Engine Design
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Connecting Rods

Connecting Rods

This is the bit that connects the knee bone to the thigh bone that connects to the???? Well kinda.

It’s the bit that transfers rotational movement into reciprocal movement .Are they important? Absolutely. They push or resist against the compression forces as the piston is forced downwards in the compression stroke and are stretched and have to resist the forces trying to pull the piston through the top of the engine on its upwards stroke. The very same forces that put strain on the bolts holding the beam and cap together.

So what makes a good rod? Depends upon use. Most street engines today have rods manufactured by sintering powdered metal or baking this powder at extreme temperatures. Quite strong under compression but not so good in tension and twisting.

Weight plays a huge part in how a rod can add to an engines performance and fuel mileage. It takes energy to push and pull a heavy rod. The engines ability to rev faster is affected by the rods weight. This effect on performance, along with strength, should be considered when choosing a rod. If we take the Porsche GT3 engine, they come from Porsche with forged titanium rods. These are included for no other reason but to help the engine accelerate quicker. Why do many put steel rods in these engines when they upgrade them? I can only assume they do this for cost cutting reasons. And possibly, they haven’t given any thought to the downgrading of performance. Maybe they don’t understand. As a customer, you need to understand the negative effect on performance a steel rod has verses a lighter titanium rod. If strength is your only concern, saw off a piece of railway line and use that. That will never bend, stretch or twist. But it will take away performance.

Some will tell you titanium rods fail. If someone can send me one that failed because it’s made from titanium it will be the first one I have ever seen in the last 40 years! This sort of engineering based upon fear and ignorance costs the buyer performance. Get the picture?

So, design plays a huge part in any rods ability to withstand all of the forces applied to it under running conditions. It’s ability to hold the big end round maintaining the correct clamping load on the main bearing shells. This is critical so the correct bearing to crankshaft journal clearance is maintained keeping the oil film consistent between these two parts. Also, at the other end of the rod, making sure the pin bushing stays round for exactly the same reasons and parallel to the crankshaft plane.

There are many different shapes and sizes. Beam designs can be A or H shaped and there are some others that are used as well. The A and H patterned rods are the most common. These two designs offer the strongest solutions when tested in any FEA simulation. Many of the other designs do not test as well, but are ok in some applications where critical requirements can be compromised for a lower cost.

In forced induction applications steel rods are the most common. Strength is extremely important as the forces in these engines are greater. We service some engines that make over 750Bhp that have titanium rods.  And, in all of the years we have serviced these engines, I have never seen one fail. Yes, they require regular maintenance, but this is the cost you pay for performance. Most of these particular engines are never pushed as hard today as they were back when they were raced professionally. Although, the same can be said for most street and club raced engines. Again, it’s the fear of the unknown. There is no excuse not to know today. FEA, CAD simulation technology exists today that gives accurate answers to accurate questions. If your engine is being built without this knowledge and you are compromising the engines performance due to the parts included, STOP! Ask why and demand the same or better than stock. It’s your engine, it’s your choice and it’s your money.