Rod Length Ratios

There are as many theories about rod lengths as there are any other subject that deals with racing.

As strokes get longer, rod lengths get shorter and at some port in time, this will create a problem. We don't know if we've crossed the line yet, but we're definitely leaning on it. At the same time, there's a point where rod lengths can get too long for a particular stroke and we're leaning on that line also.

Rod length ratios are calcualted by dividing the rod length by the stroke.
Example: a 5.000" rod length divided by a 3.000" stroke equals a 1.67 rod ratio.

For a good illustration of what increasing the rod length does for and engine, we'll use a 350 Chevy. A stock 5.7" rod divided by a stock 3.480" stroke gives us a 1.637 rod ration. Now, put a 6.00" rod in it with the same stroke and the ration inreases to 1.724 and the engine produced more power and rpm. This is a known fact that's been around for some 25 years.

A stock, 5 hp. Briggs & Stratton engine uses a 3.875" rod and has a 2.437" stroke which it's at top dead center (TDC) and this will make more power.
The second most important thing is that it improves the leverage the piston exerts on the crank journal and this also increases power.

Another feature of using a longer rod is that it creates a much friendlier environment ofr the pistion, cylinder and crankshaft to operate in.
Consider this: The piston is moving up and down in the cylinder trying to make a crankshaft rotate in a cirlce. When the pistion is in a down stroke, the resistance of the crankshaft is trying to push it out the front of the block and in an up stroke, with the resistance of compression, the crankshaft is trying to push it out the back of the block.
Our recent test engine had a 4.225" rod with a 3.000" stroke equaling a 1.408 rod ratio and we believe we may have gone beyond the short rod ratio limit but, the engine made a bucket full of power and survived even at 9,3000 rpm.

Rest assured that one day we'll reach the point of sheer stupidity.

Article From Arc Racing.com - July 18, 1998