4-stroke piston rings. 3 vs 2

[Blown 331]

I dont really know what to think about this. I copied this explanation from another forum and it make sense but I still don't like the idea of loosing a compression ring, what do you think? I'm thinking about trying it out. It's for a 450R but physics and mechanics are the same no matter what the motor. There are knowledgeable people on here and I'm looking for opinions. Also the guy who wrote this makes a 2 ring aftermarket piston, supposedly the best. $250 for just a piston! I can post the specs and features for his piston too if anyone is interested in how a piston can cost $250. Kinda long.
But if 2 rings are better than 3 why do auto's and just about every other 4 stroke have 3?

2 rings vs 3 rings

Well there are a few variables to consider but the 2-ringer is my favorite..by far!

for longevity there is no difference between the 2 styles. the top ring in either package still has to take the major combustion\compression hit, so having a ring after it doesnt help it's life expectancy at all, and likely the opposite is true. when the top ring is done, you're just plain done


if there WAS a difference to be seen in wear, the top ring of a 2-ringer would be likely to have a longer life due to getting a little better oil supply that the second compression ring would normally "scrape" away


if there was blow-by past the top ring of a 3 ring style piston, there would be pressure trapped between the top and second rings...bad news....that would have a seriously detrimental effect on the ability orf the top ring to keep its seal against the bottom of the ring groove... so first a little blow-by, then lots more soon after from pressure trapped below the top ring....and blow-by quickly burns off the protecting oil barrier between the ring and cylinder wall.

if a piston is made with an "accumulator groove" below the top ring (like VCP) it's purpose is to have a smallreservoir so that a small amount of blowby isn't enough to build pressure below the top ring to the point of unseating it from the bottom of the groove

oil rings wont hold as much pressure below a compression ring as another compression ring would ... that gas would end up more quickly as crankcase pressure. i spec'd for an accumulator groove on all the HammerHedz anyways

next let's think about actual ring seal

as compression goes higher, naturally the job of containing all that pressure becomes greater. you could combat with rings having more outward (radial) tension on the cylinder wall..... but that would add friction...and then heat

OR:

you could use a normal (or less) tension ring and then add some lateral gas pressure ports on the top of the top ring land. this would allow the pressures of compression and combustion to actually add tension to the rings seal when it needs
it and still operate at lower tension the rest of the cycle....again see HammerHedz

then there are special materials, and processes, that can be used to make ring life longer. the ring pack on the HammerHedz is a thinner than normal, upgraded nitrided set that will last longer than a normal ring pack even with the added sealing pressures the lateral gas ports provide

BTW thinner \lighter rings seal better at high RPM because they have less inertia trying to pull them off the bottom of the ring land when they hit TDC at full run.. think about it


so..... for a 2-ringer spec'd out like the Hammers......

a little more oil is likely to reach the face of a single upgraded compression ring ...
a bit more lube than the top ring would get with a 3-ringer for sure

then the lateral gas ports help with the ring seal and prevent blow-by from burning off the oil shield

...further insured seal by an accumulator groove preventing build up of pressure beneath the ring land


oh yeah... the reason 2-ringers are being developed (oem crf) is to have less power robbing ring friction and accompanying heat..... more power via friction loss is always a good deal!

[The Goat]

bumpity bump.

is that why the new 250 hondas smoke even though they are 4 stroke? designed to use oil?

[Billy Golightly]

as compression goes higher, naturally the job of containing all that pressure becomes greater. you could combat with rings having more outward (radial) tension on the cylinder wall..... but that would add friction...and then heat

OR:

you could use a normal (or less) tension ring and then add some lateral gas pressure ports on the top of the top ring land. this would allow the pressures of compression and combustion to actually add tension to the rings seal when it needs
it and still operate at lower tension the rest of the cycle....again see HammerHedz

The only problem is, this is already how all piston rings operate. spring tension against the side of the cylinder wall has almost zero to do with ring seal, its all about how the grooves are made in the piston and how that gas expansion is able to get behind the ring between it and the piston and force it out towards the wall of the cylinder.


Its been well known for many years, that you can run 2 strokes with only one ring. Almost every 2 stroke tuning book I've read mentions this. They say the friction benefits are negligible below 8k RPM, but at the speed most of these modern 4 strokes are running, thats right up their alley.