In the past two decades the metallurgy in engine blocks has changed—and not by a small amount. Cylinders are now much harder, and with modern honing practices they accommodate rings that are much thinner, lighter and lower in tension, often with exotic coatings.
In addition, lubricating oils have been modified significantly to reduce friction and viscosity. All of this has contributed to new power generated by the engine block, but to take advantage of it, new honing procedures have to be adopted. The techniques employed for the past 20 years are rapidly becoming unsustainable.
Compare the production finish and standards of the engine cylinders of modern mass-produced automobiles. Yesteryear’s GM, Ford and Chrysler were typically rated at 18 to 22 Ra. Today’s GM LS, Ford Modular and Chrysler Hemi engines measure 8 to 12 Ra. But that’s not all, the car factories anticipate further reductions to 6 Ra in the near future.
Modern Cylinder Honing Measurements: Rpk, Rk and Rvk
Harder engine blocks and affordable profilometers—the devices that precisely measure the surface finish of the cylinders—have changed honing procedures immensely and with it our understanding of oil retention and ring seal. Even the measuring standard Ra (arithmetic average of roughness) is becoming obsolescent, being replaced by three more effective standards: Rpk (peak roughness), Rk (core roughness) and Rvk (valley roughness found below the core roughness).
“Almost always,” says Tom Boucher of Boucher’s Racing Engines, “people are honing much rougher than they think. When you have a rough bore it retains too much oil for modern light-tension low-drag racing rings and they cannot seal correctly and consequently burn oil because of inadequate honing.” On the other hand, if oil retention is insufficient, metal-to-metal scuffing occurs. Thus the objective is to have just the correct amount of oil retention in the cylinder walls after the ring scrapes it down, and this is achievable with a profilometer. Though no one can advise the correct honing stone, because there are too many variables, they can stipulate the surface finish for which you are aiming. Alas, without a profilometer, which is now affordable for most pockets, it would be impossible, akin to porting a cylinder head without a flow bench.
Comparing Honing Numbers: Pro Stock, NASCAR, Top Fuel and More
NHRA Pro Stock drag racing cars use compression rings of around 0.7mm (0.027 inch) thick that generate 0.5 ft-lb of tension, while the oil control rings produce 3 to 6 ft-lb of tension. Light and with little capacity to scrape away oil, the film retained in the cylinder wall is modest. Typical cylinder bore finish readings of Pro Stock or Comp Eliminator, or NASCAR engines are as follows:
Rpk 4 to 6, Rk 18 to 22 and Rvk 18 to 32.
In striking contrast, Top Fuel engines, which endure extremely high temperatures and cylinder pressures, use 1/16 to 2.5mm (0.062 inch to 0.95 inch) rings that generate 5.5 to almost 10 ft-lb of tension. Their cylinder bore finishes have a rougher hone, measuring Rpk 15 to 20, Rk 45 to 55 and Rvk 55 to 75. These honing standards applied to a Pro Stock engine would probably allow oil to enter the headers when the cars were started. For further comparison, Sprint Car, Dirt Late Model and Super Comp drag engines use honing finishes between Top Fuel and Pro Stock: Rpk 8 to 10, Rk 25 to 30 and Rvk 35 to 40.
It’s of little significance that in Pro Stock there can be some light metal-to-metal scuffing. But in Top Fuel, where gallons of nitromethane are poured through the engine, protecting the oil film from being washed away is priority. Rings ride on a film of oil—known as a moderate boundary contact layer—and if the oil film is lost, so too is the ability to seal the ring to the bore. Inevitably hot gases escape past the rings, the oil film evaporates on the cylinder walls and a sad fate known as “black death” is the grim reality.
To counter this condition, lots of oil is retained on the cylinder walls of Top Fuel engines, not just to act as a ring seal, but also to keep the parts cool. Top Fuel oil control rings are big and generate 25 to 28 ft-lb of tension with lots of scraping capability.
Correlation Between Cylinder Wall Prep, Oil Retention and Ring Specs
Back at the cylinder wall, they speak of a plateau finish. This is the surface on which the ring makes contact, and it should be flat and smooth. But in between each plateau or peak there is a valley. And it’s here where things change for Fuel engines, alcohol motors and big blower engines. In fact, any high-powered race engine with significant cylinder pressures requires greater valley depth.
Perhaps a more interesting comparison relates to today’s hot rod engines and those from the past. The stock, small-block Chevrolets of the ’70s and ’80s used 5/64 compression rings, which produced around 7 ft-lb of tension, while the oil control ring made around 20 ft-lb. In contrast, the compression ring of a modern LS engine makes around 2 ft-lb of tension and the oil control ring around 7 ft-lbs. As a result, the new, lighter tension rings have less drag and less oil is retained on the cylinder wall. Unlike the racer who’s seeking power increases, the original equipment manufacturer’s principal interest in cylinder wall finish is motivated by the prospects of lower tailpipe emissions and better fuel economy.
Interestingly, in the early days of the LS Chevrolets, GM encountered oil-burning troubles. “At that time,” says Keith Jones of Total Seal, “they as well as Ford with their five-liter engines and Mod motors, had adopted 1.5 x 1.5 x 3mm ring packages, which contributed much lighter drag than previously. Of course, we now know lighter drag rings require a different hone finish—the normal hone that had suited the 5/64 and 3/16 combinations was no longer effective.” Today these engines have much smoother cylinder finishes with much less oil retention on the cylinder walls, but at that time they could consume a quart of oil every 1,000 miles.
Facing a Similar Challenge
In some ways, today’s cylinder preparation is reminiscent of yesteryear’s racing pushrod. Trend’s Bob Fox discovered that a heavier pushrod represented the way ahead, at a time when engineers wanted lightness. A decade later lightweight, deflecting pushrods had become obsolete. Similarly, at one time only NASA could afford a decent profilometer, which prevented our race engine builders from demonstrating their true brilliance. Now these sophisticated measuring devices sell for less than $2,000; they’re even available on E-bay. So, inevitably, like the stiffer pushrod, exacting honing practices will become the new standard.
Compare how ring tensions have diminished throughout the years. A typical small-block Chevrolet or Ford ran 5/64 x 5/64 3/16-inch ring packs for 30-plus years. Today they run 1.2 x 1.2 x 2.0mm or similar.
Some very long stroke engines will run a steeper angle, as steep as 60 degrees in an attempt to move oil to the top of the bore. Steeper angles tend to hasten oil removal from the cylinder wall. Also, if the crosshatch angle becomes too steep it creates blow-by and high ring rotation speeds, causing them to align adversely on the piston.
The measuring standard known as Ra averages the peak and valley displacement from a mean line, but provides no information about the height of the peaks and the depth of the valleys or the ability of the material to bear a load. Two different surfaces may have a similar Ra, yet have two functionally different characteristics.
Ra is replaced by the following:
Rpk: Reduced peak height measures the portion of the surfaces protruding peaks that will wear during initial loading.
Rk: Core roughness depth is used to describe the portion of the surface that will support the majority of the load.
Rvk: Reduced valley depth (valleys projecting through the roughness core profile) is the oil retaining portion or the troughs machined into the cylinder surface.
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Boucher’s Racing Engines
Text by Alfie Bilk