Inside Harley’s Bagger Racing Engine

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Taking a close look at Harley-Davidson’s 131R engine used in its factory King of the Baggers racing effort. (Harley-Davidson/Brian J. Nelson/)

First big obvious thing about the Harley-Davidson King of the Baggers racer bike is its strong, deep sound. Its syncopation comes from its irregular firing intervals, which result from the engine’s since-the-beginning 45-degree Vee angle. On the track its usual operating rev band is 6,000–7,000 and it sounds remarkably like one of the classic big radial aircraft engines of World War II—powerful and authoritative.

M-8 131R crate engines are delivered to product development where they are modified for KOTB use. Bore and stroke are 4.310 x 4.5 inches. That stroke was created for the M-8 114 engine. “M-8″ tells us these are “Milwaukee-Eight” engines, meaning they have a single chain-driven four-lobe camshaft operating four valves per cylinder through pushrods by means of newly designed and longer rockers unburdened by old-time screw-and-nut adjusters. Equal valve operation is guaranteed by holding dimensional tolerances. 131R heads are CNC-ported and combustion chambers are fully machined, giving 10.7 compression. The cam supplied is SE8-517. Valves are 1mm oversize.

These heads have dual ignition with a centrally mounted solenoid-driven timed decompression valve to assist starting.

Not so long ago the realists in NHRA Pro Stock Auto got really tired of trying to design camshafts whose wiggly shape would compensate for the constant flexing of the camshaft itself between its bearings, of the pushrods, of the rockers. Instead, they focused on improving control by making the parts much stiffer. In the old days stiffer meant heavier, and heavier meant valve float. Be that as it may, today’s approach to pushrod valve control is to use really stiff components. And it has worked.

Related: From Road Glide to Roadrace

Forged pistons in a 4.310 bore stroke through 4.5 inches in the 131R. (Harley-Davidson/Brian J. Nelson/)

When engineers need to know the details of valve motion at speed, they go to the “Spintron” which drives the valve train with an electric motor while monitoring valve position to high accuracy by light interferometry. Analysis reveals the natural frequencies of the flexing parts.

That 4.5-inch stroke is a long way to go at 7,000 rpm. Makes me think of England’s Manx Norton racing 500 singles, which began to break crankpins without warning in the 1950s, as peak revs reached 7,000. Their stroke was 78.4mm, or 3.086 inches—only 69 percent of that of this 131R. Let’s next compare piston accelerations at 7,000 revs. For the Manx it was 2,638 times the acceleration of gravity (2,638 Gs), while for this KOTB Harley it is 46 percent greater at 3,847 Gs. When I asked a team engineer about crank reliability I was told, “Really good. Really, really good.” That’s because of two differences between modern and classic Harley engines. First, large-diameter straight crankpins pressed in place replaced the spindly taper-and-nut pins used 25 years ago. This greatly increases strength and raises the “elephant ears flapping” frequency of the flywheels and crankpin vibrating as a giant tuning fork. And second, the fatigue resistance of bearing roller materials has been hugely increased since the ‘50s, driven by the needs of aircraft gas turbines.

On the other hand, because rules allow achieving the displacement limit by any desired combination of bore and stroke, team principal Jason Kehl said, “We may play with it.”

This engine’s peak piston acceleration at 7,000 rpm is far from excessive, considering that F1 engines of 15 years ago were pushing 10,000 Gs.

Looking at a cutaway 131R engine displayed outside the race tent all I could see was an arc of the flywheels and the upper parts of the forged steel rods (which are still of classic fork-and-blade construction). That engineer, following my gaze, continued to assure me that rod design has been greatly improved by application of Finite Element Analysis, in seeking to reduce stress concentrations. Somewhere, on some engineer’s computer, there is a false-color stress comparison of old and new rods. We’d all love to see it.

The production 131R has a single gear-driven balancer that reduces primary shaking force about 75 percent. It is deleted from the race engine.

When I picked up a 131R forged piston from the table on which the cutaway was resting, it had the smoothly curving organic shape I have come to expect from FEA-designed parts. I can only call it beautiful—something that John Britten would have admired. It is a very short “ashtray” design, its skirts coated with a dark anti-friction break-in coating. The light weight of such pistons is fundamental to reliable operation at 7,000 with so long a stroke.

Normally, such a short-skirted piston could not be expected to maintain the cool crown temperature that is essential to preventing an engine from detonating (knock, or detonation, is a destructive abnormal form of combustion provoked by exposure of the unburned mixture to high temperature). Light modern pistons coexist with air-cooling in this engine because of oil cooling from jets in the crankcase, aimed up at the undersides of the piston crowns. The essential details of this engine are thoroughly engineered by modern methods. The traditional appearance—as a 45-degree air-cooled pushrod V-twin—is maintained.

Related: Why King of the Baggers Racing Is So Popular

Holding a 131R forged piston reveals a short skirt and organic shape. (Harley-Davidson/Brian J. Nelson/)

The pistons used in the KOTB bikes are machined from the same forging as the piston I saw, which had only the slightest fly-cuts in its crown for valve clearance. The race pistons surely offer more compression (the class operates on a spec fuel, VP’s T4) and perhaps have deeper fly-cuts to allow valve lifts greater than the 0.515 inch of the SE8-517 cam.

Higher lift? Classic production torque curves were shed-roof shaped. The high peak of the roof was very high torque at low revs, sloping down as the engine revved up because of what engine pioneer Harry Ricardo called “wire-drawing.” The higher the engine revs, the harder it is to pull enough air through an intake system dimensioned to deliver peak torque at lower revs.

That has changed somewhat in recent years, moving toward a “haystack” torque curve peaking in the 3,000 to 3,500 rev range where on-ramp acceleration and passing take place. Below and above that peak, less torque is needed.

For a racing engine, the need for power pushes everything to the right, to higher rpm. I was told the KOTB engine spends a lot of its time between 6,000 and redline at 7,000. That in turn requires increased valve lift (and, likely, later intake valve closure and increased overlap) as  means to allow a good cylinder charge to be taken in in less time (at 3,500 revs, the intake event takes maybe 0.009 second, but at 6,000 that dwindles to 0.006 second). Something has to compensate for the shorter time.

Rules require use of a stock gearbox. When I wrote out the ratios it was immediately clear that sixth is a touring overdrive in which rpm at a given road speed falls 19 percent to reduce vibration. That big gap means it’s not going to be used much on a roadrace course because that much rev drop at upshift pulls the revs down enough to compromise acceleration. Conversation with team members revealed that first is little used after the start. What is the result? In effect, this class is using a four-speed gearbox. That in itself places some limitations on how narrowly the engine can be tuned.

If we decide the engine makes 160 hp at 7,000 rpm, that implies a stroke-averaged net combustion pressure (or bmep, as engineers term it) of 138 psi. This is about two-thirds of what is achieved in a mature purpose-designed racing engine, and it makes sense because the KOTB class is young. There’s a lot of tuning territory left to exploit. The more power you make, the more cooling you need. Keep an eye on oil cooler sizes.

The present engine is served by a single giant throttle body and a billet CNC-contoured Y-manifold about the size of a softball. One possibility, should more power be required, would be what was done to the XR-750 years ago—giving each cylinder its own separate and straight-in intake system (where would you put it?). In the present system, the single throttle body faces to the right and a bullet-shaped air filter assembly points straight ahead.

Related: Harley-Davidson Prepares for Daytona

A massive bullet-shaped air intake feeds the 131R’s single throttle body. (Harley-Davidson/Brian J. Nelson/)

It has never been easy to control temperatures in air-cooled engines, which naturally run hotter on hot days and cooler on cool ones. When I asked one of the crew if they’d been able to get their engines to detonate, he replied, “Oh yes! Thermal management has required some serious work.”

I had seen welded-on cooling fin extensions on the 2022 bike, but they are no longer used. I was told, “Getting air to the rear head is crucial.” A substantial air scoop on the left side serves that purpose (you can’t cool anything with air already heated by passing through the hot fins of another cylinder). Team principal Kehl noted that the usual place for a racebike cooler is under the fairing nose, but on this bike that would block airflow to both heads. Therefore the large cooler is located up in the frame-mounted fairing, replacing the lights. Think of the cooler as an “infrared headlight.” I was pleased to see rock screens on these coolers. Racebikes kick up little stones from track surfaces—you don’t want them hitting a heat exchanger core or getting between a drive belt and pulley at 250 feet per second.

An oil cooler sits where the headlights would normally be found inside the Road Glide fairing. (Harley-Davidson/Brian J. Nelson/)

Another classic air-cooled problem is cylinder head distortion. When metals are hot and operate under considerable stress, gradual yielding can occur even at temperatures nowhere near melting point. This is called “creep.” If creep slightly displaces or ovalizes exhaust valve seats, seal is lost and bad events follow. Two supplemental cooling schemes are possible on 131R—the so-called “Twin Cooled” (normal air-cooling plus engine coolant circulated through a heart-shaped passage surrounding the paired exhaust valve seats) and the oil-cooled (oil pumped at high speed through a passage between the exhaust valve pairs). The team’s testing revealed that the cooling needs of the KOTB engine were better served by the latter, pulling heat directly out of the most heat-affected region.

You can recognize a Twin Cooled engine by the presence of water radiators and the pairs of black coolant pipes fastened to the exhaust side of each head.

Harley-Davidson has incorporated lessons learned in King of the Baggers racing in its new 135ci Screamin’ Eagle Stage IV crate engine. (Harley-Davidson/Brian J. Nelson/)

Revealed on March 1 was a further enlarged and powered-up Screamin’ Eagle Stage IV crate engine, the 135R, whose crankpin is moved outward 0.0625 of an inch to give a 4.625-inch stroke and a 135ci displacement. Why mention that here? The reason is that the team is very pleased that their KOTB R&D was quickly applied to this new product. Displacement has increased only 3 percent but power and torque have risen 10 percent. That’s a claimed 130 hp and 143 lb.-ft. of torque at the rear wheel. Remember when they used to say that “racing improves the breed”?

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