Rods and Rod to Stroke Ratios

by GhostMachine


Connecting rods were an overlooked part of buildups not too long ago but with the recent long rod craze you may be asking if you should be running long rods???First lets cover the basics. In the image you will see the black lines indicating the center of both the large and small ends of the rod. This is the length of the rod. If you have a 5.7″ or 6″ rod it is measured at these points as are all other rods.

Rod to stroke ratio is a simple ratio. If the rod is 5.7″ and the stroke of the crank is 3.48″ you have a 1.6379 rod to stroke ratio. In theory the higher this ratio the more power it makes. The reasoning behind this longer rod is more power comes from a couple of aspects. One; Is that if you go back to basics of a simple machine the pistons, crank and rods are basically a compound lever. The up and down movement of the piston translates into rotational motion of the crank.

In an engine a longer rod can make more rotational TQ from the same piston force because it’s a longer lever!

Two; Due to the angles involved a longer rod is less angular than a shorter one and side wall loading is less. What this means is that the piston is pushed more up and down the bore than to the side of the block thus reducing friction and increasing HP.

As you can see in the below example that rasing the piston pin height changes the rod angle noticeably. You will also notice that the piston top doesn’t change. Many people believe that longer rods make an engine a stroker but this is simply not so. The only thing that can change displacement in an engine is the bore or the stroke.

Three; The pistons speeds change with rod length. The piston has more dwell at TDC and BDC. The slower it gets at TDC the more pressure builds up at the ignition point. The piston also accelerates away from TDC and BDC quicker making the intake compression and exhaust strokes more turbulent thus making more power.

So what will fit and how do I figure it out???

Here is a simple way to find out what you need to know about fitting rods.

The deck height of a small block is usually around 9.025″ from the deck to the crank centerline. The “Blueprint” figure is 9.000″. In order to work this you need to know the stroke of the crank and length of desired rod. You can figure the compression height you would need to make this work. For example a stock 350 would be 3.48 stroke and 5.7″ rod. If you add half the stroke and the rod together you get 7.44″ If you know it has to fit a 9″ space you subtract the 7.44 from 9 and get 1.56″ witch is the compression height of a stock 305/350/400 piston. This is also why the 400 has a short rod. GM didn’t want to raise the pin in the piston so they shortened the rod to 5.56″. If you take 1/2 the stroke of 3.75 or 1.875 and add it to 1.56 compression height and 5.56 rod length you get 8.995 or 9″. You can also shorten the stroke too. Say you have 1.56 pistons and want a 6.125 rod. You can figure this out by adding them together and subtracting 9″ and the result is 1/2 the stroke so double the figure and this is the stroke you can use with stock 350 pistons in a 350 block with 6.125 rods. 1.56+6.215=7.685-9=1.315*2=2.63″ stroke. Of course nobody wants that small a stroke. If you go the other way and install 6.125 rods onto a 3.25″ stroke crank for a 327 you will need 1.250 compression height pistons to make them fit.



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