I was poking around on some good engine build sites, and ran across the info about piston speed and it's effect on peak engine power. I took that info and went to some of the ex-TRD fellows and learned more about it. The bottom line is that piston speed decides how fast the air is drawn through the intake port (peak speed is the kicker, apprarently). If the ports are small (which boosts flow speed into the cylinder at lower RPM....good for trucks), the airflow becomes too high when RPM gets high. By porting the inlet (bigger) we increase the volume available, while keeping the airflow speed down (same effect as the inlet for supersonic jets).
If the airflow reaches Mach 0.5 (I think they actually hit 0.522 on some engines), the reversion wave (return pressure pulse from closed intake valve) meets the incoming air at about the same speed. This puts the intake port over Mach 1.0, and the air (at that point and also momentarily) becomes non-compressible (it stops). Because horsepower is (in the end) limited by RPM (normal aspirated engines), the stroke has to get very short to keep the piston speed down. Lower piston speed keeps the peak airflow speed from going past 0.5 Mach, for a given port and valve size.
thx, JimL
I was told that the NASCAR engines are butting up against the Mach number, running in the 9200 RPM (or a little higher) range.
My engines are extremely short stroke (the 500 is only 2.047" and the 673s are only 2.48"), which lets me spin them pretty high WITH the new larger ports and valves. Until I learned about the Mach number, I didn't realize how much small increases in valve size could matter.....it adds available RPM, and that starts multiplying the Horsepower calculation.
I kinda' jumbled the sentences....there are folks on here a lot better at this engine stuff, and perhaps they'll straighten me out a bit. Another lesson about learning lessons too late....spent too many years playing with motorcycles and guitars.... to bother learning what I needed!