So don't build a stroked big block, the rod ratio is bad.

Its been a while since I study up on this(15+yrs) but if I remember right longer rods are more proun to detonation or pre-ignition then shorter rods. Keep that in mind as well.

Yes, and that is why alot of the builders in Engine Masters go with shorter rods if they can. More compression, less chance for detonation, and a little more torque. Remember, one off builds mainly for best average power from 3k-6500,

In another thread, I mentioned how longer stroke smaller bore, needs more camshaft, than shorter stroke, larger bore, even though they may net the same cubic inch. In otherwords, say we are comparing a 4.560 bore x 4.375 stroke to a 4.5 stroke, 4.5 bore. Both measure out to about 572ci .

It has to do with piston speed. . At 6000 engine RPM, the 4.375 stroke, will have a piston speed of 4375FPM. The 4.5 stroke, will have a piston speed of 4500 FPM.

As Darin morgans' quote mentioned, regard the rod length, there could be a situation where the cam timing may need to be changed, as a result of the rod length. its all relative to what the piston is doing. Valve timing, igntion timing, intake flow, head flow, and so on. The piston movement is what sets the rules kinda.

Then there is the whole piston speed concept in regards to reliability. If you're building a long stroke BBC, and want to turn some RPM, you want light pistons, very good rods, rod bolts, main caps, piston pins, and so on. Why? again because of piston speed. Forget rod length, that has very very little to do with this. Stroke, and bore (big piston) is what can limit your reliability/rpm potential. This is why short stroke engines have always done better with high rpm.

Piston speeds at 6000 RPM with a BBC
3.76=3760
4.00=4000
4.25=4250
4.375=4375
4.50=4500
4.625=4625

ADK61 mentioned a 522ci, 4 inch stroke combo. Good example. You can rev that 522ci, to around 7000 RPM, and have about the same piston speed, as a 4.625 stroke engine at 6000rpm. Switch to a 3.76 stroke, around now 7400 RPM for same piston speed.

Camshaft wise, they would both theoretically need the same cam. The cam that will support a 604ci turning 6000rpm duration wise, is the same cam, that will support a 522ci turning 7000RPM.

A 454 turning 6000rpm, can get away with alot less high doller parts in the bottom end, than a 632ci turning 6000rpm, before chit breaks. For years, guys used to run the old GM dimple rods, and heavy trw pistons, and rev the chit out of them in 427's and 454's. Stock blocks, ductile caps, sometimes even 2 bolt mains, etc. Strap in a 4.5 stroke crank, some heavy pistons, stock rods, in a 2 bolt production block, and try spinning that to 7000 plus. The oil pan will likely get reshaped in a hurry. Kinda why I don't believe in HP ratings for connecting rods, or even cranks. Theres way more too it.

Joe the piston speeds you mention are average piston speeds that are based only off of stroke length and RPM. Rod/stroke ratio details the acceleration profile the piston actually sees from tdc to bdc at a given RPM.

To cut to the chase if your builder is well established take his advice. I don't think you listed what your combo is??? That might help as others here may have same build with either good or bad results and would likely chime in.

If this is in regards to your 489 it's a no brainier. 4.250/6.385.

Agree, but what kind of numbers are we talking about? Sure, you can compare a F1 car R/S ratio, to a big block chevy, and see substantial changes in piston velocity, but what about, a 1.53 R/S vs say, a 1.49, or 1.46 R/S ratio in a big block chevy?

At peak piston speed, say in a 4.5 stroke, with a 6.535, at 6000rpm, is 7,478 FT PM. Change to a 6.700 rod, and peak piston speed, goes down to 7459. Now change that to a 4.375 stroke, with 6.535 rod. Peak piston speed 7250 with a 6.535 rod, and 7232 ft per min with the 6.700 rod. The 1/8" of stroke changed peak piston speed, by 228 FT per minute. The rod ratio change, changed it by 19 FT per min.

Three variables affect piston movement: Crank Angle, Stroke, and Rod Length.

Four variables affect piston velocity (and acceleration): angular velocity, Stroke, and Rod Length.

The mean speed of the piston is determined by RPM and stroke.

Mean Piston Speed = 2* stroke * RPM/12 (ft/minute)

This number is simply the calculated average speed of the piston. If the piston(s) were moving in 1 direction (or in a perfect circle) at one speed this would be that speed (circular velocity). This simple calculation can be helpful in quickly comparing average piston speeds and the affects of stroke on piston velocity. Unfortunately this calculation is often misused by the uniformed and used as proof that Rod Length (Rod Stroke Ratio) does not affect piston speed. We know this is not the case.

To understand the motion of a piston it is helpful to visualize the actually problem. Below we have a diagram of a piston, rod, and crank.

P is the y axis position of the piston (blue line)
L is the rod length (black line)
R is ½ the engine stroke (yellow)
q is the crank angle
a is rod angularity




Figure 1: Piston, Rod, & Crank Diagram.



We know the stroke and rod length so for any given crank angle we can determine the piston position using the law of cosines.

P2+R2-2 RP cos(q) = L2 (Law of Cosines)

So P = R cos(q) + (L2 - R2 sin (q)2) 1/2

To find velocity and acceleration of the piston we need to convert the crank angle (q) to angular velocity (w) and time (t).

q = wt
w = 2p(RPM) (radians/minute)

for simplicity sake RPM = U

w = 2pU (radians/minute)

This gives us an equation for position as:

P(t) = R cos(2pUt) + (L2 - R2 sin (2pUt )2) 1/2

To determine the position at q degrees (p) insert a time of 60s/((U)(360/q))

To determine velocity and acceleration we need to derive the position formula.

V= d/(dt)(Position)
A= d/(dt)(Velocity)


V(t) = -R sin(2pu)pu - 2 R2sin(2pU)cos(2pU)pU
(L2 - R2 sin (2pUt )2) 1/2

A(t) = -4 sin(2pu)p2u2 - 4 R4sin(2pU)2cos(2pU)2p2U2 - 4 R2cos(2pU)p2U2 + 4 R2sin(2pU)p2U2
(L2-R2sin(2pUt)2) 3/2 (L2-R2sin(2pUt)2)1/2 (L2-R2sin(2pUt)2) 1/2



OK, so what does this calculus mean?

It means BOTH stroke and rod length affect velocity and acceleration of the piston.

If you increase stroke, you increase piston velocity and acceleration.
If you reduce rod length, you increase piston velocity and acceleration

Rod to bore angles.

4.5 stroke

6.700 = 19.622*
6.535 = 20.139*
6.385 = 20.633*

4.375 Stroke

6.700 = 19.056*
6.535 = 19.556*
6.385 = 20.385*

4" stroke.
6.135 =19.026* (stock 454/502)
6.385 = 18.254
6.535 = 17.821
6.700 = 17.368

From Flatlander racing, The piston rod diagram didn't paste over.