Re: port velocity?
[Re: mopar dave]
#1622159
05/21/14 05:35 PM
05/21/14 05:35 PM
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polyspheric
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The speeds quoted are, of course, averages, and not an accurate picture of port conditions even in the (theoretical) range of maximum piston-derived vacuum (somewhere near both the ICL and the highest piston velocity - typically in the mid-70s depending on the rod ratio). With a strong exhaust pulse, the vacuum across the chamber during overlap is stronger than piston vacuum. The static conditions in the chamber between high and low compression (small vs. large chamber volume) and high and low rod ratio are fairly different. In relative terms, cross-chamber flow is "lazy" with a really long rod (like n=2) and low compression. This effect is almost harmless if the port is too small (vs. displacement), and very annoying if the port is too big. The reverse condition (short rod, high compression) is significantly less affected by this factor.
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Re: port velocity?
[Re: mopar dave]
#1622161
05/22/14 01:24 PM
05/22/14 01:24 PM
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polyspheric
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Not quite. As the stroke is reduced, or the rod is lengthened, or both, piston motion around TDC slows down. Piston motion around BDC speeds up.
Longer stroke, shorter rod etc. = reverse.
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Re: port velocity?
[Re: HotRodDave]
#1622167
05/22/14 05:40 PM
05/22/14 05:40 PM
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polyspheric
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Sorry, "yes and no" is simply wrong. A longer stroke (with no other changes) always increases piston speed around TDC and decreases it around BDC. There is no opinion on this, it's plane geometry, and it was settled 2,500 years ago by Euclid and Pythagoras.
So, when is the piston speed the same at TDC and BDC? When the stroke is zero, or the rod length is infinite.
A 4.50" stroke with 7.10" rods (n=1.578) will have a velocity curve similar to a 4.25" stroke with stock 440 rods: faster than a stock 440 @ TDC, slower @ BDC. Unless the heads are very good, this is the opposite of what you want to keep the VE as high as possible. Where practical, the rod ratio should go up when displacement is increased, regardless of how this is achieved.
For the stock 440 rod ratio with 4.50" stroke, the rod must be 8.11" long - which is why it's not done.
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Re: port velocity?
[Re: polyspheric]
#1622168
05/22/14 06:30 PM
05/22/14 06:30 PM
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HotRodDave
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evidently my measurement set up must not be very accurate then, old dial indicator on the piston top? Should be correct down to a couple thou.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: port velocity?
[Re: HotRodDave]
#1622170
05/22/14 08:46 PM
05/22/14 08:46 PM
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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 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.
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.
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.
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.
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.
I have officially stepped away from the "long rod" religion.
R.
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Re: port velocity?
[Re: Sport440]
#1622173
05/22/14 11:42 PM
05/22/14 11:42 PM
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Port velocity may drop off, and even reverse, without actually tracking piston motion.
The entire rod ratio thingy is a conundrum. The difference in piston motion and position among all the rod ratios you would reasonably use in an engine (from roughly n=1.4 to 2.1) is almost imperceptible, yet the results are not merely obvious but wildly disproportionate to any calculations. How can such a small change give such drastic results? Yet, they've been proven by tests for many decades. Example: Harley-Davidson significantly increased the rod length (but not the stroke) in 1937 to improve peak power in a low RPM, low compression engine with small ports and mild cam.
There is a similar and converse effect @ BDC: a short rod covers a longer piston travel path (in inches) during the same amount of crank rotation (in degrees). What this does it capture a larger percentage of the full stroke length with the same intake valve closing point. This (not overlap, or LSA, or duration) is what changes DCR with rod ratio changes: shorter rod, longer stroke, or both always has slightly higher CCP.
Last edited by polyspheric; 05/22/14 11:43 PM.
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Re: port velocity?
[Re: mopar dave]
#1622175
05/23/14 11:44 AM
05/23/14 11:44 AM
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Quote:
ok, good info guys. it looks like the rod length and not so much the stroke is what we need to be concerned with to keep good port velocity? when its stated short rod long rod, what are you considering short and long? I use 6.123 sb rod. would these be considered long or short? thanks
I use the 6.125 chevy rods in basically all my builds and I call them mid to long.. the ford SB rods are short.... if you went to a 6.2 I would call them long in a SBM... but thats JMO ![](/ubbthreads/images/graemlins/wave.gif)
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Re: port velocity?
[Re: mopar dave]
#1622176
05/23/14 11:44 AM
05/23/14 11:44 AM
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polyspheric
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The effect on piston motion is exactly the same for identical changes in rod length and stroke length measured as percent. However, rod length has no direct effect on gas speed - stroke does. For many engines, maximum rod length is limited by deck height - you can't get your rod ratio back unless you use a smaller stroke increase.
Get the idea - there are no simple answers?
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Re: port velocity?
[Re: polyspheric]
#1622177
05/23/14 11:56 AM
05/23/14 11:56 AM
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mopar dave
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yes,just when I thought I was understanding. ok both stroke and rod length can effect port velocity. can you tell me what rod length would work best in a sb with 4" stroke and using an indy 360-1 head with 2.87 csa@13.5:1 compression?
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