Rod Ratio - what is "good"?

[defiance]

So I'm still working on my "desktop rebuild" (you know, the kind you do over and over on your desk on a piece of paper, but it never actually gets to the engine...    Although I did get a new block (my old one was going to need .055 over, the new one has never been bored, so I'll be able to get by with 30!  yippee   Plus, I should actually be able to order the stroker kit for it on December 4th (extra paycheck that month )

Anyway, though, so I'm continuing to read up while I'm waiting, and I keep seeing mention of "good" vs. "bad" rod ratios.  I know what a rod ratio IS, that's easy - but what does it MEAN?  Did some quick searches, and it looks like maybe a higher rod ratio has the piston accellerate more slowly at the bottom and top end, reducing stress...?  I see there are also discussions about side loading...?  What's the bottom line?  Also, when it really comes down to it, what's a "good" rod ratio, and what's a "bad" one??

[Challenger340]

Different ratios = different characteristics. 

This could turn into a really LONG thread ! There really isn't a "bottom line", way too many factors to consider.

Historically, for rpm, High ratios 1.8 and up, are considered great revvers, while low ratios 1.5 and down are considered disastrous !

But, that said, it's dependant not just on Ratio as it is a myriad of factors; wall thickness, head flow, bobweight, etc., etc. on any Rod ratio "in between ".

What specifically are you looking at, and considering build-wise ?

[defiance]

So in short, for the most part. higher ratio = better?

I'm considering a 440source stroker kit.  Originally I'd been considering a 500 (496) kit, but as I started looking over the specs, I noticed that the 512 kit actually had a higher rod ratio because it uses 7.1" bbc rods.  So with that in mind, I really couldn't think of a reason *not* to go with the 512...?

If it matters to rod ratio, the rest of the engine as planned - '69 block - 30 over - victor heads - mild porting & valve job - block decked to 0 with pistons - 24cc dished for 10.5/1 compression - hyd roller cam, probably just under 300 advertised duration, around 600 lift, waiting 'til the rest of the combo is firm before I settle on this one definitely - billet caps - girdle.

[Challenger340]

I like higher ratios, personal preference for Mopar engines. If "better" equates to long and flatter torque/hp curves, then yes.

[firefighter3931]

If "better" equates to long and flatter torque/hp curves, then yes.

Exactly. The reason440's run so well is the 1.80 rod ratio.....nice flat torque curve. The better rod ratio alllows a port limited engine to perform better....better than it would with a smaller rod ratio. Stock BB heads have small ports, relatively speaking so the superior rod ratio is more or less a necessity.



Ron

[Bandit72]

do 451 strokers have a good rod ratio?

[defiance]

According to 440source, 451's are 1.802 rod ratio - which, from above, is apparently very good.

[Nacho-RT74]

do 451 strokers have a good rod ratio?

According to 440source, 451's are 1.802 rod ratio - which, from above, is apparently very good.

with 400 or 440 rods ?

[Chryco Psycho]

440 rods maintain the stock rod ratio of 1.82

[is_it_EVER_done?]

Sorry, but I have to disagree with everyone. Rod ratio in theory can have an effect on many things, but in the dynamics of an actual running engine, it has no real relevance. This has been tested to death and proven many times on the dyno. In fact using a long rod at the expense of a short piston is detrimental to power as the piston rock tends to unseat the rings. That's why pure race engines use gas ports to help hold the rings against the cylinder wall - something that is totally unusable in a street - street/strip engine.

Side loading tests have shown that a full point of rod ratio decrease (ie. 1.7 to 1.6) only increases the cylinder wall loading by about 3%, so the cylinder wall loading argument has been proven to be a myth, as a 1 point increase in compression has a more dramatic impact on the cylinder walls.

The ideal for a street engine is a compromise between the rod ratio and piston heighth. My opinion is that the good old tried and true 500 inch RB with its approx., 1.6 rod ratio, and  1.8 C.H. piston is a more stable, powerfull, and long lasting engine than one that has a short, unstable piston.

One must think back many years to when this discussion had merit. Even less than 20 years ago, all we had was either stock rods, viciously expensive aftermarket steel rods, or (as today) relatively inexpensive aluminum rods. Since the stock rods, even when prepaired to the max with polishing, the best rod bolts for the time, and all the other tricks, were the weak link in any engine so it made perfect sense to reduce piston weight to the bare minimum to reduce the stress on the rods, and/or go to one of the alternative rod choices.

Now we have DIRT CHEAP Rods and cranks that are far stronger than anything we can hope to throw at them with stock blocks, so the necessity of a short (rocking) piston is ancient history. We are making more power than pro stockers were a few decades ago, and doing it with durability that they couldn't even dream of.

My point is that unless you are building a full tilt race engine - which would include an aftermarket block, there is nothing to be gained by sacrificing piston length for rod ratio. Any claimed gains have been disproven as myth.

[Ghoste]

 

[i c e b l u e]

One thing that hasn't been brought up about a stroker engine and rod/stroke ratio is dwell time and the position of the piston durring different times of the stroke. I have done A lot of research lately on stroker engines and have found many articles on this, here they are.

http://victorylibrary.com/mopar/rod-tech-c.htm

http://www.fordmuscle.com/archives/2003/09/stroker/ (I know its a ford site but a very good article, this topic applies to all makes)

http://www.stahlheaders.com/Lit_Rod%20Length.htm

All of these articles a very informative and the middle link is much easier to comprehend if you are new to this idea it offers good diagrams to help explain the tech. But I agree with most of the others that a Larger rod/stroke ratio is more desireable. According to the article 1st many high performance engine builders consider a 1.75 to 1 ratio ideal. But again there is plenty of info in the above links

[Nacho-RT74]

Sorry, but I have to disagree with everyone. Rod ratio in theory can have an effect on many things, but in the dynamics of an actual running engine, it has no real relevance. This has been tested to death and proven many times on the dyno. In fact using a long rod at the expense of a short piston is detrimental to power as the piston rock tends to unseat the rings. That's why pure race engines use gas ports to help hold the rings against the cylinder wall - something that is totally unusable in a street - street/strip engine.

Side loading tests have shown that a full point of rod ratio decrease (ie. 1.7 to 1.6) only increases the cylinder wall loading by about 3%, so the cylinder wall loading argument has been proven to be a myth, as a 1 point increase in compression has a more dramatic impact on the cylinder walls.

The ideal for a street engine is a compromise between the rod ratio and piston heighth. My opinion is that the good old tried and true 500 inch RB with its approx., 1.6 rod ratio, and  1.8 C.H. piston is a more stable, powerfull, and long lasting engine than one that has a short, unstable piston.

One must think back many years to when this discussion had merit. Even less than 20 years ago, all we had was either stock rods, viciously expensive aftermarket steel rods, or (as today) relatively inexpensive aluminum rods. Since the stock rods, even when prepaired to the max with polishing, the best rod bolts for the time, and all the other tricks, were the weak link in any engine so it made perfect sense to reduce piston weight to the bare minimum to reduce the stress on the rods, and/or go to one of the alternative rod choices.

Now we have DIRT CHEAP Rods and cranks that are far stronger than anything we can hope to throw at them with stock blocks, so the necessity of a short (rocking) piston is ancient history. We are making more power than pro stockers were a few decades ago, and doing it with durability that they couldn't even dream of.

My point is that unless you are building a full tilt race engine - which would include an aftermarket block, there is nothing to be gained by sacrificing piston length for rod ratio. Any claimed gains have been disproven as myth.

so as you state, 400 rods on 451 stroker won't be to much different than using 440 rods

does somebody know what's the 400 rods ratio on that stroke ?

Sorry, I know it looks I'm highjacking the thread, but doesn't only looks... I AM DOING IT

[Runner]

while i dont have a sound opinion on this, i once asked a fellow mopar racer that does alot of state fo the art piston work for pro mod cars about rod ratios,   he told me he doesnt get to caught up in that, and went on to say that a rods job is conect the piston to the crank.