The engine will make basically the same peak power with either the 3.79 or 4.00 crank but the power curve will be different. The 4.0 crank will move the torque peak down a little bit. PipeMax will give you enough numbers to make some decisions.

If you have an existing chassis then you might use that to make a decision. It can get expensive if you have to change gears, tires, transmission, etc. for the new torque peak.

For bracket racing the longer stroke is usually a good idea since it lowers the engine speed which allows the use of less expensive parts for the valvtrain.

The speeds quoted are, of course, averages, and not an accurate picture of port conditions even in the (theoretical) range of maximum piston-derived vacuum (somewhere near both the ICL and the highest piston velocity - typically in the mid-70s depending on the rod ratio).
With a strong exhaust pulse, the vacuum across the chamber during overlap is stronger than piston vacuum. The static conditions in the chamber between high and low compression (small vs. large chamber volume) and high and low rod ratio are fairly different. In relative terms, cross-chamber flow is "lazy" with a really long rod (like n=2) and low compression. This effect is almost harmless if the port is too small (vs. displacement), and very annoying if the port is too big. The reverse condition (short rod, high compression) is significantly less affected by this factor.

as soon as I get a chance to run all my numbers thru pipemax I will. maybe tomorrow. as of right now with what i'v researched looks like i'll be stayin with the 4" stroke and a 6.125 rod. seems long strokes with long rods make lazy pistons at TDC.

Not quite.
As the stroke is reduced, or the rod is lengthened, or both, piston motion around TDC slows down. Piston motion around BDC speeds up.

Longer stroke, shorter rod etc. = reverse.

yeah,but on intake stroke their lazy.

All this talk has me wondering how my combo would do. The heads are CNC 325 440-1 that flow 370 intake at .700 and the ex flow at each lift is pretty much constant at 83%. 4.500 bore, 4.25 stroke, 7.1 rods, 13.9 CR. Indy R2 cam. How does the rod length affect the power curve? I chose the longer rod for the reduced side load on the cyl walls.

don't have time to run your numbers right now,but looking at your combo, I would say you have a very strong running motor. long stroke and long rod slow the piston from TDC on intake stroke.

ok, what numbers should I pay attention to on the pipemax results to pick best stoke and rod length for my cylinder head?

Sorry, "yes and no" is simply wrong.
A longer stroke (with no other changes) always increases piston speed around TDC and decreases it around BDC.
There is no opinion on this, it's plane geometry, and it was settled 2,500 years ago by Euclid and Pythagoras.

So, when is the piston speed the same at TDC and BDC?
When the stroke is zero, or the rod length is infinite.

A 4.50" stroke with 7.10" rods (n=1.578) will have a velocity curve similar to a 4.25" stroke with stock 440 rods: faster than a stock 440 @ TDC, slower @ BDC.
Unless the heads are very good, this is the opposite of what you want to keep the VE as high as possible. Where practical, the rod ratio should go up when displacement is increased, regardless of how this is achieved.

For the stock 440 rod ratio with 4.50" stroke, the rod must be 8.11" long - which is why it's not done.

evidently my measurement set up must not be very accurate then, old dial indicator on the piston top? Should be correct down to a couple thou.

BTW the longer rod gives a few thou. less travel at BDC and TDC on the 4 inch motor, I can not try it on the short stroke motor because it is chrysler rods. The travel at (20 degrees either way) TDC and BDC is measuring the same but what do I know

I knew that Calculus would come in handy some day.

The TRUE FACT of the matter is that at TDC and BDC, for that instant where the piston is at the maximum distance either up or down, velocity is ZERO.

Acceleration is maximum at those points, and as those ancient Greeks would have figured out, eventually, the acceleration at TDC is greater than at BDC. Put another way, the piston displacement vs. time graph is not a perfect sine wave.

We're mostly worried about the piston action at TDC. That's where the power is made or the intake charge is trying to get in.

Because a longer rod causes lesser acceleration at the top of the stroke, the piston spends more degrees very close to TDC with a long rod than a short one.

For years and years the prevailing theory was to lengthen the rod to get that "perfect" ratio of rod to stroke, 1.8 was one number thrown around. And at 9500 rpm, the theory seems to work, because that's how NASCAR engines are built, and they'd kill their Grandma for 10 extra hp. Rod lengths of around 6.2 to 6.3, combined with strokes around 3.3, compute out to roughly 1.9. So it works for them.

But we're for the most part not talking about building NASCAR engines.

One of the most interesting things about the Enginemasters competition is that the winners for the last several years have had REALLY Short R/S ratios. This year the winning engine and the runner up's stroke was something like 4.7, rod length 6.658 for R/S of 1.4 or so. This is a hp/cubic inch competition, averaged over an rpm range picked to more closely represent street cars, so if there were huge disadvantages with extra side loading on the pistons, you'd think they wouldn't be doing something as "stupid". A few years ago Jon Kaase built a short rod ford 400 and won. This is in a block with deck height of 10.29 or so inches. He used 4" stroke and 6", IIRC, rods. R/S of 1.5.

Also a few years ago, it was reported that he had built a mountain motor with extremely long rods to see if it would work better and he found no power advantage over his normal R/S of 1.38.

I have to feel that the advantage is in the quicker piston acceleration away from TDC. Heads are flowing better and better. Maybe the quicker acceleration away from TDC gets the mixture flowing faster sooner for more cylinder filling. That's my best explanation at this point.

I have officially stepped away from the "long rod" religion.

R.

I wonder if the much shorter rods results in a lower bob weight? I get what you're all saying about helping the heads flow, but if Kaase's short rod motors also benefited from less rotating mass it needs to be factored in when considering what that configuration does for hp potential.

Port velocity may drop off, and even reverse, without actually tracking piston motion.

The entire rod ratio thingy is a conundrum. The difference in piston motion and position among all the rod ratios you would reasonably use in an engine (from roughly n=1.4 to 2.1) is almost imperceptible, yet the results are not merely obvious but wildly disproportionate to any calculations.
How can such a small change give such drastic results? Yet, they've been proven by tests for many decades.
Example: Harley-Davidson significantly increased the rod length (but not the stroke) in 1937 to improve peak power in a low RPM, low compression engine with small ports and mild cam.

There is a similar and converse effect @ BDC: a short rod covers a longer piston travel path (in inches) during the same amount of crank rotation (in degrees).
What this does it capture a larger percentage of the full stroke length with the same intake valve closing point. This (not overlap, or LSA, or duration) is what changes DCR with rod ratio changes: shorter rod, longer stroke, or both always has slightly higher CCP.

ok, good info guys. it looks like the rod length and not so much the stroke is what we need to be concerned with to keep good port velocity?
when its stated short rod long rod, what are you considering short and long?
I use 6.123 sb rod. would these be considered long or short? thanks

The effect on piston motion is exactly the same for identical changes in rod length and stroke length measured as percent.
However, rod length has no direct effect on gas speed - stroke does.
For many engines, maximum rod length is limited by deck height - you can't get your rod ratio back unless you use a smaller stroke increase.

Get the idea - there are no simple answers?

yes,just when I thought I was understanding. ok both stroke and rod length can effect port velocity.
can you tell me what rod length would work best in a sb with 4" stroke and using an indy 360-1 head with 2.87 csa@13.5:1 compression?