Thread: Compression to Cam Ratio ???

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K, someone please explain to me when you say you need "X" amount of compression to support a certain amount of lift in a cam, How do you arrive at this.? This is the missing link in alot of it making sense to me. I realize this will take some typing, so thanks very much ahead of time for your efforts to take this one on.

Overlap and centerline always dictatate your values when it comes to "How much compression" shall I run. Cubic inches will help settle things down. Whan a small-blk racer (driver/owner) has to deal with these issues, they're searching and experimenting. A smaller cubed motor needs a touch more compression to stay with what the bigger cubed motors need to achieve idle quality and rpm performance that bigger motors enjoy. Many cams that "share" the same specs as a big inch cam runs need to "squeeze" a bit more compression to "feel" right to get qualities that are similiar. Experience teaches us all to learn where we need to get our specs right. Overlap and ramp speeds dictate so much to the application. Lift and duration are not all that is important. Cam gringers are pretty generic and even a phone call will not give the answers you're looking for. Dig in a little bit with looking for the grind will produceyour desired results. Too many mouse motors are doing the job and too many hobbiests are scratching heads" wondering, "what if".

Originally Posted by jimmyjeep

K, someone please explain to me when you say you need "X" amount of compression to support a certain amount of lift in a cam, How do you arrive at this.? This is the missing link in alot of it making sense to me. I realize this will take some typing, so thanks very much ahead of time for your efforts to take this one on.

Try camshaftinnovations.com Jay is the guru when it comes to custom camshaft grinds. He will work with you.

It actually has alot to do with the intake valve closing point ground into the cam as much as lift if you're talking compression.
'usable combustion pressure' doesn't take place until the intake closes and ignition happens. A dozen or so factors play a part in this,ICP,intake design,degrees of actual lift,and other factors,and 'usable combustion pressure' has a different EFFECT thru out the RPM range.
If your static compression ratio is to low for the desired ratio for a paticular cam then the actual fuel/air that is captured (at certain RPM's) will be low or less than 'optimum' for the power potential of the motor.
Other words the cylinders efficiency will be reduced to fill with the optimum volume and reduced power results.
Theoretically,,,the motor may still come close,,,at a certain RPM,,, but will suffer power loss outside that RPM,but sometimes this doesn't matter. This is a point that many argue,,,but trust me,either low RPM or high RPM performance will not be optimum thru out the 'desired' RPM operating area.
I'm talking along the lines of a race motor designed for maximum power thru out a certain RPM range,the round track guys can explain better than I can though,just my 2cents.