2 Piston rings Vs. 3 Piston Rings

 
 THIS IS A VIEW OF A DISCUSSION I FOUND A LONG TIME AGO IN A GOOGLE SEARCH THAT I FOUND USEFUL IN UNDERSTANDING MORE ABOUT PISTON RING TENSION AND WHAT I HAD DONE WRONG IN SOME PREVIOUS ENGINE BUILDING ATTEMPTS. I DIDN’T WRITE IT HOPE YOU FIND IT USEFUL.
 
 
 
Here is my take on 2 rings vs 3 rings-
A typical ring system works like this…
top ring: Sealing and heat transfer.  This ring is what seals the cylinder on the power and intake strokes.  It also transfers heat from the piston crown to the cylinder wall and cooling system.  The ring groove in the piston has a huge impact on how well this all works.
middle ring: Oil removal…period.  The middle rings sole purpose is to remove oil from the cylinder wall.  The taper face design of most middle rings is not suitable for sealing pressure and is expressly designed to remove oil on the down stroke.
bottom ring: Oil removal.  This ring removes the bulk of the oil from in between the piston and the cylinder wall.  The ring tension and return holes in the ring groove will determine how efficient this ring does its job.
In a 2 ring piston you loose much of your oil control ability because you have removed your middle oil scraper ring.  This has no effect on sealing because that is the job of the top ring only, and you still have that one in place.  If you have chamber oiling issues due to the lack of a middle ring and have to increase the bottom ring tension, you will likely end up with more drag than a properly designed 3 ring system.  A reduced radial thickness middle ring, such as a Napier style ring, produces very low drag on the cylinder but removes oil very effectively.  This allows you to lower your bottom ring tension and end up with a ‘dry’ ring package that has fairly low total drag.  Best of both worlds there.
One application I know that uses 2 ring systems successfully is high performance 4 stroke motorcycle engines.  The oil system in a modern 4 stroke motorcycle is far superior to a V-8 engine.  This results in less oil for the piston to deal with and the opportunity to remove a middle scraper ring with no adverse effects.  V-8 engines with large crank strokes and poorly designed oil management result in a large amount of oil flying around in the bores and on the pistons.  This situation requires more effort to remove oil from the cylinders to keep it from the chambers.
For a 2 ring package to work well, in my experience,  a very good oil system is required as well as an engine that can tolerate some oil in the chambers.  And there just isn’t much you can do to help the oil system in a V-8 unless you go to a very efficient pan and multi scavenge dry sump system with lots of vacuum.  Even then it is not as good as the motorcycle design. If you end up with a 20-25lb bottom ring to pull double duty,   why not just use a 6lb bottom ring and an .043×145 napier middle ring.  That would have less drag that the 2 ring combination.
I’m not sure what the high end guys are really doing but I can say that I will be sticking with .043-.150 to .170 top rings, .043-.145 napier middle rings, and 3.0mm 6-8lb bottom rings for our drag race stuff. Even .8mmx.8mmx2.0mm has been working great.  I don’t see myself revisiting a 2 ring design anytime son.
If you are going to try this approach I would suggest using a normal top ring deal and bumping the bottom ring tension up a bit, maybe even going up in size.  .043 top x 3/16 bottom perhaps?  Use a vacuum pump, build a good oil pan with a kickout & scraper system, etc.  Might increase your results with all those thing aiding it along.
 
For many years CAT ran a two ring (single compression ring) piston on the 3208 diesel engine. The top ring was a keystone design and the oil ring a one piece cast ring with a spiral wire expander.
There were many, many thousands of these engines produced in both NA and turbo, with compression ratios as high as 20:1 on NA engines. When maintained, these engines gave good service life with this ring package.
Bingo.  Notice in this example, the second ring is a rigid device, not the loose wipers normally found in 2 ring “gas” buring/spark plug ignited engines.
I can only speak to my experience.  In power adder engines, the lower the radial tension of the second ring the worse the hp, blow by and overall oil control.  Keep in mind i never go larger than .043 on the second.
The best experience I have seen is with a Total Seal CI second ring with a fairly big end gap.  Close up the end gap too much and everything starts to go south.  Open up the end gap and everything gets better to a point, then too much end gap hurts too.
The only thing I can figure out is, the second ring provides a cushion pocket for the blow by coming past the first ring.  If the second ring has too tight a seal, then blow by gas (what I consider windage at this point) builds up underneath the top ring and unseats it.
Also, if the second ring has too loose a seal, the the blow by shoots in all directions once it clears the top ring (again, I consider windage at this point), again, unsettling the top ring some.
 
-All of the serious engines I have built I have used the two ring deal. The piston makers do not like it, but most of these engines made quite a bit more power than they should. I know there is a lot of HP in the gas porting, top ring package, and the vacuum pump. I like to do things that noone else does. I like real big cams too.
JOE SHERMAn racing engines
 
I will not dispute anyone’s advice on this subject. Years ago Manley marketed a piston called the S.R.T. (single ring type). T.R.W. also offered a single ring piston. It never caught on, be it, resistance to change by engine builders or it just didn’t work. The top was genarally a head land design. The head land was basically a big dykes ring with a special expander behind the ring and the top of the ring was square to the piston flat. It was a concept that in which it’s time hadn’t come. It also had a 3/16″ oil ring. Engine builders would use them, but the diehards, would use a1/8″ oil ring coupled with 1/16″second ring in the oil ring land. In essense defeating the purpose. At that time rod length and or large C.I.s was not an issue. The theory of the time was positioning the top ring as high as possible to promote cooling. The ring face was larger than a 5/64″ ring, with low radial tension and an expander was put behind the ring. To reduce drag the second ring was eliminated. The .043 top ring and gas ports were also being experimented with. When the smaller ring proved more beneficial for high RPM applications with the conventional 3 ring package. The head land 2 ring design fell by the wayside. With todays ring and piston technology along with large C.I.’s it might be time to rethink the 2 ring design.

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