Question for Engine Builders/Engine Mathletes
Thread Starter
Instructor
Joined: Aug 2012
Posts: 113
Likes: 19
Question for Engine Builders/Engine Mathletes
When building an engine, doesn't deck height minus stack height give you the piston-to-deck clearance?
For example, for a stock L72 engine, the deck height is supposedly 9.8" NOMINAL.
The compression height is 1.770" (+/-.002"), the rod length is 6.135" (+/-.002") and the crank arm length is 1.883". Adding these 3 together equals 9.788" (+/-.004). This is what's known as the stack height, correct?
Subtracting the 9.788" stack height from the 9.8" deck height means the piston-to-deck clearance is .012", i.e. .012" in the hole.
Am I doing this right or am I forgetting something, like bearing crush, high-rpm elongation, etc.?
I'm asking because I have heard that unmolested big blocks left the plant with 9.800" deck heights and pistons that were .020" in the hole, but when I did the math, it just didn't add up (pun intended).
Thanks to any and all who can shed light on this academic exercise.
Sam
For example, for a stock L72 engine, the deck height is supposedly 9.8" NOMINAL.
The compression height is 1.770" (+/-.002"), the rod length is 6.135" (+/-.002") and the crank arm length is 1.883". Adding these 3 together equals 9.788" (+/-.004). This is what's known as the stack height, correct?
Subtracting the 9.788" stack height from the 9.8" deck height means the piston-to-deck clearance is .012", i.e. .012" in the hole.
Am I doing this right or am I forgetting something, like bearing crush, high-rpm elongation, etc.?
I'm asking because I have heard that unmolested big blocks left the plant with 9.800" deck heights and pistons that were .020" in the hole, but when I did the math, it just didn't add up (pun intended).
Thanks to any and all who can shed light on this academic exercise.
Sam
Racer
Joined: Jun 2022
Posts: 482
Likes: 202
Yes, you are adding correctly, but very rarely will you find a stock block that isn't a little tall on the deck height. The pistons are almost always a little deeper in the hole than advertised, and also a little lower compression than what the factory stated. On most of my big block performance builds, I usually ask my machinist to set everything up so that the pistons are 0.005 below deck. Easy to do by decking the block if you're not working on a matching number engine. usually will take a set of custom pistons if you don't want to deck the block.
Thread Starter
Instructor
Joined: Aug 2012
Posts: 113
Likes: 19
Thanks for responding! I've heard of unmolested BBC deck heights as high as 9.815 from the Tonawonda factory.
However, when you combine that with the .021" compressed steel shim gasket and the absolute minimum stack height of 9.781" (1.768 compression height + 6.130 rod length + 1.883 crank arm length), one might be surprised to learn the compression ratio leaving the Tonawonda plant is still well above the advertised 11:1 (yes, I entered all the relevant data including piston dome cc, combustion chamber cc, etc. into various online CR calculators).
Using the median/average specs for these 3 components, the compression ratio is 11.6. No wonder they recommended using the highest octane fuel available!
However, when you combine that with the .021" compressed steel shim gasket and the absolute minimum stack height of 9.781" (1.768 compression height + 6.130 rod length + 1.883 crank arm length), one might be surprised to learn the compression ratio leaving the Tonawonda plant is still well above the advertised 11:1 (yes, I entered all the relevant data including piston dome cc, combustion chamber cc, etc. into various online CR calculators).
Using the median/average specs for these 3 components, the compression ratio is 11.6. No wonder they recommended using the highest octane fuel available!
