340 Stroker Rotating Assembly
I'm curious what benifit a 6.125 rod brings to a 4" crank other than a shelf piston? Looking at moving up to a 4" stroke, I was still looking to run a 6.200- 6.300 rod to get a shorter piston, less wight and less friction.
The thing is guys get wound up in this. And what Smokey said 50 years ago is still true.
Use the longest rod you can fit. Is that a hard and fast rule? Nope. We live in a Chevy world and engine building has been infected by that.
If you were building a 340 stroke engine I’d tell you don’t make the rods any longer. It’s already at 1.8x ratio.
when going to these long stroke cranks you can’t get the rod too long. Even with a 6.4 rod you are still a 1.69 ratio.
Regardless of what the web and YouTube says, rod to stroke ratio matters.
Is there a perfect ratio? Nope. But that ratio affects cam timing. A bit more than most might think.
As an example, as the rod to stroke ratio gets lower, the spread between intake and exhaust lobe duration gets bigger. As stroke length goes up compared to bore diameter, that spread goes up.
Cam timing SHOULD reflect those changes.
As an example, if someone thinks a 340 Chrysler takes the same lobes as a 350 Chevy I’d say they need to look at the math they are using.
I have a 3.79 crank in the shop. Two of them. I looked at the 6.4 inch rods. That’s a 1.6887 ratio. That’s pretty close to what Pro Stock currently runs. I know none of us have that much induction. IMO that’s a pretty low ratio.
A 6.125/4 inch crank is 1.53 so not only is the cam timing requirement different due to displacement but also due to rod ratio.
It all matters to varying degrees but it all matters. I don’t know what you have for induction but on cylinder head limited stuff I don’t like a 4 inch stroke. That’s .240 longer than a 427 Chevy with a significantly smaller bore.
If you like inverted power and torque curves (I don’t like it) then run a longer stroke. And by that I mean it makes the same or more torque than power. I know some guys love to touch the throttle and blow the tires off. I’m not into that. It makes people think they make more power than they do.