540 RAT

Yes, even though this topic has been beaten a number of times, it’s connecting rod time again, and here’s why. It was brought to my attention that one of our magazines had a tech article awhile back that had a section on connecting rods. It was written by someone I’ll only identify as Mr. K, and he knows who he is. In that section about rods, he used some of my words that came out of the “Rod Strength Analysis” that I did a few years ago, some of which I’ve posted in the past. He also of course used some of his own words. But, while he was straddling the fence, trying to be neutral and not offend advertisers, he contradicted some of what he’d put in that piece, which ended up confusing people. I have been asked about all that, and asked to post something to clear up the confusion he created.

So, that is why I’m posting the following info, which is an UPDATED version of some of what I’ve posted in the past about connecting rods. If you are interested in the details, read on. Otherwise, close out now while you still can.

I’ll say right up front that I do NOT sell connecting rods, so I have no vested interest in what rods people buy and run. People can of course do whatever they want. But, there is so much misinformation, misunderstanding and confusion about connecting rod design, that I’ve put together a brief overview for those who are interested in knowing the Engineering FACTS, rather than relying on the incorrect info that is so common on the Internet and elsewhere.

It is best to avoid H-Beam rods in general, no matter who makes them, and no matter who else uses them. Because as you will see below, an H-Beam rod is never the best choice. They were originally made by someone who “thought” they might be better and/or cheaper to make, without benefit of any Engineering analysis. So, the maker didn’t even know what the H-Beam shortcomings were. Then other makers copied them, and eventually people started to think they must be good because they kept showing up. And because they looked different than stock rods, some figured they must be trick parts that are better.

But, you will only find the H-Beam style being used in the aftermarket Automotive Industry where it is common for companies to create parts without using any Degreed Engineers. A lot of the aftermarket companies “just make stuff” without even knowing what they are doing. No competent Degreed Mechanical Engineer would ever design an H-Beam rod, because an H-Beam rod is a textbook case of how NOT to design a connecting rod. So, buyer beware.

A rod’s max compression loads are determined by the amount of HP being made. It’s a simple matter of the higher the HP, the higher compression loading on the rod. And an Engineering “FACT” (NOT opinion or theory) is that the I-Beam rod design has about twice the strength in compression, compared to a comparable H-Beam rod. So, that makes an I-Beam rod a far better choice for any application, and particularly for those at higher performance levels, such as those making over 1000 HP.

But, a rod’s max tension loads are determined by the mass of the parts involved, the rod length, the stroke length, and the max rpm. That’s it. The max tension loads will never change, no matter if you throw Nitrous, a Turbo, or Blower at it, as long as the short block and redline don’t change. That max tension loading occurs at TDC on the exhaust stroke. And that has absolutely nothing what so ever to do with the amount of HP being made. In order to change the max tension loading, you’d have to change the short block configuration and/or the redline. Both types of rods have similar tension capability, since that is only a product of the beams cross-sectional area.

In High Performance engines, connecting rod “compression loading” is ALWAYS considerably higher than the “tension loading”. Here’s an example using an 800HP, 540ci BBC with a 7,000 rpm redline:

Max compression loading on the rod is about 21,000 lbs or 10.5 tons.

Max tension loading is only around 11,000 lbs or 5.5 tons.

So, as you can see in this particular example, the compression loading is about twice as high as the tension loading. But, if the HP increases, the compression loading will also increase. And “THAT IS WHY” a rod’s compression loading capability is important to consider when you are in the market for a new set of rods for a High Performance engine.

An I-Beam rod made from high quality material such as 4340 forged steel will provide plenty of “Margin of Safety” with regard to compression strength. But, a comparable H-Beam rod’s margin of safety can be iffy, and it only gets worse as the HP levels go up. For an H-Beam to catch up to the compression strength of an otherwise comparable I-Beam, the H-Beam would need to be FAR heavier than the lighter, stronger and more efficient I-Beam design. So, by using I-Beam rods, you will have the capability to increase the HP later on, without worrying about the rods being strong enough to handle the extra HP.

The superiority of the I-Beam, is why it is the structural beam design of choice for countless Professional Engineering applications. So, the next time you need a set of rods, you might want to do yourself a favor, and only consider I-Beam rods which are a significant UPGRADE over H-Beams. And this is why you see I-Beam rods in countless OEM engines, including the Supercharged Corvette, which were designed by actual Degreed Engineers who knew what they are doing.

BOTTOM LINE: No matter what anyone tells you, there is simply NO good reason to ever use an H-Beam rod. So, it makes no sense to buy H-Beams when the clearly superior I-Beams are readily available.

If you are still having a hard time accepting all this, consider the following:

Lunati’s recommendation for their rods:

• H-Beam Rods - ideal for High Performance street & mild race engines.

• Pro Series I-Beam Rods – perfect for Street Rods, Street-Strip Engines and all-out Race Engines

• Pro Mod I-Beam Rods - perfect for any racer needing an ultra-strong I-beam design

They also say that every Lunati connecting rod is forged from premium quality 4340 alloy steel for strength.

So, as you can see, Lunati knows what they are doing, mirrored what I said above, and got it right about H-Beams, I-Beams and forgings.

And speaking of that topic, no one “needs” a billet rod either. Forged rods have desirable grain structure and desirable residual compressive stresses, but billet rods DO NOT. Forged parts are always better than billet parts. For example, all fracture critical jet aircraft parts are forged, NOT billet. Billet parts are simply cheaper to manufacture in small quantities, even though machining time will be higher. Because billet parts do not require the horribly expensive forging presses and dies. But, when parts are produced in high enough mass quantities to spread out the cost of the forging presses and dies, then forged parts can end up being both superior and more affordable, because forgings don’t need as much final machining time.

For even more info, stay tuned for my upcoming write-up on selecting rod bolts.

540 RAT
Member SAE (Society of Automotive Engineers)

632C2

Rick, great write-up - as usual. Ever since my mishap with the H-beam Carrillo rods in my 632 I have taken your advice and stuck with I-beam rods. So far, so good! Again, I appreciate the time you take to help out on these forums. It's nice to have an actual engineer giving fact-based information.

Steve

htown81vette

I tend to agree (by experience). I had some forged H beam rods in my LS motor, and the rod snapped in half at 6500 rpm, destroying my motor. Of course, it is almost impossible to determine the root cause, but use I beams for now on...

MotorHead

Something to consider in the selection for high HP engines is which rod weighs more ??
I am also not buying the idea no mechanical engineer has ever designed an H beam rod and it was just invented by a backyard mechanic because it looked like it might be stronger

Street Rat

Interesting read. Thanks for the post.

63mako

Carrillo makes likely the best rods available. They know what they are doing. Here is a quote from them.
"Straight H-Beam
The stoutest beam configuration available. It is capable of handling the most abusive forces in an engine"
Their "A-Beam" looks like a modified I beam and is for lower powered applications.
http://cp-carrillo.com/Tech/RodTech/...6/Default.aspx
Any rod will fail at a given point or due to fatigue.

ajrothm

I recently made 901 rwhp with Callie's Compstar 4340 H beam rods..(their entry level line).... That's roughly 1050-1075 crank HP... I spin it to 7200 rpms.... No issues at all.. And there are people making even more power then that on the same rods... No doubt they are good for 1300-1400 crank HP...

So if H beams are SOOOO bad, these seem to work pretty well for 90% of the LS crowd..

I do use the cheap Scat 4340 I beams in my BBC and Crower A beams in my turbo Hayabusa motor, I have had no issues with any of these rods.. I build a lot of Hayabusa motors with Carrillo H beams as well and never had a problem in the 500-600rwhp range (out of 1300cc spinning at 12,000 rpms mind you)..

drwet

Excellent article. Thanks for posting it.

My only question has to do with the significance of compressive strength of rods. I`m no expert but (other than some composites) most materials have much greater strength in compression than in tension. I guess the question should be do rods fail more often in compression. or in tension.

gkull

Quality rods are rated no matter what the beam looks like. In days past when i was building 360 sprint leased motors I saw lots of broken motors. Especially when they started going above 15:1 C/R for thes Alki motors. Oliver, Callies, Manley............ They learned that light weight low gram bushed rods just snapped up around 8700 rpm. The rules required steel and no titanium.

From pure engineering back ground. It might be true that a I beam design of equal weight might exceed a same weight H beam. But if a manufacturer rates either design at a specific HP I would go with it.

My friend has never broke an aluminum rod with over 50 psi of boost on alki. and 542 ci He just changes them often The are made by BME

CA-Legal-Vette

Good read. Thanks for taking the time to write it up

TheSkunkWorks

OMG, is this still being "debated"?

Among other rods I've run back during my drag racing days, I've had Crower billet I-beams, as well as aluminum BME's. That said, I have no qualms whatsoever about my decision to put H-beams in my 427 BBC redux, or any other high-perf or racing mill, as long as they're sufficiently rated.

FastEddy

In the process of rebuilding my 540 after 10 years of abuse in my "Vette for the water".

The Callies H beams are going back in.... hopefully for another 10....