Numerical comparison
 

Attached is a graph of the comparative wall loading as a percentage of the combustion force on the piston and the loading on the crankshaft at each degree of rotation. It represents a common 4.250” stroke comparing 6.385”_6.535”_and 6.700” center to center rod lengths.

As seen the crank loading is so close that the three curves are indistinguishable in the overlay. The wall loading variance moves from 0 to a peak of less than 3 percent variance at 90º, back to 0.
Mean piston speed of the three iterations is 2833.33 for all.

Max piston velocity is

6.385”_4690 FPM

6.535”_4680.3 FPM

6.700”_ 4669.66 FPM

That is a variance of .4 of 1% (4 tenths of 1%)

Rod angle

6.385”_19.44º

6.583”_18.98º

6.700”_18.49º

That is a variance of less than (.95) 1º degree.

Now an interesting trade off presents itself when weighing the downsides of raising your decks anywhere from .400” (10.2”) to 1.835” (11.635”)

Bob

Steve,
The 572”/804 HP presented by Brian is an outstanding 4.375”, short deck package that is fully capable of eclipsing the 850 mark. The heads are prepped by Tony Mamo (Mamoized) of AFR and fully capable of moving enough air to support your goals with more cam, vacuum and a revised induction package.

If you’re going 10.2” and 4.500”, Wette Vette has that covered.
598 CID
Dart 355 heads and tunnel ram by Darin Morgan
A true 10.82:1 compression ratio runs on pump gas 93 octane.
10” vacuum
34º total timing
150º water at 24 PSI
More to follow.

Bob

I'm just trying to learn here, would the graph look any different using the 4.75 crank in a 10.2 block ?

Gary,
The comparison is meant to illustrate the sum of change (or lack thereof) based on rod center to center dimensions in a given (stroke) engine displacement. I can plot any 3, what are we looking at? A 4.750” with what rod c to c? Better yet, we can start a new thread keeping the OP’s on track.

Bob

The side loading deal is a non issue. When I choose a long rod, it's usually to move the pin up the piston, which makes a more stable, and lighter piston, and also to keep the piston from being pulled out of the bottom of the bore, which does accelerate skirt wear. I mostly use a 4.375 stroke, and don't see any additional bore or skirt wear between that and say a 4.5" stroke combo with a 6.535, or 6.7 rod. When I get a chance I'll take some pics of the bottom of the pistons on a 4.750, 4.500, and 4.3750 stroke engine so you can see the difference in how far out of the bore the piston gets yanked with the various strokes.

Other benefits of a shorter stroke are: reduced windage, reduced reciprocating weight, reduced pumping losses, and less frictional losses. On the tall deck deals, I believe Bob could help explain the compromises in regard to line of sight on the induction side, bore stability, and valve train stability.
You're up Bob. :)

Just wondering how to calculate the side wall load difference between your 4.25 crank example and a 4.75 crank with 6.635, 6.535 rod. Thanks

all these years i thought that the stroke controled piston speed at a given rpm and also dwell time,it looks by the info bob posted that rod length makes almost 0 difference.

Excellent information. I know every builder has a few favorite combinations when it comes to bore/stroke/rod length. What is yours, as far as a marine endurance build?

Also, at what point does your liking of a raised pin height, become a worry with forced induction, or does it?

So what are you saying?

im saying that rod length has no real difference in piston speed from a measurable difference.a few feet per second when you are talking about thousands of feet per second is not going to matter,but i did learn something,and that is that rod length can change piston speed ever so slightly.