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Old 06-26-2011, 01:53 PM
McGurk's Avatar
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Dynamic Compression Ratio Calculator 4 U


This is my first post in this forum so please allow me to introduce myself. My forum name is McGurk and I have been retired from racing for over thirty years. I raced a Pro Comp dragster powred by a 364ci blown alcohol small block back in the late 70's. I sold the car in 1979 and I have not been to a drag race since then.

After thirty+ years I decided to build another hot rod, but it will never be raced. My hot rod is a 32 Ford coupe that will be powered with a 427ci small block Chevy. The engine has all of the killer parts so it should be quite a ride when it's completed. With the low octane pump gas I was concerned about the compression ratio, both static and dynamic.

I have never been concerned about dynamic compression before because all of my experience was with blown alcohol engines. The only way that I knew how to calculate the DCR was to first draw the mechanical aspect of it in my Mastercam CAD/CAM program to determine the position of the piston when the intake valve closed. After doing so it dawned on me that I could automate the calculation by calling on one of my other skills which is writing software.

Dynamic Compression Ratio, DCR, is a topic that many engine builders, both young and old (like myself), have never dealt with before so they may consider it to be over their heads. No shame in that because I was in that position couple of months ago.

As a token of friendship I have attached a zipped copy of my DCR Calculator. All values are considered to be positive with the exception of the deck height which can be positive or negative to allow for builds where the piston pops out of the block. If working between friends or a client you can send an e-mail message directly from the calculator. There will automatically be a picture of the calculator attached to the e-mail message. If you wish to pass the calculator to another party then there's a check box that when checked will attach a copy of the DCR Calculator to your e-mail message.

NOTE: I needed to know what the compression would be prior to ordering the cylinder heads and pistons so I used a 5% margin of error to account for the void above the upper piston ring and between the piston and cylinder wall.

If you want an exact compression calculation then you must do the following:

1: Measure the cylinder bore diameter.
((cylinder bore diameter / 2) x 3.14159265358979) = cylinder surface area

2: Measure diameter of piston above the top piston ring.
((piston diameter / 2) x 3.14159265358979) = piston surface area

3: Measure length of the top piston ring land.

4: ((cylinder bore surface area - piston surface area) x length of top piston ring land) = volume.

5: Add (volume x 16.387) to volume of combustion chamber.
Attached Files
File Type: zip DCR Calculator.zip (1.48 MB, 276 views)

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Old 07-04-2011, 12:04 AM
richard stewart 3rd's Avatar
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Welcome to Hotrodders.com Glad to have you with us

I down loaded the Calculator. Thank You!
I'll start playing with it tomorrow, I'm tooo tired now.
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