Old Bike Australasia: Honda’s Blowhard - Honda CX650 Turbo
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Old Bike Australasia: Honda’s Blowhard - Honda CX650 Turbo

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By OldBike - 16 June 2014
Story and photos Jim Scaysbrook

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 pressurised air continues to be delivered to the engine even when the throttle is closed, can be overcome by a pressure release valve between the compressor and the inlet valve – a ‘dump’ valve in common parlance. 

ABOVE RIGHT Black-painted mufflers are unique to the 650. ABOVE LEFT Front forks have Honda’s TRAC anti-dive system. Com-star wheels are similar to those first seen on the CB250/400T some years earlier. LEFT Rear hub contains a shockabsorbing cush drive to further smooth out power delivery. RIGHT Comprehensive dashboard was state-of-the art for the day with fuel and temperature gauges and a number of warning systems.

While car, truck (notably diesel) and aircraft manufacturers beavered away with solutions to refine the turbo concept to their use, so did engineers involved in motorcycle production. 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 after-market 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. Alas, only about 500 were built and sold before a new Californian law prohibited motorcycle dealers to sell bikes with exhaust systems modified from standard. 

Meanwhile, Honda, Suzuki and Yamaha were all sniffing around 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 understressed 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. In car practice, 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 maintenancefree. 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. 

Honda CX650 Turbo

Which brings us to 1982, and Honda’s entry into the turbo market with the CX500 Turbo. It is generally held that Honda had designed the original CX500 with turbocharging in mind, and had built the necessary robustness into the crankcases. However the cases are one of the few items that were retained from the CX500 – the chassis itself being all-new. Compression was dropped to 7.2:1 and the turbo set at 19 psi of boost, kicking in above 5,000 rpm. In another milestone for Honda, the CX500 Turbo featured fuel injection in place of carburettors. Four pressure sensors and several temperature sensors conveyed signals to the CFI (Computerised Fuel Injection) on-board computer which then calculated and administered the correct amount of fuel. Unlike today’s systems, the ignition and injection components were not integrated, resulting in a fairly bulky and complex arrangement that was nonetheless the forerunner of the injection systems used on later Honda motorcycles and cars. 

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, and the engine needed to be removed to repair the unit. Starter motors were another cause for complaint, the solution being to replace the CX650 motor with an original CX500 job 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.

It would seem that Honda had long-term plans for the 650 turbo, because it allocated a large number of complete bikes to dealer service schools in the US and Canada to train mechanics in the ways of forcedinduction engines. Reportedly, 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. 

John Fretten samples the fruits of his labour.

A local 650 

Unlike the CX500 Turbo, the 650 version did not make it Down Under, so John Fretten’s CX650T is a rare bird indeed in these parts. Styling-wise, the 650 and 500 are very similar, with the mono-shock (Pro Link) rear end replacing the original CX500’s conventional twin shocks. Up front were 37 mm air-assisted front forks with Honda’s TRAC anti-dive system. Honda’s unique Com-star pressed alloy wheels with the ‘spokes’ riveted to the rims, allow tubeless tyres to be used. On the 650T, the front forks received internal modifications, while the single rear shock had a manually-operated damping control system in addition to the air pre-load. 

John’s machine came from US and has been subjected to a meticulous restoration. “I like to do everything myself apart from the paintwork and plating,“ says John, who is a trained mechanic in addition to his many years in the motorcycle trade as a dealer and importer. His current stable numbers around a dozen bikes which are regularly seen in shows and rallies. John’s experience with Honda turbos goes back to the seminal CX500T. “I pulled apart the very first CX500T in Australia, under close supervision from the Japanese engineers from Honda,“ John recalls. “It had a loose dowel inside the engine that was causing a racket, but luckily had not caused any other damage”. His CX650T is absolutely pristine – there’s simply no other word to describe it – and the opportunity to take it for a spin was too good to resist.

I have ridden several of the original CX500s, and the outstanding impression is of how well thought out the riding position is; perhaps that’s how those intrepid couriers could spend so long in the saddle. The CX650T is certainly part of this family. The seat is plush and well shaped, and all the controls fall easily to hand. Even the fairing seems to be shaped perfectly, something that was decidedly not the case on OE fairings in the early ‘eighties on most other bikes. There’s a big, easy to read array of instruments to keep you informed of what’s going on, not just the usual speedo and tacho but fuel and temperature gauges and a centre section of warning lights, including one to tell you when the fuel system is not functioning as intended. John maintains that Honda were pretty serious about establishing the turbo concept into a major range in itself, and this extended to having a red/white/blue motif carried through to items such as the fuel tank cap and the ignition key. 

Thumbing the starter button brings the engine instantly to life, but it likes to be warmed up properly before it settles down to a regular beat. As with most turbocharged engines, the exhaust note is not that enthralling – more of a deep rumble that the distinctive rasp associated with the normally-aspirated CX500. Engaging first gear is an almost silent operation – the gear lever actually operates in an east-west fashion as a rocker, but the rider is completely unaware of this peculiarity – it feels just like a conventional pivoting job. Once under way and below about 5,000 rpm, the performance doesn’t feel particularly special – after all at this point it’s in effect just a low-compression OHV engine – but when the turbo kicks in things begin to happen with much increased urgency. This acceleration conveys an entirely different sensation than does a normally-aspirated engine when it’s stoked – the turbo just keeps ramming in the mixture in a seamless rush. This is not to say that the bike becomes unwieldy, in fact I was pleasantly surprised at how easy it was to keep the engine working at its happiest point, but if you need to go somewhere quickly, just open the throttle and hang on! The flexibility and the smooth transmission between the turbo and non-turbo power is something that Honda worked very hard on – and those who are familiar with the smaller CX500 Turbo say there’s no comparison between the two. 

The bike handles beautifully, thanks in no small part to the modern Bridgestone tyres, and the suspension at both ends seems to me to be a nearperfect relationship between the firmness needed for precision handling and for comfort. Certainly, the front brakes are extremely effective and vice-free, and even the anti-dive front end, a concept universally abandoned within a few years except by BMW, does a great job under braking. 

The turbo concept was short-lived in motorcycle terms, but today is making a dramatic comeback in the car world, and you can’t help wondering if it isn’t time to revisit the method for two wheels. Combined with 21st century engine management, it would seem to be ideal for bikes.  

Protect your Honda. Call Shannons Insurance on 13 46 46 to get a quote today.