Lets talk rod to stroke ratio's...

[c-reed]

  I remember back in the 90's when the small block stroker scene hit, i was really into the foxbody mustangs at the time. Every kit that you would see advertised for sale would list its rod/stroke ratio. But nowadays you don't even hear about it anymore. Why is that. Is it not as big a deal as it was once thought to be? Or is it because that most kits sold are marketed more toward high performance, assuming a higher cfm flowing top end.
  I'm no expert but it is my understanding that a longer rod tends to make better power when used with "bigger" heads, intake, cam...etc. and the shorter rods perform better with more velocity at a lower rpm. Is this correct?
If so, what about other applications like towing were you want more power down low or performance builds using factory small port heads and/or intakes. Wouldn't they benefit from a shorter rod?
 For instance I've been thinking about a 428 street build, standard bore and a 4.125 stroke using a 6.535 or 6.385 BBC rod, a relatively square engine. Using smaller port heads and intake for good velocity (TFS, or maybe stock iron) , not too radical of a HR cam and a good intake to match (streetmaster/ dominator or ?). How would the shorter rod perform vs the 6.7 or 6.8

[blykins]

I don't use "IMO" much because I usually try to speak based on experience.  However, this is one of those topics where there's a lot of correct answers. 

IMO, "rod/stroke" ratio doesn't carry as much weight as choosing the correct rod length for the displacement and induction.   

Also, IMO, a large, slow moving intake port will not like a long rod because the piston spends more time at TDC, when it needs to be moving to "draw on" the intake port.   Some camshafts will help with this, but I don't like long rods on a large, lower velocity, intake port. 

In most cases, I use a long rod and a short piston.  That's all relative, of course, depending on the combination and the application.   In some instances you can't get away from it, but in most I don't like a heavy piston.   I did a road race 289 and used a 2.875" crankshaft with a 5.700" rod.  It used a 200 cfm ported factory cylinder head and made almost 400 lb-ft of torque with it, with 445 hp @ 7000.   I had to do a lot of machine work to make that combination work because of the rod journal dimensions, but I feel that it was worth it.

What would I use on yours?  I like 6.800" rods with that combination.   Gets the piston down to a 1.288-1.300" compression height, gives stability, but makes the piston lighter.  Also, on some crankshafts, the counterweight will get into the wrist pin bosses with a short rod.  The last two 4.125" stroke engines that I've built, that's what they got.  I also use a Molnar rod, which is a ton lighter than most of the I/H beam rods out there.  Makes for a very lightweight rotating assembly. 

[plovett]

It makes almost no difference.  Smokey Yunick's book is the most misunderstood and misquoted book after the bible.

I have read his deal on rod length and it only applies to a very specific situation.  And even then it scratching for the last available hp, as in a high dollar racing scene.

I would worry more about piston heights and weights, due to rod length.

JMO,

paulie

[GerryP]

There are like 10 people on the face of the earth who can maximize the tune up based upon rod ratio.  I'm going out on a limb here and assume you are not one of those 10 people.  I'll admit, I am not.

It's really one of those bench racing topics.  Everyone knows about rod ratio but, again, they can talk about it only in a theoretical sense...you know...how a piston accelerates or decelerates at different positions in the cylinder and the dwell periods and all that super critical stuff 'n' junk.  Practical application belongs to those 10 folks.  About the only thing we can address in our knowledge base is that a longer rod reduces load on the thrust side of the cylinder.  It can also make for a lighter piston with a shorter compression height.  Just don't run into the rings for the sake of rod ratio.  That's not a good trade off.

So, with that in mind, the prevailing opinion for the past 40 years has been to put the longest rod you can into the engine.  Of course, you have to consider the engine's architecture.  There are cams, cylinder walls, and piston compression height limitations to consider.

And if you can't run a longer rod, or just don't want to go to the trouble for such a small gain, then put yourself in the company of the Pro Mod guys running those 1,000ci mountain motors with impossibly short rods.

[c-reed]

There are like 10 people on the face of the earth who can maximize the tune up based upon rod ratio.  I'm going out on a limb here and assume you are not one of those 10 people.

HA! You are correct sir, I am not.

[machoneman]

Six year old posts to review:

http://fepower.net/simplemachinesforum/index.php?topic=1461.msg12803#msg12803

[mbrunson427]

I built myself a spreadsheet and graph for this a while back, because I'm curious like that. The sheet compared rod length and what it did to piston position/velocity/acceleration vs crank angle. When I compared between a 6.7" stroker rod and 6.8" stroker rod, I had to make the graph full-screen and zoom in ridiculously far to see any semblance of difference between either one of the graphs. Bumping the rod length to 7" I could finally see some separation in characteristics, still not a large amount.

[jayb]

I think rod ratio is a lot more important when it comes to long term engine endurance.  If you have a low rod ratio, the force against the cylinder wall from the piston is greater than it would be with a high rod ratio, and so an engine with a low rod ratio will wear the cylinder wall and rings more quickly.  This is one reason why you see high rod ratios in production engines; they are designed to last over 200K miles.  If you only need to go 30K or 50K miles between rebuilds, you can certainly tolerate a lower rod ratio.

For a performance engine like most of us are trying to build, you are usually trading off rod ratio with cubic inches.  For example, you can build a 427 with stock stroke and stock rod length, and get a rod ratio of 1.71.  On the other hand, you can build a 482" 427 stroker, with a 4.25" stroke and a 6.700" rod, and get a rod ratio of 1.57.  Which will make more power, given equivalent performance parts?  Of course the larger engine will; cubic inches always wins.

I'm currently putting together what has to be the most god awful engine for rod ratio ever conceived.  4.75" stroke, 6.625" rod length, rod ratio of 1.39.  It has the longest rods that will fit, but still a very low rod ratio; nevertheless it will certainly make well over 1000 HP.  It is replacing the 4.60" stroke, 6.700" reciprocating assembly, that had a rod ratio of 1.45 and made between 930 and 1005 horsepower, depending on cams and induction.  I spun it regularly to 7800 RPM at the track. 

My advice - put the longest rods you can reasonably use in the engine, but don't get overly concerned about rod ratio; in terms of performance it won't have a big effect.

[allrightmike]

   When 347 small block ford stroker cranks came into prominence we saw quite a lot of talk about stroke to rod length issues because this combo had a very short rod and engine life was a concern. Might be why not so much being printed these days. IMO.
Mike.

[c-reed]

a quote from Jay in the 6 year old link machoneman posted:

"For what its worth, I am a firm believer that rod ratio is not a particularly important parameter for most performance engines.  If you are running in a class where cubic inches are limited and/or stroke is limited, there may be some benefit to certain rod ratio and head combinations.  For all other applications, cubic inches will trump the effect of rod ratios.  As an extreme example, my 585" SOHC has a 4.6" stroke and a 6.625" rod, making for a rod ratio of 1.44, which is extremely low in anybody's book.  That engine made 960 HP and revved to the limiter (7600) faster and more aggressively than any other engine I've ever had.  On the track I had to set the shift light at 6800 RPM in order to make the 1-2 shift before the engine hit the limiter. "

My question was not so much about changing cubic inches with stroke but only the two variables of rod length and compression height. I understand about changing the rod angle and piston weight. I was really referring to how it would "perform" not about the most power between the two.
  Jay mentioned how fast his 585 SOHC revved. If you could make a street engine rev without changing gear ratios and target your induction in that given RPM range is more of what i was getting at, but it seems the overall consensus is to not worry about it and use the longest rod readily available. Just dreaming at work really, it's how I get through the day

[plovett]

Use the longest rod you can until the piston gets too short.  I like taller pistons than is currently in fashion, maybe in the 1.35-1.55" range.  I wouldn't want to go to either extreme.  A Mopar 440 has excessively tall and heavy pistons.   347 Fords have pistons that are pretty short.  I know people run both extremes successfully, but I think the best is in the middle.   A fairly short and light piston with the ring not in the pin is what I like to see.   

If I had to pick a ideal number it would be around 1.35" or 1.40".   That is short enough to make the piston light and long enough to have the rings out of the pin, and give good piston stability.  The pistons in my next engine will likely be around 1.58".   That is just how the parts that I could find and afford went together.   I am okay with that. 

JMO,

paulie

[c-reed]

And just for more clarification i wasn't talking about targeting a specific ratio #, it was more of a longer rod vs a shorter rod question. I probably could have worded it better.

[blykins]

I do a lot of engines with oil rail supports, which means the pin intersects the oil rings.  The only time I have suffered any consequence from that is when the support has spun for whatever reason.  Other than that, I see no difference from the engines that have the pin/ring intersection.   With as many 347's that are rolling around, the weaknesses should have been found with that setup by now.   One of the previous editors of Modified Mustangs (Marc Christ) told me that he had over 100k miles on his Fox Mustang with a 347, with no oil consumption or issues. 

I do believe a lot of the voodoo about intersected rings has stemmed from internet bench racing.   The sheer number of rotating assembly combinations out there between Ford/Chevy/LS/etc. would be staggering. 

A stable piston does make me feel better though, even if it's just a warm and snuggly feeling inside.  I've had pistons in here from .990" pin height to 1.900" pin height and even though I've torn down lots of engines with 1.040-1.090" compression heights with no perceived wear, it just seems to me that the piston has to be rocking more than in an engine with a taller piston.  On FE's, we really don't have to worry about that sorta thing. 

[plovett]

I do a lot of engines with oil rail supports, which means the pin intersects the oil rings.  The only time I have suffered any consequence from that is when the support has spun for whatever reason.  Other than that, I see no difference from the engines that have the pin/ring intersection.   With as many 347's that are rolling around, the weaknesses should have been found with that setup by now.   One of the previous editors of Modified Mustangs (Marc Christ) told me that he had over 100k miles on his Fox Mustang with a 347, with no oil consumption or issues. 

I do believe a lot of the voodoo about intersected rings has stemmed from internet bench racing.   The sheer number of rotating assembly combinations out there between Ford/Chevy/LS/etc. would be staggering. 

A stable piston does make me feel better though, even if it's just a warm and snuggly feeling inside.  I've had pistons in here from .990" pin height to 1.900" pin height and even though I've torn down lots of engines with 1.040-1.090" compression heights with no perceived wear, it just seems to me that the piston has to be rocking more than in an engine with a taller piston.  On FE's, we really don't have to worry about that sorta thing.

That is all true, Brent.  I've never built or even used a 347.   I just don't "like" super tall or super short pistons.   No doubt they can be made to run good and be reliable.   I just have a height range that I think is best overall.  You work with the parts you have, but if I could wave a magic wand, I would always be around 1.35".   Good combination of light weight and stability.   And no support rails. 

JMO,

paulie

[My427stang]

8:1 is the only rod ratio I really care about, eight rods to one crankshaft LOL. I do try to use the longest rod I can to minimize piston weight while balancing it withthe most stability I can with skirt length.  But in the end I don’t put a lot of time in it on my street builds

ON EDIT: Corrected the gibberish....LOL I originally did this response via phone....never again