Role

The connecting rod's job is to take the power given from the top of the piston, and transmit it to the crankshaft. The stress put upon the connecting rod is extreme, as it not only has the force from combustion to contend with, but also the inertia from reciprocation. So not only does it have thousands of pounds of pressure pressing down on it, the rod also has the effect of stretching from BDC to TDC on the exhaust stroke. Not a simple task, yet most of us ignore this very important piece of metal in our motors.

Types

The stock rods in the 60V6 motors are forged steel, with a GM rating of 7000 RPM. Although they are forged, they still have some defects from the factory. The surface has what are called "inclusions". An inclusion would be the bumps on the surface. These bumps are the weak points, and through shot peening (the process of propelling small steel shot through an air gun towards metal, compressing the surface and relieving crack formation), you can reduce/eliminate these impurities. Use of stock rods is actually perfectly fine for almost every 60V6 buildup. Going to a higher RPM would be the main reason to upgrade, or if you are increasing power by a LOT.

Aftermarket forged rods are the next step up. Generally made from a better material to begin with and have more thorough inspections. They still have the impurities that the stock rods have, only far less in quantity. These rods also come with better connectors, such as ARP bolts and caps.

Billet steel rods do not have the inclusions found in the forged rods, which make them less likely to form cracks, which is a good thing. The downside to billet rods is the decrease strength at the crank end of the rod. This is explained via grain flow. You can think of it like a piece of wood, where it is easy to split in the direction of the grain, but nearly impossible across the grain. Forged rods have a better grain around the crank end than billet rods do. The tradeoff isn't that bad, as having a pure rod that is resistant to crack would be a better characteristic.

Fully machined forged steel rods are still forged rods, but they have been machined to eliminate the surface impurities. You get the grain flow for strength, and the perfect surface so cracking the rod isn't going to be a factor either. Not bad at all.

Aluminum. Lighter, but bulkier to maintain the strength of a steel rod. This would be nice to have, but there are some drawbacks, which will keep these out of my daily driver. Aluminum expands faster than steel, which means a cold motor can cause a spun bearing when the rod has expanded but the crank has not. Aluminum rods must also be removed and measured to be sure that they are still the proper length. Basically, don't even bother unless it is an all out racing 60V6.

Titanium. Bend over, this is going to hurt. It's lightweight and strong as hell. The only real drawback is that they nick easy if you mishandle them, which will ruin them. Otherwise, if you got the money and the need, titanium is the way to go.

Conclusion on types

(my opinion, get as many as you can before making ANY decision)

Stick with stock for all bolt-ons, or even a boost upgrade. If you are going for a lot of boost, you may consider the aftermarket forged rods, or just shot peening the stock rods and going with the ARP fasteners. High RPM motor? Get the fully machined forged rods. NOS, SC, TURBO, High RPM, and Alcohol? Email me, cause I think you are nuts. Titanium.

Length

The stock length for most 60V6 motors is 5.700" (the 3500 has a rod length of 5.9"). There are a few reasons why you may want to change the rod length in your motor. Before getting into it though, remember to change your pistons as well, or you will have issues. Some other areas to think about are:

Rod/stroke ratios: Take the length of the rod (5.7) and divide by the stroke (3.31 for the 3.1, 3.4 and 3.5 motors, 2.99 for 2.8). This doesn't have an effect on the motor by itself, but it can be used against some guidelines. It depends on whom you talk to and what you read, as there isn't a clear-cut answer to what is the best R/S ratio. Some say 1.75, others say 1.80. Reading the links below will confuse you and educate you on this topic pretty well.

Dwell: How long the piston is at TDC. This is where the power is, and having longer dwell is a good thing. This is why the higher rod/stroke is better for more power. There are other issues however, which is why the R/S isn't clear-cut.

Side load: Amount of pressure/friction on the cylinder walls. Longer rods = less friction = more power. Longer rods sound like a good idea still.

Deck height: When going with a longer rod, you have to make sure your deck height will allow for it. A piston change may be required, so make sure you have enough room for the piston on top of the rod, or you may end up with a rod that is too long.

Wrist pin (piston): Custom pistons will need the wrist pin location changed to accommodate the longer rod so that the piston isn't stick out into where the head is located.

*More will be added as questions come up relating to connecting rods. Got a question, post it on the forums.

Links

Grape Ape Racing .pdf file full of info
Rod Length Relationships