How to calculate compression ratio?

[robert0762]

i have a 4 mill stock cyl . 64.50 bore, ex port is 28 mm below deck. And i am running 23 cc domes corrected (i think thats the rite word)

[J-Madd]

According to this-->click you'll need deck height also.

[robert0762]

what would the deck height be on some stock cyls

[J-Madd]

what would the deck height be on some stock cyls

 

I don't know. I'm in the same boat with one of my 421 Cubs. There is a way to measure it it;s a pain in the ass due to the domed piston, imo.

[tedd1]

I don't know. I'm in the same boat with one of my 421 Cubs. There is a way to measure it it;s a pain in the ass due to the domed piston, imo.

 

Lay a straight edge across your cylinder, front to back.

Roll the crank till piston contacts straight edge.

Use the bottom of your dial calipers to measure the distance between the edge of the piston and the top of the cylinder. Measure on the side above the wrist pin.

This measurement will be a negative number when you enter it into the formula. The amount the piston rises above the cylinder make the dome volume smaller...

[J-Madd]

Lay a straight edge across your cylinder, front to back.

Roll the crank till piston contacts straight edge.

Use the bottom of your dial calipers to measure the distance between the edge of the piston and the top of the cylinder. Measure on the side above the wrist pin.

This measurement will be a negative number when you enter it into the formula. The amount the piston rises above the cylinder make the dome volume smaller...

 

 

You didn't mention anything about the piston being at TDC. It has to be at TDC to get deckheight. Here is how to get true TDC.Click

[No Limit Powersports]

If your wanting to calculate trapped compression you will have to know the exhaust height and the volume of the head and the volume of the dome on the piston as well as deck height. Deck height on a stock cylinder is in relation to what base gasket you are using there are too many variables to just say it is .0xxx Compression ratio are read as 15:1 read 15 to 1 which means that its taking compressing the volume of 15 units into 1 unit. Calculating the dome of the piston can be tricky and you have to know the specs of the piston. The dome rise on a 513 piston is .16" Basically you have to take the volume of the cylinder at the point where the piston closes the exhaust and divide that by the volume of the cylinder at tdc...lmao easy enough

[J-Madd]

If your wanting to calculate trapped compression you will have to know the exhaust height and the volume of the head and the volume of the dome on the piston as well as deck height. Deck height on a stock cylinder is in relation to what base gasket you are using there are too many variables to just say it is .0xxx Compression ratio are read as 15:1 read 15 to 1 which means that its taking compressing the volume of 15 units into 1 unit. Calculating the dome of the piston can be tricky and you have to know the specs of the piston. The dome rise on a 513 piston is .16" Basically you have to take the volume of the cylinder at the point where the piston closes the exhaust and divide that by the volume of the cylinder at tdc...lmao easy enough

 

 

I knew that calculator link I posted had to be too simple . There is no place to enter exhaust height, and it also looks like it assumes a flat top piston.

[No Limit Powersports]

I knew that calculator link I posted had to be too simple . There is no place to enter exhaust height, and it also looks like it assumes a flat top piston.

 

most likely for a 4 stroke but it dont accomodate for a head gasket.