High compression 205 psi average on all cylinders

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tom becker
Posts: 53
Joined: Wed Jul 19, 2017 6:25 pm

Re: High compression 205 psi average on all cylinders

Post by tom becker »

Kevin, The engine is tore down, If you can get me the measurement of the cylinder from the top (head end) to the bottom where the flange (gasket end) that hits the block for a 64 cylinder. The one I have out is 4.087. Thanks Tom
WinginEngineer
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Re: High compression 205 psi average on all cylinders

Post by WinginEngineer »

4.087 on the money for mine. I'd say jugs are out...

Time to measure your combustion chamber in the heads.

You can do it wil some plexi glass discs and fluid volume like the pros do, or you can try to measure the cut depth of the gasket surface and I could compare it to my set, but my heads are NOT 64's so i don't know if that would be an accurate comparison.
Kevin - Phoenix/Mesa, AZ
1962 Corvair 700
tom becker
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Re: High compression 205 psi average on all cylinders

Post by tom becker »

I am going to a check as soon as I pick up a syringe and plastic. Kevin will let you know what I fine out. Thanks TOM
66vairguy
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Re: High compression 205 psi average on all cylinders

Post by 66vairguy »

Tom -- O.K. I dug out some parts ----- I agree the cylinders measure fine.

NEXT - Lets look at the pistons since you have them out. Measure from the top of the wrist pin hole to the top of the piston. I checked some TRW forged pistons, been used by folks for years. The distance I measured was about 1.184".
tom becker
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Re: High compression 205 psi average on all cylinders

Post by tom becker »

The pistons or on spec--1.186, In Clark's catalog, page 7, from the head surface to the gasket surface is .570 to .580 and I have ,569 ok there. Have you looked at the pictures of the cylinders? Have you got an explanation for the line or grove or ever seen it before? It's about .070 down from the gasket edge. TOM
WinginEngineer
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Re: High compression 205 psi average on all cylinders

Post by WinginEngineer »

It's like a huricane outside right now so I can't get to the other car and take a picture of the heads, but yours look just like both of mine (minus the valve staking on mine).

You said the crank is a corvair spec crank, so the ONLY other 2 dimensions that could affect volume are the rods, which are highly unlikely but possible, and the block itself. But it'd be pretty odd to mill the block down... just sayin'

You should check them regardless, but it sounds like you're on to figuring out how you're getting extra air in there.

All that said, is 205 psi really THAT bad?

I mean the corvair runs pretty low pressure, but my suburban is running 190 and doesn't have any detonation. Now I do have a hot cam and cut heads, but it MAY be ok at 205.

Are you getting oil in the cylinders at all? Maybe raw fuel from a leaky carb? Liquid is incompressible and the volume of any liquid comes out the chamber volume, which would increase the pressure...
Kevin - Phoenix/Mesa, AZ
1962 Corvair 700
66vairguy
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Re: High compression 205 psi average on all cylinders

Post by 66vairguy »

Tom - as long as the engine is out I'd check the cam timing. Crank gear and cam gear keys do shear.
tom becker
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Re: High compression 205 psi average on all cylinders

Post by tom becker »

Every thing is correct on the engine (crank, pistons etc) The only thing I have not checked is the cam itself. From day one after putting the engine together I had the high compression. I have had this engine apart several times and no oil in the cylinders until this last time and there was a little. I have never noticed a gas smell when running or after. I even looked into the carbs while running and stop and see no signs of raw gas. Can a wrong cam cause high compression? TOM
66vairguy
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Re: High compression 205 psi average on all cylinders

Post by 66vairguy »

Tom - I doubt any Corvair cam would make that much difference in compression pressure, but I've seen issues with a cam that was way off or were a valve did not work and affected just one cylinder (exhaust valve didn't open).

I always degree a cam, but of course you need to know which cam is in the engine to get the timing card numbers. Occasionally folks find a cam or cam gear were the keyway isn't in the correct position. The crank gear woodruff keys do occasionally shear and the gear slips changing cam timing.

If the cam degree check is good, then it's back to square one. You said you tried two different compression gauges. Where they the same brand?
notched
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Location: Southern New Jersey

Re: High compression 205 psi average on all cylinders

Post by notched »

Cam timing certainly can cause high cranking compression especially if it is off a tooth or more. I don't think the original poster specified what kind of camshaft was being used. I know the Crower I have in my new engine was off 16 degrees (each tooth on a Corvair cam gear is 7 degrees). So just being off 7 degrees could do it. I degree every Corvair engine now as aftermarket camshafts can be suspect as well as sheared crank gear keys. These things can wreak havoc on cam timing and the operation of the engine.
1966 Corsa turbo
1969 Monza convertible
1987 Buick Grand National
tom becker
Posts: 53
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Re: High compression 205 psi average on all cylinders

Post by tom becker »

Clark's agree the cylinders are ok, based on the heads I removed they were for a 102 HP engine and possibly the cam could be for 102 or lower. The pressure gauge brand name is Milton, I did change the gauge and it read to same. The engine is still in the car, only the right head and cylinders were removed, when I change to a third gauge I will recheck the left side, when the engine gets removed the testing is over. I would think if the cam is off I tooth I would have rough idle. Also to enplane a little more, When I would drive the car, it ran ok , then it shifted into 2nd still ok. Only when I tried to accelerate and not pushing to the floor to drop it into low, there seemed to be little or no power and very little increase in speed, as through no fuel. The fuel pump, new and the carbs rebuilt and balanced. When it comes to cam gear timing to the crank even through to marks on the crank is poor at best moving the cam teeth one way or the other is way off and I rechecked the keys in the cam and crank to 90* as the book shows. After the compression check I will be removing the engine for the car. some say just get another engine , I my have to but not until I find what the problems. TOM
Wagon Master
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Re: High compression 205 psi average on all cylinders

Post by Wagon Master »

It'd be a lil awkward but not impossible to degree the cam while still in the car.
You could check the cams position......just not adjust it.
WinginEngineer
Posts: 150
Joined: Sat Sep 03, 2016 3:31 pm

Re: High compression 205 psi average on all cylinders

Post by WinginEngineer »

There's a slight overlap at the beginning of the intake stroke where the exhaust valve is still slightly open as the intake cracks open. Thoerotically if the cam was off a tooth or two you could lose that overlap putting more air in the cylinder than intended.

Now that's theory. Whether it would actually run like that is another question, because if that were happen it would likewise be opening the exhaust valve early after the power stroke, which one would think would cause a flameout/backfire condition.

It's a stretch, but what have you got to lose by checking, right?
Kevin - Phoenix/Mesa, AZ
1962 Corvair 700
notched
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Location: Southern New Jersey

Re: High compression 205 psi average on all cylinders

Post by notched »

They will run with the cam timing off in either direction and idle quite well. I had a 66 turbo that was like that. It started ran and idled well but had absolutely NO power. This was the culprit on mine.
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1966 Corsa turbo
1969 Monza convertible
1987 Buick Grand National
WinginEngineer
Posts: 150
Joined: Sat Sep 03, 2016 3:31 pm

Re: High compression 205 psi average on all cylinders

Post by WinginEngineer »

I should clarify that the overlap would not go away but instead occur while the piston is moving back down thereby pulling intake gases from both valves, causing a lot more gases than designed for and less flammable gasses than required for proper performance. It would act like its running lean but without the extra heat.

The alternative would be that it occurs while the piston is still moving up, which would actually result in LESS gasses making it into the chamber... so that would likely not cause your issue.

In other words you're looking for your cam timing to be retarded for your particular issue.

Sorry, just wanted to clarify cuz I only about 3 hours of sleep last night and my first post didn't make as much sense as I thought it did...
Kevin - Phoenix/Mesa, AZ
1962 Corvair 700
tom becker
Posts: 53
Joined: Wed Jul 19, 2017 6:25 pm

Re: High compression 205 psi average on all cylinders

Post by tom becker »

The cam gear I took out when I got the looked just like that, a little worse. Tomorrow I will doing another pressure check with a third gauge to make sure the first gauge is correct. I'm going to remove the engine a do an indicator check of the cam lobes to see what HP and year it is. The picture of the cam gear, what would cause this to happen. TOM
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