Compression ratio

1

12valvetater

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Nov 4, 2009
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My new engine has 380psi of compression when spun with a starter. It has .050" valve reliefs in 0600 cpl pistons that have been decrowned. It also has had .020" taken off the deck and has a .010" thicker head gasket. It has never fired so the rings aren't sealed 100%. Is there a formula I can use to find out what my cr is from psi seen in the combustion chamber? I am told stock compression should be 425-450psi.
 
Have the ring been seated yet in your new motor? I would think to many other factors going on to get compression ratio from the gauge. You would be better off using maths.
 
Well that's pretty much what I'm gathering from stumbling around the internet and searching through here. My main issue is that I don't know the volume of my bowl and I don't know how much material was removed when the pistons were decrowned.
 
For some reason 380 seems high for having .030 more area in the cylinder and .050 reliefs. What engine? Cr, 12v? If you know the cc of the bowl originally you could estimate what's left and use a cr calculator to get a rough estimate on your ratio. Only true way is to do a liquid test.
 
The compression pressure might be a bit artificially higher than normal from assembly lube.

.020 off the deck and only a .010 gasket is going be a net increase in compression. Surely you can find out how much was cut off the crowns?
 
12valvetater said:
My new engine has 380psi of compression when spun with a starter. It has .050" valve reliefs in 0600 cpl pistons that have been decrowned. It also has had .020" taken off the deck and has a .010" thicker head gasket. It has never fired so the rings aren't sealed 100%. Is there a formula I can use to find out what my cr is from psi seen in the combustion chamber? I am told stock compression should be 425-450psi.
Click to expand...

.020 off the deck and .010 thicker gasket leaves .010 in the ballance in other words your .010 tighter than when you started.......Any way that 380 psi# would translate to about 16-17 CR

Not being broke in means things will change some after it runs and the rings set. I would run it and recheck it.
 
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Ok thank you guys. I have a 188/220 cam in it and when I put everything together it seemed awfully close so I had the pistons cut and now it seems like I have a huge amount of clearance more than I was anticipating. At this point I'm going to run it whatever it is I would just like a ballpark idea on how it'll act before hand. I will try and get the pn for those pistons they are .020 cummins 0600 cpl.
 
12valvetater said:
My new engine has 380psi of compression when spun with a starter. It has .050" valve reliefs in 0600 cpl pistons that have been decrowned. It also has had .020" taken off the deck and has a .010" thicker head gasket. It has never fired so the rings aren't sealed 100%. Is there a formula I can use to find out what my cr is from psi seen in the combustion chamber? I am told stock compression should be 425-450psi.
Click to expand...

DISTURBED said:
For some reason 380 seems high for having .030 more area in the cylinder and .050 reliefs. What engine? Cr, 12v? If you know the cc of the bowl originally you could estimate what's left and use a cr calculator to get a rough estimate on your ratio. Only true way is to do a liquid test.
Click to expand...

Sorry miss read that. I thought you took .020 off the piston. So you have .040ish piston protrusion? And .010 over gasket. What's your piston to head clearance?
 
I measured a crushed .020 gasket and came up with .090". I don't know if it has sprung back at all from when it was clamped.
 
Started the engine and ran it through it's first heat cycle today. It seems to start pretty easy and doesn't blow any noticeable white smoke. I guess I'll see how it is when the exhaust is hooked up and it isn't 95 degrees out.
 
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