The rod length subject is not a strong area for me, BUT, . . . I always underdstood that a SHORT rod produced a greater rod angle relative to the crank and produced way more torque due to the leverage. It is called "Mechanical Advantage" This is why the 347 stroker can easily produce 400 ftlbs of torque for such a small engine but should not be reved to high due to the associated piston speed. Therefore, . . . shorter rod length = more rod angle = more torque !!!! ????

This is 180 degrees off from what he wants to do. Am I missing something
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I dunno, seems we are seeing/reading totally different things as what I've seen/read so far, points to just the opposite that you have stated....

the 347 stroker uses a 5.4 in long rod with a 3.4 in. long stroke giving a rod/ratio of 1.59......these motors are notorouis for side loading and wearing the sides of the cylinder bore....because of the extreme rod angle, as the piston is coming up in the bore, the rod is trying to force the piston out of the side of the block, causing faster than normal wear, generating more friction and heat,never heard/seen/read any hard data that this will make more torque,have you??? I'm familiar with the term "Mechanical Advantage", but you can also get to a point with it where you start losing advantage instead of gaining........"Diminishing Return" I think is what it is called......

for comparison, the folowing rod/ratios

331 stroker------1.66
347 stroker------1.59
351-W-----------1.71
351-W long rod--1.88
400--------------1.65

and best I can figure for big blocks;

427----------1.72
460----------1.72

I'd love to see/read any published data you have come across about the short rod/extreme rod angle/more torque theory.....
I do enjoy the research almost as much as building and running engines...

David

The L/S ratio for 331s and 347s can be improved greatly my moving the pin up in the piston behind the oil ring, but we're talking custom pistons, aka, high price. You can get away with less than a 1.7 ratio, but I avoid anything under 1.7:1 for a racing engine. Not only is side loading greater, instaneous pistion velocity and forces on the pistons are greatly increased. Piston wear and failure are more of a threat that is cylinder-wall failure.

It seems the discussion is absent of head concerns. While there are now a bunch of SBF heads available at one time a long rod helped compensate for poor head flow. I'm curious as to how that plays out now with the better heads. I think any suggestion that it is a waste of time is pretty arbitrary. I hope he does the build. The dynamics of the engine are related and disparaging comments are mostly based on "beliefs" and little hands on experience. Been around long enough to see about every "myth" about HP/Torque proven erroneous.
Good luck, I'd still check Speedomotive. They are less than $1000 for the kit and could likely tell you what pistons they use. I mean as a prospective buyer, isn't that something you would want to know....:)?

I saw on TV or read where the NASCAR teams run two different rod lengths in the same engine, based on the track. I do not recall which did what, but they claimed one rod length gives them an advantage accellerating out of the corners on the short tracks. The other is better for the long straight aways. If true, they have to be able to measure a difference or they wouldn't bother.

I did do some reading in my engine spec books, in bed the other night. I calculated what wrist pin height would be needed for this combination. No factory piston will fit the bill. I calculated the pin height for several strokers and none matched up. The 331 came the closest. I cannot recall the numbers, but I think it was close enough to possibly work with some machining. I wouldn't trust any of this as I did the math in my head and then slept on it.

E = MC Squared.


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This article claims NO difference!! ??
 

http://www.chevyhiperformance.com/te...ips/index.html

I am getting a headache from this !!!!


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Most Chevy books give me a headache too:):)

Ford 400C rod..........................= 6.580"
Ford 351w crank (1/2 stroke......)= 1.750"
Piston for 331w stroker, pin height= 1.165"
Total stack height of combination.= 9.495"
Late model 351w deck height.......= 9.500"

As an aside to this I built a 200cu in straight six Falcon to compete alongside a 202 cu in Holden straight six many years ago. Holden was 3.25" stroke with a 5.25" rod ( 1.61/1 Rod Ratio), Falcon was 3.125 stroke with 6.25 rod (2.0/1) rod ratio.
Series tech guys insisted we initially run the same cam specs, with this the Falcon idled like a baby where the Holden had a distinct lopey idle...just as the article you posted suggests- with the long rod the piston is 'parked' @ TDC during the cam overlap phase that the scavenge effect is virtually killed off at low RPM, now while some might say this was a negative it worked fine in this car & it was competitive from day one...later I plotted out the piston dwell @ TDC versus valve overlap & transferred this on to a 'new' cam profile to suit the Falcon. The Holden guys were not amused:):)
I should point out this was the 'old' cast in head inlet manifold Falcon engine, not the later seperate intake setup.

So, in all the long rod 351 should idle more smoothly than its short rod version given the same cam /head etc, but have a bit extra at the top end.

Not only running different rod lengths for different tracks, they also use different bore/strokes combinations for different tracks as well, all staying within the 358 cu.in. limit........
Short tracks,bigger bore,shorter stroke for quick accelaration off the corners,were the rpm range is greater: long tracks, smaller bore, longer stroke were the rpm range is smaller......

Before the compression rule (I think it is 12 to 1 now), Yates Racing was running engines at Talladega/Daytona in the 17 to 1 compression ratio!!!!!!!!!!!!!!!!

David