From discussions I've read by professional high performance engine builders they pretty much feel the rod ratio is one of the least important considerations when designing an engine. A lot of factory performance engines have ratios that are "theoretically" not good. What they worry about more is having to use a short-skirted piston that will rock and not be stable in the cylinder which will cause the rings to not seal properly.
i have a long rod 302 that i run now,went that route to make up for the crappy e7's i had but with the gt40's i am not sure how much it helps.seemed to run better on the e7's with the longer rod though.that and now i just need pistons and crank if i decide to go for a 347.
Given two IDENTICAL engines except for rod length, and there is a major difference in rod length, The Long rod will produce more torque. This is because the angle of the rod allows the piston to put more of the force to the crankshaft and less to the cylinder wall but also because the amount of force transmitted to the crank is relative to the tangent of the angle involved. If you are pushing a wagon with the steering arm (backwards) the closer to level with the ground you are pushing the less force it takes to move the wagon. If you are pushing straight down on the arm then all you are doing is pushing the wagon into the ground and it doesn't move. (Tan 90 = infinity so all force is lost) At 45* the Tan =1 so 1 part goes to push it down and 1 part goes to moving the wagon. At 0* Tan=0 so all the force goes to moving the wagon. This is a very simplified example that totally disregards the losses involved but it shows why a long rod is better than a short rod with the same displacement and configuration. If you have massive heads and all the best stuff on the engine the long rod will still produce more torque. Torque times RPM =HP so you get more HP with the long rod too.
That's the reason it sticks in my head. Generally Smokey was a genius with this stuff, and he strived for as close to 2:1 rod ratio as possible, but I doubt he let that override stroke and piston concerns.
The way I like to put the relation between torque and HP is: Torque is how much muscle you have, HP is how fast you can use your strength. (Thinking in terms of comparing your engine to an athelete.) This is the simpliest way I have heard it put, and I may have even picked this up from Smokey... Using this you can easily see how a diesel is so very strong, but doesn't have a great deal of HP. It has tons of 'muscle', but can't use it super fast. It also shows how you can have a 500 HP rice burner, but with only 120# of torque. It isn't very strong, but it can use what little it has very quickly.
Check this out! Boy does this tech tip tie into this thread really well. That is a great link there. I am still trying to wrap my brain around this one!
strange i had a local track guy tell me his dragger 302 ran better with the pistons backwards, i think i understand how that makes the piston sit higher in the cylinder like it was on a longer rod?
I took it as the piston pin was offset to the inside or outside of the block a tiny amount to lessen piston rock in the bore when cold. Therefore if you turn it around, the piston would have the offset in the other direction, which then reduces rod angularity, but allows the cold piston to rock even more, which is generally harmless. (annoying)
Weight Counts Too Hi All, I see many reasons based on physics brought forth here regarding the superior power making capabilities of the long rod engine. And, yet throughout NHRA Competition Eliminator in the small block world the winning trend is towards way "oversquare" engines running the absolutely minimum weight rotating assemblies. The long rod advocates assume breathing restrictions in their thinking whether they mention it or not. On the contrary, breathing restrictions rarely dictate peak RPM limitations in today's top power producing small blocks. Mechanical integrity of the system seems to be the limitation. Take a Dart 4.125" bore Boss 302 block and destroke the engine to 2.75 or 2.625 inches using a Bryant or Winberg ultra-light crank with appropriate pistons and you are spinning unbelievable rpms. By the way, use Jesel valve gear as well. This stuff costs a fortune, but stays together when lesser engines with very decent parts will come apart. For better or worse, the era of the thirty-five thousand dollar plus N/A small block is upon us in drag racing. And that dollar figure may be a tad low. Best Regards, David
