In the middle of choosing my new pistons to replace the ones with valve imprints in them.

If I go with a JE extreme duty nitrous flattop with 4cc reliefs it puts me at 12.8:1 SCR with .039 gaskets or 12.4:1 SCR with stock gaskets.

Dont know what all is needed but the cam is ~240/250 @ .050 and .568/.568 lift. If other info is needed please PM me. Thanks.

 

DCR is usually figured using advertised duration, not duration @ 0.050", so those numbers are needed along with the LSA and the intake center line. Also the combustion chamber volume, gasket bore, and rod length.

Rich Krause

 

Pm me with every thing you know about your engine bore,stroke.rod length,block deck height.Chamber CC's,adv duration,intake closing degree's,How far down the piston is the top ring, ect-ect.
Iwill figure it on my program and tell you exactly what it is.

 

Yep, we also need deck height. If you want to be fancy, the clearance of the piston in the bore and the height above the top ring are needed too!

Rich

 

Rich,

I did tell him exactly,not close like some online calculators do.



EDIT-There are 2+CC's there in most pistons but some consider it a safety margin. I like to see the exact figure on paper.

 

..... and you need to know combustion chamber volume with the same spark plug you intend to use. Need to cc the chambers to be sure.

-Mindgame

 

dia of head gasket fire ring works in also.

 

Fergot that one too

Rich

 

Easy to do when you're sitting at a computer Rich. I do it all the time.

We're just throwing stuff around here anyways. But it does bring up a good point.... if your building an engine, it's probably a good idea to take your time, write everything down on paper and crunch the numbers a few times just to be sure. It's easy to miss things.

-Mindgame

 

Ditto--Do that every day,and it gets worse with age.

 

Wise words.

Perhaps even more than a few times.

 

Just wondering..
Since a lot of people seem to want this sort of info, does anyone have the actual formulas to figure it up? If so, I would like to write up a quicky VB app to do the crunching for us. I think it might make a cool addition to the site as a free d-load. Dontcha think?

Dave C.

 

Man,that would be nice.There is all kind of info on line to save 30 minutes typing.You could paste it direct. Just type in Dynamic compression and wallaaa.

 

I belive its the same as static compression ratio, but instead of total displacement... use the total volume of the cylinder when the Intake Valve closes (which is why you need the cam specs, the rod length, and the stroke).

Now if you want to factor in VE, wavelength tuning (for mild SC effects), and other factors that's also possible, but much more involved. The "as run" Dynamic CR is a real nightmare to work out, but the traditional concept of Dynamic CR just assumes 1 atmosphere pressure @ WOT (VE=1) to assume the mixture isn't compressing until the valve closes.

If it's too high... slip in a bigger cam

 

DOH! Someone beat me to it

Would one of you guys download this thing and check it's accuracy? I'm on XP and it installed and ran fine without the VB runtimes...

Dynamic Compression Calc

Dave C.