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I knew that Calculus would come in handy some day.

The TRUE FACT of the matter is that at TDC and BDC, for that instant where the piston is at the maximum distance either up or down, velocity is ZERO.

Acceleration is maximum as (it moves away from )at those points, and as those ancient Greeks would have figured out, eventually, the acceleration at TDC is greater than at BDC. Put another way, the piston displacement vs. time graph is not a perfect sine wave.

We're mostly worried about the piston action at TDC. That's where the power is made or the intake charge is trying to get in.this is called "DWELL" and it effects alot of things especially chamber burn/shape and port efficiency and length.

Because a longer rod causes lesser acceleration at the top of the stroke, the piston spends more degrees very close to TDC with a long rod than a short one.again this is called "dwell" and it has positive and negative attributes..they have to be factored..and there are some advantages

For years and years the prevailing theory was to lengthen the rod to get that "perfect" ratio of rod to stroke, 1.8 was one number thrown around. And at 9500 rpm, the theory seems to work, because that's how NASCAR engines are built, and they'd kill their Grandma for 10 extra hp. Rod lengths of around 6.2 to 6.3, combined with strokes around 3.3, compute out to roughly 1.9. So it works for them.

But we're for the most part not talking about building NASCAR engines.the Kaase short stroke motor was an exercise in this direction

One of the most interesting things about the Enginemasters competition is that the winners for the last several years have had REALLY Short R/S ratios. This year the winning engine and the runner up's stroke was something like 4.7, rod length 6.658 for R/S of 1.4 or so. This is a hp/cubic inch competition, averaged over an rpm range picked to more closely represent street cars, so if there were huge disadvantages with extra side loading on the pistons, you'd think they wouldn't be doing something as "stupid". A few years ago Jon Kaase built a short rod ford 400 and won. This is in a block with deck height of 10.29 or so inches. He used 4" stroke and 6", IIRC, rods. R/S of 1.5.

Also a few years ago, it was reported that he had built a mountain motor with extremely long rods to see if it would work better and he found no power advantage over his normal R/S of 1.38. dynos are not race cars or race tracks..the answer to this sinareo was they ran better in the car down the track..vrs dyno...to a degree

I have to feel that the advantage is in the quicker piston acceleration away from TDC. Heads are flowing better and better. Maybe the quicker acceleration away from TDC gets the mixture flowing faster sooner for more cylinder filling. That's my best explanation at this point.not necessarily...aftermarket heads designed for this YES but not if your using stock/architecture heads

I have officially stepped away from the "long rod" religion. as long as you know the cause and effect you build it to suite your needs

R.

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