Lets talk rod to stroke ratio's...

[gt350hr]

It makes almost no difference.  Smokey Yunick's book is the most misunderstood and misquoted book after the bible.

I have read his deal on rod length and it only applies to a very specific situation.  And even then it scratching for the last available hp, as in a high dollar racing scene.

I would worry more about piston heights and weights, due to rod length.

JMO,

paulie

     +1  I cornered Smokey ( rip) at the PRI show ( in an elevator) and asked him how he could make a statement like "use the longest possible rod'' and he replied that was taken out of context and pertained to a specific non ported SBC head /bore/stroke/ class combination. HE said rod length can make or break a combination , and about that time we were at his floor.
   Randy

[Falcon67]

I'd add to plovett's comment that for 99.9% of us, it makes no difference.  Bench racing, racing rod rations, etc.  Light weight piston makes RPM easier on the rod.  Other than that, use what fits IMHO. 

[frnkeore]

I like and respect Smokey, a lot (may he RIP), he even worked with Fords a little and was a true innovator but, he did say, in one of his books, that he thought that the reason you could get more HP from a higher rod/stroke ratio, was because there was a longer dwell time, at TDC, building more pressure, for the down stroke.

Personally, I think, if there is more HP for the longer rod, it's because of less pressure on the piston to cylinder wall contact and less friction.

I know the rod length to stroke ratio has been around since, at least the '20's (probably before the turn of the 19th century) but, I look at it as a maximum angle.

A 317 Linc (3.5/7.062) has a 2.018/1 ratio, the angle is 14.35 degree, Jays 4.75/ 6.625 combo 1.395/1 and that angle is 21 deg.

What I look at for longevity is what NASCAR runs and that's  16.5 - 17 deg (9,300 rpm/500 mi) and for what will reliably work, I look a sprint cars 19.5 deg. and try to keep in those areas, striving for the lower angle and longer rod. Also, you can't deny that the sprint cars basic 1.015 C/H works on bores over 4".

Because what we talk about, is very subjective and opinion based mostly, I would like to see some dyno tests based on those perimeters, the FE would be a good test bed for it but, that old 317/368 Y block Lincoln with it's 10.94 deck height could yield  more info but, at a much higher price.

[gt350hr]

  Power from dwell time is ridiculous . A piston that is not in motion is doing anything . If an increased burn time of a couple of nano seconds increases HP I've never seen any proof of it. Rod length in some engines can make significant changes. Being in the aftermarket performance piston industry for 20+ years has let me see thousands of different stroke / rod / head combinations and the cams that go with them. I have made rod length suggestions that "woke up" some combinations like alcohol sprint cars with SBCs and All Pro 13* heads. Rod length changes will move the torque curve around .
The FE is one where rod length doesn't make much difference except maybe in a tunnel port or cammer.
   Randy

[Joe-JDC]

The long rod does change timing requirements on engines that don't have a very modern combustion chamber.  The better the combustion chamber, the less difference it makes.  I have dyno tested different engines with same crankshafts, heads, but different rods and pistons, and found that timing can change as much as 4* for optimum power and torque.  I have also tested the same shortblock with as many as four different sets of heads, and found the timing needed was the same with the same rods and pistons with different compression.  Personally, I feel the rod length has more to do with timing than folks can imagine because they never really tested back to back changes.  When you go from a 6.300" rod to 6.750" rod in the same engine, you will find things that you never thought of and some real head scratching for answers.  The more I think I know, the more I get surprised in real back to back testing.  Joe-JDC

[frnkeore]

Joe, do you have the data from those dyno tests? I'd love to see how it changes things.

[plovett]

The long rod does change timing requirements on engines that don't have a very modern combustion chamber.  The better the combustion chamber, the less difference it makes.  I have dyno tested different engines with same crankshafts, heads, but different rods and pistons, and found that timing can change as much as 4* for optimum power and torque.  I have also tested the same shortblock with as many as four different sets of heads, and found the timing needed was the same with the same rods and pistons with different compression.  Personally, I feel the rod length has more to do with timing than folks can imagine because they never really tested back to back changes.  When you go from a 6.300" rod to 6.750" rod in the same engine, you will find things that you never thought of and some real head scratching for answers.  The more I think I know, the more I get surprised in real back to back testing.  Joe-JDC

In my opinion, that is one of the most pertinent posts regarding rod/stroke ratios that I have ever seen.  Ignition timing seems like it would be affected by minute changes in piston velocity/position much more than other variables.  I have never been into the rod/ratio argument, but ignition timing could measurably be affected by it.

JMO,

paulie

[My427stang]

The long rod does change timing requirements on engines that don't have a very modern combustion chamber.  The better the combustion chamber, the less difference it makes.  I have dyno tested different engines with same crankshafts, heads, but different rods and pistons, and found that timing can change as much as 4* for optimum power and torque.  I have also tested the same shortblock with as many as four different sets of heads, and found the timing needed was the same with the same rods and pistons with different compression.  Personally, I feel the rod length has more to do with timing than folks can imagine because they never really tested back to back changes.  When you go from a 6.300" rod to 6.750" rod in the same engine, you will find things that you never thought of and some real head scratching for answers.  The more I think I know, the more I get surprised in real back to back testing.  Joe-JDC

Joe, can we assume longer rod needed less total in your tests? 

[Joe-JDC]

Yes, less lead time.  The events happen quicker with the shorter rod, requiring more lead with older combustion chambers.  Joe-JDC

[My427stang]

Yes, less lead time.  The events happen quicker with the shorter rod, requiring more lead with older combustion chambers.  Joe-JDC

Makes perfect sense, increased dwell time at TDC with the longer rod means the flame needs to travel across a smaller 3 dimensional chamber compared to a shorter rod that is dropping slightly quicker.  Thanks

[Drew Pojedinec]

Funny, I was just about to ask:
“With better fast burn chambers, wouldn’t dwell time matter less?”

Thanks for answering that before I asked it Joe.

[frnkeore]

I have dyno tested different engines with same crankshafts, heads, but different rods and pistons, and found that timing can change as much as 4* for optimum power and torque.  I have also tested the same shortblock with as many as four different sets of heads, and found the timing needed was the same with the same rods and pistons with different compression.  When you go from a 6.300" rod to 6.750" rod in the same engine, you will find things that you never thought of and some real head scratching for answers.  The more I think I know, the more I get surprised in real back to back testing.  Joe-JDC

With the timing change did the torque change in those applications and what I think everyone wants to know (at least me), did the longer rod, increase torque?

[Joe-JDC]

I will have to see if I can find the dyno sheets/graphs, but if memory serves me correctly, there was a shift in torque peak.  A manifold change, or spacer change, or camshaft degree change, header collector change, will also affect the torque peak or torque averages.  It then becomes a tuning quest to find the best of those changes for the final combination that repeats pull after pull.  Joe-JDC

[frnkeore]

Thank you, Joe.

Regarding the "shift" was it in ft/lb or rpm?

[Joe-JDC]

Ok, a couple of points to consider.  I went back and asked my friend Ted Eaton to confirm my observations.  Ted has his own dyno facility, and balances engines, builds FEs, and has raced FEs almost as long as I have.  He has a 64 Thunderbolt Clone High Riser FE, 66Fairlane with FEs, and has years of dyno experience.  He said that the longer rod does in fact require less timing, which helps with lessening detonation issues, and the shorter rods actually improve torque slightly since the rod moves the crankshaft off TDC quicker with the initial combustion explosion.  It is difficult to quantify since very few back to back dyno tests have been done for this actual comparison.  The long rod does help with reducing side loading of the piston skirts, and helps with ring longevity due to detonation reduction issues.  He will try to check his dyno sheets to see what it did with the torque curves when he gets some free time. 

What most folks forget is that torque is a measure of work performed, and it can be manipulated with camshaft degreeing, intake manifold changes, carburetor changes, carb spacers, timing changes, valve lash changes, and header tubing, length, primary tube size, tri-Y, four into one, collector size, merge size, and collector extensions, and probably more that I can't  think of right now.  It is not purely a function of cubic inches, or stroke, or rod length, or compression.  If it were, these other modifications would not affect torque, but they do.  Joe-JDC