Rod Ratio

We all hear a lot f advice about rod ratio - usually variations on the old "use the longest rod that will fit."  Gene Berg actually recommended the opposite, on the argument that it will increase torque in the usable range.

Why a high rod ratio? There was an excellent explanation on the net I will have to find,but basically he made a model with a rod ratio of 1 for the extreme of short rod and an infinite rod for the long.

Remembering these are impossible extremes for the sake of illustration: 

The low rod ratio model:
Starting from TDC, the piston will start moving immediately and will rapidly reach it's lowest point when the crank reaches 90 degrees ATDC. It will then stay at this point until the crank reaches 90 degrees BTDC.

Advantages here are - the more rapid movement of the piston in the early part of the stroke creates greater vacuum and higher intake velocity. At lower engine speeds, this pulls harder on the carburetors allowing them to operate in a better part of their curve, and the increased intake mixture speed will have more inertia, so will keep flowing longer after the piston ceases it's effective movement.

Disadvantages here are - the rod reaches an extreme angle, in the example 90 degrees, which is cocking the piston and pushing it against one side of the cylinder. This increases friction, wear and can interfere with ring operation. The increased piston speed at particular engine speeds limits the maximum rpm for a given set of components.  This also increases the velocity of the intake mixture, meaning large ports are required for a given rpm, though this is not linear, as the inertial fill can make up for some of this.

The high rod ratio model:
Starting again at TDC, the piston will start moving rather slowly (as the top part of the circle is almost level,) and this time will take the entire 180 degrees to travel to the bottom of the cylinder where it will immediately begin the return journey.

Advantages here are - as the piston uses the entire stroke to travel the length of the cylinder, it does not pull the air as fast, meaning the engine can reach a higher rpm before the limit of the intake system is reached. The rod in the theoretical example remains vertical, removing the side loads.The maximum piston speed is reduced, as is the change in piston speed at the top and bottom of the stroke.

Disadvantages here are - while the piston moves throughout the entire stroke, it doesn't move very fast in the early part of the stroke so does little work, then the maximum speed is lower, so produces less inertial fill later in the stroke when the piston is again moving more slowly.  So while we will get higher peak rpm, hence power, we will struggle to fill the cylinder at lower engine speeds.

Which brings us to the most important part of the question What rod ratio should I use?

In the real world, we are going to largely limited by the physical characteristics of our engine and engine bay- no-one is suggesting a stroker is not a good idea - even if you are not tempted by the torque, you will eventually need to go this route to achieve power through cubes.  This will largely determine your rod ratio - eg an 82mm stroke with stock rods will give you about 1.67, down from the stock 1.99. If we make the engine a little wider, we can fit a 5.5" rod which brings our ratio up to 1.7 - how much difference will this .03 difference make - not much. Either through calculations or driving, we will not find much difference, except in an all-out strip monster and even then it will only appear at the top end.

So choose a rod that suits your engine - personally I'd aim for minimal or no cylinder spacers and an appropriate copper "Head Gasket" as you will get the best sealing at both ends of the cylinder this way.

Save your "Porsche Length" rods for your stock bore stroker, where you can't get 'B' pistons, and longer rods for longer strokes where you have no choice.  If you want to fit an 86mm stroke into your road-going car, you are going to have to attack the body to make it fit!