[dda72]

I run 4.5 stroke with a rod shorter than 6.7" at 8000 RPM.

[DINO1]

My belife is keeping the piston balanced on the pin is more important than worrying about side loading in a long stroke motor. You will always have side loading, but controlling the side load is more important. the top of the piston above the pin is much heavier than the lower part below the pin. So moving the pin upwards is going to balance it out better. My vote goes for the longer rod, keeps the piston in the bore at the bottom of the stroke, therefore keeping it supported. This is resiprocating mass, it is almost gyroscopic, so balance is very important. Letting to much of the piston skirt out of the hole at the bottom of the stroke is going to let things get all out of whack, therefore scuffing the skirt. Spark plug location, flame travel and quench need to also be factored in. If the initial hit from the explosion at TDC is not exactly centered it will cock the piston right off the bat. so balance is all the more important, if not counter balance towards the top of the piston.

[BruceWilkie]

Piston moves the distance of the stroke regardless. Rod length and piston pin placement only needs to be enough to keep piston from hitting the crank on the bottom and cylinder head on the top. Ring placement needs to be factored in as well. A tenth or 2 of rod length doesnt matter as much as good ring placement and clearance.

Pistons do not need to "balance" on the pin. (Factory offsets the pins for "sound" control.) How do you balance at 45 degree angle?? Centrifugal force, gravity, friction and reciprocating movement cocks the pistons regardless. Piston rings, wall clearance and skirt design, even oil, help keep the pistons from fluttering. Most pistons are top heavy.
Offset dish... valve notches... offset domes... balance on the pin like a gyro?... at 45 degree angle too?... wth????? How does that work?

Few car engines have a dead center spark and fewer than one have dead even combustion load across the chamber.
(even engines designed for that theory have no guarantee it actually works that way)

Hate to criticize someones beliefs but just too many things that just aint going to happen or just dont matter.

[Torment]

Originally Posted by BruceWilkie

Piston moves the distance of the stroke regardless. Rod length and piston pin placement only needs to be enough to keep piston from hitting the crank on the bottom and cylinder head on the top. Ring placement needs to be factored in as well. A tenth or 2 of rod length doesnt matter as much as good ring placement and clearance.

X2!

[lreGTO65]

If the piston is rocking enough at the top of the cylinder to be detrimental to the engine then something else is wrong.. Detonation knocked the skirts in, the piston to wall clearance was too great.

We typically try and run the longest rod we can..Some of the "kits" I have seen the rod and piston combination seem less than stellar in my opinion. I am doing a 4.500 stroke engine right now and am working with Diamond to come up with a good piston/pin/ring package together for it.

[goquick]

If it is a street/race engine, put a 6.625' or 6.635"rod in it. If it is to be a high mile street car, put a 6.7 rod in it. Ideal rod for it would be a 6.5" if a race engine.

[DINO1]

Long rod is good, keeps the side load off the piston skirt. Piston to rod stroke is more balanced. The more piston skirt below the pin will make the piston more stable in the bore. JUST SAYING THIS IN LAYMANS TERMS.

[1qikta]

Originally Posted by Cliff R

After watching Jay Delaigle's "professionally" assembled short block wear itself out in short order, I would run a shorter rod with more piston skirt and a lower pin location. Jay's engine was the first 4.5" stroked Pontiac engine I actually had some association with. The high pin location and long rod deal skuffed the cylinder walls up pretty hard. It also didn't stay sealed up very long.

Personally, after building a few SBC 400's with 5.56" rods and 3.75" strokes, and watching them make fantastic power without the first issue anyplace in the piston/cylinder wall/ ring pack area, I lost interest in trying to get long rods into any of these engines.

We read about "side loading", and other issues associtated with using short rods. After having one of these 400's last 10 seasons of drag racing and it was in perfect shape when we tore it down for inspection, I'm not afraid of using short rods in a Pontiac engine with the longer stroke crankshafts.

Shorter rods also reduce reciprocating and spinning weight. Whether they bring any high rpm limitations into the "recipe" seems to be a matter of opinion, rather than fact. I've also seen folks run the piss out of BBC's with 4" strokes and 6.135" rods for years, without the first hint of a problem in that area.......Cliff

This was the beginning of the end. After only a hand full of passes and about 3 mos from the build.

http://www.youtube.com/watch?v=jf1t80K3fh0