Piston ring end gap (your thoughts)

[ProgressiveRacing]

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.

 

[ivymike]

1) True
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.

 

[vwrcr]

Reher- Morrison has a very good tech forum that they write and one thing that they cover is on gas porting. In their eyes, it is a great addition to their combination. And, like all engine components, it is a combination. In the drag engines that I have built over the years, I have used gas porting on thick and thin ring combinations and found it to work on all. I have never had any problems with the holes filling up with garbage and have found that they seem to almost instantly seal.

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...

 

[ShaunT]

Ring technology has come a long long way. The piston makers had to significantly increase their quality and maintain a much higher standard of tolerance and flatness because the rings today are much more true and consistent in every way.
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)

 

[strokersix]

The radial clearence of piston to ring fit should also be checked without exception. I run from anything more than .002.

ShaunT- do you mean radial clearance here or axial? Please explain.

Mike

 

[ShaunT]

If you take a ring and stick it in the ring land, you should not be able to fit a feeler guage thicker than.0015-.002 into the land. I am talking about vertical clearance, the amount of up and down play the ring has in the ring land.

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

 

[strokersix]

Thanks for the clarification Shaun. I figured that's what you meant but wasn't sure.

Mike