TZ750 REVISITED: The Engine

Yamaha's TZ750A engine arrived in 1974 making a claimed 90 horsepower, and was developed in fair-sized steps to make the 120 hp that today's dynos report for the D-model and later.

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TZ750 REVISITED: The Chassis

Technical Editor Kevin Cameron exams what Yamaha's TZ750 race bike accomplished in its day that lead to the many developments of today.

This was a transverse inline four with a difference. In one sense, we can trace that architecture back to 1923, when recent engineering grads Carlo Gianini and Piero Remor sought from it a shorter wheelbase than existing fours (such as the FN, with its engine installed the long way), and equal presentation of all cylinders to cooling air.

The difference is in how the crankshaft is made and power delivered. Yamaha already had facilities for making parallel-twin crankshafts, making it convenient for them to give their 750 two such cranks, taking the drive from their inner ends.

Two-stroke cranks are pressed-together so that their roller rods and big-end roller-and-cage assemblies can be one-piece rather than split. It being a shaky proposition to try to transmit torque the full length of a crank held together by press-fits, there is a long history of driving from the center. Gilera and MV did it in the 1950s, Honda did it with their fours, fives, and sixes, and Mercedes-Benz did it with the long-and-floppy cranks of their W196 straight-eight Grand Prix car (albeit not with pressed joints, but rather with Hirth radial-spline couplers).

Each of the TZ750’s two cranks has a thin drive gear on its inner end, both of them engaging a single double-width gear on the inner end of a jackshaft. That shaft, driving accessories on its way to the right of the engine (water pump, gearbox oil pump, and ignition shaft), also carries a pinion which drives the substantial dry clutch.

Engine assembly is like that of nearly all Japanese engines of the 1960s and ’70s–via a horizontally-split crankcase into which cranks and both gearbox shafts are neatly set. Did some unsung Honda production engineer of the late ‘50s see this as ideal for rapid, everything-visible assembly? I recall my own delight at first seeing such an engine, after the complexities of English and German powerplants, with their tapers, heat shrink fits, and other idiosyncrasies.

But, like everything we humans dream up, this system brought compromise–in this case, making engines quite long. Yamaha would later tackle this excess length exactly as Giorgio Parodi and Carlo Guzzi had done beginning in 1920–by stacking the two gearbox shafts one above the other. Since then, as each maker of high-power sportbikes discovered the same need for a longer swingarm, stacked gearbox shafts have been adopted so universally that the resulting high-mounted clutch has become the new orthodoxy.

Yamaha surely knew of Honda’s experience with inline fours, beginning in 1959. A flat-crank four (180-degree crankpin spacing) consists of two 180-degree twins, each of them rocking opposite to the other, generating a bending force that tries to break the engine in half at its center. When there proved to be no way to stop cylinder base gasket leakage on that first RC-160 Honda four of 1959, Honda learned how to cast the upper crankcase and cylinder block in one piece – greatly stiffening the engine and eliminating the troublesome gasket.

That was impossible with the 500 and 750 two-strokes Yamaha were building. Two-stroke cylinders have to be replaced frequently, and their complex porting and bore hard-plating (it was chrome on ’70s TZs), made integral construction with the upper case impossible.

And so it is that, after two or three seasons, leakage appears under your TZ750 engine. You replace the gasket under the gearbox sump plate but the leakage continues, leaving the unpalatable conclusion that your lower crankcase is cracked.

Now the crankshaft. In about 1971 Yamaha began to produce modular twins with a 54mm stroke, and this production system gave us the R5, RD 250 and 350, TZ 250 and 350, and TZ750–all with the same stroke and six-piece pressed-together construction. There were four flywheel discs, joined in pairs on separate crankpins, and with the pairs joined by pressing them together over the paired inner main bearings and center seal. I have rebuilt boxes of such cranks and can attest that they are easy to work with. Occasionally you get an abused crank wheel that prevents trueing to the desired accuracy, but generally all is well.

But as engines strained to stay on the ever-faster pace in racing, rpm rose, and with it, stress on all those pressed joints. Early crankpins had large central lightening holes, so two easy steps of tightening the pressed joints were to make the hole smaller, and when that was no longer enough, to use the solid 1A1 crankpin from the RD400. The resulting tighter press-fits increased flywheel stress–especially in the one inner wheel which had two press-fits–dsa mainshaft and a crankpin. Those wheels began to crack, and if not discovered in time could open up inside your engine like a pair of steel brake shoes. So we began to Magnaflux-inspect even brand-new flywheels for such cracks.

This was not a mistake in design. It is the normal process that occurs in every engine whose power is progressively boosted. Originally, durability is good, but as power rises, durability falls. Finally, the design is said to be obsolete and its outstanding faults are remedied in the next design.

Accordingly, the TZ500 engine of 1980 was given an all-new crank construction. Three of the original five press-fits were eliminated by forging the two inner flywheels in one piece with their connecting mainshaft, and making the crankpins in unit with the outer flywheels. Fracture-split main bearings and a split seal were assembled over the “dumbbell”. This was naturally a much more expensive construction.

Did I mention a gearbox oil pump? Why yes. Rather than just accept the drag of a gearbox up to its hips in oil, Yamaha engineer Noriyuki Hata in 1959 eliminated most oil drag by directing a small jet of pumped gear oil onto each meshing gear pair, with the gearbox oil level far below. Although four-stroke production bikes share the same oil between engine and gearbox, today’s low viscosity engine oils make poor gear lubricants. To make such sharing possible, gears are made larger in diameter and given wider teeth to reduce the pressure to something lo-vis oil can handle. Now I hear that one or more new designs in MotoGP use separate gear oil, whose greater load capacity makes it possible for smaller, lighter gears to survive.

Pulling a TZ750 cylinder reveals more compromise. Because port area increases slowly with stroke, Yamaha might have gained 7% in exhaust area by switching from 66.4 x 54mm bore and stroke to “square” 62 x 62 dimensions. But the tooling to make 54mm stroke flywheels was in place. Honda reportedly evaluated the 6 percent port area gain from switching from their normal 54 x 54.5mm to the old Bultaco numbers of 51.5 x 60; the engine (if it existed) was not raced. Titles-winning two-stroke engineer Jan Thiel has said engines respond very well to increased exhaust area, but Yamaha were content in TZ750 with a single large oval exhaust port–no T-port with troublesome center divider, no sub-exhausts. Despite such lack of sophistication, the big TZ got by–nine consecutive wins in the Daytona 200.

And the transfer ports! Scandalous! With a large ‘A’ main pair and afterthought-sized ‘B’ secondary transfers, the cylinder looked late-1960s. To say nothing of the fact that putting all those cylinders in-line left no room for the sweeping, large-radius transfer loops that work best.

So Yamaha shifted gears in 1981, building compact V4 GP engines that were only two cylinders wide, thereby allowing use of high-flowing larger-radius transfer ducts.

The TZ750 engine used the same dinky steel reed intake valves throughout its ten-year career, but in 1982 Honda showed that much larger reeds (brought in from the MX department!) could give greater power. Large-area reeds quickly became the norm for 500GP bikes, primarily because they made engines compact, with intakes located in the V between the two banks of two cylinders. Yet in the fullness of time, Aprilia’s disc-valve 125 GP singles developed to make over 53 hp, leaving behind the best of the reed-valve 125s at just 47 hp. Either number would make a helluva 500, but reeds solved the 500’s packaging problem.

Like the other TZs, the 750 was water-cooled, but its tiny radiator let it push water when starts were held a minute too long (at the time, we thought AMA officials loved four-strokes), and pushed water temp to a power-wasting 90C or above while racing. Best power with modern two-strokes has come at 55-60C because any hotter and the intake charge is heated excessively, losing density. Some engines even cooled the transfer loops!

Were a 750 two-stroke race engine to be designed using what is known today, it would be relatively easy to make 250-hp at a moderate 10,500 rpm. Sparkling performance for the sport-minded rider.