The Duo-Dyna engine in the Honda 1300 Coupe

Excerpt from Car and Driver magazine June 1970 written by Patrick Neville (permission pending)

Curiously, the very conservative approach in stressing materials is combined with the wildly imaginative in layout - especially in the engine, which is certain to be rememberd as a precedent-setter. Fundamentally, it's a motorcycle engine very shrewdly adapted for use in an automobile, and it bears surface similarities to not only earlier Honda bike engines, but to the NSU 1200. However, close examination proves that the Honda 1300 engine is unique in at least one of its features - and the rest are highly unusual.

One would have to describe the engine's cooling system as unique, as it more or less resembles a water-cooled engine except that air instead of water is pumped through the "water-jacket". And there is another difference in that the inside of the water-jacket is full of cooling fins. The whole thing is enough to make a foundryman weep, for the head is a one-piece sand casting that includes not only the expected finning but the cooling duct shroud as well. Air flow out of the blower (located on the end of the crankshaft) is divided, with part going into the base of the block and exhausting at the top, and the rest going to the cylinder head.

All of the major engine castings are of aluminium - including the sump, (which also serves as mainbearing caps around the all-roller bearing crankshaft). And here we find one of Honda's rare inconsistencies, for their 4-cylinder motorcycle engine has a plain-bearing crankshaft. Oddly too, all of the rest of Honda's roller-crank engines carry their oil in the sump, while the 1300 automobile engine has a dry sump.

The cooling blower, and drives for the overhead camshaft and oil pumps, are attached (in that order) at one end of the crankshaft, with the power being taken from the opposite end. The engine is mounted transversely in the chassis, like a BLMC Mini, and as in the Mini there is an overhung diaphragm-spring clutch and a sleeve gear that transfers the drive back to an all-synchro, 4-speed gearbox. From there, short shafts and constant-velocity joints lead out to the front wheels.

The single overhead camshaft is driven through a single-stage chain, and this is made to do a number of jobs besides turning the camshaft. One end of the camshaft has a pair of skew gears to drive the distributor and tachometer, and at the other end there is a cast-aluminium pulley with a vee-belt leading off to the alternator. At the "front" of the cylinder head is cast-iron exhaust manifold that gathers together the end and center pairs of cylinders, and then these dump into a single outlet pipe. The arrangement is all very racing-car, except that the cast-iron manifolding must weigh at least 20 pounds. This free-flow exhaust is matched, on the 115 h.p. series-99 engine, with four carburetors - one for each cylinder - on the intake side. The 77-series Honda 1300 uses the same Keihin variable-venturi caburetor, but in the 77 one feeds all four cylinders.