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Old 10-05-2012, 08:05 PM
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Originally Posted by birkey View Post
Advice needed for beginner, achieving high rpm seems to be one question on my mind. I understand lighter valvetrain, and rotating assembly along with forced induction, long rods, moderately wide lsa, and single plane manifold along with other factors properly synchronized together will produce fairly high rpm. Now the real question is how does d.c.r. factor in to all this. Is there any certain ratio that people aim for when building these motors?
Is a low compression motor going to have a better chance of achieving higher rpm than a high compression motor?
You don't need forced induction to rpm a motor.
You don't need long rods to rpm a motor.
You don't need high or low DCR to rpm a motor
Referring to DCR, you figure this when you're planning the motor, before you ever buy a part. It is DCR that determines the fuel you will have to use to prevent detonation. Another way of looking at DCR is cylinder pressure. If you're gonna make a lot of cylinder pressure (high DCR), then you will need to use a fuel that will be resistant to detonation such as racing gasoline or alcohol. If you use a lower cylinder pressure (low DCR), the motor may run on unleaded regular gasoline without a whimper.

Problem is, there are many different calculators to figure DCR and all of them are a little different. You need to zero in on one calculator and use it all the time so that you know the results are valid. I have used the calc on Keith Black Pistons site for years and know that for a pump gas motor, I will want somewhere between 8.0:1 and 8.5:1 DCR. I adjust it by altering the static compression ratio (SCR) and the intake closing point of the camshaft.
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