Reply to "18,000 RPM"

Power/revs/torque is always a trade-off.

Revs produce hp. But not necessarily torque.

High revs is mostly down to managing inertia. Long stroke crank means that more weight is away from the bottom end causing more forces on the bottom end. Long crank also means that the con rods have to work at more of an angle – more movement, more inertia, more force – especially side force which means that you want a deep skirt piston to stop it skewing in the bores. Thus, long stroke means heavier piston, so more stop/go forces on the rods and crank.

Short stroke gives less force on bottom end – but bigger bore gives heavier piston, and more difficulties with scavenging. Higher revs also mean that the piston oscillates (accelerates and decelerates) more often – causing more stop/go forces on the rods and crank.

Too big a bore/short a stroke gives the possibility that the combustion explosion will still be happening when the piston starts the next upstroke.

In the Valve train, single valves actually flow individually better than multi valves, but a single valve has a lot of weight going back and fore with only one (maybe duplex) valve spring. Multi valves have lighter valves with a valve spring each – hence, higher rev capability before valve bounce.

BTW, engine configuration plays quite a part too. V8’s are “inherently out of balance” which produces the V8 “rumble” Straight 6’s are in balance, and v12’s even better – but this gives a longer crank which is more susceptible to whip and wind up – V engines also have smaller bottom end bearings due to space constraints.

So, for big revs you need:
Short stroke crank/Lightweight pistons and rods (Ratio designed exactly for the required rev capability)

Lightweight multivalves with strong valvesprings (I’m a great exponent of the Cross rotary valve system here)

Engine “in balance” with large bottom end bearings and short crank

It goes without saying that the materials chosen play a huge part too.

And I haven’t mentioned high compression ratio and high octane fuel…..

Phil