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Old 02-27-2006, 02:48 PM   #1
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Piston rod ratio speed lkajsdflsdflkj wtf?

ok..... sorry for the weird topic name, but i cant remember exactly wat it was called that i wanted to ask about (u guys know i suck at explaining).

anyhow, a few months ago i read an article bout some guy that spend a weekend with a semi-pro racing team. he gave a very thourough walkthrough about how the team got the car together and tested it out.

at one point, some senior came in. apparently, he is an extremely well known man that, apparently, really knows his damn stuff. while the team (consisting of fairly young guys, late 20's and early 30's) had to depend on looking at computers for half an hour, this man came in and had a look at some screen for a few seconds, and said, "the rod speed ratio is too fast. need to fix it." i know wat i probably said was wrong, but it was something about "rod ratio" and speed. if i could find the magazine i would type out the exact quote.

anyhow, anyone who reads that and understands care to explain to me wat was it that he was actually talking about and how he figured it out?

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Old 02-28-2006, 02:43 AM   #2
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He was talking about the rod/stroke ratio. A higher rod/stroke ratio will yield a slower peak piston speed than a smaller rod/stroke ratio. Slower piston speeds means less stress on the reciprocating components.
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Old 02-28-2006, 03:24 AM   #3
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Yes one of the things to watch for is oil sheer. Other obvious things are sidewall loading, dynamic compression, mechanical advantage, exhaust gas rarefaction and inversion, blowdown, cam events, swirl, etc.

Long rods cause the piston to have a slower velocity around TDC and faster around BDC, so you need to look carefully at your exhaust ports and blowdown event otherwise you can lose some power pumping out the exhaust gases. Tend to give a higher peak power and can sustain higher revs because of a lower piston acceleration and less strain on the low end mechanicals.

Short rods give you a better angle in relation to the crank pin for mechanical advantage, but you will need stronger mechanicals as a result. They put a fair amount of stress on the cylinder walls. Blowdown timing is typically earlier and you will need nice big exhaust ports so that the slower piston speed is still enough to pump out the residual gas.

The "senior" probably looked at the ratio and decided it was either outside the 1.6 to 1.9 limits, or if he was after a short RR outside the 1.6 to 1.8 limits.
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Last edited by Wally : 02-28-2006 at 03:37 AM.
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