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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 ratio is the relationship between the length of the connecting rod and the stroke of the motor. Generally speaking, it is desirable to have a higher rod ratio than a lower ratio. You start running into a lot of ware issues when the rod ratio is below 1.55 and a stock 302 ratio of 1.69 is considered OK for a street motor. Most stroker motors have a lower rod ratio due to the longer stroke and limited room for a longer rod. Again, generally speaking, it is easier to increase HP with more displacement on a NA motor then any other modification. There is a direct relationship between cylinder pressure, displacement and HP. A long rod motor increases cylinder pressure with out affecting compression ratio and displacement.
Why would one want to build a long rod motor?
1) If you run in a class where displacement is limited then you can gain extra HP by increasing the rod ratio.
2) For a given displacement, you can increase the efficiency of the motor. You can make more HP on the same grade of fuel and maintain or even get better MPG.
Again, it is easier to make more HP with more displacement but I’ve selected to build a more efficient 302. As soon as I get my car back together, I’ll chassi dyno it and post the results and MPG results.

Hope this helps
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