For years and years, many of the world’s cruising and touring bikes soldiered on with traditional engines having two valves per cylinder while the rest of motorcycling switched to four valves. Having four valves came to be associated with go-faster sportbikes and high rpm. Crotch-rocket stuff, not for mature riders who know the score.
Two valves—if anybody today still knows what valves are—came to be viewed as, “Good enough for my dad and his dad before him, so they’re good enough for me.”
Then, relatively recently, a wave of change has swept through big-inch touring engines, affecting BMW first, then Honda's Gold Wing and Harley-Davidson's Big Twins. What apostasy is this? Once you understand the evolutionary pressures on these engines, and the real differences between two valves and four, it all makes good sense.
When I was a lad, few riders went on long tours. Bikes then were far from the reliability of the present day, and the Interstate Highway System had not yet been built. But today’s riders expect their big tourers to roll right up one side of the Rocky Mountains and down the other, two-up, with luggage, sound system, and even a trailer behind, at speeds of 75 mph and up. Power to spare for passing. That has driven constant growth in engine displacements. The old 61s became 74s, then 74s became 80s, and today here we are with touring motorcycles having bigger engines than many economy cars.
To make their much-loved traditional air-cooled engines reliably keep pace with highway traffic and liquid-cooled Gold Wings, BMW and Harley-Davidson have had to add strategic liquid-cooling to the hottest parts of their cylinder heads in order to prevent valve leakage from heat-driven head distortion. That improved cooling has made it possible to boost torque by raising compression ratio; the hotter an engine runs, the more vulnerable it is to the destructive abnormal combustion called “knock” or “detonation.” Harley-Davidson’s 117ci CVO Road Glide engine is now at 10.2:1 compression and BMW’s R 1250 RT at 12.5:1.
The makers have pushed up displacement and compression ratio yet riders continue to seek more power and torque to easily accelerate impressive loaded weights at interstate speeds. Riders don’t want to sacrifice any low-speed torque to get it.
More power was never any problem; almost 40 years ago, the late Jerry Branch revved a souped-up Harley-Davidson Big Twin to 7,600 rpm (think of the vibration!) and got 100 hp. But the engine lost its bottom-end torque in the process. Horsepower is easy; the real trick is to combine more power without loss of the desired strong low-speed torque.
Branch got that power from a 1980s two-valve V-twin mainly by using the long cam timings of racing engines, but when you do that you are just trading away bottom-end to get top-end. Where does that leave us? It leaves us needing more engine airflow to make highway horsepower but with no way to get it through two valves per cylinder.
So, one by one, the makers of big touring rigs have switched to four valves, which thrive on the short valve timings necessary to deliver tractor-like bottom torque, yet with them can provide extra flow area to supply every bit of the air an engine needs all the way to the top of its normal powerband.
Everything in engineering is a compromise. When it comes to airflow per square inch of valve-head area, a single intake valve in a hemi chamber is still king. But the two intakes in a flat four-valve chamber deliver more overall airflow because they have so much total valve-head area.
The old rule that the fewer the holes you make in a combustion chamber, the less trouble you’ll have from cylinder-head warping or cracking made two valves the right choice for air-cooled engines, whether they were Harley-Davidson Big Twins or combat aircraft engines from World War II. That’s why you don’t see a lot of successful purely air-cooled four-valve engines in production.
The details? When you place two exhaust valves side by side in one chamber, the narrow bridge of cylinder-head metal between them gets very hot being heated from both sides and its constant expansion and contraction makes cracking there quite common. But when you add local liquid-cooling, those paired exhaust valves comfortably settle into reliable operation and life is good.
Therefore, fear not, the conversion of big touring engines from two valves per cylinder to four has been made to further improve the combination of tractor-like low-end with interstate-gobbling cruise power.
