1983 Honda CX650 Turbo

The CX650 Turbo replaced the CX500 Turbo produced a year earlier. The CX650 Turbo was only manufactured for one year. When the CX500 Turbo suffered from low-speed power issues and turbo-lag, the engine was enlarged to 647 cc to help provide more power at lower rpm. The compression was increased to 7.8:1 and the maximum boost pressure lowered to make a smoother transition from off-boost to on-boost. The fuel injection system was revised and the rear shock received a manually operated damping control mechanism in addition to the “air” pre-load of the previous model. The turbocharger compressor wheel was increased to 51 mm while it’s exhaust wheel was sized at 50 mm. The waste gate was set limit boost to 16.4 psi.

The CX650 differs little from its predecessor cosmetically, other than in colors and badging. To reduce cost, the 650’s fairing was made from ABS plastic as opposed to the 500 Turbo’s GRP (glass fiber reinforced plastic).

The CX650 Turbo’s liquid cooled, four-stroke, longitudinal 60-degree V-twin with 4 valves per cylinder produced 100 horsepower @ 8000 rpm and 68.5 lb.-ft. of torque at 5000 rpm. The bike was capable of quarter-mile times of 11.9 seconds at 112.4 mph with a top speed of more than 140 mph.

The wet, multi-plate clutch controlled a 5-speed gearbox with a shaft final drive. Average fuel consumption was 47 miles per gallon.

In 1983, turbocharging was the wave of the motorcycle future. And while the Honda CX650 Turbo was arguably the best of the turbocharged motorcycles that roared down the roads in the early Eighties, it is now one of the rarest production Hondas ever, with only 1,777 built and fewer than 1,200 imported to the U.S. and Canada.

Although Honda was the world’s largest motorcycle manufacturer in the 1970s, the company was very conscious of competitors breathing down its neck, and it was looking for ways to stay ahead of the pack. One way to do this was to stay ahead of the technological curve.

In 1978, Honda introduced the CX500, a bike it said was designed to be quiet, efficient, low emission and maintenance free. The CX had a 4-valve-per-cylinder, pushrod operated V-twin engine, shaft drive, a 5-speed gearbox, water cooling, and Honda’s new ComStar wheels shod with tubeless tires — the first tubeless tires designed for a production motorcycle.

Although the CX500 wasn’t particularly exciting to ride, it proved (after a cam chain tensioner glitch was ironed out) to be reliable and user friendly. It sold well, especially in Europe, where tax and licensing laws encouraged people to buy smaller bikes, and it became the favorite mount of British motorcycle couriers for many years.

Honda, however, wanted to show it could do more than simply build user-friendly, durable motorcycles. Its engineers, conscious of the need for innovation, thought the CX might be used as a platform for something more exciting. After some experiments with supercharging, they came up with the idea of turbocharging the CX. A turbocharger significantly increases engine performance without adding a lot of weight, and at first glance it seemed an ideal way to increase performance.

Basically a mini turbine, a turbocharger is powered by the engine’s own exhaust gases. Exhaust gases flowing through the turbo turn a small rotor, which in turn drives another rotor to deliver air under pressure to the intake manifold. This creates a denser air charge, allowing a larger fuel/air charge in each cylinder, and hence more power. The challenge is making the extra boost of power easily controllable. As one contemporary writer said about the Honda CX650T, “The power comes on so suddenly that you’d best be pointed in the desired direction, because THAT is where you’ll be heading with great alacrity.”

The first Honda turbo, the CX500TC, was announced in 1981 and appeared in 1982. It not only sported a turbocharger, but also a complex fuel injection system and a pair of onboard computers. Like all CXs, the engine was a stressed member of the frame, while the chassis sported an integrated fairing, a huge headlight, dual disc brakes in front and a single disc in the rear.

Yet despite Honda’s best efforts, bugs remained. Period testers found that, although the CX500 Turbo was blindingly fast on boost, easily reaching 125 mph, it was expensive, thirsty and suffered from a phenomenon known as turbo lag, meaning it would hesitate before the turbo spooled up and sent it lunging ahead. Getting a CX500 around a set of twisties could be challenging.

Honda went back to the drawing board and came up with the CX650T for the 1983 model year. The engine was bumped up to 674 cc and the compression ratio increased from 7.2:1 to 7.8:1. The computer controls were simplified and the gearing ratios were adjusted, with a wider gap between fourth and fifth. The result was a bike that, while equally impressive on boost, was a lot easier to ride when transitioning from off-boost to on-boost.

But in 1983, the bottom dropped out of the economy, and people stopped buying motorcycles. To make matters worse for Japanese importers, Harley-Davidson successfully petitioned for a tariff on all Japanese imports of 700cc and larger. The U.S. economy got going again in 1984 and 1985, but motorcycle sales stayed slow. It made no sense to continue to produce any but the most popular models, so the CX650T was taken out of production.

Cosmetically, the CX650 Turbo differs little from its predecessor other than in colors and badging. However, in a cost-cutting exercise, Honda manufactured the 650 Turbo fairing from ABS plastic as opposed to the 500 turbo's GRP (glass reinforced plastic).

Turbocharging Background

The concept of force-feeding had actually been around for almost a century by this stage; the inimitable Gottlieb Daimler having invented (and patented) a process using a gear-driven pump to cram air into a normal internal combustion engine in 1885. Daimler’s concept however is more aligned to supercharging than turbocharging; the former using a device that is mechanically-driven to force extra air into the engine, while the latter relies on exhaust gases to drive a turbine that is then used for the same purpose. The turbo concept is generally attributed to Alfred Buchi, a Swiss engineer who patented the idea of using a compressor driven by exhaust gases in 1905.

Like many engineering advances, both supercharging and turbocharging came in for extensive development during wartime, and specifically in the field of aviation where the problem of overcoming the shortage of breath suffered by normally-aspirated engines at high altitude was a major limiting factor to performance, and hence military superiority. By the time of the Second World War, several US bombers including the B-17 (Flying Fortress) and the B-24 (Liberator) used turbocharged engines.

Whereas supercharging actually requires mechanical power to drive the unit before it can begin its process of forced induction, the turbo relies on energy already developed (the exhaust gases) to spin the compressor. However, it inherently suffers from the extremely high temperatures under which it operates, and from the fact that the forced intake air is at a higher temperature than ambient, with less volumetric efficiency. One way to overcome the latter is by artificially cooling the incoming air, most commonly by means of an intercooler. To reduce the effects of the turbo ‘lag’, a valve, or ‘wastegate’ can be used to regulate the flow of air to the turbine, while another inherent design problem, whereby pressurized air continues to be delivered to the engine even when the throttle is closed, can be overcome by a pressure release (dump) valve between the compressor and the inlet valve.

By 1978, Kawasaki had a limited production version of the Z1R, called the TC, featuring a turbocharged engine. Strictly speaking, this was not a factory product but the result of a liaison between Kawasaki and the US Turbo Cycle Corporation, using an ATP aftermarket turbo kit. To keep the engine together, a fairly mild setting for the wastegate (maximum 8 pounds boost) was used, but it still produced stonking performance. Only about 500 were built and sold before a new California law prohibited motorcycle dealers to sell bikes with exhaust systems modified from standard.

Meanwhile, Honda, Suzuki and Yamaha were all playing around with turbos as a means to extract bulk horsepower without the need for a total engine redesign. Honda struck first, with the release of the CX500T in 1982 – the first factory-produced turbocharged motorcycle. The basis for the unit was the tried and proven, reliable and thus-far under-stressed CX500 v-twin, using a liquid-cooled overhead valve motor with the cylinders set across the frame, Moto Guzzi style – Honda’s first v-twin. With a modest 50 hp on tap and quite a few kilos to haul, the CX500 was no rocket ship, but its 10.0:1 compression ratio, ultra short stroke and 4-valve heads with a narrow angle 80-degree vee produced a sweet engine that loved to rev up to the 9,700 maximum and was pleasant to ride. Comparisons with Guzzi’s twin saw the Honda come out on top in almost every respect. In typical Honda fashion, there were design elements that would have ended up in the ‘too-hard’ basket for other manufacturers, such as the heads that were rotated inwards through 22-degrees to keep the carburettors from contacting the rider’s knees, and to make the whole package more compact. The barrels were integral with the crankcase, while the transmission broke new ground with the clutch at the front of the motor and the cartridge-style gearbox below. Because the transmission outputs in the opposite direction to the crankshaft rotation, the torque reaction on acceleration inherent with the v-twin design was largely cancelled out. Bevel-driven, oil-bath shaft drive with a cush drive in the rear hub further smoothed things out and was virtually maintenance free. Should the battery go flat, the bike could be push-started thanks to the separate CDI magneto.

In fact, apart from the cam chain tensioner which was rather fragile, the CX was fairly bullet-proofed provided the oil was changed regularly – just one reason why it became the machine of choice for legions of motorcycle couriers notably in Britain and Australia, clocking up millions of kilometres in what is perhaps the toughest of environments for a working motorcycle.

The CX500 Turbo lasted just one year, with 5,343 bikes produced, and in its place came the 674 cc CX650T. Along with the increase in capacity came a substantial boost in power–up from the 500’s 77 hp to 97 hp. In an attempt to moderate the lag between on and off phases of the turbo, compression was increased slightly to 7.8:1 while maximum boost pressure was lowered. The biggest change was to the fuel-injection system–technology for which was progressing in leaps and bounds in the early 1980s. One of the criticisms of the 500 Turbo–the internal gear ratios, had been addressed by widening the gap between fourth and fifth (top), and a worthwhile five kilos had been pruned from the all-up weight, much of this coming from the use of ABS plastic instead of glass-fibre for the fairing.

Unlike the 500 Turbo, which was sold around the world, the CX650T was primarily aimed at the North American market and was not sold in Australia. That same year (1983) Harley-Davidson successfully applied for a tariff on all imported Japanese motorcycles of greater than 700 cc capacity, but although this did not apply to the CX650T, there were other problems that led to its demise after only twelve months in production. Alternators proved to have a short life, often burning out at 15,000 to 20,000 miles, and the engine needed to be removed to repair the unit. Starter motors were another cause for complaint. The engine kicks back and breaks the starter clutch springs. The starter clutch rollers grind on the crankshaft, and you soon find yourself without a bottom end. The solution being to replace the CX650 motor with an original CX500 starter that went straight in and was totally reliable. There were also reports of premature wear inside the cylinder heads, and that was no simple fix.

According to information in Turbo News and On Boost newsletters, of the approximately 1,200 CX650 Turbos imported into the U.S. and Canada, half supposedly went to Honda tech schools and junior college mechanics programs. Honda specified in the donation agreement that these service school bikes were supposed to have been destroyed after their job was done, but quite a few seem to have escaped the crusher and crept onto the second-hand market. Just 1777 examples of the CX650T were built before Honda pulled the plug on the model and its venture into turbocharged motorcycles.

Press Comments on the CX650T

“The problem with the CX-T is that it’s impossible to resist constantly dipping into the boost. And anytime you light up the boost gauge on this bike, speed limits become a joke.”— Cycle Guide, March 1983

“Off boost, the CX650T chuffs its way down the road like a mildly tuned 650 twin. On boost, it accelerates like an F-4 being blasted off the flight deck of the USS Enterprise by a steam catapult.”— Rider, October 1983

Honda CX650 Turbo General Specifications

Years produced: 1983

Claimed power: 100 hp @ 8,000 rpm 70 lb/ft @ 5000 rpm

Top speed: 140.4 mph (period test)

Quarter mile: 11.9 seconds @ 112 mph

Engine type: 674 cc OHV, turbocharged, liquid-cooled 80o V-twin with 4-valves per cylinder

Bore and stroke: 82.5mm x 63mm

Compression ratio: 7.8:1

Fueling: EFI

Driveline: Five-speed transmission, wet multi-plate clutch, shaft final drive, Comstar wheels

Weight (wet): 573 lb

Price when new: $4,998

Fuel Capacity: 5.3 gal.

MPG: 45.8 mpg (period test)

Front tire: 100/90-18

Rear tire: 120/90-17

Front brakes: Dual rotors with twin-piston calipers

Rear Brakes: Single rotor with twin-piston caliper

Chassis: Tubular steel frame

Wheelbase: 58.9 inches

Seat height: 31.1 inches

Front suspension: Showa conventional forks with TRAC anti-dive

Rear suspension: Honda Pro-Link rear with Showa air pressurized shock