Was cruising the internet for some info on forged internals and came across this article over at Badasscars.com gives a decent look at what happens to stock internals and the stresses they endure in HP applications... Just food for thought so enjoy!

Do I really need forged rods and forged pistons?


That's a BIG question, and it depends on what you are running and how you treat (or abuse) your engine. Let's start with the rods first. First of all, ALL rods are made out of forged steel. They certainly aren't made out of cast iron, or they'd simply be too brittle and would snap in half. With that, most factory rods are fairly strong. In the older days, we used to polish the beams of factory rods to get rid of "stress risers", and install stronger SPS rod bolts and run them in 500HP race engines. There have been some pretty high reving factory engines with stock rods, such as the famed Ford BOSS 302’s, the early Chevy 327’s and the really high reving Chevy 302’s that came in the first generation Z/28‘s. All of which were notorious 8,000 and 9,000RPM zingers. So with all of those coming off the show room floor with 11:1+ compression and being capable of spinning in excess of 8,000RPM, it pretty much goes to show that a stock rod (in some cases) can withstand some pretty nasty abuse. These days though, we are making MUCH more power than we did decades ago. And with that, there are more stresses and loads being put on the internal parts, so sometimes forged alloy steel rods are a better choice for certain applications.

Now I‘m sure most of you know someone that‘s thrown a rod out the side of the block. Well, the truth of the matter is, it usually wasn’t the rod‘s fault at all. Here’s what happens in reality 90+% of the time; the pistons hit TDC and get yanked back down within a fraction of a second. The more RPM you spin the engine, (and the heavier the pistons) the more kinetic energy builds-up which in return, yanks on the rod cap harder and harder. Increasing the stroke is another way of increasing the load on the rod cap when the piston hits TDC, stops and turns the other direction again in a nano-second. The loads are tremendous. Remember this too; whenever you increase stroke, you also increase piston speed because the piston is traveling a greater distance per revolution.. By increasing the piston speed you again add to the kinetic energy even more.

If you took a small sledge hammer and pushed it up in the air as far as you could, and then yanked it back down. You’d feel some serious stress on your arm. Now if you did it as fast as you could, you’d probably pull your arm out of its socket from the built-up kinetic energy from the speed of your arm and the weight of the hammer. Well, the rods and pistons are going through the exact same thing. If you make that sledge hammer even heavier and did it even faster, you would increase the stress and loads on your shoulder and arm, and of course, serious bodily injury WILL occur. Well, this same scenario is exactly what’s happening with the rod, which is acting like your arm.

As the piston stops it’s upward motion and then changes directions back down again, the one thing that is taking most of the stress is the rod cap, (assuming the piston is strong enough to hold the wrist pin in place). What holds the rod cap tightly in place? The rod bolts. Bolts are only so strong and can only take just so much stress before they stretch and break. When a rod bolt stretches, it can no longer keep the rod cap tightly in place, and when that happens, the bearing comes loose and you “spin a bearing”. When you spin a bearing, you create LOTS of friction and heat and even more stress which yanks on the rod cap with a hammer-like motion, which eventually rips the cap right off.

Most of the time, the rod cap gets yanked-off when the piston has just hit TDC, and starts on it’s way back down again, which in turn, smacks the piston into the head. Once the rod cap gets yanked-off, the piston and remaining part of the rod just sit there at the top of the cylinder until the crank swings around again on the next revolution and guess what? It smacks the bottom part of the rod and throws it out the side of the block. So it’s usually not the rod’s fault for snapping. It’s usually due to rod bolt stretch which ends in ultimate failure.

Now, why don’t we just install new, stronger rod bolts in all of our rods and stay with stock rods? We do in mild engines, but in a serious engine making serious power, we always use after market forged rods, because after market rods are made out of a stronger material, and in most cases, they are lighter, which relieves some of the kinetic energy and loads. Keep in mind the ONLY thing keeping the rod attached to the piston is the wrist pin. Although wrist pins are made out of tool steel and are ultra strong, the bottom part of the piston that “houses” the wrist pin is usually pretty thin. The heavier the piston and rod, and the more RPM you spin the engine, the more it is trying to yank the wrist pin out the bottom of the piston. Yanking wrist pins out is common in cast pistons and hypereutectic pistons when they are used for racing purposes, especially in circle track racing. That’s another reason why forged pistons are the choice for racing, so the wrist pin doesn’t get yanked out the bottom of the piston.

So what about the new PM (Powdered Metal) rods that GM is using in their crate motors? Powdered metal is a forging process that takes metal particles and mashes them all together. Kind of like fiber board used in cheap furniture. The problem with this stuff is it’s hard and brittle. Most rods will take a bit of stretching or flexing with no real harm. Hard metal though is brittle and can’t take stretching or flexing because it will snap. PM rods are OK for mild engines, but at high RPMs and under high stresses, they tend to snap. We use factory rods in most mild engines, and we always fit them with super strong ARP bolts to increase the strength of the rod. We also use many types of forged steel rods (5140 steel, 4340 steel, etc) on our more serious engines. It just depends on what the engine is going to be put through.

That only covers a tiny fraction of rods and such, but I think you’re getting the idea anyway. Now onto pistons. ALL pistons are made out of aluminum. Most low performance and stock pistons are cast aluminum. There are different alloys of aluminum which all have different characteristics. Hypereutectic pistons are simply a stronger type of alloy with a higher silicon content, which helps bond the aluminum particles together more tightly, which in return, makes the piston stronger. Some forged race pistons have a high silicon content and other alloys have a non-silicon content. It just depends on the expansion rate relative to heat and stress levels the piston is going to endure. Some pistons need a wide clearance because they expand so much, and other’s can get away with tighter cylinder wall clearances because the alloy doesn’t expand as much in heat. Remember.. horsepower makes heat. This is a pretty involved subject so I’ll keep it short and simple. Forged pistons should ALWAYS be used in nitrous applications, supercharged applications, serious street performance applications and racing applications. Cast pistons should be used in stock applications or hypereutectic cast pistons can be used in most street performance applications where nitrous or supercharging isn’t in the future. Just remember, the lighter the piston, the less kinetic energy and stress gets put on the rod. Of course there’s a LOT more to it than that, but this is free info and I ain’t got all day to teach everyone this stuff.

 

I couldn't read past this. Sort of discounts what the rest of the article says.

 

 

They could have elaborated a bit on different alloys, but that could be a whole other article.