hoffman900 wrote: ↑Thu Jan 13, 2022 10:02 pm
Bringing this one back from the dead.
I heard Mark Cronquist of JGR mention today about closing the intake valve "early"* on restrictor engines / limited compression engines to build cylinder pressure, but he has learned with the less rules JGR Baja truck engines that you can close it too early.
I can imagine what is happening here, but anyone with experience elaborate on this? Mike?
*early is relative to what you would one would think the engine wants for the rpm, I am assuming.
Welcome to the 90's
Back in the late 80's and 90's we had a lot of limited compression classes in circle track racing. Many in the 9-9.5:1.
To increase power, we needed to increase cylinder pressure. By increasing the lift, reducing the duration, and closing the intake valve sooner, we could increase the charge velocity into the cylinder, and increase the ram air effect. This increased the amount of mass into the cylinder, increasing the cylinder pressure, and increasing the power. It was very common for these engines to be running a 98-100 Intake Centerline.
We took what we learned from the limited compression engines, and used in in the restricted carb and restrictor plate engines. When the restricted inlet engine is above the point where the carb or restrictor plate starts restricting, the mass of the intake charge is reduced, and that reduces the cylinder pressure, so the higher lift, shorter duration, and earlier intake closing helped the power.
The issue was below the point the carb or restrictor plate started restricting. Below that point, the engine is seeing no reduction in mass of the intake charge. In these lower RPM's that early intake closing is way too early, and the cylinder pressure is too high.
Look at it this way, at high RPM's the engine thought it was running in Denver Colorado, but at low RPM's, it thought it was running in San Diego. We had to come up with a compromise for cam timing.