pushrod vs. O/H cammer RPM ranges

[Mach 1]

Most of us are aware of the tendency of overhead cam engines to have less torque down low and make their horse power higher up in the RPM range, and that pushrod engines will make more torque down low and make their horse power lower in the rpm range.

What I am wondering is WHY?

How does cam placement affect these characteristics?

[PKRWUD]

Don't quote me, but my understanding is it's all about where the engine can make the most power reliably. An OHC engine will be reliable at higher rpms than a pushrod engine for the simple fact that there are less moving parts between the cam and the valves. The more parts involved, the more physics plays a limiting part in the potential. That being said, the engineers that design the charachteristics of the camshafts do so accordingly. For higher horsepower, a higher rpm power band is preferred, so the engineers would design the cam so that it helps to develop the power band in the highest rpm range that is considered reliable. Since OHC engines have a higher reliable rpm range, it follows that that is where there peak power will be found. Since it's difficult to design a cam that provides power from off idle through redline, there is bound to be a weaker range as well as a stronger range. Since the OHC stronger range is higher than the pushrod engines, it makes sense that their lower rpm range would be less powerful. For example, let's say, for illustration purposes, that the cam has a power range of 4000 rpms, with the strongest point in the middle. If your engine has a reliable rpm range peak of 6000 rpms, you would design the cam to come in at 2000 rpms, peak at 4000 rpms, and peter out at 6000 rpms. The range from zero to 2000 rpms would be considered it's weakest point. Now, if you took a cam with that same 4000 rpm range, and put it in an engine with a reliable rpm range peak of 8000 rpms, you would design it so that it came in at 4000 rpms, peaked at 6000 rpms, and petered out by 8000 rpms. This would make it's range from zero to 4000 rpms it's weakest point. Thus, the first engine, with a lower reliable rpm range peak, would get it's power at a lower rpm than the engine with a higher reliable rpm range.

Make any sense?

Take care,
~Chris

[Mach 1]

makes sense, but even cammer engines with a red line, or RPM operating range that is the same as a pushrod engines, still make thier power higher up. I am comparing engines of different dispalcements though. What you said makes sense.

later