340 Piston and Valve Question
Thanks. I just got the heads back from the machine shop and they CC'd at 66. I know the pistons from the factory were .014 out of the hole and the deck was milled .010 so theres the .025 out. May I ask procedure for 1/2 down fill? I believe that the pistons are 10.5 to 1 but that will be determined exactly by the gasket thickness? Thanks
It’s easier said than typed. But I’ll give it a shot.
Bore squared times the stroke length will give you the volume of any cylinder. So in your cascade it’s 4.070 times 4.070 times 12.87 (the number to convert cubic inches to CC’s) and take that number and multiply it by your stroke (or the thickness of the gasket or whatever the depth of the cylinder you are measuring is. And that number is the total volume of your cylinder.
4.070*4.070*12.87=213.19026*3.313=706.29933
That is IF you have a true flat top piston, with no valve reliefs and a zero deck.
Now you get the number 1 cylinder at absolute TDC and put your dial indicator on the piston and lower it .600 (hang with me as I know I said .500 but you can use any piston depth you want, as long as any dome is lower than the deck). Take some grease and seal use it to seal around the top of the piston. That’s why I said go an extra .100 deep. Once you get the grease in there (dial indicator is still on the piston) turn the crank clockwise (direction of rotation in the car just like it runs) until you are at exactly .500 down. Use your clear plexiglass plate with a hole in it and your graduated burrette and slowly fill the cylinder until it’s full and read how many CC’s it took to do that. That will tell you your exact dome or dish volume, including valve notches. I can’t say what yours is. You have to measure it.
I have a .187 dome on mine, and I know my numbers so I’ll post the, here so you can see how the math works.
My cylinder volume is exactly 695.92543 which is less than yours because my bore is a skosh smaller. That’s why you measure. Sooooo, now we need to know what the volume is at .500 down. Same math.
4.04*4.04*12.87=*.500=105.02949 and again, that’s with a flat top and no valve notches. My .500 downfill was exactly 99 CC’s, which means my dome displaces exactly 6.02949 CC’s. Damn this takes some typing.
Ok, now we have a cylinder volume, and a 1/2 down fill volume so we know exactly what the piston dome (in my case dome...could be a flat top with valve notches or a dish) but we ain’t done yet, unless you have a .0000 deck height. You don’t. Neither do I. But the math is the same. I’m out of the hole a measured .040 so it’s:
4.04*4.04*12.87=210.05899* 040=8.4023596 CC’s and that gets added to the dome volume. So that 14.431849 CC total dome volume. And were not done yet. You need to figure the gasket volume. For me that’s 4.185*4.185*12.87=225.40807*.036=8.1146905 CC’s because my gasket has a 4.185 bore and is .036 thick. Hope this is making sense. And my head is 71 CC’s. Now we are down to simple math.
To find your actual compression ratio the math is:
Cylinder volume at Bottom Dead Center divided by cylinder volume at Top Dead Center! Simple huh? That means we need to add up the actual cylinder volume, plus the head CC, plus the gasket. So...
695.92543+71+8.1146905=775.04012 CC’s. That is my actual total cylinder volume at BDC. Now we add up the volume at TDC, which is 71+8.1149605=79.11469 BUT WAIT...we need account for the dome volume and the positive deck height because that displaces volume at TDC. So that means 79.11469-8.1146905=71CC’s. Ok we are now down to the short strokes.
Volume at BDC is 775.04012 and the volume at TDC is 71. Therefore:
775.04012/71=awe crap...going by my memory I missed something. Hang on...gotta find my note book.
Ok, let start over with my actual numbers.
71+8.1146905-8.4023596-6.02949=64.682841 and that is the volume at TDC. Good thing I have my notes here or I’d be screwed. So...now it’s the total of volume at BDC which is:
64.682841+696.422=761.10484 and that’s the total of the volume at BDC. So...now it’s:
761.10485/64.682841=11.767619 actual, measured compression ratio. I’m doing that on pump gas and iron heads, so my butt may be sucking buttermilk but I think I can do it. So...the math above was:
Head CC plus gasket volume minus dome volume minus positive deck height for the actual volume at TDC and...
volume at TDC (64.682841) plus the swept volume (696.422) of the cylinder and that gives you your total volume at BDC and you simply divide the BDC volume by the TDC volume and that’s it.
FWIW it’s much easier to do it than to type and read it. Damn, hope I didn’t confuse you. In reality, it the measuring of a few volumes, doing some simple math and you’re done.
YR
