6.5 to 1 compression ratio?

[Rog]

Here goes:

I have a 1964 Monza 900 with a 1963 145ci engine that was running fine before dropping an exhaust valve seat.

My lowest cylinder (besides the 0psi cyl) had 80 psi and the highest was 109.
One thing led to another, and I ended up splitting the case to measure everything. I'm replacing bearings and rings.
Just got my case halves together with new bearings. I decided to check my deck height to verify my compression ratio.

Specs
I have a 145ci "YN" engine with a 5607 crank. The pistons (030) don't look original, but the wide rings indicate they are early model. I've also compared them to 2 other early model pistons I have, and the compression height looks the same.

I mocked up 2 cylinders, pistons and rods w/out rings No. 1 and 4. My deck height is .14 and .17 inches respectively. [edited]
[added] 48 cc combustion chamber
When I use the Summit racing compression calculator it shows as 6.5 to 1. This seems really low to me.

Any thoughts on this?

thanks

[jimbrandberg]

1.4 and 1.7 what?
I generally see deck height, that is the distance the piston is below the top of the cylinder, at .010 or so, sometimes a bit more maybe .016 or so.

I took apart a EM engine that had LM pistons and they sat quite a bit below the top of the cylinder. They said it ran okay but didn't have much power.

You're correct that EM pistons usually have the wider ring grooves so I'm not sure offhand what you got going on there.

[Vairone]

Is your deck height in mm?

What is your cylinder head combustion chamber volume? Without that, you can not calculate the compression ratio.

[Wittsend]

My '61 80hp engine had the pistons .010 in the hole and I forwent the cylinder to case gaskets to an additional .012. In MM .010 would be something close to .25. I cut my head lands by .017 to true them up. The gaskets were .032. My heads #588 measured 54cc's. My Compression ratio calculated out to 7.75 to 1. FWIW the quench area was in the .090's and without the head cut and the base gaskets removed it would have been close to.130!

[Wagon Master]

1.55 mm = .061".
Without champer volume, I'd just be guessing commenting further.

[Rog]

You Guys are right on the money!
Case Solved
I ASSumed my piston was em but this proved out not to be the case.

I looked up the part number on my piston.

Shows as a late model piston. I also verified by measuring the piston compression height.
Luckily, I have some LM turbo engine parts I was saving for another build. Going to use the case and crank to build a 164ci engine. I'll build the smaller engine for another Corvair of mine.

Sorry for the confusion in my original post my deck height was .17 and .14 below the top of the cylinder without shims or gaskets. Going to edit that.

Thanks for the input.

To answer another question The chambers CC'ed to 48.

[Vairone]

Sorry for the confusion in my original post my deck height was .17 and .14 below the top of the cylinder without shims or gaskets.

Did you meant to say you deck height was .017 and .014 below the top of the cylinder?

[Wagon Master]

I think somehow he got long stroke (LM) pistons mated to a short stroke crank. Difference in compression height of the pistons is .1875 so.......
The difference in stroke of the cranks is .340". Half of that is .170". That adds up.

[Vairone]

I think somehow he got long stroke (LM) pistons mated to a short stroke crank. Difference in compression height of the pistons is .1875 so.......
The difference in stroke of the cranks is .340". Half of that is .170". That adds up.

Wagon Master, you are correct. His previous post sated this, I missed it.