max-boost.co.uk skrev:
http://www.max-boost.co.uk/max-boost/MythsExtract1.htm
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4)· Con-rod to stroke ratio-exploding the myth. It’s not only the amateurs that come up with misguided opinions. A supposed “expert” in the field, who has written many books and used to post articles in engine performance magazines makes huge claims for increasing the con-rod to stroke ratio of an engine but typically fails to back it up with any quantified examples. It’s been claimed that doing this delays peak piston velocity (although also making it lower) and moving it in line with when the the cam shaft is at peak lift. Well let’s shed some light on the matter-an engine with a 81 mm stroke and a con rod length of 130 mm running at 6800 rpm will have an instantaneous piston velocity of 30.1 m/s (around peak) at 75 deg ATDC. Now increasing the con rod length to 140mm moves this peak piston velocity down to 29.94 m/s but will only delay it by barely over a crank degree! A similar neglible gain argument can be laid down over the favourable geometric angularity –he piston being driven less into the bore (thrust). If you’re an clean sheet engine designer, as long as you stay within sensible norms, I would therefore recommend that other considerations such a block height and engine weight predominate. If you’re trying to modify an existing engine I would say you’re better off spending the money else where to get the performance gain, such as the cylinder head, rather then getting custom pistons/rods.
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4)· Con-rod to stroke ratio-exploding the myth. It’s not only the amateurs that come up with misguided opinions. A supposed “expert” in the field, who has written many books and used to post articles in engine performance magazines makes huge claims for increasing the con-rod to stroke ratio of an engine but typically fails to back it up with any quantified examples. It’s been claimed that doing this delays peak piston velocity (although also making it lower) and moving it in line with when the the cam shaft is at peak lift. Well let’s shed some light on the matter-an engine with a 81 mm stroke and a con rod length of 130 mm running at 6800 rpm will have an instantaneous piston velocity of 30.1 m/s (around peak) at 75 deg ATDC. Now increasing the con rod length to 140mm moves this peak piston velocity down to 29.94 m/s but will only delay it by barely over a crank degree! A similar neglible gain argument can be laid down over the favourable geometric angularity –he piston being driven less into the bore (thrust). If you’re an clean sheet engine designer, as long as you stay within sensible norms, I would therefore recommend that other considerations such a block height and engine weight predominate. If you’re trying to modify an existing engine I would say you’re better off spending the money else where to get the performance gain, such as the cylinder head, rather then getting custom pistons/rods.
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Min logiske sans siger mig at det er forkert... Jeg tror han har fået byttet rundt på 30.1 & 29.94 m/s
Peak Piston Velocity må da være et sted i mellem bund og top.. og hvis et stempel med en længere plejlstang bruger lidt længere tid i top må det da skulle bevæge sig lidt hurtigere de andre steder for at komme lige så hurtigt rundt som den korte plejlstang ??
Her er et par gode links (som sikkert ville give mig forklaringen hvis ikke jeg skulle i seng nu... der er desværre noget der hedder arbejde
)
http://www.stahlheaders.com/Lit_Rod%20Length.htm
ser ud som om der er en del god info her
siden kan også downloades som .pdf her -> http://www.stahlheaders.com/RodLength.PDF
Og et par andre links :
http://www.engr.colostate.edu/~allan/th ... page2.html
http://www.engr.colostate.edu/~allan/th ... age1f.html
http://www.max-boost.co.uk/max-boost/MythsExtract1.htm
http://yarchive.net/metal/piston_acceleration.html