When you are building your project engine, there’s a lot of math involved. One of the biggest calculations is compression height. That’s especially true when you are using a custom piston and rod combination. While the term may be confusing, the important thing to remember is that compression height is the distance from the piston pin centerline to the piston dome.
To calculate compression height, you’ll need to use a caliper to measure the top of your pin bore to the dome. Once you have this measurement, you will be adding 1/2 of the pin bore dimension to the distance.
When assembling your engine, the compression height is vital. This will essentially allow you to factor other aspects, such as static compression or piston compression distance.
In order to calculate your engine stroke, rod length and especially your piston compression distance, you need to know the block deck height.
The block deck height is also known as the BDH of your engine. To measure your block deck height, measure the crankshaft centerline bore to the deck of your shortblock. When your pistons measure even with the block deck height, this is also known as a zero deck. It’s aggressive and will require you to do the right amount of homework to prevent your engine from crashing.
To do this you will need to calculate crankshaft stroke, connecting rod length and piston compression height. Zero deck clearance will require you to measure the top dead center position of your rod journals. To do this you must add 1/2 of the stroke to the length of your rod center-to-center length, and add it to your piston compression distance.
Once you have the calculations completed, this number must match the block deck height.
If you are working on an engine with a deck height of 9.240″, you will be adding 1/2 crank stroke plus rod length and your piston CD.
If you are using a crankshaft with a stroke of 4.000″, the first number to start with is a 2.000″ stroke reference.
Using this number you can come up with the rod and piston combination that will add up to 9.240″. For example when you are using a connecting rod with a 6.125″ length, the piston compression distance must come to 1.115″. To review this calculation, look at the BDH formula below.
BDH = (Stroke ÷ 2) + (rod length + piston cd) 2.000″ + 6.125″ + 1.115″ = 9.240″
Don’t forget that you must measure the cylinder head gasket as well. Let’s say you are using a deck with a block height of 9.234″. This will place the piston dome 0.006″ above the block deck, measuring a head gasket that’s 0.045″ thick your piston dome will be put 0.039″ below the cylinder head deck.
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