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Chris Cogan · 12-07-2007, 02:13 PM

Hey guys,
I got a little bored this morning and decided to be a geek for awhile and investigate this topic further. Keep in mind that ((stroke x rpm) / 6) is the equation for mean "average" piston speed in feet per minute. So rod length does not affect average piston speed. However, piston speed is sinusoidal and when you look at what is happening to instantaneous piston speed at every degree of crank rotation, you will see that piston speed is affected by rod length. Using Mr. Leong's cranktrain parameters at 8000 rpm, a 5.94" rod will have a peak piston speed of 8753 fpm @ 74 degrees leaving and approaching TDC. a 6.16" rod will have a peak piston speed of 8726 fpm also @ 74 degrees. Average piston speed for both rod lengths is the same at 5306 fpm. So if you picture a sinusoidal curve, the piston speed of the shorter rod is greater than the piston speed of the longer rod from 0 to 90 degrees, slower from 91 to 179, equal at 180, slower from 181 to 269, equal at 270, faster again from 271 back to 0. So with a shorter rod, the piston accelerates toward and away from TDC faster. this is a very slight change, but if you are trying to pluck frog hairs as most of us racers are, it should not be overlooked.

Again using Mr. Leong's parameters, consider this. At 8000 rpm with a 5.94 rod, the piston speed is greater than 6000 fpm for 170 degrees of every revolution.(47.2% of the time). At 9000 rpm with a 5.94 rod, the piston speed is greater than 6000 fpm for 192 degrees of every revolution. (53.3% of the time) And at 9000 rpm, the peak piston speed is 9847 fpm for the 5.94" rod and 9816 fpm for the 6.16" rod. So as rpm increases, the percentage of time that the engine sees piston speeds greater than the average also increases. Sorry if I bored the hell out of anybody, but I was curious to understand what's going on. Take care!

Chris Cogan
Kaizen Motorsports

12-07-2007, 02:13 PM	#6
Chris Cogan Junior Member Join Date: Apr 2007 Location: Defiance, OH Posts: 47 Likes: 0 Liked 0 Times in 0 Posts	Re: Rod length? Piston speed? Hey guys, I got a little bored this morning and decided to be a geek for awhile and investigate this topic further. Keep in mind that ((stroke x rpm) / 6) is the equation for mean "average" piston speed in feet per minute. So rod length does not affect average piston speed. However, piston speed is sinusoidal and when you look at what is happening to instantaneous piston speed at every degree of crank rotation, you will see that piston speed is affected by rod length. Using Mr. Leong's cranktrain parameters at 8000 rpm, a 5.94" rod will have a peak piston speed of 8753 fpm @ 74 degrees leaving and approaching TDC. a 6.16" rod will have a peak piston speed of 8726 fpm also @ 74 degrees. Average piston speed for both rod lengths is the same at 5306 fpm. So if you picture a sinusoidal curve, the piston speed of the shorter rod is greater than the piston speed of the longer rod from 0 to 90 degrees, slower from 91 to 179, equal at 180, slower from 181 to 269, equal at 270, faster again from 271 back to 0. So with a shorter rod, the piston accelerates toward and away from TDC faster. this is a very slight change, but if you are trying to pluck frog hairs as most of us racers are, it should not be overlooked. Again using Mr. Leong's parameters, consider this. At 8000 rpm with a 5.94 rod, the piston speed is greater than 6000 fpm for 170 degrees of every revolution.(47.2% of the time). At 9000 rpm with a 5.94 rod, the piston speed is greater than 6000 fpm for 192 degrees of every revolution. (53.3% of the time) And at 9000 rpm, the peak piston speed is 9847 fpm for the 5.94" rod and 9816 fpm for the 6.16" rod. So as rpm increases, the percentage of time that the engine sees piston speeds greater than the average also increases. Sorry if I bored the hell out of anybody, but I was curious to understand what's going on. Take care! Chris Cogan Kaizen Motorsports