Jan. - Feb. 2001 Lane Racing And Rodding Article By Jim Kaekel, Jr.
A discovery in the area of piston ring sealing is creating a bit of controversy among engine builders. It concerns proper ring end gaps for the second compression ring. Engineers at Speed-Pro claim higher horsepower can be achieved by actually widening the second ring end gap to a minimum of .020" to .022" for moderate performance usage, considerably different from previous guidelines of .012" to .014". All-out racing end gaps would be slightly larger. To understand their reasoning on this, we first must examine the basic function of each piston ring.
The Top Compression Ring must deliver consistent compression sealing to maximize power output and control "blow-by", gases which leak past the top ring. The Second Compression Ring, referred to as a compression ring, is actually a scraper which removes oil from the cylinder wall and keeps it out of the combustion chamber. The third, or Oil Ring, consists of an expander ring and two rails which control oil by drawing it off the cylinder walls and returning it to the crankcase via small drain-back slots in the rear wall of the oil ring grooves.
Pressure Differential, the difference in pressure above and below the top ring keeps the top ring seated and properly sealed against the cylinder wall, according to Speed-Pro. As long as a very large percentage of pressure exists above the top ring and a negligible amount below, the ring seal will be superior. Speed-Pro engineers and most industry experts agree that a significant amount of gases leaking past the top ring will become "trapped" between the top and second rings, causing Inter Ring Pressure and a rapid fluttering of the top ring which affects its ability to seal combustion.
Opinions differ as to the remedy for "ring flutter." Speed-Pro feels that a greater second ring end gap creates a larger "escape path" for combustion gases. This idea is used in current GM LS-1 engines. Other industry experts aren't quite ready to jump on this band wagon.
Piston manufacturers, including Ross, Wiseco and JE, reduce inter-ring pressure and ring flutter by machining pressure balance or "accumulator" grooves between the top and second ring lands to increase the effective area between the top and second ring. "All of these ideas may exist in a given application", according to J.E.'s Randy Gillis, "but even with all of the engine variables, ring flutter should be a concern only in excess of 8,000 RPM." Gillis doesn't see the advantage to widening the second ring gap over traditional recommendations. Brian Nutter of Wiseco doesn't feel that flutter is much of an issue until the engine reaches 8500 RPM. "The second ring runs at much lower temperatures than the top ring," said Nutter, "and requires no additional end gap clearance.
The entire theory of opening up the second ring end gap does not coincide with gapless second ring technology. Total Seal and Childs & Albert gapless second ring designs both claim to virtually eliminate blow-by and increase horsepower and both also have huge followings within the engine building industry.
Total Seal's two-piece Gapless ring consists of a primary ring with a machined "L" step to accept a special rail which resembles an oil ring rail and is inserted within the machined step with its end gap 180° from the end gap of the primary ring eliminating any gap.
The Childs And Albert Zero Gap second ring is a one-piece ring with an overlapping step gap which the manufacturers claim is superior because there is no gap (as there is in the Total Seal insert) to eventually rotate and line itself up with the gap in the other ring.
Nearly all industry experts agree that "ring flutter" does exist Where they are not in total agreement is the specific engine speed at which flutter occurs, or upon just what a suitable remedy might be.
We've outlined the three theories of second ring end gap here, and, not unlike skinning a cat, there's more than one way to create more horsepower during the engine building process.
What it all boils down to in the long run is personal preference and what experience has shown will work best in a given application.
Whether you're a believer in this new concept discovered by Speed-Pro, or in gapless technology, or in traditionally-gapped second rings, careful preparation and machine work will still be the most important factors in a successful race engine, regardless of ring selection.