At 10:21 PM 7/11/2002 -0700, DB wrote:
>Maybe it's too obvious to point out, but no one has yet mentioned that long
>stroke means that the lever arm length through which the crank twists the
>driveline is 1/2 the stroke, so an engine with 10% longer stroke, all other
>things being equal, will have a 10% higher torque rating.
DB, good point. Also, increasing 5% the bore increases piston area by 10%,
increasing gas pressure force on the fixed crank torque arm (for the same
gas pressure) by 10%, increasing torque by 10%. The result is roughly the
same. So, you can say that the increase in displacement is what really
gives you more torque, regardless how you do it (boring or stroking or a
combination). By maintaining a good bore/stroke ratio (larger than one)
you get benefits as discussed before in terms of volumentric efficiency
(among others), allowing the engine to "produce" higher levels of gas
pressures at higher speeds, and thus torque.
>My understanding is that they specifically stroke it, as opposed to boring
>it for mega torque motors.
>I have a Hot Rod on the subject lying around here somewhere.
>Rob
Rob. I think the limits of stroking an engine are much wider than
boring. In other words, if they tried to bore the engine, they can only go
so far. There are limits in stroking too, but I think there is a lot more
room for increasing the displacement that way. This is obviously a very
compromised method of increasing displacement, because (in addition to
compromising your bore/stroke ratio) you also have to reduce connecting rod
length, which is not a great design change (I guess if all you care is a
bit more torque for your weekend hot rod, that's OK). There is a standard
misconception out there that stroke gives you torque, and certainly the Hot
Rod writers may may be influenced by that (these guys do not know much
about engine theory, but granted, they often know what works and what
doesn't, if not why).
D^2
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