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Old 11-11-2007, 07:56 PM   #1
art leong
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Default Rod length? Piston speed?

A question about rod length, piston speed.
I'm running a 6.160 rod. the stock length is 5.94. The stroke is 3.98.
Will the longer rod.
A= make more lower end torque?
B= make more power in the higher rpm ranges?
C= not make a hill of beans difference?
This is for a 4 cylinder car with an automatic (2600 pounds), I need all the torque I can get.
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Old 11-11-2007, 11:00 PM   #2
Chris Cogan
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Default Re: Rod length? Piston speed?

Hi Art,
I think that most people will tell you that the answer is C, but if I were you I would run the shorter rod. For two simple reasons: One, a shorter rod is a lighter rod. Two, a shorter rod is more forgiving of detontation. This is just my 2 cents based on my experiences with large cubic inch engines. I have seen 700 plus cubic inch engines regularly turn 8000 rpm with rod to stroke ratios between 1.37 - 1.41 and when dealing with high compression ratios and or nitrous oxide, you want the piston to accelerate away from top dead as quickly as possible. IHRA pro stock engines run relatively short rods for this very reason. Again, just my 2 cents. Take care!

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Old 11-11-2007, 11:03 PM   #3
Chris Hill
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Default Re: Rod length? Piston speed?

My gut answer is C, not a hill of beans.

Your changing from a stock rod ratio of 1.49 to 1.55. You generally need to make a major change of say 0.25 or greater to see any difference, but my reasoning in only therotical and not tried on the track or dyno. Any gain on airflow will probally be offset by additional rotating weight for probally a net zero change. I think engine friction should go down with longer rod, but this pretty small compared to increase in rotating mass.

But with a rod/stroke ratio of that low, your going to sucking REAL hard on the port the first 70 degrees of crank angle during the intake stroke.
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Old 11-12-2007, 12:18 PM   #4
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Default Re: Rod length? Piston speed?

You'll benefit across the board if you optimize your cam profile to take advantage of the ratio, but it's going to help your torque more than anything.

Higher pin location is good, I'll assume you haven't offset the pins.
Shorter skirt, lighter piston....
Better cylinder filling and evacuation, IF the cam is profiled for it.....
Should be able to run less ignition lead, adding to cutting down on the load during the work cycle.

It's all good......the rod weight at the beam will make the least difference.
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Old 12-06-2007, 01:44 PM   #5
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Default Re: Rod length? Piston speed?

http://www.gofastnews.com/board/tech...ill-going.html You want to see piston speed, check out this article
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Old 12-07-2007, 02:13 PM   #6
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Default 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!

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Old 12-07-2007, 02:24 PM   #7
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Default Re: Rod length? Piston speed?

Sorry, in my last paragraph I meant to say, "So as rpm increases, the percentage of time that the engine sees piston speeds greater than 6000 fpm also increases."


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Old 04-11-2008, 04:18 PM   #8
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Default Re: Rod length? Piston speed?

Do you use a computer program to do your calculations, or do you just use a calculator? It takes me forever to do it on a TI-86, and I can't print out the results.
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Old 04-13-2008, 09:11 AM   #9
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Default Re: Rod length? Piston speed?

Yes, I use Excel to do the math. Excel is great because it allows you to analyze the data in many different ways.(graphically, statistically, etc) So all I did in this case was create four columns, Crank Angle, Piston Position, Piston Speed and Piston Acceleration. Then I created cells for stroke, rod length, rpm and rod/stroke ratio (this saves a few parenthesis in the equations). Crank Angle goes from 0 to 719 degrees. Then copy in your equations for piston position, piston speed and piston acceleration at crank angle = 0 and then copy those cells down to the 719 cell and there you have it. Hope this helps. Take care.

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Old 04-13-2008, 11:47 AM   #10
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Default Re: Rod length? Piston speed?

Just for fun, what happens when you offset the piston pin?
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