SCAN FOR MORE INFO - S&S Cycle

SCAN FOR MORE INFO

ContentsMAY/JUNE 20128ON THE COVERVolume 22 • Number 4 • Our 183rd IssueThis issue of IronWorks brings you not one but twostriking Shovelheads. One was built to give away, theother intended as a prized rider and keeper. PureHell, built by partners in a new shop called Moto RodDesign, was a solid team effort. In contrast JeffCochran’s solitary study of the Shovelhead allowed theopportunity to employ long-held components that werewaiting for the right moment to shine. Both machinesshow what a pile of disparate parts can become whenthey’re treated with care, thoughtfully altered, and assembledwith intent. Dig in!FEATURE BIKESPure Hell Shovel......................................8Pure heaven for its winnerBobber on the Bottle .............................18Pared down perfectionHank Young Panhead .............................22Honor thy fatherJeff’s Silver Bullet .................................26Twin Cam tributeSpeed Century ......................................34From Argentina’s first custom shopSpeedking Shovel ..................................44Jeff Cochran’s personal rideSnowracer............................................72Getting custom tractionIW GARAGEProGuard Protectors .............................50Save that chromeMicro Swirl Edging ................................52Banish the windEdge Throwover Bags ............................54Function rulesAbout Avon...........................................56How to choose the right tiresShop Time with Mike Witt ......................58Finding the Missing LinkeRusted Nuts Tech .................................60DIY ParkerizingS&S Cycle Story....................................62Engine Science continuedFEATURE STORIESMotor Bike Expo ...................................30Two-wheeled world viewKnuckle Start-up ...................................40Helping the Fossil fly againWyoTech/OCC Bike Tour....................... 48Paul Sr. lends a handDEPARTMENTSDriveline Buyer’s Guide...........................64Getting power to the pavementSeasoned Citizens .................................76Indian Bobber: a crafty compositeIW Eye .................................................82Worth a thousand wordsREGULARSMarilyn Stemp ........................................6Inked for lifeSam Kanish ..........................................12Fun on Friday the 13thBert Baker ...........................................14Different is goodRick Fairless .........................................16Keep it down, will ya?Ed Youngblood ......................................70Welcome: MotoHistory in Print

The S&S StorySTORY AND PHOTOS COMPLIMENTS OF S&S CYCLES&S Engine Science: Part 2Sucking the squeeze, and banging the blowIn the last issue, wetalked to S&S PrincipalEngineer Roy Meyerabout structural and physicalconsiderations that weneed to respect when buildingbig engines. In this article,he delves more deeplyinto what it takes to reallymake that big engine performlike you want it to. -ed.A big inch engine has a lotCombustion efficiency and the amount of spark lead required are greatly affected by combustionchamber design. The shovel style head on the left has a fairly quiescent chamber,virtually unchanged since the 1930’s, and typically can run with 35° to 40° of spark lead.The S&S Super Stock® head features an aggressive chamber with a lot of turbulence. Typicalspark lead is 28°.Compare the bore size of the stock Harley-Davison® EFI throttle body on the left to thebore size of the S&S 58mm throttle body on the right. Bigger hole, more air. More airmore power. Bigger carbs and throttle bodies help to achieve higher volumetric efficiency.of potential, and the actualperformance depends onhow efficiently we achievethat potential. The two majorefficiency factors to considerare combustion efficiency andvolumetric efficiency. Combustionefficiency refers tohow much of the fuel is actuallyburned, and that dependslargely on how well it’s vaporized.Liquid fuel does notburn; fuel can only burn as avapor. Vaporization of the fueldepends on the design of theintake, cylinder heads, andcylinder. In general, modernengines with appropriatecamming and a properlysized intake system havepretty good combustion efficiencyat about 90-95%. Soit seems that vaporization ofthe fuel shouldn’t be a major concern. However,with too large an intake and thewrong cam, vaporization can be poor at allbut high rpms.The air fuel ratio (AFR) refers to theratio of the mass of air to the mass of fuelin the intake charge. Stoichiometry is thescience of optimum ratios in chemical reactions.That’s where we get the term stoichiometricmixture. A perfect stoichiometricmixture for pure gasoline is 14.7:1.This ratio will theoretically allow all theavailable oxygen to react with all the availablegasoline in an ideal, “perfect” engine.Since our engines aren’t perfect and ourgas isn’t pure, the actual AFR is slightlylower (richer). Performance engines mayrun an AFR as low as 12.5:1. A slightlyrich mixture doesn’t hurt performancevery much, since you use up all the availableoxygen and any unburned fuel is justexpelled out the exhaust. A lean mixturehurts performance because there is excessoxygen available and you could beburning more fuel. Incidentally, while thestoichiometric mixture for gasoline is14.7:1, stoich for ethanolis 9.0:1 and nitromethaneis 1.7:1. That explains whyalcohol and nitro dragstersget such lousy mileagefrom the fuel they use.Ignition timing also effectsengine efficiency. Dependingon the compression ratio,rpm, and the amount of turbulencein the combustionchamber, the amount ofspark lead (advance) requiredvaries a lot.High compression andan aggressive, turbulentchamber design will requireless spark lead because theflame moves faster anddoes not require as muchtime to complete the burn.In fact these engines willknock if too much advanceis applied. At high rpmmore spark advance is requiredto give the flamemore time to burn the availablefuel. If spark lead isnot sufficient (retarded) thefuel may still be burningwhen it goes out the exhaustport. Not only doesthis cost performance, italso causes overheating.That energy is not beingused to turn the crankshaft.It’s just heating up the exhaust port.Anyone for blue exhaust pipes?Volumetric efficiency is defined as theratio of engine displacement to the volumeof air the engine can actually take in. It reallyonly matters at wide open throttle(WOT) and at high rpm. If an S&S 124inch engine is idling, the throttle is almostclosed, and it’s drawing in very little air.Therefore the volumetric efficiency is verylow at this rpm, as is power output. Athigh rpm and WOT, the engine is crankingout some serious ponies, and volumetric62 IronWorks May/June 2012

efficiency is much higher. How high? If themanifold pressure is the same as atmosphericpressure when this engine isscreaming at WOT, volumetric efficiency isvery close to 100%. If manifold pressureis less than atmospheric, volumetric efficiencyis less than 100%, indicating a restrictiveintake system or one too smallfor the engine. That’s why S&S makeslarger carburetors and throttle bodies,and why we offer cylinder heads with highflowing ports.With optimal valve timing it is possibleto achieve volumetric efficiency over100%. That means you’re stuffing moreair into the engine on each intake strokethan its displacement volume. That’s exactlywhat a turbocharger or superchargerdoes, but more than 100% volumetricefficiency is achievable in naturally aspiratedengines through cam timing, intake design,and exhaust design. To understandhow this works you need to realize that anengine is a dynamic system. Air doesn’tsmoothly flow into the air cleaner and outof the mufflers. The flow of air is intermittent,and energetic events are involved.As a result, harmonics, resonances, andpressure pulses are generated that effecthow the engine runs at a specific rpm.Physical dimensions and material characteristicsof the engine components causethem to resonate at specific frequencies.Finally, air has mass. That means it hasinertia and momentum! It takes energy tomake it move, and once it’s moving, itkeeps moving.How can we put all that to work? Firstof all, on the exhaust stroke, you have apipe full of air rushing away from the cylinderhead. Even when the piston reachestop dead center, the air keeps movingwhich creates a slight vacuum in thechamber. If the intake valve is open at thispoint, air from the manifold will flow intothe chamber before the intake stroke evenbegins. On the intake stroke, air rushinginto the cylinder through the intake tract,will continue to flow even after the pistonreaches BDC and starts back up for thecompression stroke. All it takes is to keepthe intake valve open for a little whilelonger and we get some extra air in thecylinder. Both of these occur at a specificrpm, and generally it’s a high rpm, butthat’s how it is possible to achieve morethan 100% volumetric efficiency.In the next article, Engine Science Part3, we’ll explore how all of this theoryworks in the real world. We don’t havetotal control, but we’ll talk about how tomake a silk purse out of a sow’s ear, so tospeak. IW*RESOURCES&S Cycle Inc.www.sscycle.comIronWorks May/June 2012Reader Service No. 3463