SBC Connecting Rod Length
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Years ago we made some 6.375, 2-ring, .748 wristpin pistons for a customer. These weighed 298 grams, but the driver said it was a turd on restarts and getting out of the pits. It would really march if they could stay in the upper rpm green flag laps. Eventually Nascar put a rod length limit of 6.250 to discourage "exotic" practices. They won the championship but with 6.250 combo.
DCal
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rod
On these engine, were they run with an "X" length rod and then switched to a longer rod, maintaining the same compression, etc???
Is it possible that the weight of the piston has more to do with it holding on, then the rod length???
Knowing the small difference in crank/piston location between rods i am having a hard time understanding what is really making the engine hold on longer.
I had a conversation a few years ago about this subject with a guy that ran a RED, big block Mopar. He had raced the car for 10 or 12 years, 2 or 3 days a week. He told me they tried every available rod length combo, spent a bunch of money doing it and it never made the car ET or MPH any better.. Never made it worse either.
Keith
Is it possible that the weight of the piston has more to do with it holding on, then the rod length???
Knowing the small difference in crank/piston location between rods i am having a hard time understanding what is really making the engine hold on longer.
I had a conversation a few years ago about this subject with a guy that ran a RED, big block Mopar. He had raced the car for 10 or 12 years, 2 or 3 days a week. He told me they tried every available rod length combo, spent a bunch of money doing it and it never made the car ET or MPH any better.. Never made it worse either.
Keith
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Re: rod
If that were true we would have to change the cams look how the engines breathes with the longer rods and the dwell times at TDCk-star wrote:On these engine, were they run with an "X" length rod and then switched to a longer rod, maintaining the same compression, etc???
Is it possible that the weight of the piston has more to do with it holding on, then the rod length???
Keith
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Re: SBC Connecting Rod Length
What are you doing with it? How much RPM? On a street or street strip engine I wouldn't worry too much about it, buy whichever is better on your budget. Find other areas to spend the surplus money.GotSomeRust wrote:I was told that I should use a 6 inches connecting rod in my Chevy 350 instead of the stock length of 5.7 inches. What are the advantages/disadvantages of using a 6 inch? Thanks in advance for the info...
Mark Whitener
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Wouldn't this be more of a factor pertaining to "limited" intake capability, in that the longer rod tolerates poor air flow/restriction better then the short rod.CNC BLOCKS wrote:We see a big differance on the 2 bararrel circle track engines going from a 5.7 to 6.250 rods and from what I have seen they male peak power about the same RPM but the 6.250 rod will carry alot longer then the short rod engines.
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If adequate flow is available would you see the same results?
Isky has a nice chart for this exact rod question 5.7 to 6.0
http://www.iskycams.com/techtips.php#2005
note the pistons location changes by less then .020 at the most...from one ratio to the other.
interesting...
[/url]
http://www.iskycams.com/techtips.php#2005
note the pistons location changes by less then .020 at the most...from one ratio to the other.
interesting...
[/url]
The max difference in lift of that 4° shorter cam that the long rod motor likes is about .020" as well.stealth wrote:Isky has a nice chart for this exact rod question 5.7 to 6.0
http://www.iskycams.com/techtips.php#2005
note the pistons location changes by less then .020 at the most...from one ratio to the other.
interesting...
[/url]
Interesting, INDEED.
Last edited by dacaman12 on Thu May 21, 2009 12:09 pm, edited 1 time in total.
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Thanks everyone for the info. I just brought this question up because a local engine builder is the one who suggested it to me. I'd never really heard of it before, but I'm not much of a mechanic so that explains that. He said that he builds all of his motors (mostly circle track engines) that way. I figured he knew what he was talking about, but wanted to get a little more info... Thanks again.
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We along with most circle track engine builders use the longer rods to cut weight out of the pistons and it does result in more power past peak power. Not sure if its the speed change as much as weight.
Look on page 13 in the ARP book,they have a chart showing the load against a rod cap at various rpms. example @2000 rpm 2500 lbs 4000 rpm its 9800 lbs 6000 rpm and its 20,000 and 8000 its 35,000 lbs.
Some piston in that engine needs to overcome the weight of stopping the small end of the rod,piston,rings,and pin before it can produce one usable H.P. Bill
Look on page 13 in the ARP book,they have a chart showing the load against a rod cap at various rpms. example @2000 rpm 2500 lbs 4000 rpm its 9800 lbs 6000 rpm and its 20,000 and 8000 its 35,000 lbs.
Some piston in that engine needs to overcome the weight of stopping the small end of the rod,piston,rings,and pin before it can produce one usable H.P. Bill
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828-286-0780