Turbo Kings: Honda CX500, CX650, Yamaha XJ650, Suzuki XN85, Kawasaki GPz750

Date 09.8.2011

Presented by
  • Motorcycle Trader

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Turbo Kings:
Honda CX500, CX650, Yamaha XJ650, Suzuki XN85, Kawasaki GPz750

One of my favourite fantasies is that the lads in the R&D department of your favourite factory are sitting around on Friday night, enjoying a few sociable schnapps/ sakis/ vinos/ G&Ts/ bourbons/ whatever, when one says to their neighbour at the increasingly tipsy gathering, “Wouldn’t it be great if we…”. The rest of the crew chimes in with wildly enthusiastic additions to the idea, and, before you know it, someone’s pressed the ‘go’ button on the production line and the latest transport of delight has rolled into the showrooms. That’s when the hangover kicks in. A number of motorcycles in my shed can only be explained by this phenomenon, and they know who they are.

It might also help to understand the spectacularly rapid rise and sinking without trace of the turbo kings from all four of the big Japanese firms. They were, in order of appearance, Honda’s CX500 (1981, followed in 1983 by the 650), Yamaha’s XJ650 (1982), Suzuki’s XN85 (1982) and Kawasaki’s GPz750 (1983). In reality, they lasted barely two years and were never revisited as a catalogue item.

Turbocharging is one of those terribly attractive ideas, from an engineering standpoint. You use the energy in waste gas from the powerplant to spin a turbine that pressurises the intake, which in turn assists the engine in making a lot more power. Something for almost nothing. The reality is, of course, more complex.

You’d think it’s an idea just made for motorcycles. But due to a mix of factors – which we’ll delve into a little later – the idea comprehensively bombed.

 

SO WHO DO WE BLAME?

Hard to know, really. Though I think Honda gets the nod as it was the first to show a working version, in late 1980, based on, of all things, the CX500 twin. As Ian Falloon explained in his Hindsight piece in MT issue #224, the liquid-cooled V-twin pushrod CX engine was designed from day one with a turbo in mind.

Sure, lots of people had adapted turbos to road bikes, but the results tended to be slow on the bottom end, ludicrously fast up top, and borderline unrideable in the middle. At this point in history, turbo kits for small engines were in their infancy, so any backyard effort was struggling to produce good results.

Along came Honda with a cunning plan, which was to begin with a likeable but not terribly muscular twin and turn it into an autobahn stormer of high order. Its three competitors soon joined the bandwagon.

The actual turbo units chosen came from three makers. Honda and Suzuki opted for IHI, Yamaha (which built its first turbo powerplant – a 5lt V8 – in 1970) a Mitsubishi unit and Kawasaki employed Hitachi. It was the CX500 that won the prize for apparent complexity, thanks mainly to a highly sophisticated sensor and engine management design. The location of the turbo itself varied wildly between makes.

Honda had its unit in front of the vee, the Yamaha’s behind and below the gearbox, Suzuki mounted its party piece behind the head above the transmission, while the Kawasaki’s was in front of the head.

And here’s another twist in the tail – Honda, Suzuki and Kawasaki went for fuel injection, but Yamaha stuck with a bank of pressurised CV carbs.

Power outputs also varied. Honda claimed just 60.3kW (82hp) from its remarkably high-boost (19psi – about double the norm) turbo, Yamaha 61.8kW (84hp), Suzuki 65.5kW (89), while the Kawasaki claimed a mighty 80.2kW (109hp).

None were in any danger of blowing away, with weights generally in the 230 to 240kg (dry) range – or about the same as a litre-plus machine.

Reading the commentary of the day, you can see ongoing debates about the virtues or otherwise of turbos, along with arguments over what tuning methods were best. For example, was low piston compression matched to high turbo pressure superior to the opposite? Should one be concerned about having a long or short run between the exhaust manifold and the turbo, and what were its effects? It did little to reassure the punters.

 

ALL ABOARD

So what were they like in the saddle? Truthfully, any modern mid-sized sportsbike would have them for lunch in a straight line and particularly around a corner. In their day, they got very mixed reviews.

The writers really wanted to like these machines but there was just a bit too much Weird Harold going on for too little gain.

Honda’s CX turned into a totally different handling package in turbo form, which is just as well. It was also bigger, heavier, and was notorious for a longer than ideal lag between when you cranked the throttle at lower engine speeds and when the numerous extra horses joined in. Turbo lag had always been considered undesirable in a car, but it’s even less attractive on a motorcycle – particularly if the party started halfway through a turn.

Get it out onto the open road and it was a joy. Long legged, very comfortable and supremely stable, it had great potential as a fast mile-eater.

Yamaha’s XJ quickly developed a reputation for being more nimble than the Honda. Less ‘out there’ in concept and configuration, it appealed to those with a loyalty to Japanese multis who were looking for something with some extra bragging rights. The catch was it, like its colleagues, went faster in the pub than it did on the road. A better all-rounder than the CX, it was barely a performance match for a true big-bore multi.

Suzuki’s XN85 started with an oddball name that came with, for the day, a cutting-edge chassis. The package included a 16in front wheel (just like the GP bikes of the era), the first use of the firm’s successful Full Floater rising-rate rear-end in a big road bike, plus a number of other goodies such as built-in anti-dive up front. It actually handled pretty well and was closest in feel to the XJ – very much a mid-sized multi (albeit a heavy one) with big bike urge once you got the tacho needle buried into the fun zone.

Kawasaki’s GPz ended up being the most conventional-looking and best performing of the breed. Based on the biggest engine, it had the pick of the low and mid-range performance, along with a serious top end. It was no quicker than the GPz1100 of the day but was quite different in feel. The engine package was silkier and much sexier at high speed, while it felt a little more stable – something attributed to the slightly different fairing design.

As an all-round ride, the Kawasaki was easily the king of the turbo heap.

 

WHY DID THEY FAIL?

The sad fact is buyers stayed away in droves. Sure we all loved the idea of turbos, in theory. But when it came down to handing over the readies, well, most of us shuffled our feet and mumbled something about buying a nice, ordinary litre bike, but thanks anyway.

There is a mix of reasons.

The motorcycle market is widely held to be conservative to the point of suspicious when it comes to avant-garde technology, and it just won’t buy it in big numbers. For example, we all went “ooh, aah” at Honda’s sexy CBX1000 six, then bought the far more prosaic 900 fours by the truckload (and some of us are still regretting that decision).

The mid-sized bikes that used turbos didn’t really perform spectacularly better (sometimes worse) than the more conventional 1100s of the day.

Long-term reliability and maintenance was viewed as being a complete unknown potentially filled with wallet-eating dragons.

Timing was also an issue. Turbochargers in any application were still regarded as pretty exotic in those days, though a decade later they were commonplace in the automotive world and treated with far less suspicion.

In many ways the rapid rise and fall of the factory turbos was a great shame. For a while there, saying you had one gave you serious bragging rights. And they added real interest to the motorcycling diet.

Even now, I reckon a tidy factory turbo would look pretty good in the shed…