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reprinted from Automotive Engineering, September 1993

Honda's race-bred connecting rod bearing

Honda's new 1.8-liter VTEC engine for the updated, top-of-the-range Integra model attains an extremely high piston speed, 23.3 m/s at 8000 rpm, which matches that of the company's Formula One racing engine. This far exceeds a normally accepted ceiling of 21 m/s for a high-performance road car engine. "If I were to cite a single item that made this high piston speed possible, it is metallurgy," confides a senior engine designer, ["]specifically the connecting rod bearing and the connecting rod material."

To enhance the new Integra's sporty image in a fiercely competetive class, the development team concluded that it would need a high-output, high-revving engine of 1.8-L capacity, aiming at 75 kW/L (132 kW, JIS net) and the capability to rev to as high as 8000 rpm with ample reserves of durability and reliability.

Honda has two L4 engine families that could produce a 1.8-liter displacement. One is the "Accord" block, with 94-mm bore pitch and, currently, 85-mm bore, obtaining 2.0-L with a 88-mm stroke and 2.2 with a 95-mm stroke. This engine family traces its origin from a 1.8-L capacity with a 81.5-mm bore, which should be capable of attaining the targeted high power and rpm. The snag was that would have been 16 mm too wide and 10 kg too heavy for the projected Integra.

The other engine family considered was the "Civic" performance engine group identified by the prefix "B," with 90-mm bore pitch. The B16A, DOHC, variable-valve-timing/-lift VTEC, 1595-cc unit was the top engine of the previous Integra series. This famility also had a 1.8-L version, the standard unit for the U.S. Integra, and in DOHC, 16-valve, non-VTEC form it powers the Japanese-market-only Domani sedan. The B18A, 1834-cc unit has a long stroke of 89 mm and 81-mm bore, which would have increased piston speed to a stratospheric 24-m/sec, which was excessive and simply unattainable.

So a new aluminum block was designed for the new engine, designated B18C, for the third-generation Integra, with 85-mm bore and 87.2-mm stroke, obtaining 1787 cc. The block has siamesed, cast-iron, dry liners cast in. Nos. 2, 3, and 4 lower bearing caps/carriers are tied by an aluminum bridge to reinforce this critical area.

The VTEC cylinder head is newly cast for the B18C; however, its valvetrain is shared with the B18A unit, including 33-mm-diameter intake and 28-mm-diameter exhaust valves, either having a 5.5-mm lift. The VTEC is combined with a variable-length induction system that effectively produces three torque curves which the engine rides on in its rpm range. For the promise of combining ample low- and mid-range torque and high-speed power, the B16A VTEC was sometimes criticized for its shortage of low-end pulling power. The bigger B18C has rectified this shortcoming with a flatter torque curve–a combination of three curves–which peaks at 6200 rpm, 1000 rpm lower than the smaller unit's, and at 3000 rpm the engine produces 5% higher torque.

Two and a half years ago, when the engine team began developing the new 1.8-L VTEC engine, it had to limit its highest sustainable speed to 7500 rpm and maximum power output to 125 kW. "Our initial reaction was, 'The senior Prelude's 2.0-L VTEC's limit is 7400. So it should be high enough,'" says the senior engineer. Obstacles were known to us. "Stroking of the 1.6-L engine by 19% to obtain 1.8-L would have been accompanied by a 20% increase in load of such vital components as the crankshaft. Our data on the 1.6-L's crankshaft indicated that it would not stand up to that kind of load. Nor would the connecting-rod bearing metal." Widening the bearing metal would have made it withstand the load, but that would have further reduced the crankshaft's strength, which had to accommodate the wider bearings within a set length. Attainable and allowable piston speed is really determined by the fine balance between the crankshaft and connecting-rod bearing performances.

At about the same time, at another corner of Honda's Wako R&D Center, a group of advanced engine designers and engineers were striving to get another engine to combine high-rpm power and reliability under very demanding operating conditions–Formula One racing. Lack of reliability had for some time been plaguing the naturally aspirated, 3.5-L, V12 engine.

To remedy the problem, a team of Honda metallurgists/engineers created a highly seizure-resistant overlay on the bearing's sliding surface using a unique electrodepositing of tetra-methyl lead, Pb (h00). The highly oriented Pb surface has a composition of myriad minuscule pyramids, which possesses outstanding "wettability" or lubricant-retaining properties. Honda claims it has given a 30% or higher increase in the anti-seizure parameter, PV, than a surface with conventional deposits.

In August 1991, Soichiro Honda passed away. The Wako engineers' way of expressing homage to the late founder was to win the next Formula One race–the Hungarian Grand Prix of that year. The new highly oriented crystal bearings were used in the Honda V12 which propelled a McLaren racer to its long overdue victory.

The beauty of this bearing was that it was cost-competetive. It was subsequently adopted in the Legend's new longitudinal V6 engine. In the B18C, it enabled the engine designers to reduce the connecting-rod bearing width from the B16A's 19.5 to 17.5 mm. Two millimeters shaved off each connecting rod journal is added to the crankshaft webs flanking it, giving the crankshaft the extra strength it needed.

Furthermore, with the new bearing material, Honda was able to revert to a low-vicosity, low-friction lubricant (the smaller B16A VTEC is specified with a higher-viscosity one), that contributes to improved fuel economy.

Another vital item that received Honda metallurgists' attention was the connecting rod. The forged steel alloy rod's chrome content was increased and its carbon content reduced, increasing the alloy's strength by as much as 26%. The longer-stroke rod's weight is the same as the B16A's shorter rod.

Jack Yamaguchi