I have been trying to calculate static and dynamic compression ratios for the last few days. I am using these online calculators and they are each giving their own different answers. SCR seems to be pretty close +/- .1, but DCR varies up to .6 from each other. I went to the machine shop today and the machinist used his own calculator which gave me yet another set of different SCR and DCR numbers.

Does anyone have the bare equations for doing the math on these ratios? I'd love to just use a calculator but when I one shows a DCR for pump gas and the other shows a DCR far beyond pump limits, it makes me wonder if either are right. Measure twice, cut once, only I've measure like 48 times and each time it's different

I have to get this figured out to buy pistons for this motor.

I've used KB silvolite, Wallace racing, and a few others online. Wallace corrects DCR for altitude but is .5 to .6 from KB Silvolite's. Then other sites are different also.

-Dustin-

 

Static comp=(swept volume+chamber volume+gasket volume+piston dish+amount in the hole)/(chamber volume+gasket volume+piston dish+amount in the hole)

Crevice volume is typically ignored if figuring mathematically (area between piston and cyl wall - it will decrease as piston gets hot). If measuring actual piston dish volume with a greased up bore in the cylinder, the grease will take away some of the crevice volume but it is a negligible amount.
For DCR use the same formula as above but measure swept volume from where intake valve closes to TDC

 

So I'm assuming everything must be converted into CC's right? I have most of those values except swept volume and gasket volume. I'm assuming you'd have to have a cylindrical volume formula that would change everything into cc's?

I guess what I'm saying is I don't possess the know how to calculate that stuff, so I'll probably just use online calculators for now.

-Dustin-

 

It's a ratio, you can use any units you want as long as they are consistent. There are plenty of automated on-line calculators to make it easy.

Rich

 

Thank's Rich, it's been awhile since my last math class. Problem I am having is the variance from one site to the other. Then it becomes here say on how much DCR the LT1's can actually take.

-Dustin-

 

You can use this basic formula t figure a number of things

.7854 x bore x bore x height= volume

.7854 x bore x bore x stroke x number of cylinder = engine displacement

if bore and stroke are given in inches, to convert to cc multiply by 16.39

.7854 x bore x bore x stroke x 16.39= swept volume in engine
.7854 x bore x bore x distance piston is in the hole x 16.39= deck volume
.7854 x gasket bore x gasket bore x gasket thicknes x 16.39=gasket volume

chamber cc and piston dish cc will have to be actually measured or known numbers

if you remember back to your high school math, then you remember that the volume of a cylinder is
pi x r^2 x height or
3.14 x r x r x h or
3.14 x d/2 x d/2 x h or
[3.14 x d x d x h]/4 or
.7854 x d x d x h
that is where that comes from

 

BTW if you are really trying to figure out your dcr, then unless you have a program that can simulate the oprening and closing points of your valve in relation to crankshaft degrees it will be difficult to get an accurate number. rod length has a big bearing on piston location in the rotation of an engine so keeping the cam in a steady state but changing rod length will change dcr. You're best off degreeing in your cam, finding when the intake valve closes then using a depth mic to determine how far in the hole the piston actaully is. (Don't forget to subtract the deck clearance at TDC)