Editing Basics


With any fuel you run, the cylinder pressure that the motor generates is limited by the anti-detonation qualities of the fuel. 

I speak almost every day on one thread or another, about how I use only the KB dynamic compression ratio calculator. If you use the same one all the time, then you begin to establish a relationship of what will work and what components work best together toward a common goal. In my experience, a DCR of 8.0:1 to 8.5:1 will work with pump gas, allowing you to dial in all the spark timing that the motor will need to operate efficiently. As alluded to by Bogie on another thread, you may be able to raise the DCR to 9.0:1 or higher with fuel that will withstand the tendency to detonate before the plug fires. More cylinder pressure means more hp, but you must control detonation.

That's what I'm doing when I recommend a range of camshafts to someone, I have gone to the calculator and entered all the info, including the closing point of the intake valve. That closing point, along with the static compression ratio, determine the amount of squeeze in the cylinder....or "cylinder pressure" that is generated before the plug fires. The squeeze does not begin, in any motor, until the intake valve closes and is on its seat. That is what seals the cylinder at that point in the 4-cycle process and cylinder pressure continues to increase as the piston comes from BDC to TDC. So, you can alter the cylinder pressure by altering the point where the intake valve takes its seat. 

That is where most amateur builders make their mistake. They don't know or understand what I have just said and will often opt for the nastiest cam they can buy to pop into their 8.50:1 SCR motor. The low SCR, combined with a very late intake closing on this killer cam, makes very low cylinder pressure (low DCR) and results in a motor that won't pull the hat off your head. NOW HEAR THIS....THE CAM ITSELF DOES NOT MAKE THE POWER ALL BY ITSELF....IT MUST BE PART OF A COMBINATION THAT IS DESIGNED TO DO A JOB IN A CERTAIN RANGE OF RPM'S. That's why I continue to harp on the fact that the cam should be one of the last items bought for a motor, along with the torque converter. You need to determine what fuel you will have to operate on, then build for a static compression ratio that will be friendly to that particular fuel, then match the cam to the SCR to establish the proper DCR. Then, the cam's operating range will determine the stall you need in your torque converter.  

In Crane Cams literature, they explain that the limit of cylinder pressure when using pump gas is somewhere around 165 psi, but I have witnessed fellows running over 200 psi on pump gas. You just have to make sure everything in the chamber is smooth, with no sharp edges that could glow red and touch off a round of pre-ignition. You also would not use PCV or EGR with such a motor. Goes without saying that the ignition timing must be spot on. It is my opinion that a tight squish (0.035" to 0.045") will allow higher pressures also. As the piston comes up to mate with the underside of the head, the mixture in that space is compressed and blown across the chamber. This eliminates dead or lazy spots in the chamber and homogenizes the entire mixture, giving it a much better chance of burning completely and making max power from the combination without detonation. The tighter the squish, the lesser grade of fuel you can run.