Piston ring end gap (your thoughts)

Well , ProgressiveRacing, I only do race engines because I hate to work on my regular stuff. To the point, I have been setting my ring gaps the same for years with no problems I can think of. I tried a "gapless" second ring once and it was a waste of money and time. No performance gains and some reliability and wear issues that went unresolved. They may work on drag engines but not on my road race engines. I have varried the ring gap from rebuild to rebuild and, I personally, saw no change in power output. It's entirely possible that there was an improvement that the chassis dyno that I use was not sophisticated enough to detect.

On a related issue, pistons with the back of top ring land ported to the combustion chamber. I have seen them (Cosworth) and I know folkes who have used them but, I have never heard if they were worth the effort. Anybody else have any thoughts? Was this just an effort to get "Dykes ring" performance without all the special machined pistons?

Rod

Perhaps you can clear this up for me:

In your first paragraph, you say that mfr's specs usually call for a larger second ring gap than first ring gap. In your second paragraph, you say the opposite.

In your first paragraph, you say that this guy wants the top ring gap to be larger, and in the second paragraph you say the opposite.

So what did the guy really say, and which way do you remember the specs going?

Ok, a little goof on my part.... to err is human.

Speed Pro recommendations:
*Top Ring*
Moderate Performance: .004 x bore diameter
Drag Racing & Oval track: .0045 x bore diameter
Nitrous Street: .006 x bore
Nitrous Drag: .008 x bore
Supercharged: .006 x bore

*2nd ring*
Moderate Perf: .003 x bore
Drag & Oval track: .0035 x bore
Nitrous Street: .005 x bore
Nitrous Drag: .0065 x bore
Supercharged: .005 x bore

So, there's where I stand in folowing the manufacturer's spec. I have found that other issues play a more significant part in ring seal but I'd like to keep the discussion narrowed to just the gap issue (pardon the pun). I would assume that this is a heat issue. The top ring is obviously going to see more heat than the 2nd ring and therefore needs a gap that will still seal well at maximum operating temperature. The 2nd ring has some of the same job as well as double duty as a wiper but the gap is less because it "sees" less heat. Any thoughts as to why you might want more gap on the 2nd ring in relation to the 1st? Why would the manufacturer make such a recommendation if this did not put us closer to "optimum" given the operating conditions?
Thanks again.

Allen Foulson

It is my experience that OEMs spec a larger cold nominal second gap than first gap, by a ratio of about 1.5 to 2.0:1. I'm looking at a set of ring drawings right now, for an upcoming product of one of the US OEMs (no names today), and the cold nominal gap ratio is 1.55:1. The top ring gap is .0030 x bore and the second ring gap is .0046 x bore.

In general, you want the hot second ring gap to be larger than the hot first ring gap, so that the gases that escape past the top ring mostly escape past the second ring. The two biggest motivations for this are (AFAIK):
* blow-back is bad for emissions & oil consumption
* you'd like to keep the top ring seated as much as possible, and having a long blow-back period means you'll have a long period when the top ring is at the top instead of the bottom of the groove.
Achieving the above requires that the top and second ring gaps be sized according to how much the rings and bore are expected to expand in operation, which will vary from app to app.

Your statement about the top ring seeing more heat seems fair enough, and is probably a bigger factor in high-performance applications than in OEM applications. It has been my experience that while the top ring gets slightly warmer than the second ring, the difference is not as large as you might expect, and the top ring sees (on the average) a slightly larger bore than the second ring sees (although they're about the same at BDC where butting may occur).

My experience all goes out the window, however, when you start talking about extremely high liner temperatures, and filled-in cooling jackets. As you hinted before, the operating conditions for the intended application need to be addressed. The OEMs spec out what they believe to be the best gap combination for the conditions they expect their engine to see, and hot-rodders do the same. Their answers may not agree, but their operating conditions probably don't match either.

Obviously the only answer that matters is the one that gives you the best performance in operation. For a mildly-modified engine, I would hesitate to use the specs you mention above, if the vehicle will be smog-checked, because they sound like a recipe for hydrocarbon emissions.

Best Regards,
Isaac

Rod,

In regard to the vented top groove, I think that there probably is some merit to this. With extremely tight ring side clearances (axial clearance from the sides of the ring to the sides of the groove), the top groove takes longer to pressurize, and doesn't reach as high of a pressure as it would with greater clearance. There is the possibility that the gas pressure at the ring face (plus oil film, etc) will overcome the gas pressure at the back of the ring (plus ring tension, etc), and cause the ring to pull away from the cylinder wall. The cylinder will "burp" some gas when this happens, reducing power output. Additionally, if there is too much flow past the ring face, the second ring will see much greater pressure than it normally would, and friction & wear will be increased. Venting or porting the top groove allows the groove to pressurize, even with a very tight ring-to-groove fit.

On the other side of the coin, there is no reason that I can think of to have more gas pressure behind the ring than the precise amount that it takes to keep the ring in contact with the liner. Any more pressure than the bare minimum to maintain contact will just result in more friction (less power) and wear.

The thing we see in racing engines is that when the engine speed goes up the inertia of the top ring increases to the point where it wants to lift off of the bottom of the land We machine our gas ports in more of a fashion to hold the ring down rather than out First saw this on the dyno with a blowby meter connected the meter would be fairly stable until about 7000 rpm then it would make a sudden climb next time around on this particular engine we gas ported the pistons ..12 .040 holes around the top of the piston but not into the back of the land .01 to .020 out from the back
and the problem went away can't remember the exact numbers but the added bebefit was a hp increase in the upper rpm

Tom

As a point of interest, axial ring flutter is fairly common even at low speeds. Regardless of engine speed, the top ring will pop to the top of the groove at least once during each cycle, during the blow-back period, when the second land pressure exceeds the cylinder pressure. The trick is to keep the ring from going up & down over and over again, and have one clean "pop" to the top of the groove and back down. I had thought (perhaps incorrectly) that the biggest concern with this flutter behavior was increased groove wear.

I suppose I should correct my earlier statement:

"Any more pressure than the bare minimum to maintain contact will just result in more friction (less power) and wear"

When I said the above, I wasn't thinking about ring conformability. If the cylinder liner is particularly wavy, then having higher groove pressures will force the top ring further into the troughs, and cut down on gas flow past the ring face. This will still incur wear & friction penalties, but the improved sealing should be worth it (a perfectly cylindrical cylinder would be nicer, but some addage about icewater comes to mind).

I have noted, over years of waching this cause and effect of sealing, a somwhat differant slant. The ring gap does not appear to be the 'Holy Grail' concerning 'sealing'. Notice I did not say 'ring sealing'. Everone knows the basics here although, concider the heat spread across the top of a piston first. To get to my particular understanding, I had to first get my vendor to do what I asked just a little at a time, toward what the piston & engine was showing. Believe me, I have had them do this for me under protest, for they have engineering lines to stay with-in & they don't like to vary from those untill they sense you may 'know what youre doing'. But if you have the want to 'go out there' on your own, you can learn from it.
If you dare tightening up the crevace area up top and below, you shall find the piston allways runs with less clearance between the wall and the exhaust side of the piston. The land clearnces increase, the pin bore has more growth, and the 'seal' drops. You have to hunt for this. Now finding the way around this to 'Cruch'it, is the key. Control the heat into that portion as a priority. When you can get the land to more or less 'stay put', you shall gain power regardless of ring gap. It is 'PISTON SEALING'. This is very current pro stock drag race proceedure and in the high end 'Cup' teams. Placement of gas ports, land cam, thermal coating and where, blowdown speed, all for the top ring. When you feel better about what you see, then you can move onto the 2nd ring & onto the oil assembly. You cannot do this suddenly, it takes money, people and some good common sense. ---- Ponder it.

Gentlemen,
I appreciate all the time and information given thus far. I'm a young engine builder so I'm trying to learn as much as I can and I try never to discount an idea that's different than my own. I don't feel that that's conducive to much learning.

Isaac,
Your explanation gives me a much better understanding of what's going on with piston rings. I've always just oversimplified things in thinking of the pressure differential and heat and not really understanding all the dynamics that take place there. Any papers or books you could recommend on the subject?

Tom,
On those tests, were you using a titanium nitrate coated ring or one of the harder ring varieties? And what ring thickness? I'm gathering from this that you weren't necessarily having a radial tension problem, just a problem with uncontrolled ring motion to the top of the land.... hence the "flutter".

Hasbro,
Very interesting information. Which piston manufacturer do you prefer to work with?

Thanks again,
Allen

Allen,

Unfortunately, I've never come across a good book on the subject. I've taken a few courses put together by piston/ring manufacturers, and I've never really been satisified with the content therein (positive twist on the top ring, negative twist on the second ring, scrape oil downward and avoid blow-back, have a nice day). All of what I know comes from working with people who have been doing this (piston and ring design / analysis) for a while, and trying not to screw up too badly myself. I do have the advantage, at work, of having software available to me for piston & ring pack dynamic analysis, which can yield great insights into what is going on inside the engine, as well as a number of highly qualified senior engineers & physicists to consult.

In/re simulations, calibration is the tricky part, of course, because often the only measurements available to compare with the simulation are oil consumption and blow-by figures. Once in long while we get the opportunity to work with a fully instrumented piston, but on those occasions the instruments rarely work satisfactorily anyway.

I suppose I should forward the question on to anyone else who is reading... are you aware of a good text on the subject of ring pack design? (preferably something written in the last 5 years)

Regards,
Isaac

Allen
The rings were not coated but the garden variety Speed Pro file fit plasma moly The particular engine I spoke of had a 1/16 top and second ring with a 4.375 bore dia On bigger bore dia we prefer .043 top rings to further reduce the ring weight and now as I read IvyMike's post I wonder if we aren't improving the conformability also We have just recently experimented with using a 1/16 end mill and maching the gas ports laterally rather than through the top of the pistons i.e. machining a half round in the upper surface of the top land this has shown good results and is a lot easier process.....no more broken .040 drill bits

Tom

as usual, I forgot all about important details in an earlier post... like when I said "top ring will pop to the top of the groove at least once during each cycle, during the blow-back period, when the second land pressure exceeds the cylinder pressure." I forgot to mention that this usually happens close to TDC exhaust, when the cyl pressure is low, and the inertia load is helping the ring up, or at least not holding it down so much... details, right? Ah, the wonders of late night posting.

This paper discusses the use of a larger 2nd ring gap to prevent pressure building up under the top ring and causing it to lift. I wonder if this has become standard practice now or if it is still being debated.

McCormick, Anderson, Mayhew, Rychlewski, New Developments in Piston Rings for the Modern Diesel Engine, SAE 750769

John Woodward

Why is there a second ring them? if you have only 1 top ring, there will be no pressure buildup, and on the exhaust stroke the vacuum of the engine will try and keep the ring down?

because the second ring still adds a flow restriction between the combustion chamber and the crankcase, and blow-by is reduced. Blow-by would be unacceptably high without the second ring, if a conventional top ring gap is used. Aside from increased expense, I don't know why overlapping gap (or similar) top ring designs aren't used.

Also, the second ring is a significant player in engine oil control. It scrapes a large amount of oil left behind by the oil control ring. I'm not sure whether the top ring could assume this role if the second ring was removed. You could potentially add another oil control ring in place of the second compression ring, but I don't see the point of that.

I wonder how an overlapping gap top ring set lower in the piston, and a dry sump system that held very low crankcase pressures would work for a race engine where cost, emmissions and engine life are not as important as maximum seal with minimum friction

Regards
pat

Top ring lower in the piston is good for reduced UTRRP liner temp (less oil coking), but I'm not sure what else it gets you for racing. Would you care to elaborate?

I've been cranking the brain lever for nearly a year now (since about the time that this thread originated), trying to find a way to get good emissions & lower friction with only two rings, and I haven't come up with anything I really like yet. If I do, I'll let you know (right after filing the patent paperwork for my employer).

I believe from anecdotal evidence, that gapless top rings are prone to breakage, and I wondered if it was?
1) True
2) Due to high temps near the combustion area
3) Due to the high and suddenly variable gas pressures close to the top of the piston.

I wondered if being further down the side might be a bit cooler and the pressure surges might be dampened a bit.

I know a trick used by the guys that did all Barry Sheens engines, and I believe some for Mick Dohans engines, but it was told to me in confidence. That was some time ago, so I will see if he is prepared to disclose it now.

It revolves around details of a 2 ring only set up, Dykes rings and Nikasil? bores. Some specific details sounded very radical, but the guy was no fool.



Regards
pat