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The best way to approach the subject ofoptimal cylinder wall surface is tounderstand some of the basics:

1. We attempt to seal compression andsuction with piston rings

2. Ring, piston and cylinder wall designand material play a big role in howand what type of surface yougenerate

3. The conditions under which you race(dirt, salt, sand, etc.).

4. The type of working fluid(s) you use(fuel, gasoline, gas, etc.)

5. Metrology

First, when we discuss the conditionof a race engine we reference acompression ratio or leak down value. Ineach case, an attempt is made to find theculprit that robs us of expensive andvaluable compression. But, keep in mind;we only compress what air/fuel we takeinto the cylinder during filling. The age-old example of calling our normallyaspirated gas and diesel burning enginesair pumps is not far from the truth. Ifrings fail to seal under suction (filling),we will not draw the maximum air/fuelvolume into the cylinder andperformance suffers. This holds true forpositive displacement induction systemsas well. If the rings won't seal onsuction, they certainly will not seal when

you kick a few additional atmospheresinto each cylinder.

Second, the combinations of ring,piston and cylinder wall material isstaggering. You literally have a choicefor any contingency. In our world, castiron cylinder walls, hypereutectic,hypoeutectic and forged aluminum alloypistons are commonly used for racing.Hypoeutectic not so much but I still seethem where rules dictate. Four commonracing piston rings are: ductile and greycast iron, tool steel and stainless steel.Throw in nitride, chrome and bothplasma moly and moly-filled coatingsand you get the picture. No singlesurface will work for them all.

While on the subject of rings, itshould be noted that many ring facedesigns exist. There is the ubiquitousbeveled face, radius (barrel) face andstraight face. In all cases, pre-lappedand/or double lapped ring faces are veryimportant to qualify. Most quality ringpacks come with lapped faces so thisshould not be a worry.

Thirdly, certain ring and coatingmaterials lend themselves to specifictrack surfaces. At Bonneville and ElMirage, where caustic and abrasive trackelements can find their way into theengine, it may be better to considernitride or hard chrome face rings. Theseharder surfaces resist wear and holdsliding seal properties longer. Contraryto popular opinion, these rings take the

same surface finish as other commonracing ring packs so long as top ringfaces are pre-lapped for a light tight sealin a straight, round cylinder bore.

Moly-coated/filled rings are used forhigh ring face conformability and arebest suited where cleaner air exists. Theplasma moly rings are more forgivingand do handle heat well. It is no wonderwhy these are the #1 choice among manyengine builders. However, if the surfacefinish is not right, these rings candegrade rapidly and may not deliveroptimal performance over the period of aweek of racing.

Fourth, the working fluid and I don'thave to tell you how many choices youhave here. If we have learned anything itis there is nothing simple about gasoline.What should be considered is how yourchosen fluid works in the engine. Does ithave high detergency? Will it causeunusual tuning problems? Is itcompatible with all engine components itcomes in contact with? Indirectly,answers to these questions play a part inwhich ring pack you select and in turn,what surface finish you put on thecylinder wall.

Lastly, metrology and here is wherethe wheels fall off. For most of us, asingle surface parameter is used (if usedat all) to determine surface quality. Thatparameter is Ra. Ra or Rms is nothingmore than an arithmetic average (or rootmean square) of all peaks and valleys

Surface Qualities forPerformance EnginesBY JOHN GOODMAN

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recorded in a single sample lengthfrom a surface measuring tool called aprofileometer. Ra attempts to tell youhow rough the surface is. Too roughand the surface will destroy rings. Toosmooth and the rings may not seat. Raalone is of little practical use and nomore valuable than rubbing a pennyacross the cylinder wall to determinesurface roughness and/or quality. Atthis point, things get interesting.

OK, Ra is not a good indicator ofactual surface roughness so what is?As it turns out, an ISO standard of Rksurface measurement parameters fillsthe bill nicely. Rk attempts tosegregate peaks, core roughness andvalleys. So, for the first time, you cansee how rough the bearing area of asurface is without much mathematicalinterference from valleys (Rvk). Alltogether, they identify surfaces bothgood and bad. I won't get into everyparameter of Rk but we will look atRpk (peaks), Rk (core roughness. Thisis the area of surface that willultimately bear load.), Rvk (valleys)and Vo (potential volume of oilretained in the surface). Armed withvalues for these four measurements,you can come very close tounderstanding the most criticalelements of a honed cylinder wallsurface.

Most of us have heard aboutplateau honing. Essentially, thissurface is purposely honed to greatlyreduce peaks while leaving the deepervalleys for oil retention. If donecorrectly, the peaks are eliminated andcore roughness is prepared formaximum load bearing capability.This type of surface demands pre-lapped ring faces because there are nopeaks left with which to carry out anuncontrolled ring lapping task. Nowthink about what was just said. If youleave a peaky surface and this surfaceis required to finish lapping ring faces(hey, we need that light tight seal if weare to maximize our compression andsuction efficiencies, yes?), where doyou think worn ring and cylinder wallmaterial goes? More about that later;common sense dictates that we do notwant an uncontrolled machiningprocess (using peaks to lap ring faces)to take place in our race engines orour passenger car engines for thatmatter. We should strive to finish honea cylinder wall to remove all peaks

and micro burrs. What is left behindresembles a run-in engine. How is aplateau finish accomplished? Well,there is more than one way to do thisbut an acceptable method has youhone about .003" stock with a 80-100grit stone, .0015" stock with a 280-320 grit stone, .0005" with a 400-600grit stone and finish this off with a320 grit abrasive filled nylon filamentbrush. An 800 grit cork bond stonecan replace the brush tool if you likethem better. The 80-100 grit abrasiveremoves the bulk of stock. It alsoleaves sufficiently deep valleys thatyou will likely not remove entirelywith upstream honing operations. Thisis ok. The 280-320 grit abrasiveprepares the core roughness of thesurface and regulates the amount andvalley depth.

The 400-600 grit abrasive preparesthe plateau or load bearing attributesof the surface. Finally, the 320 gritbrush (or 800 grit cork bond stone)removes all surface defects left behindfrom honing operations. Now, here iswhere you can get creative. You canuse the 280-320 grit abrasives to alterthe Rvk measurement to leave more orless valley depending on what you aretrying to accomplish. If you have adiesel engine, I would build into thesurface considerably more Rvk (valley)than I would for a gasoline burningengine. Cylinder pressures are muchhigher for diesel engines and theysuffer cold start piston scuff if there isinsufficient oil retention on the wallsurface. Consequently, I would alsowant to see higher Vo (volume of oilretention) numbers coming off myprofileometer. I did use this type ofsurface on a gasoline burner a fewtimes. The surface looks like trash tothe naked eye but boy, did that enginemake power. By leaving a greatersurface area of the cylinder in valley, Iwas able to reduce surface areaavailable to the ring, thereby greatlyreducing friction in the highestfriction-producing area of the engine.Just be prepared for oil consumption.If this engine was to be used forendurance, lowering Rvk and Vowould be vital. You want all thebenefits of reduced peak count butgreater control of oil consumption. Itdoes you no good to build a killerengine that consumes more oil that isrequired to finish a race. The Indy 500

It is not a black art!Surface finish isunderstandable,definable,measureable,controllable andeasily subject to analysis.

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SURFACE QUALITIES OFPERFORMANCE ENGINESBY JOHN GOODMAN

is such a race and the teams are not allowed to add oilonce the race starts. Now you should begin to see whyit is so important to select rings, pistons and cylinderwall surfaces that match the venue in which you race.One size does not fit all.

Well, as it turns out, there is another family ofsurfaces that is not a plateau. It is called peak honeand is further broken down into smooth peak andconventional peak. This type of surface is prettystraight forward with exception that Rvk and Vo aredramatically reduced from what is considered anormal plateau. In fact, it is quite possible to bring theVo down so low that there is virtually no oil retentionavailable to rings or pistons. Only a good deal of hot-run data can point you to that line where to little Vo isdangerous to the longevity of the engine. While we aretalking about smooth peak surfaces, it is myopportunity to say a few derogatory remarks aboutpolishing cylinder walls. At first blush, this wouldseem a good thing. But think about what you aredoing.

Remember all those surface defects generated in anormal honing process? You know the ones youremoved with brush and cork bond honing? Well, thatdefect layer is full of other unwanted debris that comesfrom spent abrasive crystals, unknown particles in thehoning fluid, and whatever you are using to polishwith. You may end up with a mirror finish but it willbe one of the most destructive finishes you could haveproduced for your rings. Within a few rotations of thecrankshaft, this unstable polished layer is removed.Particles are released from the surface and imbeddedinto piston skirts and ring faces. Continued running ofthe engine increases the damage caused by theseabrasive particles until rings fail completely. If you arelucky and the engine doesn't destroy itself, all thatpremature wear of sliding seal components go directlyinto your oil supply and is pumped to every nook andcranny of the engine. Just think about all thoseabrasive bits in your soft bearing material, constantlywearing on crank and cam journal surfaces. It isn’t apretty sight.

I have probably rambled on far too long to be ofany benefit to you but please approach cylinder wallsurface with as much respect and investigation as youdo with other critical areas of the engine. IT IS NOTA BLACK ART! Surface finish is understandable,definable, measurable, controllable and easily subjectto analysis. Choose wisely and you will have troublefree and almost break-in free race engines withincreased reliability and performance.

One final word; No matter how careful you are in the selection of rings and preparation of cylinderbores, if you do not make straight, round bores yourrings will not seal properly, you will lose power andexperience copious amounts of oil consumption. Pay attention to details! Almost good is not goodenough.�

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