Rod Length for Performance Engines

by John Connolly

So what is the relationship between power and rod length? Any other sage advice on building a 2110 that will not likely see 5000 or more RPM? For instance, heads and carbs?

With a rpm powerband that you need, use the shortest rod you can get. Stock or even Porsche length.

All else being equal, a longer rod moves the powerbanD UP higher in the rpm range, and a shorter rod is torquey at the low rpm band.

If you have some spare parts, assemble this (or imagine it if you can see in 3 space well): an assembled engine with a long rod on one journal, and a short one on the other. Deck height is the same (adjusted, of course). Now, as the cylinder fires, and the piston moves down say, .060", you will see the short rod engine moves the crank MORE crank degrees because of the short rod, and the long rod moves barely at all. Imagine a rod length of 0, and a rod length of infinity to better understand this.

As a result, the long rod pistons "dwells" at TDC longer, and allows the pressure to build up. Obviously at high rpm this is good for maintaining power since the short rod engine would have the piston halfway down its stroke in no time (no push)! Conversely, the short rod engine at low rpms and high loads will not ping and the engine has "leverage" to spin the crank, where the long rod engine "stalls".

Since the piston "timing" is different on the two respective engines they have different characteristics regarding camshaft selection and timing requirements. Also, the shorter engine will "pull and push" the charge thru the ports harder, running out of "breath" at a lower rpm while the long rod engine keeps pulling! VW managed to incorporate this characteristic on the T-4 engines which have a VERY short rod, which is ideal for the heavy bus. Obviously high rpms are a "no no" on this engine without a rod change (and length change). Some high rpm engines use insanely long rods to "slow" the piston down to keep parts together and power up at these rpms. The biggest drawback is engine size, since we can only move the piston pin up so far (the ring stack gets in the way) to keep things close to the same size.

Lastly, rod bearing life is shorter on short rod engines, and so is piston and ring life (due to the side loads associated with the "extreme" angles the short rod engine's see).

Hope this makes some sense. This is one aspect of engine knowledge that most people are clueless about.

On a 2110 you can NOT use the stock rebuilt T-1 rod without asking for rod failure. This rod is only safe to a 78mm stroke. If you go to a better rod, don't be afraid to crank it to 86mm.

For your engine, use the Engle 110 cam (SCAT C-35 is the same), and a 40 X 35.5 head (good one).

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