About Suzuki’s New Parallel Twin

[Hugh Janus]

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Everybody’s doing it, and that now includes Suzuki, who released (after a long gestation) its 776cc parallel-twin engine of 84 x 70mm bore and stroke. As reviewed on this site by Ben Purvis, parallel twins have strong attractions in the present era: affordability, low parts count, lightness, and a compactness that gives flexibility in positioning the engine in different chassis.

Suzuki has a new parallel twin used in two models, seen here in the GSX-8S. (Suzuki/)

Mr. Purvis also points out that this engine’s ancestor was shown as long ago as 2013 as the 588cc turbocharged Recursion. The word “recursion” means “return,” referring perhaps to the 1950–1970 popularity of British parallel twins like the Triumphs, BSAs, and Nortons of that era.

After the economic disaster of 2008, makers cast about for novelties that might catch on in the resulting down market. Automobiles were then adopting smaller, more economical engines that were turbocharged to provide zippy performance. Could this work in bikes? Most evidence was negative. Turbo bikes are fast in the quarter-mile, but so far no one has been able to give a turbo bike the fine torque-to-tire matching needed to confidently stay with unsupercharged engines on real roads.

Thinking has now swung away from quirky and back to basics: How do we bring the well-loved qualities of traditional motorcycles to market at prices people are now able to pay?

Yamaha sampled this direction in the ‘90s with TDM850, but BMW started the current trend in 2008 with its F-series. Others have joined since.

Suzuki’s 776cc parallel twin. (Suzuki/)

Aren’t such twins a big comedown in performance from the high-horsepower four-cylinder sportbikes of the previous era? Not necessarily. A big handicap of those 16,000 rpm rockets was that in order to sell, they had to win Supersport races; in order to win, they had to be given spiky, hard-to-ride powerbands that required constant up- and downshifting. They were very far from being twist-and-go bikes.

All manufacturers have conducted what we might call “drivability studies,” because in 1990s focus group testing found that average riders overwhelmingly preferred high rideability bikes that delivered usable torque everywhere, rather than just above 10,000 or 12,000 rpm. High rideability makes us all into better riders by asking less of us.

Today’s new parallel twins benefit from this work, as they uniformly have wide, smooth, surprise-free powerbands that give riders confidence. Such engines accelerate much more strongly at real-world rpm than Supersport bikes ever did. Suzuki, as we shall see, has done things a bit differently in its version.

I have previously referred to a recent trend that I call the “Euro 5 powerband.” It is flat and easy to ride without having to bury the tach, because to meet Euro 5 emissions limits, large valve overlap and very long valve timings in general have had to be given up in favor of more valve lift. The fact that this also delivers high driveability is a happy accident.

Suzuki’s new 776 twin is conventional in that it has double overhead cams with narrow valve angle, giving a compact, fast-burning combustion chamber.

It moves away from the 25-year sportbike trend toward ever-shorter strokes and bigger bores because it is designed to accelerate most strongly at revs riders actually use rather than at revs capable of winning those close Daytona 600 races. Its ratio of bore to stroke is just 1.2. Compare this with MotoGP, which is limited by rule to 1.68. Or with the most extreme of Formula 1 engines, way out at 2.3! Because it doesn’t need valves big enough for operation at 16,000, the bore can be made smaller. This speeds combustion by shortening flame travel and it also improves economy by reducing the heat-loss surface area of combustion chambers and piston crowns.

Are you skeptical? Back when World Superbike was mainly a contest between big Ducati twins and everyone else’s fours, why was it that the fours always had much bigger radiators than the twins? We know it wasn’t that the fours were more powerful, because Ducati was then doing most of the winning. The reason was that two big cylinders lost heat through a smaller total area of combustion chamber and piston crown than did the fours.

A smaller bore also reduces the total piston circumference that must be sealed by piston rings: in this 776 it is 20.8 inches, while in the last of the GSX-R750 fours (having roughly the same displacement) it is 34.6 inches, or 66 percent greater.

Suzuki’s new parallel twin utilizes twin counterbalancers. (Suzuki/)

Why worry about this? Emissions agencies do the worrying, because during a cylinder’s compression stroke, unburned fuel-air mixture is forced into piston ring crevice space (cylinder pressure is what “inflates” the rings, making them seal). The steep rise of combustion pressure after the ignition spark keeps that mixture trapped there until much later in the engine cycle—too late for it to make any contribution to power. But out it comes during the following strokes, straight into the exhaust pipe as unburned hydrocarbons; meters in the air pollution labs swing toward the red. For equal displacement, the two bigger cylinders of a twin generate less of this than the more numerous but smaller cylinders of a four.

Emissions agencies aren’t the only ones watching. The insurance industry, without whose product we can’t ride, has learned to charge us extra if our chosen ride is a pavement-wrinkler. Therefore it makes sense to own a machine whose image attracts less of their attention.

Going on with the usual list of powertrain features, we find the expected six-speed gearbox with a dual-action slipper/assist clutch and an up-or-down quickshifter. The fuel system is digital port injection by dual 42mm throttle bodies plus two 10-hole injectors. The two crankpins are separated by 270 degrees because, frankly, people find the resulting syncopated exhaust sound more to their liking than the snore of even firing intervals from a classic 360-degree British parallel twin.

Back when British was king, anyone who complained about numb fingers, feet, and butt was told, “Vibration will make you tough!” Because numb body appendages no longer appeal, the 776 has two crank-speed contrarotating balancers with eccentric weights.

Long intake pipes could help give Suzuki’s 776cc parallel twin a midrange bump in torque. (Suzuki/)

When a single-cylinder engine is balanced to 50 percent of the mass of its reciprocating parts—piston, rings, wristpin, and small end of the con-rod—the resulting imbalance is a constant force rotating opposite to the crankshaft. That being so, provision of an equal and opposite force, also rotating opposite to the crank, will cancel primary shaking force. That is what these two balance shafts do. Are you tempted to complain that these extra rotating parts will make the engine slow-revving (like the old Cosworth-Norton definitely was)? Or will you conclude, as I did, that the presence of the balance shafts allowed corresponding reduction in the mass of the crankshaft itself?

I will not list the usual suite of electronics having to do with engine modes, anti-wheelspin, and so on. Every maker has them now, each with its own forgettable corporate acronym.

What Suzuki has done differently is coyly revealed in nondimensional power and torque curves (they give no actual numbers—just the shapes of the curves). In place of the expected flat torque of a Euro 5–compliant engine, we find instead a nice hump of extra torque right in the middle of the range. What could be creating this?

We know that torque is boosted by higher compression ratio; this engine’s CR of 12.8 is high but not extreme for its bore size. And we know that raising compression benefits torque at all rpm, not just in a narrow zone. So what is causing that hump in the 776′s torque?

Scrolling down through Suzuki’s release, we come to two images of its rather long 6-liter (not very big) intake airbox, positioned behind the engine. Much of its interior volume is taken up by much longer-than-usual intake pipes. Thanks to all these past years of thinly disguised race engines with the shortest possible intake pipes, we’re not used to this. But those long intake pipes may be generating that nice hump in the 776′s torque curve, right where we’d like to find it when accelerating up on-ramps or during passing. Drivability!

Source

[Buckster]

270 degree Pedro crank.