SBC Connecting Rod Length

Post by **3V Performance** » Wed May 20, 2009 11:20 am

Like Joe said.

It will not fall off as fast after peak.

Longer rod engines tend to like shorter dur cams. From a 6" to a 6.250 we can run 4* less.

DCal · Post by **DCal** » Wed May 20, 2009 11:38 am

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

k-star · Post by **k-star** » Wed May 20, 2009 12:46 pm

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

CARRITAS RACING ENGINES · Wed May 20, 2009 1:08 pm

SEEN THE SAME THING AS 3V AND CNC.

1989TransAm · Post by **1989TransAm** » Wed May 20, 2009 1:25 pm

"What I have seen with the longer rod is that the engine does not nose over as bad above the HP peak--"

I have seen this posted many times on the internet. Based on these experiences that is why I went with a 6.125" rod in my current SBC build. Should get more power "under the curve."

CNC BLOCKS · Post by **CNC BLOCKS** » Wed May 20, 2009 2:14 pm

k-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

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 TDC

Post by **jmarkaudio** » Wed May 20, 2009 2:57 pm

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...

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.

stealth · Post by **stealth** » Thu May 21, 2009 11:17 am

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.
.

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.

If adequate flow is available would you see the same results?

stealth · Post by **stealth** » Thu May 21, 2009 11:35 am

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...
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dacaman12 · Post by **dacaman12** » Thu May 21, 2009 12:06 pm

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...
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The max difference in lift of that 4° shorter cam that the long rod motor likes is about .020" as well.

Interesting, INDEED.

GotSomeRust · Post by **GotSomeRust** » Thu May 21, 2009 12:08 pm

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.

Post by **Stan Weiss** » Thu May 21, 2009 12:16 pm

It really does take a lot of rod length change to show up as much piston travel change.

Hendrens Racing Engines · Thu May 21, 2009 6:31 pm

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

Don Terrill’s Speed-Talk

SBC Connecting Rod Length

rod

Re: rod

Re: SBC Connecting Rod Length