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rod ratios

548 Views 4 Replies 4 Participants Last post by  camachinist
what exacatly are rod ratios? and whats a good ratio to stay in?
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Rod ratios and their effects are subjective when applied to an engine, mostly with a stroker engine. It goes something like this:

The rod ratio is determined by the length of the rod as compared to the stroke of the crank and a longer rod is considered to have more HP potential because the piston stays at the top of the stroke for a longer period of time. That therefore enables the engine to take advantage of the combustion burn for a longer ammount of time. In simple terms = the same burn over a longer ammount of time will give more HP.

All that sounds like the holy grail of engine building, and when building a NASCAR engine or a Pro stock, well, maybe it is. The problem is that changing just the rod ratio from a standard to a long rod motor will net maybe 2 or 3 percent, big stuff in an Indy engine, but nothing worth noting on the street.

In practical terms a stroker engine will have a shorter rod and make a bunch more HP. The reason is that you have increased the Cubic Inches and that is what makes the HP. So you shorten the rod with a stroker and loose 3 percent because of that, and add 30 percent because of the increase of Cubic Inches (and other factors like voluumetric efficiency).

So, will a better rod ratio make more horsepower? Sure. Is it economicaly feasable? Not unless you are on an unlimited racing budget and it is the last place left that you have to go to make more horsepower.

One last thing, the longer the rod you run the farther it pushes the wrist pin up into the piston (design-wise), at least with a finite crank CL to deck height. Past a certain point and that will severely compromise the stability of the piston in the bore, unseal the rings, and rock the piston in the bore creating accelerated wear and a higher chance of catastrofic failure at high RPM. On the flip side of that, Too long of stroke will give a much higher side load on the pistons in the bore, also rock the pistons, and wear the bores out much quicker.

So is it worth it to buy a long rod 302 or 351 kit? hahahahahaha, (oh, sorry)................... *G*.
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