Thread: DCR's??.

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Any engine, put together with proper parts and care, will operate well enough to take grandma to bingo and back. It is not necessary that you know the static compression ratio or the dynamic compression ratio if you are not interested in blueprinting the motor and taking maximum performance from it.

Static compression ratio (SCR) is just what it says, static. Static means still, at rest. So, SCR has little meaning other than an initial indication of the type of fuel that you may have to use in the motor to prevent detonation. The missing ingredients are rod length, stroke and the intake valve closing point. You must understand at this point that compression does not begin in the motor until the intake valve is on its seat with the piston coming up the bore on its compression stroke. As long as the intake valve is open, even if the piston is coming up the bore, there will be no compression. The piston will simply push the mixture that was just drawn into the cylinder, back out of the cylinder through the open intake valve. So this is where you either build a motor or you don't. If you use a high SCR with a short cam, the intake valve will close too early and trap more mixture than the fuel can stand, so the motor detonates. If you use a high SCR with a long cam, the intake valve will close later and allow the motor to push a little of the charge back out the valve so that cylinder pressure is within the limits of the fuel used. The idea is to close the valve at exactly the right moment in relation to the position of the piston, so that you capture just the right amount of mixture that will make max power without detonating on the fuel used. If you don't know how to figure the DCR, then the camshaft you choose and screw into the motor is just a guess. Now, after a fellow has built a number of motors, he gets an idea of what will work and what will not work, but newbies don't have that luxury.

Dynamic compression ratio (DCR) takes into account the static compression ratio, the rod length, the stroke of the motor and the intake valve closing point, so it gives us a more complete picture of how the motor will perform and will also allow us to choose exactly the right cam to accomplish what we wish to accomplish with this motor. For instance, if we use the same DCR calculator all the time, then we will get a handle on what we can get by with and what we can't. Don't try to use first one calculator and then another. They're not the same. Settle in on one calculator and use it exclusively and you will know exactly what cam timing figures will work with the SCR you are using and you'll know that the motor won't rattle on the fuel that you buy down at the corner station. I've used the one on Keith Black's site for decades and know that I can recommend a camshaft that will generate 8.0:1 up to 8.5:1 DCR and operate well on just about any brand of pump gas. There are fellows I know who will push the KB calculator to nearly 9.0:1 DCR and get away with it on pump gas, but I'm content to cap the number at 8.5:1. I'd rather err on the side of caution in the event I get a bad load of fuel.

If none of this makes sense to you, PM me and I'll give you my cell number so we can talk it through.