General engine questions: Compression ratio and Displacement

[jkarlsso@metabolex.com (Jonas K.)]

On 6/12/02 5:28 AM, "Dan Bubb" <jdbubb@ix.netcom.com> wrote:

 
> No! Changing CR will have only a small impact on the volume or mass of
> fuel and air the engine breathes, so this statement is not correct.

Actually this is wrong. Higher compression will increase the efficiency of
the motor (as an air pump) at lower RPM. Greater efficiency = more torque.

With a higher CR, the combustion chamber gets "emptied more" at the exhaust
stroke, so for the intake stroke there is more suction. Ever play with a
syringe? Without the needle, of course. For low CR it's like having the
plunger 1/4" from the end, and putting your finger over the end while
pulling out the plunger 2". For high CR, have the plunger nearly all the way
up, then pull out the plunger 2". There is much more vacuum generated with
the High CR model. 

Similarly, the high CR engine will suck in more air each intake stroke. And
also there will be a lower percentage of old exhaust in the chamber.
However at high RPM, the fill/drain happens so fast that the CR has less
effect. There is a nearly continuous flow of air pumped by the piston.

This is why the 1.8l High CR 8v motors have so much more pull at low RPM
than the JH. And also why, even though the JH has the crappy single outlet
EM, the HP difference is only 10hp (mostly attributable to the larger intake
runners and the better EM of the A2 cars).


HTH, 
Jonas