I think we are saying basically the same thing, my only disagreement is that a
longer lever arm and a longer power stroke will provide more torque from the
same BMEP.
The size of the port is moot when the valve is closed. A smaller exhaust port
will delay the pressure drop in the cylinder when the valve opens and thus
positively impact torque.
Also if you run the "Bonkers high revvin" Honda through the HP vs Torque
calculation you will notice that the torque/HP number is not particularly
impressive. That is specifically because it is a short stroke , high revving
engine.
Bill Lawrence
MarkoNTL wrote:
> ynotink wrote:
> >> Long-stroke doesn't mean low-speed torque, got nothing to do with it
> >> at all actually.
> > Actually it does for the reason stated. The force on the piston (BMEP)
> > multiplied by the additional stroke length determines the additional
> > torque to be expected from a longer stroke.
> Other things come into play there my friend, take a look at that miniguide.
>
> > I don't think the size of the ports in itself affects the potential
> > torque. The tendency to combine greater valve overlaps with larger
> > ports to improve cylinder filling and increase horsepower output
> > works to decrease torque capacity. This is because opening the intake
> > earlier and closing the exhaust later decreases the duration of the
> > power stroke.
> >
> > There are several other factors in the torque/power interaction that
> > are lost in the mists of my mind.
> Well go find them before you say what does and does not affect torque...
>
> torque = cylinder filling. Get more air and fuel into the engine and you'll
> get mroe torque. The difference betweeen a low-rev torque monster and a
> high-rev power-monster is where the engine is 'tuned' for torque. The cam
> profile, the valve and port sizes, and the manifolding arrangments determine
> this. Long 'runners' on manifolds, narrow ports, small valves, mild cam with
> little overlap all mean a torque peak at low revs. Short runners, big ports,
> big valves, bonkers cam mean high-rev torque.
>
> As it happens, most long-stroke engines do tend to have small-valves, small
> ports, long runners and mild cams, which is why the make low-rev torque; but
> if you were to put those same small valves, small ports, long runner, mild
> cams onto a short-stroke wide-bore engine you would make exactly the same
> low-rev torque. So square or over-square has 'owt to do with where it makes
> its peak torque.
>
> "The tendency to combine greater valve overlaps with larger ports to improve
> cylinder filling and increase horsepower output works to decrease torque
> capacity."
>
> It moves the torque-peak up the rev-range, so the smae torque moved higher
> up makes more power. Honda S2000, THE bonkers high-revving road car, 240bhp
> from 2 litres, 140something lbft peak torque. Land-Rover Discovery
> pi (a 2 litre petrol version we got in the UK), 131bhp from 2 litres,
> 136lbft peak torque. Torque is basically the same, not decreased, just in a
> different place..
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