Originally Posted by Silver Surfer
I thought fast ramp rates were better? This is what makes the XE and Voodoo line of cams so good right? And also what makes roller cams better too — fast ramp rates. Can you explain a little more?
The XE and Voodoo have short fast ramps of modern design cams. The short-coming of fast ramps is wear, it takes careful break-in and these days a ZDDP additive to preserve them.
The older design cams use a long ramp, this is typical of the muscle car era cams that have this really cool sounding idle but really don't and didn't produce amazing power for their stated durations. Plus they are designed to work with the very high compression ratios of the era which are not practical with unleaded fuel, at least not affordable unleaded fuel, of today.
Where the long ramp gets you into trouble especially with modern compression ratio limits is that they hold the intake valve off the seat for a longer time so the piston as it rises in the cylinder pushes the mixture it drew in on the down stroke back into the intake system. This leaves less mixture, by weight or density, in the cylinder, thus it makes less force on the piston. While the seat clearance isn't very high at this point you have to consider that the backward flow though the valve is considerably higher than the inflow for this lift would have been because the piston is pushing the mixture out with considerable force, much more force than atmospheric pressure pushes mixture into the cylinder chasing the descending piston. So this reversion loss is more than a minor one.
The whole concept behind the late closing intake is that as the RPMs raise there comes a point where the mixture entering the engine develops enough speed which translates to kinetic energy that it will continue to flow into the cylinder at these low ramp lifts against the outward push forces caused by the rising piston. From this critical RPM up to the breathing red line, the engine develops a surprising amount of volumetric efficiency and the power output that comes from this deep breathing. One can play with the critical RPM a bit with sizing of the porting and carburetor. This is where smaller ports, valves and venturies will raise the velocity thus the kinetic energy of the mixture sooner in the RPM band which will pull the power peaks lower and reduce the need for so much compression. The darkside of this change is that it greatly reduces the otherwise possible upper RPMs and the horsepower to be found there.
So a fast acting cam with short ramps (this may be only 10-15 degrees more compared to a slow ramp) clips off the reversion flow that happens below the critical RPM so it doesn't kill the bottom end torque as much as a long ramp cam. Another result of closing the intake sooner is that ultra high compression ratios needed to recover torque below the critical RPM are not necessary such that these fast ramp cams are more compatible with modern un-leaded fuel octane levels. To achieve good top end breathing with these cams the lift can be raised taking advantage of the larger curtain area of the open valve. This of course requires a better flowing port, but one that uses improved location and shape rather than just bigger ports because you still want to have a high velocity flow as soon as possible, but at the same time not restrict the maximum flow of the port to do it. At Chevrolet this lead to the LT1 and 4 ports which found their way into the L31 Vortec and GMPP Fastburn heads while still using production detail parts and processes and fitting within the confines of the production vehicle envelope. Certainly without these constraints one can look to the many and high power efficient GMPP and aftermarket 18 and 15 degrees heads and the many port variations this frees up that were not possible with existing production constraints of engine shape and size into the then current vehicles. The pan still has to miss objects the vehicle has to pass over for example so you can only get the engine so deep in the chassis and the chassis so close to the ground so the engine (Gen I&II) sets rather high in the chassis so the intake system gets shape compomised to fit it under the hood lines the designers want.
Roller cams make it easier to push the lifter open faster and further without tearing the lobe and lifter up. The OEMs went to rollers really for emission purposes. Just to show how things drive other things; back in the 1980s the EPA wanted to get ZDDP out of oil because it contaminated the catalytic converter is a way similar to lead in fuel did, just slower since it is carry over component in the combustion chamber rather than a direct component of fuel. So the OEMs were put in the position of running into wear problems between the lobe and lifter without this additive to which the only viable solution was Parkerizing which was also going away because of pollution issues with the process. So the choice was going to be very expensive to purchase and machine materials to make lobes and flat tappet lifters survivable in the new world or go to a roller. The less costly route was the roller. You will note that the actual timing at the valve of production roller engine such as the L31 Vortec is nothing unusual for a production truck engine, power performance was not what the OEMs went to the roller for, it was a backhanded way of solving an emissions problem.