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R36 Member
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796 Posts
Discussion Starter · #1 ·
How's it goin' fellas!

I'll be upgrading the engine internals, I'm trying to choose between H-Beam and I-Beam. Can anyone enlighten me on why I would choose one over the other? For example, HKS provides H-Beam ConRods and AMS provides I-Beam. What's the pro's and con's of each?

Thanks for your response in advance!
 

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Fast GT-R
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3,129 Posts
The discussion came up last week, here in another thread about I-Beam rods vs H-Beam rods, and which is better. So, I thought I'd post it as a stand alone discussion for those who may have missed it and may like to see it. This all came about after a 632 Mountain Motor coughed a Carillo H-Beam rod and EXPLODED on a chassis dyno. Though the owner Steve never gave out a dollar amount, others have estimated that when that motor exploded, it was like flushing $30,000.00 down the commode. So, the search was on to find an appropriate rod for the rebuild. But research had been inconclusive, because some claim that either one will do, but H-Beam makers and others say H-Beams are best, while I-Beam makers and others say I-Beams are best. They can't ALL be right.

Of course rods need to have enough absolute strength to not be snapped apart by the maximum tension forces at TDC exhaust, where the piston would like nothing better than to go right up through the head. Those tension forces are mitigated some during the opposing compression forces at TDC compression. Absolute strength is determined by the total cross-sectional area of the rod. The material being used has to have a low enough psi loading from the forces it sees, to have a positive margin of safety, if the rod is to survive a reasonable length of time. So, there is a minimum total area that will get the job done for any given rod material and engine combo.

Rods also have to be stiff enough to stand up to the enormous cylinder pressure forces it sees on the power stroke, which would like nothing better than to bend the rod into a C-shape, if the rod is to have an acceptable fatigue life (durability). If the rod bends/flexes on the power stroke, even a little, the fatigue life will be reduced by the pressure cycles causing that repeated bending/flexing, which will eventually lead to micro-cracks and ultimately total failure. And the rod does NOT have to be bent beyond it elastic limit for fatigue to take its toll, causing cracks and failure.

The bottom line is that we need an ideal rod to be both STRONG enough and STIFF enough, in order for it to last more or less indefinitely. If you don't have both things, then your rods will be a ticking time bomb and will give up the ghost sooner than you'd like, which will ruin your whole day. Aluminum rods that are used in really serious stuff, for reduced weight and for some cushioning, so to speak, are considered disposable items because of flexing reducing their fatigue life. So, for most gear heads and less than Pro Racers, the rod material of choice is a high quality steel with high quality bolts. So, from here on out, the discussion of I-Beams vs H-Beams will assume the same material and bolts are used for both types of rods, so that the only consideration between the two is strength and stiffness.

Personally, I hadn't come across any good factual analysis between both types of rods. Only opinions, personal preferences, and who's marketing what, seem to be plentiful. But nothing concrete is available to really help anyone make a good intelligent buying decision. And if that isn't bad enough, some tech info put out by certain manufacturers as well as some magazine articles coming from them, are filled with incorrect information. So much so, that it became quite apparent that most rod makers are not even engineers at all, they simply make stuff based on what's been done before. I've run across this issue countless times before with other aftermarket Hotrod/Race parts that don't fit or function as they should. That being the case, I decided it was time for me to do some analysis myself and see once and for all, where the chips fall. Keep in mind the important things here come down to AREA in square inches for absolute strength, and STIFFNESS value called "I", (and this has nothing to do with the shape of the rod, its just a name here), which is used in an equation to calculate the amount of bending/flexing. The higher the area number, the stronger the part. And the higher the stiffenss "I" value, the more durable the part, from extended faigue life. So, consider the following and decide for yourself:



But keep in mind, that even the best rod "design" could fail if it wasn't used in the right application. For example an I-Beam that is perfect for a stock 396 isn't going to live in a bad boy 572. It must be robust enough for the job at hand. Also material imperfections as well as surface nicks, can also derail your plans. So, all those things need to be kept in mind as well.
http://forums.corvet...-beam-rods.html

this might give you some idea.

BTW, who is the lucky shop? Regardless which rods you go with, choosing the right shop is way more important.
 

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R36 Member
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796 Posts
Discussion Starter · #3 ·
Excellent, thanks for the quality post Se7enGTR! The shop is TBD at the moment, but it will definitely be one of the vendors on the forum. Awaiting all of my parts to get started
 

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Fast GT-R
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3,129 Posts
Excellent, thanks for the quality post Se7enGTR! The shop is TBD at the moment, but it will definitely be one of the vendors on the forum. Awaiting all of my parts to get started
Nice. Let me know if you need my opinion to determine the lucky shop. We can definitely discuss that in pm. GL with your research.
 

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SPI Performance Consultant
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2,997 Posts
We run custom Carillos in our engines without issues, there is more than just stiffness and weight that needs to be analyzed when determining what rods to run..
 

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GTR Nerd
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5,335 Posts
There is something wrong with the diagram above. I need to sit down and look at it,and think about it. I think what does jump out at me is that we are looking at a top down cross sectional area, and a rod doesn't take loads quite the way that it is showing.

Carillos are proven parts. In many of the best engines in the world. However, any part when pushed beyond its design limits will fail. Spin a rod bearing due to knock, and it will overheat the rod. It will break the rod. I really like the Carillo rods. Its odd to say, but when you hold them, you know there is quality there. I have run a lot of different rods in engines, and when its important to have a good part, I use Carillos.
 
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