Big Joe - rod/stroke b.s.

Big Joe - "As long as the rod is 2.00 inches or more longer than the stroke, NOTHING GOOD will happen. The rod to stroke ratio is pretty much all BS."

Joe, to clarify, are you saying that the rod should be at least 2" longer than the stroke, or not more than 2"?

I've had some debates with some friends on this, and would love to have your thoughts.

From my observations, it has seemed to me that when you can run as high an octane level as you want, that a longer rod may have a little benefit.

But, with octane level limited (like in the Engine Masters competitions in Popular HotRodding), a shorter rod may help keep a motor out of detonation.

Thanks,
Lee Atkinson

That two inch thing is a real generalization. What I am really saying, the rod ratio is VERY UNIMPORTANT-- I used to build all kinds of differant rod lengths, and after several years, I could not remember which engine had which rod length,. They all seemed to run about the same. I remember back in 1990 ( Whatever ) I did a rod length buildup with Hot Rod magazine. The storter rod beat out the longer rod and boy did I ever get a lot of fan mail about that. Now days, I just put together what is easy to find and convient. I do use the longer as opposed to the shorter, maily because I like the lighter PISTONS-- I do lots of 383 Chevy engines, and one time I used the 5.70 rod because they were out of stock on the longer 6.0 rods, anyway, that engine was much WEAKER than any of the 383 Chevys I made with the 6 inch rods.. Something to think about. The rod length is important when you want to be able to rev the engine way up past the true HP peak (8000 and up )


JOE SHERMAN RACING ENGINES

Joe,

Thank you, sir!

Lee

the big benny of a longer rod in strokers is it keeps more piston up the bore at BDC , next is on restricted carb engines

Does keeping more piston up for longer cause the burn to occur for a longer duration thus effectively increasing the power of the burn? Is that the theory there?
Or is it all about the swing of the crank?

my speculations on why longer rods might hold a advantage:

* trivia: only engines over 400 cubes with rod/stroke of over 1.8:1 were the 426 mopar and 429 Ford...hmm, both had tons of potential, and were built for sitting at 7k all day in NASCAR, back when NASCAR was driving almost production vehicles. think there might be something to that coincidence?



longer rods reduce piston scuffing sideforces compared to shorter

longer rods increase amount of crank rotation at the few degrees top of stroke- possibly 'cushioning' compression speed slightly to possibly cut down on preignition a bit, and piston sits there a tad longer at TDC to build more pressure

softening acceleration at TDC/BDC would have to mean reduced force on things like wristpins- would likely mean increased durability att high rpm.

mostly though, pushing straight perpendicular to the crank at 90 degrees would be best- longer rods help with that...as a sidenote, read the police/CJ 429 had piston pins offset like 1/16 clockwise to further straighten the rod out on the power stroke...do todays engine builders do this 'all the time' now? I dunno, I just read




To be fair/ on the flipside: shorter rods approach tangential angles to the crank sooner(at higher cylinder pressures), so can build more torque(even if piston sideload far increased) so powerwise, long rod *might* hurt torque numbers at lower RPM.


not trying to start any arguements either way, but would like to hear counterpoints for thinking either way...always heard 'best is around 1.75:1' but never heard actually WHY, and moreso wondered why the 426/429 did so darn well with longer rods- something just tells me its not a coincidence, but would love to hear everyones thoughts either way

Tim

F 4V429 , got a few things backwards - one longer rods hit BTC for a shorter time . think of time , you only got so much , so things happen at different rates during the cycle .

Originally Posted by DanH

F 4V429 , got a few things backwards - one longer rods hit BTC for a shorter time . think of time , you only got so much , so things happen at different rates during the cycle .

just drew up to doublecheck...3inch stroke with 6 inch rod takes 18.84 degrees of crank to move .100

5 inch rod takes 18.47 degrees crank to drop .100

so at the same RPM the longer rod takes more rotation=more time at TDC/BDC= less shock to wristpins etc...the difference is less than I thought it would be, but still longer= softer unless the CAD is lying to me

edit...I'd checked top of stroke for above numbers- rechecked at bottom of stroke and different- Danh is correct, as he was talking BDC/I was thinking TDC(and I was assuming the crank angles were same top and bottom which they arent) the longer rod accelerates piston faster at the bdc reversal by quite a bit larger amount- 22.1 for the long rod, 22.9 degrees on the crank for the short rod, at the same .100 piston movement... looking at it again I see that up top the arc of the rod is opposite the arc of the crank, where at the bottom the arcs are on the same side of the circle- hadnt thought about that one...interesting...

Thanks for this stuff guy's, keep it up!

in the numbers crunching/bench racing just got to wondering someting else...

What if instead of using offset wristpin holes, the cylinder bore centerline were offset instead, allowing pins on center to minimize piston wear, yet still achieve straighter rod angles on the power stroke?

just wondering aloud thinking of all the aftermarket blocks out there if one was ever cast for offsetting the banks for centered pins...would be a tricky part to machine, and would probably take a funky intake manifold, but just wondering...seems like the mains could be shifted around and then adjust deck accordingly, only issues then would be valve notches possibly...and bellhousing would need enough material for the tranny being offset then...hmm, maybe not, but still- just for the sake of arguement...