High compression 205 psi average on all cylinders

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tom becker
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High compression 205 psi average on all cylinders

Unread post by tom becker » Fri Jul 21, 2017 10:26 am

I have a 1965, 110 HP engine that I have rebuilt, not knowing at the time it had parts from other years. So this is what I have. 65 block with a 4809 crank. 64 cylinders bored out 40 over with flat top pistons. The heads are also 64 with a complete valve job. The head gaskets are .032 copper and I stacked another .032 on top for a total .064 and that helped a little but still running around 200 psi. The pistons run flush to the top of the cylinders. What is causing this high pressure and how can I bring it down to where it should be. I would prefer to keep what I have already done and if not what should I do. Thanks, TOM

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toytron
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Re: High compression 205 psi average on all cylinders

Unread post by toytron » Fri Jul 21, 2017 11:43 am

I don't know but do those heads increase the squish area with flattop Pistons?

Are they heads from a 95hp engine?

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Re: High compression 205 psi average on all cylinders

Unread post by 64powerglide » Fri Jul 21, 2017 12:44 pm

Try a different pressure gauge and hope that's it. :dontknow:
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tom becker
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Re: High compression 205 psi average on all cylinders

Unread post by tom becker » Fri Jul 21, 2017 12:47 pm

I did change pressure gauge and still the same

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Re: High compression 205 psi average on all cylinders

Unread post by GasDaddy140 » Fri Jul 21, 2017 12:57 pm

Wow! That's close to 12:1 compression! Hope you find out the answer.
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tom becker
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Re: High compression 205 psi average on all cylinders

Unread post by tom becker » Fri Jul 21, 2017 2:07 pm

After reviewing Clark's catalog page 7A it says for each .010 gasket thickness adds 1.5 cc the the head volume or .221:1 compression ratio decrees. So if I add a .032 base gasket and .032 and stack .052 head gaskets that would equal 16.5 cc and 3.65 compression drop if I did my math right. you thoughts, Tom

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Re: High compression 205 psi average on all cylinders

Unread post by Jerry Whitt » Fri Jul 21, 2017 2:30 pm

Divide compression pressure by atmospheric pressure to get compression ratio.

205/14.7 at sea level, equals 13.9 compression ratio!!!!!!!!!!!!!!

That seems like a piston change is due. If not, there will be a diesel effect with ignition of the fuel before the piston is near top dead center. Knocking noise and poor performance, other damage.

By stacking gaskets, you are more likely to have a blown head gasket. Try to fix it right rather than do a "mouse job" as that "mouse" may come back and bite you.
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Re: High compression 205 psi average on all cylinders

Unread post by 66vairguy » Fri Jul 21, 2017 3:11 pm

Hmmm -- Pistons are suppose to be flat - no issue there.

You said you had the heads rebuilt. The only practical solution is to pull the heads off and do a proper CC measurement (I always verify). I suspect the heads have been cut too much at the gasket area and the squish area was not relived. I had a set of heads off a parts engine I bought - 12 to 1 compression due the fact the gasket area was cut flush with the squish step. There is also a minimum squish step to piston top distance to allow for expansion and "give" at higher RPM. Also the depth of the valve in the head affects the CC value.

Sadly there are machine shops that claim to do Corvair heads and do not know how to properly cut them.

Yes you can stack gaskets, and the preferred method is to use a single gasket at the top and put thicker gasket at the cylinder to block area. Stacking gaskets between the head and cylinder has been reported to be unreliable.

DO make sure the head gasket area was not cut into the spark plug hole!!

You might want to post on the Corvair Center forum for an opinion as they have some sharp engine guys there.

tom becker
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Re: High compression 205 psi average on all cylinders

Unread post by tom becker » Fri Jul 21, 2017 3:17 pm

Jerry thanks for the info, very helpful. didn't know how to figure ratios. Tom

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Re: High compression 205 psi average on all cylinders

Unread post by 66vairguy » Fri Jul 21, 2017 5:17 pm

Jerry example is a pressure ratio - not the same as a volume ratio that is in the shop manual.

Plenty of sites with info - here's one. https://www.summitracing.com/expertadvi ... calculator

Bob Helt's Corvair book does a great job of explaining how to calculate Corvair compression ratio.

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Re: High compression 205 psi average on all cylinders

Unread post by notched » Sat Jul 22, 2017 5:33 am

There is nothing wrong with running the piston with 0 deck height IF you have a large combustion chamber. I would say you would need at least 50cc or better. As others have said, the heads are the key to it.
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tom becker
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Re: High compression 205 psi average on all cylinders

Unread post by tom becker » Sat Jul 22, 2017 7:22 am

I would like to thank everyone for their comments. I removed the head on the right side to check out all the components I'm dealing with.The heads show no signs of machine work except for the grinding the valves. The pistons are the correct ones (top of piston to top of wrist pin, 1 3/16) The stroke is also correct. All heads for 1964 have compression of 8.25-1 for 95 HP, 8.25-1 for 150 HP and 9.25-1 for 110 HP, so no matter which head I have the compression would be correct or lower, If the makes sense. The only thing that would effect the compression would be boring out 40 over, except If someone machined the height of the cylinders. The cylinders are 4.087 from the top of the cylinder to the flange that goes against the block. With my limited information, does any know what the cylinder height should be? With all the help, were going to solve this problem I sure. Thanks Tom

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Re: High compression 205 psi average on all cylinders

Unread post by 64powerglide » Sat Jul 22, 2017 9:48 am

My 64 owners guide says the compression ratio's are 8.0:1 & 9.0:1.
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Re: High compression 205 psi average on all cylinders

Unread post by bbodie52 » Sat Jul 22, 2017 9:58 am

Image
Extremely High Compression Readings - Carbon Deposits?
:link: http://corvaircenter.com/phorum/read.ph ... 890,338176
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toytron
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Re: High compression 205 psi average on all cylinders

Unread post by toytron » Sat Jul 22, 2017 10:11 am

Hi Tom, you originally gave us the info about your engine but you did not give us the specifics on the heads. Could you give us the casting number from the end of the heads? Though parts fit throughout years changing some of them will create different configurations. When you use the owners guide or shop manual out is going under the assumption that all the parts are sock and original to the configuration that it was manufactured as. By changing a crank, or cylinder and piston or head etc. you cannot always go on the factory info. This is where we are at now. If the heads are truly 110 then we have to look elsewhere. If the heads are from another then we have something to go on. By changing to a head that has less volume especially such as a 95 head it increases the squish area which effectively increases your ratio. There are people in this and the other forum that can explain this much better than I . But if you could get the head numbers that would be the next step so we know which direction to go in this discussion. Btw the compression ratio is not just the heads as you mentioned in your last post. You stated that you had a crank from a 110 but if you don't have the head from a 110 then you could have changed the ratio. This is why people were commenting about carbon buildup. Though it seems like small amounts of carbon building up might not cause a problem you must remember that the compression ratio is just that a ratio of the volume of are from the piston being fully down in its stroke to fully up. So changing this volume by adding layers of carbon (or as it might be, a head change) will change the lower number to a smaller amount. So since it is a ratio this effectively INCREASES the larger number. So you see that changing from a head that has a pocket to a head that has a bump of land will increases the compression ratio.

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tom becker
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Re: High compression 205 psi average on all cylinders

Unread post by tom becker » Sat Jul 22, 2017 11:16 am

When I rebuilt the engine, it had #3813516 heads on both sides, and when I checked the compression it was around the same it is now. I ordered two used heads from Clark's #3819876 for a 1964, 110 HP engine. The other head is still in the block and I can't see the numbers, but I did check both the numbers in their tech book and they were for a 64, 110 HP engine. As I stated before the crank # is 4809. I haven't ask yet, but would boring out the cylinders to 40 over add the much compression? I know it's going to take the whole village to fix this child's problem. TOM

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