Very good illustration of Rod Ratio theory

FWIW using an extreme 1.5:1 vs 3:1 , this video is very well done

Sorry if it's been posted before

https://www.youtube.com/watch?v=C_YNn3ZkJmU&t=866s

What is interesting is how a short rod 'dwells' lest (relative to the same stroke) at TDC but more at BDC which with a long stroke partially explains why longer durations and earlier ICLs (vs a smaller cam in later with the same exhaust closing event) can be beneficial when you have a really good (or large) intake port. It also helps explain why under ported motors (like a 906 headed 440 or a 455 olds) benefit from a longer rod because the piston squeezes a bit longer at higher RPM/Piston speed.That helps the motor "hold on" near peak torque a bit longer on the way to the HP peak.

I love engine theory! This is a very well done video and his accent makes him sound much smarter also!

It explains very well the differences and yes you can see why cam timing needs vary a little where one works better at higher RPM and one better at low RPM... I would like to find some dyno tests of very dramatically different engines but I have only ever seen slightly different combos and of course the improvements are only very slight also and hard to infer much from the results. 1/4 inch either way seems to make very little difference but an inch or two different would not be practical in most if any engine. Bottom line I wouldn't worry much about it as long as I am within generally acceptable norms.

Engine masters had a long vs short rod ratio episode. Same displacement engine. I recall the difference being almost indistinguishable.

it would be cool to see a 7.100 rod RB 511 vs a 6.535 Rod B motor 511 with the same heads, CR, Cam and induction/exhaust. That's probably as extreme a RR difference we would see.

The easiest way to compare is to centerline the cam at the same degree and measure how far down the bore the piston is relative to the cam lift. There are programs that calculate the effective displacement (the stroke at the point of the intake closing.

It's just always been interesting to me in terms of tuning the powerband. And with a roller cam it likely make less difference than with a flat tappet,

Some interesting comments re rod ration wrt NHRA Pro Stock engines in this article HERE

I only worry about rod stroke ratio from the standpoint of reducing piston speed, and increasing the durability of the engine by reducing the piston speed. I have seen a lot of testing that proves to me that rod stroke ratio does not affect performance that much. It does however affect the life of an engine.

https://rehermorrison.com/tech-talk-10-by-the-book/

I also highly recommend Reher Morrison engine building book https://rehermorrison.com/engine-book/

Yes I know these are Chevy guys, but an engine is an engine.

I remember David's comment and essentially I agree, but as an engine builder/tuner I have learned (to a much lesser extent) that there are certain cam tuning specifics you can utilize based on the shortblock configuration and the headflow characteristics that can either enhance the powerband or, if poorly chosen, leave power "on the table". In that regard the rod length can come into play particularly with the intake closing event and maximizing piston velocity past top dead center.

And longevity (specifically side loading which obviously effects both friction and ring seal) is also a factor.

Yeah the interesting thing in that article is that the engine builder claims that even with Pro Stock heads they still need a long rod ratio since the heads just aren't good enough to support a short rod ratio. So basically he is saying that it is impossible to properly feed a short rod ratio engine since Pro Stock heads are as good as it gets.

Interesting read. Thank you

Exactly. A Pro Stock engine has a relatively short stroke compared to other engines. And they are induction limited. I get the short deck heights and such, but you still need some reasonable rod/stoke ratio. The further you get from “reasonable”, the more you have to compromise on cam timing and such.

I talk to a current Pro Stock engine builder a few times a year and we have talked about this and his sentiment I have heard echoed many times in high end drad racing engines. He said "we decide a bore and stroke to achieve 500 or less inches, we then design the piston we want in the engineand then use a rod that connects the two. The rod ratio is not a consideration"

He is oversimplifying it a bit since they can play with deck height. If they really wanted a specific rod ratio they could change the deck height. From what I've seen, they let the intake manifold design and cylinder head design dictate the deck height and then the rod ratio ends up being what it is. That is, rod ratio is very low on their priority list.

Dave Storlien (engine masters) told me that he can make more power in an NA engine with a short rod. Because the most important thing in making power is filling the cylinder. The quicker acceleration of the piston away from TDC on the intake stroke gets to sucking on the intake charge sooner, providing more momentum to the charge thru the remaining intake event. All subject to properly timing the intake valve opening to take advantage of it. And that the advantage of this improved cylinder filling surpasses the advantage of the longer TDC dwell on the power stroke of a long rod. Something to chew on...

After deciding the stroke. It's my understanding that the deck height more a function of getting the intake runners to the correct length. Not to mention the pushrods can be shorter with a lower deck, thus stiffer.
Doug

After thinking about this more it seems that with the piston moving faster before and after TDC that engine would like tighter lobe separation angles so the valve is open more while the piston is moving faster. A long rod engine is moving the piston faster at BDC so shifting the valves to be open more then would benefit that engine more all else being the same.

Most rod ratios are in a small range around 1.5:1 to 2.0:1
When you calculate piston position for each degree of rotation, the differences are pretty small.

i like to use longer than stock rods when I can, helps make those heavy b and RB pistons a lot lighter, light is right, correct

Point is in these engines as well as those in Comp etc rod ratio is NOT a consideration. It is what it is