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k-star 06-24-2004 06:07 AM

dynamic compression
Does anyone have the formula to figure out dynamic compression???

I have found some on line. But i would like to see the actually math formula to figure it out.


mstngjoe 06-24-2004 07:18 AM

Computing the DCR is done basically the same way as computing static compression. The difference being that you need to know the position of the piston in the cylinder bore at the point the intake valve closes.

Here's a link with some more detailed info:

k-star 06-24-2004 07:31 AM

I read through that early this week. I could not find the formula though. It did give you all the measurments that you need to calculate but not the final math, Unless i missed it.


mstngjoe 06-24-2004 07:49 AM

The 2004 Ford Racing Performance Parts catalog has a diagram and the equation you are looking for. Look on page #148 of the catalog.

There may be other places to find this formula, this is just the one I'm familiar with.

BTW, that is the "standard" formula that uses the volume above the piston at BDC.

As I understand it, if you want to compute the DCR, you will still have to determine the total volume above the piston at the closing point of the intake valve.

blndweasel 06-24-2004 09:23 AM

Isn't that formula in the Ford Catalog just a Static Compression Formula? It doesn't include anything for intake closing ABDC.

Based on what's involved in calculating Dynamic Compression Ratios, I'd tend to stick to a pre-configured calculator. Try out KB-Silvolite's DCR Calculator at: DCR Calculator

I've been through a couple years of college level Calculus and Physics, and I, for one, will not trust myself to do the math right with a formula that involves as much stuff as in calculating DCR.

Static compression can be fun to calculate as an excercise in learning, but DCR isn't something real enjoyable to do by hand.

just my $.02


mstngjoe 06-24-2004 09:35 AM


Originally posted by blndweasel
Isn't that formula in the Ford Catalog just a Static Compression Formula? It doesn't include anything for intake closing ABDC.........
You are correct about the formula.

In order to use it to determine the DCR, you would have to calculate the position of the piston in the bore relative to the closing of the intake valve.

Say the intake closed at 45* ABDC. You will need to figure out how far the piston moved up the bore ABDC and subtract that amount from the volume at BDC.

Knowing that figure, you could use the formula in the catalog.

I think. ;)

But I'm with you. I'd sooner trust the calculator you listed than my own math/method. :thumbup:

k-star 06-24-2004 09:38 AM

Weasel you are right that formula in the catalog is for static compression.

I have used the on line calculators and they work fine. I am just a sick person that loves to know the old school way of figuring things out.....I found one that looks close on the mopar tech library. I did not have time to sit down and work it out and see if it comes up to the same as the on line calculators.....

I remember the first time i mad a cad drawing that had compound angles i sat with a calculator and figured it all out the long way just to see if the program was right..... he... he... he....It took me a long time but guess what..... the math never changes L.O.L.


blndweasel 06-24-2004 09:40 AM

That first article you posted was really good... took some time to read through, but its very informative. I wish that he'd include more information on how to measure some of that stuff... like when he calls for the measurement of the rod below crank CL at intake closing, where is this being measured from? the theoretical center of the crank's rod journal?

That's actually the best article on DCR I've read yet... props!


k-star 06-25-2004 09:33 AM

I looked closley at that site and all the info is there. I must have overlooked it the first time... I sat down and worked it out with a calculator and compaired it to the on-line calculator at rbracing.....and the numbers come out exact!!!!!!! the only difference is the on line has a place for altitude... I just have to get that into the mix yet and i am set L.O.L.

Thanks again,keith

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