Reducing stroke, effect on torque.
Turbo Mini Forums

Anyone got any insight into this, after an hours reading my head hurts.

A high spec sprint cammed 1380 k-head engine on a 73.5mm bore and 81mm stroke seems to produce around 105 1b/ft and 160hp.

Now assume the same high spec head was fitted to a 1255cc engine achieved with a the bore of 74mm and a stroke of 73mm, with inlets and exhaust optimized. Assuming a similar hp per litre figures for the two engines the headline hp figure would be around 150, but any guess on what torque output would be?

IF you managed to get the engine to produce the 150bhp at exactly the same RPM as the engine was making the 160bhp then the expected torque figure for 150bhp would be approx 98.5 lb/ft

Peak numbers may change little.

But where those numbers are made will be different

Longer stroke will always make torque lower in the rpm range ( of course camshaft will also affect this )

Without some real fancy calculator, dynosim or something like that, hard to give actual guess numbers

But on the flipside...the reduction in capacity may affect that lower rpm torque to a degree too.

So with big cam, reduced engine size and reduced stroke...I'd say there could be a fair drop in terms of how the engine feels on the road..whether peak numbers change much or not.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Another thing to consider is the maximum piston speed, which is related to engine rpm and stroke length, the shorter the stroke the higher the rpm you can use before the piston speeds become too high. Hence things like bike engines and F1 cars have a very short stroke so they can rev higher, which allows them to make more power (the bigger bore also allows bigger valves, for a given displacement)

I have a short stroker. it makes its torque nearly 2thous higher in the rev range ( think redline of 9 vs 7), but it spins up very quickly. It is in a racer , as said , lower piston speed helps longevity in theory' Unless you were prepared to spend a lot of your time above 5000rpm, i wouldn't recommend it. Torque and power absolutes seem very similar at the top limits for the same displacement, just get there in different ways. ( ie the standard 1293 and the short stroked 1298 both can get to approx 140horsies and 110/115 footlbs (at the fly)