The R-2800 C series ran slightly faster as did later model JuMo and Daimler Benz engines, Klimov VK-107, The Allisons installed in the P-40Q,
R-2800C was a completely different engine that shared the same bore and stroke (and the starter dog?), New crankcase, new crankshaft, new con rods, new pistons.
The Klimov VK-107 took over 4 years to get into service (with such outstanding reliability {sarcasm} that it was taken out of production
twice during the postwar years while problems were fixed).
The Allisons in the P-40Q (and some other late war aircraft) had a crankshaft with 27lbs worth of counter weights added.
The French Pre war Hispanos that ran at 2500rpm instead of 2400rpm had vibration dampers added in addition to a few other modifications, like changing the conrods
Yes, later engines were developed (not just allowed) to run at higher rpm but it took improved metallurgy as in different bearings or bearing material, a better understanding of vibration and how to deal with it and almost always the engines grew in weight to handle the increased stress.
Now please note that most of these engines are NOT going to get a very big increase in power from RPM alone.
The R-2800C only turned 3.7% faster than the R-2800B. The engine in the P-40Q turned 6.6% faster than a normal P-40 engine, the P-40Qs speed came from being able to use 75in of MAP at altitude and not at sea level.
WIthout comparing engines from different times we can compare the Mercedes 1939 M163 3 liter Formula one engine. However as I have said before comparing car engines to aircraft engines leaves out the aspect that in many cases the car engines were built to an artificial displacement limit. Artificial in the sense that it was either a rule imposed by the race organizers or it was way of classifying engines for a government tax. If you are going to build a 3 liter engine you have two avenues of increased power. Increase the RPM for more power strokes per minute or increase the volumetric efficiency with better breathing or supercharging.
For the aircraft engine maker the option of just making a bigger engine was usually open, at least until they hit
real limits, like the speed of combustion in the cylinder limiting the bore of the cylinders.
The Mercedes engine used 67mm X 70mm cylinders. it used 4 valves per cylinder in a pent roof DOHC head. it used a real witches brew of fuel (86% methanol, 8.8 % acetone, 4.5% nitrobenzol and 0.8% sulphuric ether.. It ran at 2.31 Atm for manifold pressure (19 1/4lbs of boost?) and at 7500rpm it made 480hp, it used a two stage supercharger (one supercharger discharging into the inlet of the 2nd supercharger.) The BMEP was 305psi.
The Corrected piston speed was 3,370ft/min, a bit more on this later.
the engine weighed 603lbs. (dry weight) and this, while an extreme example, shows the problem with high rpm engines.
The P & W Wasp Junior which was hardly state of the art in 1939-41 weighed about 10% more (668lbs) was 16.1 liters (over 5 times the displacement) 132 X132mm cylinders, 2 valves per cylinder using push rods. It ran on 91 octane fuel and at 36.25 in hg (3 1/4 lbs?) at 2300rpm it made 450hp for take-off. It could also make 400hp at 5,000ft at 2200rpm for as long as the fuel lasted. The BMEP was 157psi and the piston speed was 1,988fp/min.
Since aircraft desingers don't give a rat's *ss about the displacement of an engine (unless they are building a race plane for certain set of race rules) and are very interested in power for weight, reliability and fuel consumption, heavy/high rpm engines never found much favor, Major Halford aside.
Piston speed was often used to compare engines at the time but it was not really a reflection of the stress or friction of the pistons and pistons rings but rather an easily computed number the reflected the stress on the rod bearings and reciprocating parts.
The Bristol Pegasus engine due to it's long stroke had one of the highest piston speeds of the time. 190mm (75in) times 2600rpm giving 3250fpm uncorrected. The corrected piston speed was 2850fpm to account for the light pistons (small diameter.)
The formula is twice the stroke in feet, times the rpm, then the mean piston speed is divided by the square root of the stroke/bore ratio, to reflect large diameter/heavy pistons and small diameter light pistons.
Major Halford went off on his small cylinder, high RPM tangent in an effort to build a powerful and fuel efficient engine given the fuels of the time. A small cylinder will cool better than a large cylinder (more cylinder wall per unit of volume) and he was hoping to use higher compression in the cylinders and more rpm to make power. Unfortunately for him (and Napiers) fuel improved faster than he could develop his engines and the large cylinder engine designers could simply boost pressure (and redo the broken parts) with little or no change in rpm.
The lots of little cylinders branch of development also had increased maintenance loads. One reason the R-4360 was so unpopular, 56 spark plugs to change.
