What is the general consensus on the maximum load capabilities of a H-beam rod versus an I-beam rod?
From my own personal experience, I've changed over from using a H-beam to a good I-beam with no regrets, and I've gotten the impression over the years that H-beams do have a lower load limit than I-beams, particularly in high cylinder pressure applications.

 

Always thought this was a good read on rods....

http://www.enginebuildermag.com/Article/81067/connecting_rods_and_bearings.aspx

 

interesting read.

 

A very interesting link.

Comparing NASCAR Cup engines to Formula One engines, by EPI Inc.

A quote from the article; At max compressive load, the stresses and buckling margins in both rods are acceptable (although 'I' beam rods have considerably more buckling margin at the same loads than 'H' beam rods of similar cross section),

There appears to be differing opinions among the experts?

 

There's been some talk that an H beam is more likely to shear at very little bend angle where an I Beam will bend alot further before shearing.

Personally I like the idea of radial I beams or even having the radius decrease from the bigend to the small end. Two opposing arches should give some add some bend resistance.

 

But what would two opposing arches do to stretch resistance? The big end going out-of-round is bad.

 

how much power we talkig here 1000+ ?

 

I was thinking of 5 or more hp/cu in.

 

What I'm curious about is what sort of load is put on the rod if the wrist pin is pinched or drags and doesn't rotate freely in the small end of the rod or the pin bore of the piston.

If abnormal combustion pressure causes this pinching of the pin at near TDC and the crank throw continues rotating through its normal path, what sort of stresses would the rod go through?
Let me try to describe this better. If I had a tight fitting wrist pin in the end of a rod and clamped the wrist pin in a vise, and then took the big end of the rod and attempted to rotate the rod about the wrist pin, what sort of stresses are we putting on the rod?
I believe we would be putting a bending force on the beam of the rod.
If this is true, and we are putting a bending force on the beam of the rod, then what beam orientation is the strongest to resist this bending deflection of the beam?
Let's add to this.
An engine rotating at 6000 rpm. At TDC, cylinder pressures become such that the pin drags and momentarily causes this slight bending force to be imparted on the beam of the rod. This would be occurring at 100 times per second. Or maybe not so regularly, but erratically.
Comparing this to another sort of occurrence that is commonly seen. A driveshaft with a tight U-joint. It's not uncommon to see a tight U-joint on a driveshaft set up a vibration in the shaft. Even a vibration that resembles a harmonic vibration.
I wonder if a vibration couldn't be set up in the beam of a con rod if the above situation were occurring? Add to the momentary drag on the wrist pin happening 100 times per second, all the other forces that are occurring with the rotating/reciprocating assembly. We know the crank is twisting and vibratiing. The crank damper is busy trying to control that, but what about the rods? The harmonics from the crankshaft also must be transferred to the rods adding to the mix of forces that are being put on them that we do clearly understand.
Could a frequency of these forces be met where it sets the beam of the rod to wobbling like a piece of spaghetti? Twisting and bending? And which axis would the bending typically be greatest?

An engine builder once told me of an experiment where someone put a strobe light on the bottom end so that they could observe what was occurring to the bottom end during high speed. Similar to the type of videos that you can see on youtube of the valvetrain. He told me that it would scare the heck out of you to see how components in the bottom end were moving around at speed and load. Frankly, I really don't think I'd be surprised.

 

You should not design the beam of the rod to compensate for a poor choice in Piston Pin diameter/ wall thickness. We have crushed normal high quality .990" diameter piston pins
under some boost pressures and have gone to the Street Hemi 1.030" diameter pin to fix the issue. No issues to date with a 2500+ hp engine.

I personally like an aluminum rod (based on design specs of a GRP or MGP rod) for high boost pressures and high HP. The aluminum rad can in some cases save a detonating engine from killing the bearings/ crank assembly.

Granted they are not a street piece but you will not have the issues requiring a GRP or MGP design rod with a 1.030" diameter pin in a street car in 99% of the cases. Good aluminum rod manufacturers can emulate the design specs mentioned above.

Tom Vaught

 

Some engine designs don't give a lot of room for an aluminum rod. Camshaft placement is low in a Buick V6, so an aluminum rod is not an option since an aluminum rod must have more bulk for it to live.

Tom brought up an important point. The piston pin is an important part of the equation. I agree completely.

 

More of an over all thought after the pin was mentioned. When designing an n/a engine I would pick a crank and then pick a piston, whatever space is left in between the rod will fill. In a turbo engine can a rod be an after thought so to speak? Not making it the weak link, but not sacrificing the other areas such as the piston pin placement?

 

This link gives some good info about rod dimension effects:

http://www.rustpuppy.org/rodstudy.htm

Tom Vaught

 

I've heard that in a Buick V6, rod length has very little affect, if any. Still, in a supercharged application, to keep cylinder wall loading to a minimum, I would shoot for the longer choices, to a point.
Again, in a supercharged application, the piston strength should come before rod length. In a n/a application, the piston can end up having a short ring package, sometimes. In a turbo piston, the ring package is not as short. Piston crowns and ring lands have to be stronger. That means more material. Piston pin placement will usually be lower.
To answer your question more simply, the rod would be picked last. Similar to how a pushrod length is picked after the valvetrain components have been picked out. If you're shooting for an off the shelf rod length, then you'll have to be aware of the available rod lengths while you're picking stroke and piston design.

 

GREAT TOPIC.....both rod type & rod length seem dependant on the exact application...aluminum for all out drag,truck, or tractor pull. I beam for class restricted circle track
usually under 500 h.p. and or 7000 rpm's. H beams for pretty much everything else.....Joe Sherman(mostly credited for pioneering the 383 chevy) did a rod length test
for a magazine back in the mid 90's....on 383 to 406 drag racing engines between 5 & 700 h.p. N.A. he found no advantage in horsepower or torque from 5.565 or 6" rods
I've assembled over a thousand engines since 1986 (drag,circle track,nitrous,turbo, and industrial truck applications) & the single most prevalent thought I have regarding
rod length is cylinder wall longevity i.e.(ring seal)!!!!! let me give you 2 examples...434cid S.B.C (4.155x4.00) w/ 6"rods,13.75:1....AFR235 cnc heads w/1050 dominator
and .672/.638 roller...never dyned but approx. 625 horse @7500 + 250 nos. In less than 60 passes this thing raised a few ring lands and even though it was hard blocked
the cylinder walls looked brutally ugly in the thrust area when rehoning. Wrist pin height was 1.00"...light piston,tight ring pack
2BBL restricted street stock engine 359 (4.040x3.500) ultralight 6" I beam rods 510grams, 330 g JE pistons w/1.2mm rings Flattops, Vortec heads, .490 lift hydraulic cam
cast iron intake and exhaust manifolds....approx.350 horse @ 6200...1.25 piston pin height....very noisy piston slap since brand new even honed w/torque plates @ .005
piston to wall clearance....After 1 season...approx 1200 dyno pulls figuring 2 per lap & 30 plus laps/night X 20 races...again the cylinder walls looked like crap in the thrust area
severely compromising ring concentricity. These particular pistons were slipper skirts however I didn't see a dramatic improvement in a similar build w/ fully skirted pistons
until I built one w/5.7" rods....much quieter and better bore longevity when refreshening. Now I know we could debate rod length(piston speed @ or near TDC) and how
it affects low or limited compression ratio classes and how that changes timing requirements and vaccum or combustion chamber pull as well as quench squish on compression

but IMO keeping the piston as tall as possible regardless of weight or rod lenght adds stability which promotes ring seal!!!!!

 

I have no personal experience here. But I have seen people on more than one occasion go to a taller deck without maxing the stroke any farther than you could have gotten in the shorter deck. Explanation same as yours.

 

personally, i use a carrillo h beam in my engine..steel rod
in a few customer engines, i have I beam billet oliver steel rods
flip a coin.... if the tune up is good, either will work

 

Same steel rod for the new goal ?

 

me ?

yes...
400 ttsbc , 76mm turbonetics - carrillo h beam
393 ttsbf , 76mm turbonetics - oliver billet I beam
469 ttbbc , 76mm precisions - oliver billet I beams

 

Yes