Piston ring end gap (your thoughts)
Piston ring end gap (your thoughts)
(OP)
Hello gents,
I was in a technical discussion about a year ago in which the question of ring end-gap came up. I made a recommendation to a gentlemen using a Speed Pro ring for a normally aspirated street/strip car. Every manufacturer spec I've ever seen calls for a smaller ring end gap at the top ring and a larger one for the 2nd.
Now, one of the gentlemen on the board who claims to be an engineer at Ricardo told me that this theory is all backwards and is a sure fire way to cause ring unseat on the top ring. Who am I to argue other than to say that the piston ring engineers obviously know more than I do on the matter and I've always felt confident in following their advice.
He also went on to state that the OEMs always use a larger 2nd ring gap.... so you're looking at a ratio of something between 1.5-2.0:1. This seems backwards from the ring manufacturer recommendations so I thought I'd see what you gentlemen have to say on the issue. Any experiences on this matter? Keep in mind that my primary interest here lies in engine performance and oil consumption as many of the engines we build are true "street" powerplants.
Thanks for your time.
I was in a technical discussion about a year ago in which the question of ring end-gap came up. I made a recommendation to a gentlemen using a Speed Pro ring for a normally aspirated street/strip car. Every manufacturer spec I've ever seen calls for a smaller ring end gap at the top ring and a larger one for the 2nd.
Now, one of the gentlemen on the board who claims to be an engineer at Ricardo told me that this theory is all backwards and is a sure fire way to cause ring unseat on the top ring. Who am I to argue other than to say that the piston ring engineers obviously know more than I do on the matter and I've always felt confident in following their advice.
He also went on to state that the OEMs always use a larger 2nd ring gap.... so you're looking at a ratio of something between 1.5-2.0:1. This seems backwards from the ring manufacturer recommendations so I thought I'd see what you gentlemen have to say on the issue. Any experiences on this matter? Keep in mind that my primary interest here lies in engine performance and oil consumption as many of the engines we build are true "street" powerplants.
Thanks for your time.





RE: Piston ring end gap (your thoughts)
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
RE: Piston ring end gap (your thoughts)
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?
RE: Piston ring end gap (your thoughts)
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
RE: Piston ring end gap (your thoughts)
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
RE: Piston ring end gap (your thoughts)
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.
RE: Piston ring end gap (your thoughts)
and the problem went away can't remember the exact numbers but the added bebefit was a hp increase in the upper rpm
Tom
RE: Piston ring end gap (your thoughts)
RE: Piston ring end gap (your thoughts)
"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).
RE: Piston ring end gap (your thoughts)
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.
RE: Piston ring end gap (your thoughts)
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
RE: Piston ring end gap (your thoughts)
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
RE: Piston ring end gap (your thoughts)
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
RE: Piston ring end gap (your thoughts)
RE: Piston ring end gap (your thoughts)
McCormick, Anderson, Mayhew, Rychlewski, New Developments in Piston Rings for the Modern Diesel Engine, SAE 750769
John Woodward
RE: Piston ring end gap (your thoughts)
RE: Piston ring end gap (your thoughts)
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.
RE: Piston ring end gap (your thoughts)
Regards
pat
RE: Piston ring end gap (your thoughts)
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).
RE: Piston ring end gap (your thoughts)
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
RE: Piston ring end gap (your thoughts)
wouldn't surprise me, although they have been available since at least 1940, and you'd think that someone would have found a "gentle" way to machine them in all that time.
2) Due to high temps near the combustion area
perhaps - the ring will be hotter if it's higher up
3) Due to the high and suddenly variable gas pressures close to the top of the piston
I'm not going to buy this one - the pressure seen by the whole top land is essentially identical (very little flow restriction), so moving the ring pack up or down 5mm won't change the pressure the top ring sees significantly.
RE: Piston ring end gap (your thoughts)
Lowering the top ring grooves is very common on nitrous and supercharged piston combinations. The high ring lands are commonly used on N/A engines allong with the .043 thick rings. I also have used gappless rings and never found a real difference.
Ring land dimensions are a science. Many pro stock engine builders would get pistons without the lands cut and do their own. Width of the land is very critical, especially on high rpm designs and most machinists work very hard to minimize this dimension. Some builder will also hand lap the rings to insure flatness as well as removing any machine marks to facilite this fit.
End gaps, as seen above, increase as the temperature and "violence" of the combustion process increases. Nitrous and supercharged motors require more endgap as well as lowered ring lands to survive. A 300 plus HP shot of nitrous is killer on standard piston configurations.
Good luck with your experimenting...
RE: Piston ring end gap (your thoughts)
The other thing that is EXTREEMLY critical is Honing and surface finish (RA&RZ) It is time for Profilometers to be as common of an item in engine builders tool boxes as tapered ring compressors. The amount of power, reliability, and longevity that I have been able to add to customers engines from this practice has been outstanding. The radial clearence of piston to ring fit should also be checked without exception. I run from anything more than .002.
The Gapless top ring with the Gold power oil ring package is a great set-up on mid priced pistons, however, they need around a 15RA and 150RZ to work correctly. That is unless you are running a NIKASIL coated cylinder. The RA can get down to 10 or even 6 and be killer.
Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)
RE: Piston ring end gap (your thoughts)
ShaunT- do you mean radial clearance here or axial? Please explain.
Mike
RE: Piston ring end gap (your thoughts)
Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)
RE: Piston ring end gap (your thoughts)
Mike