Rod ratio vs reversion
04-09-2017, 03:31 PM
#11
04-09-2017, 03:46 PM
#12
I don't believe that is correct. The dwell will be the same at either end (TDC or BDC). It is a circular/trigonometric function with angular rates.
04-09-2017, 03:52 PM
#13
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04-09-2017, 03:59 PM
#15
Would you agree though within the usable stroke/cylinder length like in a 9.8 deck block that a 6.385 rod with a piston pin height of lets say 1.270 would be better for longevity, ie ring seal than any benefits of running like a 6.535 rod with 1.120 pin height? I see in drag racing guys will use these 1.080 to 1.120 pin heights but I assume they are going in motor thats ran 100th the time for re-ring vs a offshore boat? Thanks, Smitty
For a drag car reversion isn't an issue, and tears downs are likely to occur more often (depending on builder motivation and finances), so the combination chosen is likely the one that yields max HP / results.
I'm not sure there is an absolute correct answer, but I prefer minimizing rod angles so that the rod spends more time going up and down the bore rather that pushing sideways at the extremes.
04-09-2017, 04:02 PM
#16
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04-09-2017, 04:29 PM
#17
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i think most builders of marine engines use proven combination,s that last the longest rather than reinventing the wheel and spending stupid money for little or no gain,jmo.if you want to build a 800 inch engine that changes everything as you do whatever it takes to fit it in the block and keep the rings from coming out the bottom of the bore.
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04-09-2017, 05:57 PM
#19
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Darin Morgans take on the topic
"Most people tend to overgeneralize this issue. It would be more accurate to compare different rod-to-stroke ratios, and from a mathematical stand-point, a couple thousandths of an inch of rod length doesn't really change things a lot in an engine. We've conducted tests for GM on NASCAR engines where we varied rod ratio from 1.48- to 1.85:1. In the test, mean piston speeds were in the 4,500-4,800 feet-per-second range, and we took painstaking measures to minimize variables. The result was zero diff-erence in average power and a zero difference in the shape of the horse-power curves. However, I'm not going to say there's absolutely nothing to rod ratio, and there are some pitfalls of going above and below a certain point. At anything below a 1.55:1 ratio, rod angularity is such that it will increase the side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes. Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of your induction package to increase velocity. Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. What-ever connects the two is your rod length
"Most people tend to overgeneralize this issue. It would be more accurate to compare different rod-to-stroke ratios, and from a mathematical stand-point, a couple thousandths of an inch of rod length doesn't really change things a lot in an engine. We've conducted tests for GM on NASCAR engines where we varied rod ratio from 1.48- to 1.85:1. In the test, mean piston speeds were in the 4,500-4,800 feet-per-second range, and we took painstaking measures to minimize variables. The result was zero diff-erence in average power and a zero difference in the shape of the horse-power curves. However, I'm not going to say there's absolutely nothing to rod ratio, and there are some pitfalls of going above and below a certain point. At anything below a 1.55:1 ratio, rod angularity is such that it will increase the side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes. Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of your induction package to increase velocity. Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. What-ever connects the two is your rod length
