High-Performance Connecting Rods Explained
Published by Adam Pol,
Easily one of the most overlooked engine components which are literally a driving force in the function of every engine are known as connecting rods. They are shaped much like a rod and connect two vital components together pistons, and crankshafts. To understand what a connecting rod does you need to understand how a car engine works. With fuel and air getting mixed together a spark is created by the spark plugs which ignites the fuel and as a result, a controlled explosion of a small scale takes place.
This explosion is what pushes the pistons which thanks to the engine rods make the crankshaft move which in turn makes the wheels on the car turn. This happens a lot faster and it is how every engine in all petrol or diesel cars work with some minor differences. We know that a conrod plays a major role in transferring reciprocating motion which is needed to move a vehicle. But a more important piece of information is that by replacing the stock conrods you can affect engine performance as long as you know what your engine needs.
What to Consider
Material
When it comes to the material a connecting rod is made of steel is going to be the more common option here. Why? Because steel conrods are strong and durable without being as susceptible to damage as their lighter counterparts, aluminium rods. High performance connecting rods come made of aluminium because not only are they lighter but they are also more rust resistant whilst acting like shock absorbers. This is because they give in a bit to peak loads which helps absorb some of the pressure and transfer it to other components. Aluminium is not as strong as steel though but it is more environmentally friendly.
Length
The length of a connecting rod plays a huge role in how it performs. Also known as the rod ratio is is the length of the rod divided by the stroke of the crankshaft. If you're considering making a high-performance vehicle then you need to include longer high performance connecting rods.
The reason behind this is that with a longer rod the piston dwells longer at the top dead centre of the compression stroke. This helps build up more pressure for a bit longer before the pistons start moving again. With the result being a little bit more power being added back into the engine with ever stroke which makes for both a flatter and broader torque curve.
H vs I – Beam
There are two types of connecting rods you'll find on the market H and I-beam. An H-beam conrod looks like the letter "H" when you look at it from above with the cross-section cut off. Some may say that the H-beam design is much stronger than the I-beam but this is only true for high combustion pressure with forced induction. If you have a high-revving naturally aspirated engine then the I-beam is better suited since these engines are lighter thus create less pressure. An I-beam conrod is better at handling normal forces while the stronger H-beam design is better at handling abnormal forces in the engine.
HP vs RPM
When it comes to performance, everyone is debating whether horsepower or RPMs (revolutions per minute) are more important. High RPM means there's an increase in tensile strain whilst high HP means an increase in compressive force. What you need to know is that the majority of rods fail because of a high RPM since they're pulled apart. So it's better to go for extra HP than high RPM ranges but if you want a high-revving engine then you need to increase the tensile strength which is best done with steel rods.
Bolts
Remember that the majority of conrods fails because they get pulled apart by high RPM. Well, with a solid set of bolts the rods won't get separated from the crankshaft. The push and pull force generated by high-revving engines can be resolved with rod bolts made from more premium materials. This way you get higher tensile rods for a lower price than what you would pay for if you were to install steel connecting rods.
Weight
While I-beam and H-beam profiles determine the strength of the rods together with the material they're made of, they also determine the weight of the rods. A heavy rod can make transferring the reciprocating a lot harder which in turn makes the engine work harder as well. A lighter conrod means a more fragile one which can ruin a high-performance experience in a split second. What you want here is a balance between weight and strength.
Adding parts made of stronger materials than the conrod itself adds in weight without slowing down the engine. Remember heavier parts can handle high horsepower settings and lighter parts, read rods, are made to be used in naturally aspirated high-revving engines. If you want the best of both worlds then it is wise to balance strength and weight.