What is compression distance?

[gkz]

I did a search on google/ask and found nothing but calculators; Can someone at least attempt a definition? I have a choice between 1.56" and 1.54." Thanks...

[ytram]

I think it is the distance from the center of the wrist pin to the top of the piston.

[gkz]

Ok, compensates for decking?

[glenn64vette]

Originally Posted by ytram

I think it is the distance from the center of the wrist pin to the top of the piston.

That's compression height.

[grumpyvette]

the pistons quench area or flat deck is supposed to come within a few thousands of an inch of the deck in most combos and its not unusually for the piston quench area to reach the deck or even stick out above the deck a couple thousands, this deck or quench area is a set distance from the piston PIN center line ,the dome on a high compression piston reaches well up into the combustion chamber, at times easily 3/8" or more on some big block engines

you figure the COMPRESSION HEIGHT by subtracting 1/2 of the crank stroke plus the rod length measured from piston pin center line to crank pin centerline subtracted from the blocks deck height, to get the piston compression height

EXAMPLE
on a small block chevy the deck height is 9" -9.023 if its never been deck milled
if you use a 6" rod and are building a 383 with its 3.75" stroke your dealing with a 6" plus 1.875" stroke subtracted from a 9"-9.023" deck height that results in a piston deck height of 1.125-1.130" compression height being correct

http://kb-silvolite.com/forged.php?a...tails&P_id=391



remember the QUENCH distance should be between about .037 and .044 (thats the distance between the cylinder head and the piston deck) and theres a compressed GASKET between the two so you need to measure, lets say your decks never been milled and you have a block measuring 9.021 for the deck, the piston sticks up to within .015 thousands of the deck in this example (6" rod,1.875 stroke, 1.130 compression height = 9.005 subtracted from 9.021" so the quench WITHOUT a gasket would be .015 so you need to add about a .028 thick gasket

http://kb-silvolite.com/forged.php?a...tails&P_id=352

[mrvette]

WHICH is why I don't build short blocks....I can and have done cams, lifter, heads....

ain't no friggin' way man.....NFW.....

[glenn64vette]

Originally Posted by grumpyvette

the pistons quench area or flat deck is supposed to come within a few thousands of an inch of the deck in most combos and its not unusually for the piston quench area to reach the deck or even stick out above the deck a couple thousands, this deck or quench area is a set distance from the piston PIN center line ,the dome on a high compression piston reaches well up into the combustion chamber, at times easily 3/8" or more on some big block engines

you figure the COMPRESSION HEIGHT by subtracting 1/2 of the crank stroke plus the rod length measured from piston pin center line to crank pin centerline subtracted from the blocks deck height, to get the piston compression height

EXAMPLE
on a small block chevy the deck height is 9" -9.023 if its never been deck milled
if you use a 6" rod and are building a 383 with its 3.75" stroke your dealing with a 6" plus 1.875" stroke subtracted from a 9"-9.023" deck height that results in a piston deck height of 1.125-1.130" compression height being correct

http://kb-silvolite.com/forged.php?a...tails&P_id=391



remember the QUENCH distance should be between about .037 and .044 (thats the distance between the cylinder head and the piston deck) and theres a compressed GASKET between the two so you need to measure, lets say your decks never been milled and you have a block measuring 9.021 for the deck, the piston sticks up to within .015 thousands of the deck in this example (6" rod,1.875 stroke, 1.130 compression height = 9.005 subtracted from 9.021" so the quench WITHOUT a gasket would be .015 so you need to add about a .028 thick gasket

http://kb-silvolite.com/forged.php?a...tails&P_id=352

Good stuff!
When all done with my 400 I ended up with .048 quench. I have run it closer, but with anything less than .035 I get worried. I have seen engines with less than .035 quench and the piston was PERFECTLY clean in that area, so was the head. Even a little mark on the top of the piston matching the shape of the quench area on the head. Can give you cause for concern.

[grumpyvette]

"the piston was PERFECTLY clean in that area, so was the head"

THATS a GOOD indicator that the QUENCH/SQUISH function is working correctly AS its SUPPOSED TOO! the idea is to force the fuel/air mix at very high speeds into the center of the cylinder to speed up the burn rate and lower both emmisions and increase the potential HP because youll expend far less of the cylinders pressure curve resisting the piston as it approaches TDC on the compression stroke and use a larger percentage of the pressure that results from the fuel air mix being ignited durring the power stroke where it produces usefull TORQUE

[gkz]

So is "compression height" the same as saying "compression distance?" BTW, great info.