TECHNICAL ANALYSIS: Ducati’s 821 Testastretta Engine We’ve reached a crucial turning point for Ducati’s “old reliable.”

Ducati Monster 821 engine close-up Quietly, the 821 is bringing radical change to the family of 90-degree V-twins derived from the original 851 8-Valve Desmo, which, in turn, was based on the Pantah 500 that Dr. Fabio Taglioni designed back in the mid-1970s. The Pantah was the result of a crash program to give Ducati a middleweight sportbike after the bad flop of the 350-500 GTV SOHC parallel twin. To develop the new middleweight as quickly as possible, Taglioni extensively drew from the experience collected in years of honing the 250cc Desmo and the mighty 750/900 90-degree “bevel gear” V-twin in all its variations, including the legendary Imola 1973 “short stroke” racer. Thus, in the end, the new Pantah 500 shared the 74mm x 57.4mm bore and stroke of the 250cc engine and a thermodynamic section inspired by that of the Imola 1973 racer, featuring a new-to-Ducati 60-degree included valve angle in place of the traditional 80 degrees, which helped the Pantah 500 produce well in excess of 50 hp and making it the fastest 500cc 4-stroke bike in production at the time. From the 350-500 GTV, Taglioni drew on basic experience that would change his very conservative approach to crank assembly design—the plain bearings and related forged one-piece crankshaft and cap-type rods were the optimal solution to solve the dramatic problem of multiple failures of the traditional pressed together crankshafts turning on ball and needle bearings. Still, he did not want to use too much oil flow and pressure, so he decided for a hybrid solution for the Pantah: a forged one-piece crankshaft, and cap-type rods with plain bearings but ball bearings at the crankshaft main ends.
Ducati Monster 821 engine Though it may sound awkwardly complex, there was a good reason: The main bearings had been the only failure-free components of the 750/900cc “bevel gear” twin. They were not regular ball bearings, they were aerospace class, “angular contact” in 35mm (bore) by 80mm (diameter) by 21mm (width), and when adopted on the 750/900 they were made in the US by Marlin-Rockwell. They had superb radial loads and axial thrust capacity, and thanks to their lightweight synthetic material cages they could stand extremely high rpm. With the relatively short stroke of the Pantah 500 engine, even the relatively small 35mm diameter of the main journals would create an adequate overlap with the 44mm crank journals for a nearly flex-free crankshaft. Things slightly degraded at each increase of displacement and, more particularly, the stroke, since that meant a progressive reduction of the main journal-crank journal overlap. This would cause an increased tendency of the crankshaft to flex and thus dump extra axial thrust on those magic bearings and the crankcase. The bearings held on, but the crankcases did not. At each sign of failure (there were plenty of them with the Superbike racers) Ducati beefed up the crankcases and the 8-valve Desmo soldiered on to grow into a 1998cc twin with a 67.9mm stroke. Today, the 1198 Testastretta is a strong and perfectly reliable unit. But those angular contact ball bearings are too expensive. And in the meantime, 1199, with all plain bearings, proved that the latest Clevite 112 plain bearings in combination with today’s synthetic oils can handle anything—sky high loads or equally high rpm.
Ducati Monster 821 studio 3/4 view Which brings us back to the new Ducati 821, the first of the “old” family to replace the old angular contact ball bearings with plain bearings. This is a change more radical than it might seem, and it confirms what CEO Claudio Domenicali said a couple of years ago, when he made it clear that Ducati will proceed with 2 lines of engines—the then brand new 1199 , and a constantly evolved Testastretta in various displacements. At this stage, the liquid-cooled DOHC Desmo 8-valve Testastretta comes in 821cc, 848cc and 1198cc displacements. Since the 848 already has proven to be a very solid middleweight performer, why have the fractionally smaller 821cc? The answer is found in the bore/stroke measurements and related ratios. At 94mm by 61.2mm, the 848, which now powers only the Streetfighter model, is a stroked version of the 749R, possibly the hottest member ever of the old family. Though tamed down, this enlarged version delivers a claimed 132 hp at 10,000 rpm and 69 pound-feet of torque at 9,500 rpm. As can be seen, only 500 rpm separates peak torque from peak power; that means it remains a rather peaky engine, not appropriate to power either the new Monster or the Hypermotard/Hyperstrada. So Chief Engine Development Manager Gigi Mengoli developed a less oversquare unit by adopting the 88mm bore of the old 748 teamed to a 67.5mm stroke, which is only 0.4mm shorter than that of the 1198. In the Monster, the new 821 delivers a claimed 112 hp at 9,250 rpm and 65.9 pound-feet of torque at 7,750 rpm. The 1,500-rpm range separating the power peaks is a sign of good flexibility, in line with the spirit of the Monster and Hypermotard.

Ducati 821 Testastretta CAD engine illustration This computerized image shows the 821’s engine-frame assembly, which draws directly from the experience of the 1199 chassis in the use of the engine as a fully stressed member.

Ducati 821 Testastretta CAD engine illustration The new 821 retains the basic character and layout of the original members of the family, but looks neater, refined and more rational in the design of ancillaries such as the belt covers, which feature a net-protected air intake to keep the operating temps of the belt under control.

Ducati 821 Testastretta Engine The 1198 that powers the top version of the Monster has undergone the same structural modifications to work with the same frame-engine assembly.

Ducati 821 Testastretta Engine To enable the Testastretta engines to function as stressed members of the chassis (which also has a steel trellis frame), additional engine-to-frame mounting bosses had to be added to the horizontal head (one on the top side) and the vertical head (one at the front, one at the rear). The one located on the crankcase, at the center of the vee of the cylinders, is no longer structural. It was retained because the other Ducati model powered by the 821, the Hypermotard, still uses the original trellis frame that is bolted to the bosses located on the crankcase, plus the front one and a rear upper one.

Ducati 821 Testastretta engine vertical head

Ducati 821 Testastretta engine vertical head The vertical head of the Monster 821 with its 2 additional bosses. Only the front one is used to mate the engine to the trellis frame, with a 12mm bolt. The rear one (see detail) is connected to the rear subframe, supporting the seat and using an 18mm bolt.

» Check out page 2

View all photos in gallery:
Ducati Monster 821 engine close-up. Ducati Monster 821 engine. Ducati Monster 821 studio. Engine #1 Engine #2 Engine #3 Engine #4 Engine components #1 Engine components #2 Engine components #3 Engine components #4 Engine components #5 Engine components #6 Engine components #7 Engine components #8 Engine components #9 Engine components #10 Engine components #11

Ducati 821 Testastretta engine horizontal head The horizontal head carries only one boss that mates frame to engine, and it also uses a 12mm bolt.

Ducati 821 Testastretta engine crankcase The crankcase is modified in the bore of the main bearings and related support areas. The bore goes down from the old 81mm to 45mm. The smaller bore produces a stronger case structure thanks to the thicker annular section around the main bearing seat, plus additional webbing.

Ducati 821 Testastretta engine bearings The new plain bearings versus the old 35/80/21 angular contact ball bearings. The plain bearings bring significant advantages in load capacity, mechanical quietness and additional crankshaft rigidity. They also save money in production. The main-end plain bearings do not feature the usual groove and admission hole because Taglioni has always favored axial admission lubrication circuits, which require less pressure. With axial admission, the oil flow does not need to overcome the centrifugal force found in a radial admission layout.

Ducati 821 Testastretta engine crankshaft The new crankshaft (foreground) of the Ducati 821 comes from a new forging with more accurate counterweight profiles and the new, larger main-end journals. The rest of the crankshaft layout remains unchanged. The larger main journals generate a much more rigid crankshaft because now they overlap the crank journal by a substantial 8.75mm versus the 3.75mm of the traditional layout. This translates into far less pulsating axial load dumped by the flexing crankshaft to the crankcase. Despite its more generous dimensioning, the new crankshaft, complete with its main bearings, weighs 177.8 ounces versus 198.8 for the old one.

Ducati 821 Testastretta engine connecting rods Connecting rods have remained unchanged in their structure and vital measurements, with 40mm bearing diameter and 124mm center-to-center dimensions.

Ducati 821 Testastretta engine top view The small hole adjacent to the spark plug seat is the port for secondary air. A reed valve atop of it controls flow.

Ducati 821 Testastretta engine inlet valve The secondary air enters the combustion chamber through the port of the left inlet valve. Secondary air improved stability at medium and heavy low-rpm loads, where Ducati engines typically develop a staggering response to throttle action. The addition of secondary air translated into a significantly improved combustion in those specific conditions.

Ducati 821 Testastretta engine piston top The Ducati 821 features a 12.8:1 compression ratio, a good number given the fairly clean, though elaborate, piston top. The little pocket under the sparkplug reminds me of the 6.2-liter direct-injected LS1 V-8 that powers the Corvette C7. The pocket (deep and spoon-shaped) exploits the high-pressure injection jet of gasoline to enhance the turbulence inside the combustion chamber, for faster and more complete combustion.

Ducati 821 Testastretta engine ghost view This ghost view of the Ducati 821 confirms that the toothed belt originally adopted by Taglioni for the Pantah 500 has been retained, despite the good progress that has been made with chains and oil-bath toothed belts such as that used by Ford for its Ecoboost 1.0-liter 3-cylinder.

Ducati Monster 821 engine close-up. Ducati Monster 821 engine. Ducati Monster 821 studio. Engine #1 Engine #2 Engine #3 Engine #4 Engine components #1 Engine components #2 Engine components #3 Engine components #4 Engine components #5 Engine components #6 Engine components #7 Engine components #8 Engine components #9 Engine components #10 Engine components #11