High compression 205 psi average on all cylinders

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toytron
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Re: High compression 205 psi average on all cylinders

Post by toytron »

Do you still have the head off? If so could you snap us a few pictures of the combustion area of the head and post it here?

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Re: High compression 205 psi average on all cylinders

Post by tom becker »

Ed, I'm new to this forum and not sure how to send the picture.
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Re: High compression 205 psi average on all cylinders

Post by tom becker »

Ed if you want to, email me with you email address and I can email you the pictures and them you can post them for me. tbbecker@qwestoffice.net
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Re: High compression 205 psi average on all cylinders

Post by toytron »

Tom I just sent you my email address.

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Re: High compression 205 psi average on all cylinders

Post by 64powerglide »

Just click on attachments then click add files then select the photo from your computer. Resize first to 300 to 500 K..
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Re: High compression 205 psi average on all cylinders

Post by toytron »

Here are Tom's pics.

ImageImageImageImageImageImage

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Re: High compression 205 psi average on all cylinders

Post by toytron »

Tom as 66vairguy and notched mentioned the best way is to cc the heads but you probably do not have access to what you would need for that. 66vairguy also mentioned the correct way to do your gaskets. If you need to stack then stack the base of the cylinder gaskets. You can use a combination of stacked bottom and thicker cylinder to head gasket to accomplish this.
Ccing the head would require a lab type tube. I cannot remember the correct name for it but someone here will know. Then you would need a plate the diameter of the cylinder that fits into the head made out of plexiglass or acrylic with two holes in it. One for dripping the fluid into and one for air balance to prevent a bubble forming. There are youtube videos out there that show you how it is done. But safest to say that 66vairguy is correct and that the heads might have been cut and they didn't cut the flat squish area to make up for it. The only way to confirm would be to cc the head.

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Re: High compression 205 psi average on all cylinders

Post by 66vairguy »

Tom - Yes getting the head casting numbers would help. In 64 the longer stroke was first used. Except for the 80HP heads, ALL other heads had a unique casting number per San Diego club "Junk Yard Primer", which is usually accurate. In 65 the cylinders and heads were revised for a wider gasket surface. In fact I have a few of the early 65 cylinders and they were 64 castings with a band around the top. Folks copied this to put 65 and later heads on 64 cylinders since the 64 heads are one year only and not common.

It is possible some well meaning machinist shaved the cylinder tops because of damage as you asked about. I don't have the info handy, but try the other forum as I'm sure somebody has it. You would need a long caliper with a resolution of 0.001" to verify. Typically if a cylinder gasket surface requires a lot of machining - it gets tossed. BTW - remember ALL long stroke cylinders must be notched in the bottom to clear the longer crank throws - that's 64 to 69. Also the shop manual was never corrected and says you can load pistons into the bottom of a cylinder - NO you can't on the 64-69, you must load from the top or risk breaking the rings (and I've seen it done) on 64 - 69 cylinders.
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Re: High compression 205 psi average on all cylinders

Post by tom becker »

Kind of funny, your comments about loading the pistons from the bottom. When I got the car, the seller told me it was stored for about 5 years. It would try to start but never would run. When I tore the engine done I was surprised to see it was just rebuilt. Crank ground 10 under, new bearings, cam gear and new pistons and rings. The problem I think the person who worked on it must of used a trouble light and hammer. Two of the pistons had broken rings and ring groves. the cylinder wall scored deep and the cam was driven on the gear cutting a new key way. Not solving my problem, but you comments were interesting. Thanks Tom
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Re: High compression 205 psi average on all cylinders

Post by 66vairguy »

tom becker wrote: 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
Tom - The CORSA tech guide says the 3813516 casting is a 45.9CC chamber and the 3819876 is a 50.6 CC chamber - so yes the the 3516 heads would cause higher compression, based on some old calculations - about a 0.8 increase in compression ratio. Yes "040" over raises compression, but off the top of my head, about 0.2 - not much difference. If your heads are good (50.6cc without gasket) then your suspicion about the cylinders might be justified. Doing some fast figuring (may be off) the cylinders would have to have been cut about 0.085" - almost three gasket sizes - to go from 9.0 to 11.5 compression!! This assumes the head chambers are the correct 50.6cc

At this point determine cylinder deck to base height and if that's good I'd cc the heads (you can make a plastic plate and buy a big laboratory syringe online).

Good luck.
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Re: High compression 205 psi average on all cylinders

Post by notched »

I gave this some thought yesterday.
A camshaft can affect cylinder pressure and can cause the cranking pressure to be high.
Static compression ratio is based on fixed mechanical features of the engine:
Cylinder Bore diameter, compression height of piston, combustion chamber size, head gasket thickness, etc..
A short duration cam will build more cylinder pressure than a long duration cam.
Camshaft selection is dependent on the engines static compression ratio and many guys (especially in the Corvair community) do not pay attention to this. So yes, a camshaft can effect dynamic compression ratio(cylinder pressure).
In the early years of the automobile the compression ratios were typically very low. When hot rodders started playing around with cam shafts they learned that long duration cams bleed off cylinder pressure and added compression to gain back the lost cylinder pressure.
Some times you can get away with a long duration cam with low to moderate compression ratio but a high compression engine with a short duration cam can be a pinging nightmare especially with an air cooled engine.
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Re: High compression 205 psi average on all cylinders

Post by 66vairguy »

A conservative cam will raise compression at lower RPM vs. a more aggressive cam, but not to the extent Tom originally reported.

I was also thinking that Tom said he purchased the correct 64 heads from Clark's. Now I'm wondering if he checked compression with those?

The original heads on the engine had chambers too small for a long stroke engine.
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Re: High compression 205 psi average on all cylinders

Post by tom becker »

Yes I did put the 64 heads on and the compression is 205 average on all cylinders. In early comments I made, I checked all the parts, crank, pistons stroke and etc. I am still leaning to the cylinder being the problem, If you look at the pictures of the cylinder bottom, that line or grove makes me wonder if it was cut down. I have contacted Clark's to get a measurement from the top of the cylinder to the block flange. I have tried to find the spec on these cylinders, but no luck. I may be climbing up the wrong tree, and if wrong, one more thing to rule out. I will keep you all informed of what I find out. As soon as I get a syringe I will check the combustion chamber.

AS A SIDE NOTE I live just south of Des Moines , Ia and many of you probably live many miles away. But this kind of reminds me of the good old days when all your friends would stop by the shop at night or after school and help you work on this project. Thanks TOM
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Re: High compression 205 psi average on all cylinders

Post by WinginEngineer »

Compression ratio is specifically the volume of the chamber at bdc divided by the volume of the chamber at tdc. So in terms of actual compression ratio, the ONLY thing that affects it is dimensions along the axis of the cylinder (stroke, rod length, piston height past the wrist pin, cylinder length, head chamber depth, etc...)

Actual cylinder pressure, however, has hundreds of variables that affect it.

Just picking at some highlights from this discussion: if there are no changes axial deminsions, no changes in valve size, and no changes in cam, increasing the bore will increase the volume (uniformly) which means more fuel, which means more potential energy, which.means more power, but not higher pressure (precombustion).

The pressure is driven by PV = nRT where P = pressure, V = volume, n = number of moles of gas (just call it amount of gas for simplicity), R = universal gas constant (which is CONSTANT, so nothing you do will change it, so it can be ignored for this), and T = temperature.

On the compression stroke n, R, and T don't change, but V goes WAY down causing P to go up to balance the equation.

Point is, for you, P is going up, R and T don't change, so either V is smaller than normal or n is higher than normal.

A hot cam, big valves, more open carb, forced induction, or liquid in the cylinder will all increase n.

Longer stroke, taller piston, shorter jug, shortened head will all decrease V.

Sounds like you need a factory head and factory jug to compare yours too if you KNOW the crank, rods, and pistons are right. I have both, and can measure them for you, but it would mean you gotta tear your down to measure yours....
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Re: High compression 205 psi average on all cylinders

Post by notched »

I notice that the notch at the bottom of the cylinder is not a factory cut notch. What is the deal with the cylinders as it looks like early cylinders that someone cut the notch into
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Re: High compression 205 psi average on all cylinders

Post by 66vairguy »

Good points, I assumed all the parts were original, or proper aftermarket.

Tom - Are there any numbers or identification on the pistons to suggest who made them?
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