Don't forget what we're talking about... small industrial engines 3600rpm ->5500/6000 rpm!
While it's true that a dropping torque curve can cause a higher rpm to produce less HP,
that's unlikely (I'd even say impossible) for our scenario.
Think about it as provided torque per combustion.
Now, the dropping torque curve is partly based on the amount of fuel,
the longer a filling stroke takes the more time the fuel/air mixture has to get into the combustion chamber... and the higher the rpm, the less each stroke takes of course..
so based on the flowrate that means less fuel per combustion; ergo less power (or torue here).
That's why a high flow filter and exhaust together with a rejetted carb comes in handy.
That way you're able to provide a faster fuel delivery, wich keeps the torque curve up for a little longer.
The next factor is the compression, if the valves have time to close properly,
we can assume that, since if they don't we reached our intended rev limiter (valve float)!
and finally ignition timing.
the position of the piston in the cylinder at the time when the compressed mixture is ignited is important for 'quality' of the combustion the amount of fuel being actually burned, thus the amount of expansion being created.
And since the spark has to travel from the coil to the spark plug that takes time... enough time to make a difference at higher rpms.
That's why if you want to have a high revving engine, you either have a billet flywheel with an 8°advance built in or you install timing keys.
With stock valves, it may be usefull to install a 2° or even 4° timing key to have the timing corrected if the engine's torque curve appears to be dropping premature.
keep in mind, we haven't 'added' torque with the modifications, we just altered the curve to our likings with the mods above.
And there we have still say 80-90% of the max torque @ nearly two times the rpm.
and that'd be 8-9HP @5500 or 9-10HP @6200 for a slightly modified 6.5HP engine
'sid
