fly1

What kind of rods are you guys using and why. I'm trying to determine which rods I should use. It appears that I beams are lighter which would help refuse the weight of rotating mass but are the weaker or does it matter much in a car that will see more street usage.

joe_padavano

This topic seems to have come up on a lot of automotive forums lately. All I can say is that as a structural engineer, I just can't understand the benefits of the H-beam rod. Maximum rod bending moment is in the direction normal to the piston pin, which is why every production car in history has I-beam rods. I have seen comparison analyses that show the I-beam rod is much stiffer in that direction for the same weight, or conversely for the same stiffness the I-beam would be lighter. What am I missing?

cutlassefi

I prefer the I beams too but if you're doing s BBO I don't know of anyone making an "Olds" I beam rod anyway.

380 Racer

My 425 SB has H beam rods. I think it must have been "a good deal" because I remember my builder said it didn't make much difference.

geckonz08

this is a very simple ,maybe oversimplified (eg no torsional stresses included),analysis/comparison

http://forums.corvetteforum.com/c3-t...beam-rods.html

joe_padavano

And that is exactly my point.

I-BeamvsH-Beam.jpg

The H-beam on the far right is the same weight as the I-beam but only 41% as stiff. The H-beam that's second from the right is the same stiffness as the I-beam, but much heavier. Tell me again why H-beam is better?

380 Racer

My point is that my engine builder is a very (almost ****) fussy/precise/successful engine builder. Nothing goes out of his shop that isn't right. In other words I totally trust him. If there was any difference between the H and I beam rods, I have the best of the two in my motor in his opinion.

In the last two years my SB has been pizz-pounded and if it had to spit a rod out the side because the rods are H beams, it surely would have. The engine was tore down a month ago for a freshen-up. Guess what? The rods checked out perfect. So I think all your theories and the internet don't prove anything. Dyno and track usage tell the whole story.

joe_padavano

Well, if you really believe that, then you had better not ever fly in an airplane, drive over a bridge, or ride in an elevator.

Look, I'll be the last person to criticize your engine builder. The results speak for themselves. I will say that centuries of ENGINEERING, not theories, proves the numbers I quoted above. You can dismiss that all you want, but facts are facts.

Does this mean that H-beam rods don't work? Of course not. Obviously they do. Lots of things work that are less than optimum. My whole point is that there is no ENGINEERING (as opposed to internet theories) that I've seen that show H-beams to be better, only more expensive to manufacture. Sure you can build an engine that will work with them, but I can guarantee that an I-beam rod of the same stiffness will be lighter. That's not a theory, that's fact.

Boldsmobile

Holy Crap Joe, thats a step back in time. Havent touched that stuff in 20 years.

Nick like for like x-sectional area, the I beam will be stronger.

If in your application the H is below the failure point obviously its not an issue.

However it would be interesting to calculate the strength of your current H beam rod. Than see what the corresponding area (and weight) is on an I beam with the same strength. If you could have the same strength but save 50 grams? 100 grams? would you do it?

Is there some other forces acting upon a rod that doesnt make this so?

The calculation is not theory and i am intrigued to know what factors dont make this so.

joe_padavano

I still need to deal with it nearly every day.

I will say that the calculations above are simplified and are for a constant cross section beam, NOT an actual connecting rod under dynamic loading. Still, the basic premise is valid. As with anything in the real world, however, simplification rarely produces the right answer.

First, there are at least two design constraints, bending stiffness and overall strength. There are likely other constraints, like torsion. I'll set production cost aside for the moment since in a race motor that is less of a concern. Assuming the same material properties in both rods, bending stiffness is solely determined by section shape. Strength is solely determined by cross sectional area. Clearly the I-beam is stiffer in bending. The question is, does the interface of the H-flanges at the rod bolts and the piston pin boss result in a localized strength benefit (and thus weight reduction in those complex areas) that outweighs the detrimental design of the H-beam in bending?

I'll be honest, I'm too lazy to build the NASTRAN model. I think this should be someone's undergrad thesis.

cutlassefi

Only have 1 question, why is it that when you get to a certain hp level they go over to H beams from I beams? I have never seen a high hp anything with I beams in it, not Nascar, and you'd think the weight savings would be something they would covet, not anything high hp with I beams.

Hope someone can explain that.

380 Racer

I just checked out the weight difference on the Compstar web site since those are what I have. They are made with 4340 steel.

6.200" long SJ end W/.927 pin Advertized 598 grams (actual 602)



I beam......6.125 SJ advertised 596 grams
Large journal 6.125 advertised 617 grams

I would assume we decided the .075 longer rod for a little shorter piston. The smaller journal size for less bearing speed. There isn't much difference in weight between the two. A non educated guess would be if the I beam were lengthened that .075" they would be even.

joe_padavano

NASCAR???

Don't they use a rear suspension design from a 1962 Chevy pickup?

Look, as with any engineering problem, there are a LOT of considerations - fatigue strength, internal harmonics, etc. All I can say is that the photo below is a rod from a Ferrari Formula 1 engine. When cost is no object and the engine turns 19,000 (yes, you read that right) RPM for hours, what design did they pick?

cutlassefi

Joe I respect you tremendously however I know for a fact that on the F1 stuff windage is as much of an issue as anything. H beams have more windage, think of the design and how it moves. They beleive air is trapped between the beam supports.
But remember the rods are titanium, and the small end of the rod, the piston and pin, and rings all together weigh a little over 400 grams. Pretty light, not alot of stress. And yes they turn 19,000 for hours, idle at 7,000, will shut off/stop spinning from idle (again 7,000rpm) in less than a half of a second due to so little rotating/reciprocating mass and use fuel pressure in the range of 2600psi.

I know a little about F1 too, in fact I marvel at the technology. And I don't believe for a minute that rod is from a modern day F1 Ferrari engine, wrist pin and rod journal sizes for one are way too big.

joe_padavano

I don't disagree, and in fact after searching for info on other F1 engines there appear to be a wide range of designs that include I-beam, H-beam, and other. The reality is that as short and wide as these rods are, the traditional column buckling and beam bending equations don't really apply anyway. These rods look more like the parts we design for satellites and I'm sure they are engineered the same way using the same design and analysis tools.

I'll be the first to admit that trying to compare an F1 connecting rod to an Olds rod (or a rod from any production engine) is like trying to compare Barrett Jackson prices to those in the real world. It's really not a valid comparison. On the other hand, I'm also not one to accept conventional wisdom without an understanding of why it's the right thing to do. "Experts" can be wrong (just ask any header manufacturer if their headers fit Supremes) and there is definitely a lemming mentality in the automotive world. A lot of people are just flat wrong (like the repeated assertions in various automotive forums that frame rails can't be welded). Finally, it's not usually a good idea to extrapolate from one operating environment to another. For example, NASCAR engines may use H-beam rods for whatever reason, but do we know if that reason applies to our engines?

It's probably obvious that the design of a connecting rod encompasses a number of different requirements, including compressive load, buckling, fatique lifetime, bolt failure, twist, low mass, etc. The optimum design needs to balance these requirements to the best extent possible, and as we repeatedly find out, there is usually more than one acceptable solution to an engineering problem, depending on how you make the trades. Production rods are obviously I-beam for the production cost. All other considerations are probably secondary to that.

Whether the beam section of the rod is more important than the design of the ends, and how the optimization trade plays out is not something I'm qualified to comment on without a bunch of analysis. I will suggest that the importance of the beam cross section design probably increases as the rod gets longer, however. The design of the ends - particularly the big end - probably dominates the optimization trade for short rods, but that relative importance likely changes as the beam portion gets longer and thus forms a larger part of the total mass (and concurrently becomes more susceptible to bending). Again, this sounds like a good undergrad thesis topic.

Note to Nick - I appreciate your length/weight data, but without knowing the relative strength and stiffness of each rod, it's not really possible to meaningfully compare those numbers. Just as an example, the I-beam rod may be the same weight as the same length H-beam, but if the I-beam is stiffer and/or stronger, it should be possible to make it lighter. Again, that's just an example, and without the data one can't make an informed conclusion.

I'll close by saying that I also respect the guys here who have demonstrated their ability to build and run these motors. I appreciate the experience that they share with us. Please keep it up.

Texas Jim

You guys are one up on me as I'm no engineer, but simply put, I've always understood that the "I" beam rod is supposedly a better rod f/ a high torque engine, like the 455 Olds, and an "H" beam f/ an engine that's built more f/ HP and not as much f/ torque. Supposedly the "H" beam is better f/ higher RPM. I've gotten this from info on building big cubic inch small block Chevys. (I do realize that Chevy is a four lettered word on here). But this is getting down to the "nitty gritty." F/ something that's not being run at the track f/ record times, as long as the rod will hold up, I can't see it making that much difference.

I will say that you guys who commented on this topic contributed some great info and made the topic over-all really interesting. Thanks.

D Appeldorn

if the same I put on his side as a H (same proportions) you get a differ in strenght.

best way to tell is a gauged bending test (as we do in construction work) concrete and steel/ wood. building material

you could test it with a gauged bench press ( you use to press on new bearings)

just a tought