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Old 10-08-2012, 12:18 PM
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QUOTE=birkey;1596389]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?[/QUOTE]

The answer to your last question is NO. The effort to overcome increased compression pressure by the crankshaft is minimal compared to the energy released by burning the mixture at higher pressures.

The Dynamic Compression Ratio is tied to the cam timing. Specifically it computes the actual compression lost to reverse pumping action of the piston as it rises up the bore at the point where the intake valve is closed. (The effect is that the stroke appears shorter than it actually is at lower RPMs) The problem DCR is attempting to correct is one of the engine's performance with a late closing intake which is typical of a radically timed cam. With such a cam the intake is held off its seat till late in the compression stroke. The advantage of this is strictly to greatly improve high RPM, high power output. But the bottom and mid RPMs suffer pretty large torque losses as a result. The reason all revolves around the intake mixture velocity which is used to generate mass inertia to force feed the cylinder. This only happens at high RPMs where the mixture velocity generates enough inertia to overcome the reverse pumping of the rising piston this permits holding the intake open late into this cycle. The downside is that when the mixture velocity is low the rising piston does pump mixture back into the intake. This results with the cylinder pressure being quite a bit lower than the Static Compression Ratio (SCR) would suggest. Therefore low and mid RPM power is significantly reduced.

The DCR is a calculation tool used to modify the SCR to derive a compression ratio that increases the absolute cylinder pressure in the low through mid RPM bands in-order to recover power lost to reverse pumping the intake mixtures. A rough rule of thumb is that the DCR should be about 8.5 to one with an allowance of about .5 ratio lower for open chamber, iron heads and regular fuel to about .5 ratio higher for a tight chamber, aluminum head, and premium fuel. There is quite a bit of rattle space in this simplified recommendation for different operating and induction temps; type, strength, and amounts of ignition advance; type of fuel, mixture ratio and how it's delivered carb/TBI, port injection, direct injection; vehicle weigh, gearing, transmission type, converter stall; excreta.

A caution is that you need to be careful in relating power and efficiency as related to compression ratio. There are plenty of misunderstood reports that assert that there isn't a lot of power to be gained by compression ratio increases. These reports tend to not make clear that this is true only where the increase in compression ratio is not accompanied with increases to the cam events. When the cam events at the valve are increased; a great amount of lower through the mid RPM power that could be gained will be lost if the DCR (thus the SCR) is not high enough. While the affect of the added compression is fairly low at high RPM, the gains in the middle of the power band are quite large when the compression and cam events are properly correlated.


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Old 10-08-2012, 01:26 PM
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Exactly what I wanted and more! Thank you
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