Mean Piston Speed
The best known "rule of thumb" is mean piston speed (average speed the piston travels at a fixed rpm). It goes like this:
Mean Piston Speed Result
------------------ ------
Under 3,500 ft/min Good reliability
3,500-4,000 ft/min Stressful, needs good design
Over 4,000 ft/min Very short life
Mean piston speed is easy to determine. It's based on the piston stroke and rpm.
Cm = 0.167 x L x N
Cm = mean piston speed in feet per min L = stroke in inches
N = crankshaft speed in rpm
Using the 1988 E28 M5 as an example:
E28 M5, Bore and stroke 3.68 x 3.31 in (93.4 x 84 mm) At 6,900 rpm (factory redline)
Cm = 0.167 x 3.31 x 6900 = 3,814 ft/min
At 7,200 rpm (Dinan chip redline) = 3,980 ft/min At 6,500 rpm (max constant speed) = 3,593 ft/min
Here are some other piston speeds I figured for comparison.
Redline Stroke Piston Speed
Engine (rpm) in,mm (ft/min)
-------- ------- -------- ------------
BMW 2002 6,400 3.15, 80 3,367
BMW 318i 6,500 3.19, 81 3,463
BMW 325i 6,500 2.95, 75 3,202
BMW 3.0 6,400 3.15, 80 3,367
BMW 535i 6,200 3.39, 86 3,510
MM 4000 6,000 3.70, 94 3,707
MM 3500 6,400 3.39, 86 3,623
BMW E36 M3 6,800 3.38, 85.8 3,838
BMW E28 M5 6,900 3.31, 84 3,814
BMW E34 M5 7,200 3.39, 86 4,076
CBR600 13,250 1.78, 45.2 3,939 (motorcycle)
Notice that most production cars stay below 3,500 ft/min, while the E36 M3 has the same 3,800 ft/min piston speed as the E28 M5. With its longer stroke, the E34 M5 piston speed is even higher than the screaming Honda CBR600 motorcycle! The two Metric Mechanic (MM) engines also go above 3,500 ft/min.
>From the MM Catalog Engine Specification table, MM uses
thinner (lighter) 1.5 mm (0.059 in) top rings than the standard BMW 1.75 mm (0.069 in) top rings. Lighter weight reduces vertical ring acceleration force at high rpm. MM talks about less "hammering" of the ring groove from the lighter ring, but it also reduces piston and ring overheating.