTW: Dwight - that large "screw" you mentioned on the carb is a plug in the end of one of the ports that connect from the carb base to somewhere inside and not an adjustment on any sort - If the air leak you are creating by unscrewing it make the carb run better then there is something suspect with the air/fuel ratio adjustment of your carb.........
 

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Discussion Starter · #3 ·
Thanks Jim...i did the compressioncheck with a not to expensive compressionmeter. I soon will go to a friend with a "real" compressionmeter. But thanks for your info anyway
 

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GTJIM said:
The later, low compression, Opel motor has a compression ratio rated at 7.6:1 so its compression pressures are nothing to write home about! Its "real" compression ratio is probably nearer 7:1
So 14.7 psi X 7 = 102.9 psi is the theoretical best you can expect!
Thats pretty much only true for the US spec cars. In Europe they were 9:1

But 90 isnt terrible, as Jim said, consistant numbers is more importnant than the number itself.

Charles
 

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Refugees ....

GoinManta said:
Thats pretty much only true for the US spec cars. In Europe they were 9:1Charles
Yep! But lots of the GTs' in Europe are poor little refugees who have returned home ... ;)
14.7 X 8.5 = 124.95psi theoretical - not much higher!

BTW Rini - New Zealand was named by Abel Tasman; a Dutch navigator from Zeeland!
 

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Discussion Starter · #6 ·
Ok...i thought it was a bit low. Together with the blue smoke when i start the engine i was concerned that someting might be wrong with the valves. The smoke must be another problem then...
 

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Discussion Starter · #7 · (Edited)
GTJIM said:
So 14.7 psi X 7 = 102.9 psi is the theoretical best you can expect!
May i ask how you get that calculation (mathematics..i always hate it)..
 

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compression calcs . . .

ZmokE said:
GTJIM said:
So 14.7 psi X 7 = 102.9 psi is the theoretical best you can expect!
QUOTE]

May i ask how you get that calculation (mathematics..i always hate it)..
Accepted atmospheric pressure at sea level is 14.7 psi . . . then times the compression ratio gives approximate value of expected "compression test" cylinder psi. In actual cases, freshly rebuilt, stock LC (7.6) engine will give ~115-120psi readings. Same, but HC (9.0) engine will give ~135-140psi readings. That's been my general experience in any case.

BTW, compression test should be made with all plugs removed and with throttle held wide open!
 

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Discussion Starter · #9 ·
Thanks for the explanantion.
Well...i did it all wrong then. I did the comressiontest with only one plug removed, and let my wife start the engine when i read the scale. Time for a second opinion i guess.
 

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Nog maar een keertje . . .

ZmokE said:
Thanks for the explanantion.
Well...i did it all wrong then. I did the comressiontest with only one plug removed, and let my wife start the engine when i read the scale. Time for a second opinion i guess.
Raad van een vroegere Voorburger (1954), "Doe 't dan nog maar een keertje, heh!"

English: Advice from an former Voorburg (suburb of The Hague)-er (1954), "Just do it once again, eh! (see, not only Canadians use this! :rolleyes: )"
 

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Een goede raad van een vroegere Voorburger die Nederland verlaten heeft als ik het goed begrijp?

English: A good bit of advice from a former Voorburger (lives in Voorburg) who has left the Netherlands, if I understand correctly?
 

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ZmokE said:
Een goede raad van een vroegere Voorburger die Nederland verlaten heeft als ik het goed begrijp?
Juist . . . exactly!
 

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Hey folks, a bit of English is always nice! OK, now for some Greek...

A minor comment on compression test results. First, air is NOT an "Ideal Gas" (that is, it doesn't behave "linearly" with respect to pressure and volume changes), so P1V1=P2V2 (P is Pressure, V is Volume) doesn't really apply. Air has a compressibility factor that depends on its temperature, exact starting and ending pressures, humidity, exact gas content, and such.

Even with an ideal gas, the formula "PV=nRT" would not work either, as there are other factors to consider. An Otto-cycle (pun not intended) suffers pressure drop in the intake due to the carb venturi, the throttle plate, friction in the intake, and pressure drop across the intake valve. Not to mention that an engine is not an "adiabatic" (meaning no loss of entropy, nor heat loss or gain) compressor. Simply, the thermodynamic theory has to be adjusted to reality. So the exact "value" of the result isn't as important as the variance between cylinders.

As Otto mentions, the proper way to measure compression is to have all the plugs removed (which allows the engine to spin over faster (minimizing leak down pressure losses), the throttle wide open (to minimize "pumping loss", which is the pressure drop due to the throttle plate and velocity and friction losses in the intake), and to spin it over at least 3 to 4 compression strokes.

And as has been mentioned, the most important aspect is the consistency of the pressures. They shouldn't vary more than 10% from highest to lowest.

That all said, 90 psi is too low, even for a low compression engine. Did you use a screw-in type gauge or one you have to hold against the plug hole? Can you verify the accuracy with another gauge? If the gauge checks out, definitely do the "wet test". An increase of more than 10% indicates poorly sealing rings. No increase usually means leaking valves.

Finally, the best way to determine engine condition is with a "leak down test". In short, that test allows the cylinder to be pressurized with air, and how quickly the air leaks out is a better determination of condition. And it is easier to diagnose the leak, as you can hear the path the air takes.

HTH and sorry for the bafflegab...
 

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Keith, that's about it in a nutshell. Pull the plugs, open the throttle, spin the motor until the gage quits climbing and there you are. You've got compression pressure from one cylinder, do the rest, the same way, then minor calculations for variances and if you're with 10-15% of all the readings, you've got a good engine as far as the cylinders, pistons, rings and valves are concerned. ;)
 

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Discussion Starter · #16 ·
kwilford said:
That all said, 90 psi is too low, even for a low compression engine. Did you use a screw-in type gauge or one you have to hold against the plug hole? Can you verify the accuracy with another gauge?

I only removed one sparkplug at a time, and also didn't a full throttle.
I just learned it at this forum... :cool:
It was a screw in type...and i don't have another one to test the accuracy.
But soon i will let it check by a garage..
 
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