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Ring loading variations caused by cylinder pressure changing with the throttle position? or Gap-less rings have a variation in loading due to their nature/design? |
Cylinder pressure forces rings down against the groove, and out toward the cylinder walls. Light throttle or backing off the throttle will reduce pressure to the point where inertia and inter-ring pressures take over and the top rings are not pushed down & out.
The downstroke flow of some residual combustion gasses through a gap help to move scraped oil toward the piston's drainbacks and the sump. Lacking any downward impetus, the oil will cause the second ring to eventually hydroplane and the oil can contaminate the combustion chamber. |
gapless
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It's fairly obvious what might happen with a ring gap that is to close.
BUT, what happens when the ring gap is to wide??? |
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Somewhere fuzzily back in a memory I no longer trust, I seem to recall that the flow through a gap is a function of the gap cubed. Not sure if this was a gas law or a polymer dynamics relationship, so don't hold me to that. |
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...hold's Olddogs feet to the fire... :) |
Some where in this thread someone mentioned that what pressure leaks past the first compression ring is trapped by the second compression ring. If I recall correctly they said something to the affect of the pressure trapped between the two rings, pushes the top ring off the land causing it to flutter and loose it's seal. Maybe they didn't exactly say this, but it was how I interpreted what ever was said. I think this is along the lines of what Barry_R is talking about.
What about using one gap-less and one normal compression ring? Has this been tried? I'm not sure if the top should be the gap-less or not, but it would seem the logical place. |
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My thought would be that if the leaking gases past the gaps tend to act like a pin that prevents rotation then the wider the gap the more powerful the affect. That is a totally theoretical thought based on the discussion in this thread, which I find very interesting. I'm not certain that ring rotation is necessary. It would seem to me that the rotation would help to round out the cylinders, by randomizing where the tight spots tend to wear things. However it also occurs to me that at some point in time when the thrust starts to wear the cylinders out of round, the rings are going to stop turning and tend to wear out of round to match the bore. Now I'm way out on a limb, because I never spent the time studying worn out engines and trying to understand all these things. |
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Smokey Yunick
In his book Power Secrets Yunick elborated around the gap and power loss, and 20 years ago when I read it, lost me.
It relates to a more than linear (cube?) loss depending on the gap. I shall dig out that book. I did, and he also wrote about rings spinning in the grooves (or was is Bill Jenkins?): Page 50, center column Smokey Yunick's Power secrets "If the head gaskets, the rings and the valves are all sealed tightly, there should not be very much leakage when the the high-pressure air (from a leak-down tester) is fed into the cylinder. It is not possible to gain a 100% mechanical seal in the cylinder, but if everything is in top notch condition, there should not be more than about 4% leakage. ..., if the average is over 8%, you don't have an engine,... This doesn't sound like much to worry about, but you have to remember that only 1/3 of the heat energy (pressure) developed by each cylinder is actually going to push the piston down. So, if 8% of the cylinder pressure is leaking away, you are theoretically losing 24% of the recoverable flywheel horsepower." |
Smokey was, on so many levels, well ahead of his time. Certainly one of my hero's.
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Sometimes lost in the discussions lies the fact that gapless rings are not a new invention. They have been around in various overlap joints, bevel joints, and multirail designs since at least the 20s if not earlier.
The ring package is a dynamic sealing device moving at a high rate of speed under high pressures and at high temperatures. Not an easy thing to optimize or quantify. What works in one environment (say an air compressor) is not the best in another. If you look at the wear pattern of rings at the gap you will find that they are not round. More & wider wear is evident at the tail end alongside the gap. They cut them after lapping so that the unit pressure at the gap is higher - this helps to stabilize the "tails". |
Barry,
I don't want to put words in your mouth, but I get the impression that you feel that traditional rings do a good enough job. I'm thinking the positives minus the negatives on gap-less isn't an overwhelming reason to go fix something that isn't a big problem to begin with. For high performance applications - certainly racing applications - engines don't typically run long enough to wear the bores a whole lot. I have pulled some high mileage engines down that the bores were so worn that a 0.040 over bore wouldn't straighten the bores up and tossed the block. It never occurred to me back then to stick a ring in and measure the gap, but I expect the gap would have increased by .010" to .015." I believe that the gap-less ring would be better at the end of a street engines life. Not that that necessarily makes a good case for there use. Your thought? |
I wonder if the rotation might occur mainly in the warm-up phase of operating the motor, when the piston crown has not expanded to its full diameter at operating temp, during this phase with the flame front on many motors tending to have a toroidal/circular pattern due to the spark plug placement & chamber design the gas pressure increase must tend to flow around the piston top land area therefore inducing ring rotation rather than just over the edge of the piston crown.
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Gap increases with wear by the factor of PI. So if you have .040 wear (that'd be pretty incredible from a standard bore - never seen anyting like that still runnin' ) your ring gap would have increased by +/- .120"ish...
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We did a 59 Cadillac motor once with STD size pistons where we had to skip the next overbore straight to the following... 30 over wouldn't clean the bore.
Which we only realized once first oversize pistons had arrived. Maybe we should have quietly inserted the 30 over pistons and close it up again. Can you imagine that ring gap? BTW, the rings cannot be 100% round if your bore is not exactly matched. The rings get more oval when you compress them. Unless they are made to be round when they butt. But then they might as well come exactly gapped to begin with? Yunick says they rotate at BDC and TDC |
Rings do rotate. But it is cool they work both ways LOL
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It's simple....if it's a La Nina weather cycle, the rings rotate clockwise and if it's El Nino, they go anti-clockwise. And if the weather's stable, the bores wear oval shaped and the rings don't rotate. I did a PhD on this. :)
Cheers, Glen |
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