Inside the S&S Cycle Polaris Pro Stock Motorcycle Drag-Racing Engine

Going big

s&s v-twin drag racing engine close-up
The S&S V-twin drag-racing engineSavannah Rose

Let’s make an “elephant” V-twin engine by bandsawing two cylinders off a 454 Chevy Big-Block. That’s 113.5 cubes.

Puny! The S&S Cycle Pro Stock drag-race engine measures 5-1/8-inch bore by 3-7/8-inch stroke. B-I-G. That's 159.9ci. To paraphrase Crocodile Dundee, "That's an engine!" Stare in wonder at its gaping injection throttle bodies. This hugeness, remarkably, revs past 10,000 rpm.

Three engines compete in National Hot Rod Association (NHRA) Pro Stock Motorcycle (PSM): the Vance & Hines "Harley-Davidsons," the S&S V-twins (once billed as Buells, now as Victorys), and the higher-revving GS-based Suzukis, which get "only" 107 cubes. A quick Pro Stock run in the quarter-mile is 6.750 seconds, reaching 195 to 199 mph through the lights. Engines are unsupercharged, burn gasoline fuel, and produce roughly 350 hp. Bikes in the class are limited to a 70-inch maximum wheelbase, a rear slick 10 inches wide, and a minimum weight of 550 pounds.

The original designed-in-house S&S PSM engine of 2003 sensibly used everything possible from that company’s bulging engine aftermarket parts book. This gave it four geared-together individual cams just like a Sportster, driven by an intermediate gear meshing with the timing pinion on the crankshaft and the intake cam gear of the number two cylinder. To simplify manufacturing, the cams and their drives were mounted between plates bolted into the engine’s timing case.

angelle sampey drag racing action
Angelle Sampey on the S&S Cycle Polaris Pro Stock Motorcycle at GainesvilleMatt Polito

The crankshaft was familiar Harley technology, pressing two separate flywheels onto S&S’s big 1.5-inch crankpin. On that pin were aluminum knife-and-fork connecting rods with hard-steel bearing inserts, turning on the caged needle rollers used to this day on Harley Big Twins. Separate cylinders—at first aluminum, then iron—carried the giant pistons. Any problems with such giant bores?

“Keeping them round and finding pistons,” I was told in an interview with S&S president Paul Langley and VP of Engineering Scott Sjovall.

s&s v-twin drag racing engine piston details
The light weight of these glorious giant pistons is one key to repeated runs using 10,000 or more rpm. Try to visualize the compressed mixture, jetting out of the squish zones as a piston approaches TDC.Savannah Rose

The vital heart of the matter is the valve train, for with NHRA's grant of all that displacement (160 cubes is slightly more than 2,600cc) comes a requirement that the engine have only two valves per cylinder, operated by pushrods and rocker arms as production Harley Big Twins do. Pro Stock Motorcycle lives for partisanship; spectators root either for the "Harley guys," which would also basically include the twin from Polaris/S&S, or the four-cylinder crowd. It matters not at all that there are no Harley parts in Harley-Davidson's 60-degree V-twins. All three rivals share the proven Suzuki clutch and gearbox.

The intake valves of the S&S PSM engine measure 2.750 inches and are lifted a colossal 1.1 inches by 2:1-ratio aluminum rockers, driven by watermelon-shaped cams with a lobe lift of 0.550 inch. Now get this: Ballpark valve-spring pressure at full lift is 1,200 to 1,400 pounds. Down at the level of the cams, this doubles to 2,400 to 2,800 pounds. Now add the force required to accelerate those giant valves up off their seats and you can see that big stuff happens in the timing case, thrust up through the pushrods to the rockers. Every time a cam lobe lifts its tappet, the force is greater than the weight of a four-door sedan.

s&s v-twin drag racing engine crankshaft details
Ever see a more beautiful crankshaft? One-piece construction eliminates “shifty” press-fits. Smooth, graceful shapes avoid stress concentrations, which prevents cracking. Giant main shafts are equal to the task.Savannah Rose

Think metals are rigid? This force turns them into springs. Here’s how the springs stack up: 1) the flex of the cams on their needle bearings, supported by the bolted-in-place aluminum cam plates; 2) flex in the tappet rollers, their needles or bushings, and their pivot axles; 3) as pushrods hammer upward, slight side movement occurs from the swing of the rocker—movement that can excite lateral bending—like that of a vaulting pole; and 4) the rocker, a beam in simple bending, and its pivot and supporting structure.

Every combination of a spring and a mass has natural frequencies of vibration. When you stack springs this way and pound large cyclic forces through them, resonance may develop, building up to complete failure. A tool for understanding this is the Spintron—and S&S has its own. Spintron spins your engine (or a rig representing some part of it) with a big electric motor and measures valve motion by laser interferometry.

s&s v-twin drag racing engine ally rockers details
Aluminum alloy rockers with 2:1 ratio push valves to 1.100-inch lift, against springs with hardly any coils, exerting more than 1,200 pounds open. Big forces require such big rockers and rocker stands. Not-quite-round port is surely CNC.Savannah Rose

My informants told me, “The operator can usually hear a resonance developing and stop the test before anything breaks.” And the Spintron isn’t the only tool; computer modeling refines the choices ahead of time.

The task is to accurately control the motions of valves, which weigh a substantial fraction of a pound each, while this giant engine revs to 10,500 rpm or more.

Engineers with tools like these can stabilize a valve train, but these engines, unlike the V&H, are for sale (yours for $42,500). That means the teams modify them to stay competitive. As ETs shrink and terminal speeds swell, engines work harder. Until recently an S&S engine was doing well to make four runs without needing maintenance (like the Suzukis of a few years ago, getting maybe five runs from a crank). Therefore they applied to NHRA to be allowed a basic redesign. NHRA, feeling they’ve achieved engine parity in PSM, regard redesign as “potentially destabilizing”—as when V&H were allowed to run four-valve heads, winning 15 of that year’s 16 events. To meet this objection, S&S pitched this redesign as a reliability upgrade, which it certainly is. Otherwise it would have updated the wedge-chambered squish cylinder head as well, using the latest airflow techniques. Next time!

s&s v-twin drag racing engine connecting rods details
Steel connecting rods run side by side on the crankpin. Formula 1 piston speeds and those great, big slugs require stoutness and consistency not provided by titanium.Savannah Rose

The new (still milled-from-solid) aluminum crankcase is horizontally split, not vertically as before, containing a forged one-piece steel crankshaft with a single crankpin carrying two side-by-side split-and-bolted steel plain-bearing connecting rods (just as in the S&S X-Wedge production engine). Steel, being twice as stiff as the equally strong titanium, does a better job keeping piston-to-head squish clearance consistent. Piston speed is above Formula 1 level (6,500 versus 5,000 feet per minute). For maximum rigidity, the cylinders are now integral with the upper case, with removable ductile iron liners.

Gone are the bolted-on cam plates—more rigidity required! Now the inner cam bearings seat directly in the upper case. Gearing all four cams together also led to fatigue and breakage from the impacts of tooth backlash stack-up (at Gainesville just now I was shown “gravel” of broken gear teeth and escaped needle rollers). Now the intermediate gear drives both intake cams directly, while each of them drives that cylinder’s exhaust cam.

An earlier step up in reliability was “big cams,” cams with bigger, stiffer shafts, and bearings, and larger cam lobe base circles.

s&s v-twin drag racing engine combustion chamber details
The S&S Pro Stock combustion chamber is more influenced by automotive practice - the classic "wedge" combustion chamber with two valves whose stems are close to parallel - than it is by two-valve motorcycle tradition - for example, Norton's 1940s-'70s twin with its valve stems at 58 degrees to each other in a shallow hemi chamber. This engine's huge 5 1/8" bore provides adequate room for valves with little angle between their stems; the goal is then to have enough squish area to make mixture in the combustion volume turbulent enough to light up and burn quickly - something that was often a problem with hemi chambers.Savannah Rose

The new crank spins on two large ceramic-ball bearings, and to further increase valve drive rigidity there is a third main bearing outboard of the crank’s timing pinion. When pressed-together cranks vibrate in “elephant-ears flapping” mode, flywheels shift on the crankpin or break it. Many assume welding crankpin to flywheels fixes this, but 1) welds in high-carbon steel are brittle; and 2) welding heat relaxes the crankpin press fit. The new forged one-piece crank eliminates this.

Gainesville’s Gatornational was the first official outing for this new engine, and for its relationship with Polaris. With the brand’s closure, Victory bodywork at Gainesville will require an identity adjustment.

CNC-machining centers take away everything that’s not on the drawing. Write the software, debug, load the tool carousel, fixture the billet, hit “RUN,” and go home to your family for the evening. If everything’s perfect…

“We’re glad to be working with Polaris,” Langley said.

The two Team Liberty Victory Magnum bikes of Cory Reed and Angelle Sampey were eliminated in the first round at Gainesville, while Harley-Davidson’s mature program topped the field. The bikes operated consistently and were fast despite being a product of just three months of intensive preparation. Now that reliability issues have been resolved, teams running S&S Pro Stock engines can put their effort into engine and chassis tuning.