Honda CB 750F Turbo

Turbo CB750F Super Sport
If you think a turbocharged motorcycle should be capable of more than smoky burn-outs and be as docile at 40 mph as it is powerful at 140, you're ready for the turbobike of the future.

Turbocharging is what the world is coming to. More horsepower-efficiency is the goal, and turbos offer a more elegant and direct means to that end than multiplying valves, changing combustion-chamber shapes and grinding radical cam profiles. What's more, the gain in performance is almost free, since a turbocharged engine uses its own exhaust gases to drive a compressor that force-feeds the motor its fuel-air mixture, producing more punch per power pulse.

That's why you should think of turbocharging as artificial respiration for a generation of engines suffocating in mechanical complexity.
With that vision of the future in mind, the staff of cycle guide bolted together its own version of today's ideal street turbo, using a combination of R.C. Engineering's new real-world turbo technology and Honda's good-handling CB750F. The result is not revolutionary, but it represents a major step along the path to a turbocharged tomorrow.

Because this is not a futuristic race bike, even though the Porsche-red paint pumps the adrenaline and the turbo script on the sidecovers stirs emotion within everyone from punks on RD400s to bankers in Mercedes-Benz Turbo Diesels. No, this bike is more than just another rabid refugee from the drag-strip; it's a manageable street bike, and that's a first for turbocharged motorcycles.
With a couple twists of the throttle to let the accelerator pump prime the engine, the Turbo Super Sport fires up readily, and after a few minutes it settles into the fluctuating idle peculiar to turbo bikes.

The unruly growl from the exhaust pipe and the heat cascading from the cylinder head might tempt you to think such a bike should be lit off only on special occasions; but you can snick this motorcycle into gear and embark upon an expedition into city traffic without fearing that it will load up and die at the first stoplight and melt into an expensive heap of slag by the third. The huge carburetor requires that you do not grab a handful of throttle frivolously, but on the whole you can treat this bike like a motorcycle instead of a beast and live to tell about it.

And yet the turbocharger boosts the F-model into a new dimension of hyper-speed. Crank the throttle on and the turbo comes up on boost at 3500 rpm, feeding the 16-valve 750 with enough potential energy to enable the Honda to keep pace with lit-erbikes, even with just six psi of boost. Power arrives with a predictable sustained rush that makes the blood surge to the back of your cranium and yet proves perfectly controllable. As powerful as the Turbo Super Sport is, you never feel disoriented, as if the turbo had triggered a reactor melt-down beneath you. As a result, you can put your turbo-power to use on winding roads as well as on Burnout Boulevard.

The F-model still glides through left-right-left transitions with an effortlessness that makes you feel invulnerable, and it's sensitive and willing at every turn—but now at a higher speed. The simple 750cc street bike beneath you suddenly has acquired new muscle.
It's the combination of low-speed docility and high-speed potency that makes turbocharging the topic of discussion among motorcycle engineers. Because a turbo ideally should provide an engine with flexibility unattainable by conventional means, as if its power curve had suddenly been boosted upward along its entire length. At cycle guide, we're fascinated by turbocharging for the same reason. Yet we also know that previous turbocharged motorcycles have compromised flexibility. Warm-up times were long, frequent plug changes were necessary, boost didn't arrive until the tach had swept two-thirds of its way across the dial—and once it did, the front wheel inexorably rose skyward. We weren't satisfied with turbo bikes calibrated for the racetrack. We wanted to create a turbo street bike.

We turned to R.C. Engineering because it is a company that shares our interest in street turbos. Though its initial plunge into turbocharging was inspired by the high-performance market, R.C. emphasizes repeatedly that from the moment its turbo development began two years ago, "streetability" was high on the priority list. And a review of the $1495 worth of high-boost hardware featured on the Turbo Super Sport confirms it.
To begin with, Russ Collins and his development crew, headed by Art Leach, selected the smaller FF25 Rajay turbo for R.C.'s kits instead of the FF40 model commonly used, expecting that the smaller unit would produce boost at low rpm where the street rider could take advantage of it. Also, R.C. opted for the heavy-duty model, with a sand-cast housing and twin oil-feed lines to the turbo's vulnerable main bearing.

To head off the possibility of too much boost, always a threat with small-size blowers, R.C. fitted a sophisticated adjustable wastegate into the system. The waste-gate senses intake manifold pressure and then, through a valve held in place by a spring, controls the amount of exhaust gases reaching the turbine.
Carefully controlled fuel flow is as important to a turbo engine as determining the amount of boost and the rpm at which it should arrive, so R.C. features a few tricks in its intake system.

The intake manifold has an internal deflector to distribute fuel evenly to all four cylinders from the single carburetor, while an aluminum gasket between the manifold and the cylinder head speeds engine warm-ups. At the other end of the cylinder head, the header pipes of the four-into-one exhaust system are fairly small in diameter, increasing the velocity of the exhaust gases and further adding to the turbocharger's ability to build boost at low rpm.

The linchpin of this turbo kit, though, is its new Power Master carburetor, designed by R.C. in conjunction with a development team from Keihin carburetors headed by T. Miyasaki. The problem with turbo car-buretion systems so far has been that a single carb had to serve the needs of both low-rpm performance and boosted high-rpm running. Most have failed miserably at less than 5000 rpm, offering as much coughing and burbling as smooth running. The Power Master carb attempts to solve the problem with a strong accelerator pump to prime the engine with fuel at low intake velocities, and with a high-speed enrichening circuit (commonly called a power jet) to supplement the main jet when the engine is hungry for fuel at high rpm. Moreover, the carb is insulated from both heat and vibration, so the fuel won't vaporize and froth in the large float bowl.

There were some disadvantages and advantages in mating this entire turbo system to the 750F engine. First, the Honda's connecting rods are weaker than those found in most other comparable engines, so the turbo's boost pressures had to remain low. On the other hand, the high oil pressure of a motor with a plain-bearing crank serves the needs of the turbocharger perfectly, since turbos like a generous oil supply. Also, there was no problem in fitting the bike with an oil cooler, for drilling out the cast-in fittings in the sump pan and adding a CB900C oil pump permitted the adaptation of the CB900C oil cooling system. An overflow bottle was necessary to keep the crankcase breather from blowing oil onto the rear wheel, but on the whole, the Honda engine was perfectly equipped in terms of lubrication for turbocharging.

The only serious problem lies in the fact that the header and turbine feeder pipes must be removed to service the oil filter, and since R.C. recommends renewing the oil at 1000-mile intervals, you'll face this problem frequently.
As you can tell, installing a turbo on your bike is something you shouldn't undertake casually. We didn't. Even though we intended this bike to represent a reasonably economical turbo system, we ordered the installation of stiffer valve and TURBO 750F Continued
clutch springs. Certainly the likelihood of extreme overrewing as a consequence of missed shifts impels you to keep the engine's valves under control at all costs. Furthermore, stiffer clutch springs provide the added plate-to-plate friction needed to get all of that horsepower onto the ground without burning up the clutch.

We also keep boost to six psi to insure durability.
R.C.'s kit is like previous turbo adaptations in that the exhaust pipe fouls your left leg and the K&N air filter on the right
contacts your knee; but from the moment you fire up the engine, the resemblance to other turbo kits ends. First, the exhaust pipe doesn't burn you, even if you're wearing low-cut shoes. Second, boost builds at 3500 rpm, just as advertised, and because of our six-psi decision, the surge arrives predictably. Thanks to the Honda's good breathing and free-revving character, boost and horsepower build fairly quickly in the lower gears. You won't want to risk a powershift with a quick fan of the clutch
though, since there's enough boost at hand to bury the tach needle deep into the expensive end of the red zone should you miss a shift—not unlikely thanks to the Honda's long-throw shifter.

In terms of high performance, the Turbo Super Sport registered an E.T. of 11.99 seconds at 113.8 mph, not impressive for a full-race turbo bike but significant indeed for a 750cc bike you can ride on the street. Frankly, the dragstrip won't always tell you whether a bike delivers usable power. And there's no question that the R.C. bike attains a measure of streetability far beyond any turbo street bike we've ever tested in these pages before.
Not everything is perfect with the Turbo F-model, though.

The problem lies in throttle response, the traditional bugaboo of turbocharged bikes. While the engine performs perfectly at small throttle openings and large ones, the transition between the two must be made very carefully. For all its sophisticated high- and low-speed jetting, this particular 38mm Power Master is simply too large for the 750cc engine. Whack the throttle open in any roll-on situation and the engine bogs as intake velocity slows to a crawl. Just as with any bike with a large carb, you must use the throttle discreetly, dialing it on gradually rather than dumping it open suddenly. You can still build speed quickly, but it's better to downshift than to consider whacking open the gas with impunity. As a result, you must resort to the gearbox in any emergency and think twice before pulling out into heavy traffic.

While it's possible to adapt to the R.C. carb's idiosyncrasies, we were still disappointed that the Turbo 750F wasn't quite the shattering breakthrough we'd hoped. Fortunately, R.C. is as aware of the problem as we are and is working with Keihin to produce a smaller carb in a month or two. With luck, that Keihin should make the 750F as tractable as R.C.'s Z-l turbo equipped with the huge carb.
Even though the Turbo Super Sport isn't everything we'd hoped for, it does have at least one wheel in the future. For this is a turbo bike that you can live with, one that can make you happy in the city or in the country, one that has an added dimension of performance beyond that of any other 750. Most importantly, this turbo won't destroy itself. That's why we're still enthusiastic about the Turbo Super Sport's potential. And that's why we're going to keep it around so we can experiment with it further.
You see, this bike has given us a glimpse of the future. And the future, friends, is turbocharged. •

COMPARATIVE TEST DATA:
Make Quarter-Mile, sec mph Top Speed, mph Weight, lb* Stopping Distance From 60mph, ft.
Turbo Honda CB750F 11.99/113.8 NA 523 135
Honda CB750F 12.41/106.8 128 524 134
Kawasaki KZ750 12.50/107.8 127 471 ' 134
Suzuki GS1000ET 11.98/113.9 136 528 137
Suzuki GS1100ET 11.43/118.3 140 544 116
Yamaha XS1100SG 11.94/112.6 135 572 129

Sour4ce Cycle Guide 1980