]I understand most of everything that goes on inside the engine, influences in/out, how everything works, but im not techincian by any means - but i dont understand a few terms like:

Dynamic CR
Static CR

I dont understand whats the difference, sorry CR = compression ratio, i know what compression ratio is i just dont get the static/dynamic part.

Also i dont understand what cranking pressure is. Also is a 25% increase is it true top end power start to become blunted compraed to the lower gains in cranking pressure >

 

 

static compression is the a physical measurement of the difference in cc's displaced at Bottom Dead Centre vs Top Dead Centre.
Dynamic is a measurment of the compression of the air going in. Sometimes due to the efficency of modern motors (or via a supercharger or a turbocharger) more air will go into the cylinders (than just the regular displacement) and the compression of that air is refered to as dynamic compression.

-edit-
most people deal in static compression (9:1, 11:1) etc etc, whereas dynamic compression is usually a more important number since that can be refered to as the actual cylinder pressure that a motor will see.

 

Static comp ratio is built into the motor,dynamic is the pressure the cyl sees during operation and changes continusly as piston location changes.Static never changes,but dynamic will be affected by everything you do to the motor to increase or decrease the amount of air entering the cyl.As in blower,either super or turbo,nitrious,cams or camtiming.

 

im interested too, what's cranking pressure?

 

is there any way to calculate this?
say if you know your static cr and the cams duration and lift timing etc and a set point for the piston?
just curious thanks

 

there is, but is fairly complicated and is constaintly changing...
thats why static compression is used.

 

Ask and you shall receive! http://www.rbracing-rsr.com/comprAdvHD.htm

If you swap out the cams and are making less than stock dynamic CR, you are in trouble. And don't let the difference in cranking pressure exceed 25%, like you said the power will get blunted in the high end. Aim for close to 20% increase.

 

Static CR - chamber volume/volume displaced by piston from BDC to TBC (swept volume)
Dynamic CR - chamber volume/volume displaced by piston when intake valve is fully closed, to TDC.

As long as your cam timing is fixed, the dynamic CR is constant. The later your intake cam closes, the lower your dynamic CR.

Remember, a compression ratio is simply a mathematical expression of the final volume over initial volume - final volume. It is not the same as cranking pressure, which is the actual pressure in the cylinder.

 

Dynamic CR is a function volumetric efficiency. RPM, cam timing, and whether the intake is pressurized or not all affect the VE.