Over the last few months we have had an in-depth look at the main components that go into building a performance engine. The aim has been to give you a better understanding of what is available, and eliminate the jargon so you can make better-informed decisions about the parts going into your engine.

But this series wouldn’t be complete without having a look at the humble con-rod, so let’s get up close and personal with one of the most stressed parts of any engine.

What is a Con-Rod?
As its name implies, a con-rod (connecting rod) is responsible for connecting the crankshaft to the piston. All the force generated during combustion is transferred through the con-rod and the end result is rotation of the crankshaft. Unsurprisingly, there is a lot of force involved, and the con-rod must be able to handle the massive compressive and tensile stresses without getting bent out of shape.

Con-rod failure never ends well, and usually when one breaks there isn’t a lot of useable engine left to recover. A broken con-rod will usually punch a rod-shaped hole in the side of your block and destroy just about everything else in its path. You can kiss goodbye to your block, sump and crankshaft, and in some cases even a cylinder head may end up damaged.
Since the carnage can be expensive, it goes without saying that a con-rod is not the best place to try to cut corners and save a few bucks.

It’s no wonder they break!
The poor con-rod lives in a brutal environment. As the crankshaft spins around, the con-rod is constantly being pulled by tensile loads or crushed by compressive forces. With all this going on it is no wonder that con-rod failures are common in performance engines.

The factory designed your stock con-rod to be more than capable of handling the power of your stock engine, but what happens when we want more?
Increasing engine power results in more load on the con-rod, but if we double our power, the load on the rod only increases by around 33 per cent. Much more critical is the effect of increasing the engine rpm. As engine rpm rises, the resulting load on the con-rod increases by the square of the rpm change. That means if we double the engine rpm, the load on the con-rod is increased by four times. Keep this in mind before buzzing your engine past the factory redline.

Forging a better con-rod
The majority of production engines still use cast con-rods due to their lower production costs. But cast con-rods are not great for any performance engine. While they are cheap to manufacture, their irregular grain structure and inferior metallurgy limits their strength, which can result in failures at high rpm and power levels.

In comparison to a cast con-rod, an aftermarket performance con-rod will start from a forged blank of high-quality steel.

This allows the quality of the parent steel to be very closely controlled, as well as providing a superior grain structure, which improves strength.

Feel like replacing your con-rods every 20km?

For special uses such as drag racing, aluminium con-rods are a worthwhile alternative to steel. Aluminium con-rods provide a generous weight saving compared to a steel item, which helps reduce the reciprocating weight of your engine. This pays dividends when running to very high rpm, since the inertial loading on the crankshaft is reduced.

Another advantage of the aluminium con-rod is that it is a lot softer and more forgiving than a steel item. This provides an advantage in engines that are tuned to live on the very edge of detonation. The soft aluminium material tends to absorb the massive shock loading of detonation, while a steel rod will transmit this force directly into the bearing shell, which can ultimately result in big-end bearing failures. An aluminium con-rod isn’t a fix for a poorly tuned engine, although it can buy the tuner a bit of breathing space when tuning on the limit.

There are no free lunches in this world though, and alloy rods have one major drawback: their fatigue life. Aluminium con-rods can only be used reliably for a reasonably short number of load cycles before the material fails — maybe as few as 50 passes down the drag strip. Aluminium con-rods are therefore only really suitable for drag race applications when they can be replaced frequently.

Extra care needs to be taken during engine assembly, too, as an accidental scratch on an aluminium con-rod can become a stress raiser, which can cause premature rod failure.

Since aluminium has a higher modulus of elasticity than steel, alloy rods will stretch more at high rpm. This needs to be considered in the engine design, since additional clearance between the piston and the cylinder head or valves is required.

The duck’s nuts
If money is no object then the ultimate con-rod material is undoubtedly titanium. Titanium offers a large weight saving over a traditional steel con-rod, which results in a finished weight around halfway between steel and aluminium. But titanium is stronger than aluminium and has a fatigue life comparable with steel, which means the con-rod can enjoy a long service life. Titanium alloys have become the norm in high-end racing classes such as Formula One, thanks to the material’s impressive qualities.

Design details: H-Beam versus I-Beam
While most production con-rods are an I-beam design, this is mainly for cost reasons and has little to do with strength. When strength is the main consideration an H-beam design is typically preferred, because it makes the con-rod more resistant to bending under high compressive loads. An H-beam rod is normally slightly heavier than an equivalent I-beam design, so your choice will depend on the application.
There is a lot of debate about the pros and cons of different beam designs and which is the ultimate for strength. The reality is that unless you are aiming for world-beating power, a quality aftermarket con-rod from a reputable supplier will probably suffice, regardless of the beam design.

What is rod-to-stroke ratio?
One of the key design criteria in any engine is the rod-to-stroke ratio. This ratio is the length of the con-rod divided by the crankshaft stroke. Higher rod-to-stroke ratios result in less side-loading of the piston against the cylinder wall as the angularity of the rod is reduced. A longer rod-to-stroke ratio also reduces piston acceleration and results in the piston dwelling longer at TDC, which can improve efficiency and power.

As an example, a typical road car engine may have a rod-to-stroke ratio in the vicinity of 1.5 to 1.8. On the other hand, the highly developed race engine in a Formula One car may have a rod-to-stroke ratio of around 2.5 to 2.8. With production engines it is difficult to make dramatic changes to the rod-to-stroke ratio, but it is often possible to fit the wrist pin higher in the piston and fit a slightly longer con-rod to gain a small improvement.

Bringing it all together
If you have followed the last few issues, you should now have a better understanding of all the major components of a performance engine. If you have been paying attention, you should be able to weigh up the pros and cons of different components and, most importantly, choose parts that will be suitable for your intended power level. Nobody likes a broken engine, and choosing the right parts is the first step to a reliable and powerful engine that will serve you well for years to come.

Words: Andre Simon

This article is from NZV8 issue 66. Click here to check it out.