We found this fantastic article by 540 Rat on the Speedtalk Blog.
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I-Beam or H-Beam – which rod is best?
By 540 RAT ©

I do NOT sell connecting rods, so I have no vested interest in what rods people choose to use. 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 They were originally made by someone who “thought” they might be better and/or cheaper to make, without benefit of any Engineering analysis.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 having them designed by actual 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) determined by proper buckling and crippling analysis, 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, your engine can rev quicker, and 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 such as:

• OEM automobile engines, including the Supercharged Corvette
• Aluminum rods (that aren’t a solid rectangular cross section)
• Piston aircraft engines
• High performance high rpm motorcycle engines, which put out way more HP per cubic inch and spin to much higher rpm than most of our stuff ever will.
• Heavy equipment that uses Diesel engines, such as big rigs, bulldozers, earth movers, cranes, ships, trains and other industrial engines.
• Big rig trailer frames
• Aircraft, spacecraft, and ship structural frames
• Large in-building overhead crane main support beams
• Bridge construction
• Large building construction
• Etc., etc.

The fact that I-Beams are used in these applications, to name just a few, should serve as a sanity check for those who “think” H-Beams are better. H-Beam fans need to ask themselves one question, “If the H-Beam is better, then why haven’t they been used by Degreed Engineers in these applications over these many, many decades?”

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.

BOTTOM LINE: For comparably made I-Beam and H-Beam rods which weigh the same, the I-Beam will be FAR STRONGER IN COMPRESSION, than the H-Beam. For comparably made I-Beam and H-Beam rods that have the same strength in compression, the I-Beam will be FAR LIGHTER. The Engineering facts (NOT theory and NOT opinion) are that the I-Beam rod is simply a far stronger, lighter and more efficient design than the H-Beam. So, no matter what anyone tells you, there is simply NO good reason to ever use an H-Beam rod. It makes no sense to buy H-Beams when the clearly superior I-Beams are readily available.

H-Beam users sometimes get emotional and nasty about the fact that H-Beams are inferior in every way, and that they could have made a better choice. But, emotional outbursts will NOT change the Physics involved that prove the superiority of the I-Beam rod design. Of course people can use whatever they want, and for those 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.

540 RAT

U.S. Patent Holder

Member SAE (Society of Automotive Engineers)

Member ASME (American Society of Mechanical Engineers)

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