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CC Advertisers
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05-13-2010, 10:25 AM
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CC Member
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Join Date: Sep 2002
Location: Hickory,
NC
Cobra Make, Engine: ERA 427SC w/427so, ERA GT #2002
Posts: 1,106
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Not Ranked
I just took delivery of my newly published book How to Build SB Ford Racing Engines where I discuss among many things rod length-to-stroke ratio. This ratio is best for racing engines at no less than 1.7:1.
__________________
Tom
"If you can make black marks on a straight from the time you turn out of a corner until the braking point of the next turn, then you have enough HORSEPOWER." Mark Donohue
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05-13-2010, 10:30 AM
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CC Member
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Join Date: May 2000
Location: Northern VA,
VA
Cobra Make, Engine: Classic Roadsters
Posts: 2,765
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Not Ranked
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
.
__________________
LIFE IS TOO SHORT TO WORRY ABOUT GOOD GAS MILEAGE
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Utinam logica falsa tuam philosophiam totam suffodiant!
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Last edited by CobraEd; 05-13-2010 at 10:36 AM..
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05-13-2010, 11:56 AM
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Senior Club Cobra Member
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Join Date: Jan 1999
Location: MARKSVILLE,LA.,,
Posts: 3,235
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Not Ranked
Quote:
Originally Posted by CobraEd
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
Quote:
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I just took delivery of my newly published book How to Build SB Ford Racing Engines where I discuss among many things rod length-to-stroke ratio. This ratio is best for racing engines at no less than 1.7:1.
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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
__________________
DAVID GAGNARD
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05-13-2010, 12:40 PM
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CC Member
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Join Date: Sep 2002
Location: Hickory,
NC
Cobra Make, Engine: ERA 427SC w/427so, ERA GT #2002
Posts: 1,106
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Not Ranked
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.
__________________
Tom
"If you can make black marks on a straight from the time you turn out of a corner until the braking point of the next turn, then you have enough HORSEPOWER." Mark Donohue
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05-13-2010, 01:20 PM
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CC Member
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Join Date: Jun 2003
Location: Freedomia,,
Il
Cobra Make, Engine: Coupe,Blue w/white stripes SB; Roadster, Blue w/white stripes BB w/2-4s; SPF installer/Hot Rod-Custom Car builder
Posts: 1,376
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Not Ranked
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....  ?
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WDZ
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