Some Details of the Miller Racing Engine
We recently availed ourselves of an opportunity to examine a Miller racing engine, which is of considerable interest because, although it has not been possible to establish exactly in which car it was used, it is of the type used at Indianapolis in 1925-6, and very like the engine which Lockhart used for his historic 164 m.p.h. 1 1/2-litre flying-mile record in 1927. This engine is additionally interesting because we believe that M. Ettore Bugatti purchased one and carefully examined it before introducing his Type 51, twin o.h.c. straight-eight, in 1931. The engine certainly bears close resemblance to the Bugatti and it may easily prove to be the actual unit used in the Lockhart car.
It is a straight-eight, plain-bearing engine of 55.5 x 76.2 mm. bore and stroke (1,467 c.c.) and a 2-litre crankshaft is also available. The crankcase of this Miller is virtually a light-alloy oblong box, into which the one-piece crankshaft is inserted from the rear. The crankshaft runs on five quite narrow bearings and has 1-in, thick webs with balance weights. The three centre bearings are carried in large-diameter split housings, each bolted to eight studs depending from the crankcase webs. The big-ends are plain, the journals measuring 1 3/4 x 1 5/8 in. The connecting rods are exceedingly light, circular type, with two-bolt ribbed big-ends of which the white metal is run direct on to the rod. The light-alloy pistons have flat crowns with a cut-away each side to give valve clearance, and they have considerable internal webbing, but no support for the gudgeon-pin bosses. Each has two compression rings, beneath which is a ringless deep oil-control groove, with lesser grooves down the length of the skirt. Circlips retain the gudgeon-pin.
Two Bugatti-like cylinder blocks with integral heads are secured to the crankcase, Lancia-fashion, by eight studs each, the studs passing into holes in the crankcase and their nuts being reached by removing large plates along each side of the crankcase. There is no sump, a long plate closing the bottom of the crankcase. The blocks are very slim, being 3 3/8 in. in width, and water-cover plates close the sides, while a breather tube runs up between the centre cylinders. Unlike Bugatti practice, the cylinder bores are flush with the blocks and do not spigot into the crankcase. Two semi-tulip valves with 30° seats are used per cylinder, at 90° to one another, and closed by double valve springs. The cam-cases bolt to studs protruding from the heads and are continuous for all eight valves, bridging the gaps between the two blocks. Each cam-case has nine plain bearings for the camshaft and eight vertical studs to which the cam-cover is bolted. The inlet valves are on the near side, the exhaust valves on the off-side, and they work in push-in guides. The camshafts have sharp-apexed cams, suggestive of a high boost, and they operate the valves via very Bugatti-like sliding “pistons.” They are driven by a train of gears from the front of the crankshaft, these gears being enclosed within a Y-shape timing case and running on self-aligning ball-races; all have straight teeth. Across the front of the timing case is a cradle housing a transversely-set Type AM8 Scintilla magneto, this being driven from the off-side camshaft via a gearbox containing right-angle reduction gearing. The plug leads enter a conduit which leads them to the 18-mm. plugs, one set vertically in each head.
At the rear of the engine is mounted a centrifugal supercharger driven from the rear of both the camshafts. It runs at 5 1/2 times engine-speed and is mounted partially on the ends of the cam-cases, thus forming the highest part of the engine. A short inlet pipe supports a Solex carburetter (possibly not the original) on the near side behind the blower and feeds into the blower eye. There are two inlet manifolds available. One is a light-alloy eight-branch manifold with the delivery pipe from the blower coming up beneath it, to feed in centrally, through a 1 7/8-in. dia. opening. The alternative is a manifold in which the inlet ports are siamesed externally and united at four branches, and the blower delivery brought in directly at the rear end via an opening of 2 1/16-in. dia., which corresponds to the diameter of the blower outlet. Each manifold has a curious uniflow pipe which attaches at each end and loops round outside the manifold, to provide the mixture with a continuous circulatory passage past the branches. The blower inlet eye dia. is 3 in. and the engine inlet and exhaust ports have a dia. of 1 1/2 in. (A large, ribbed Roots blower was found with the engine but does not appear to have been used with it.) The inlet manifold runs along the near side. There seems to be no provision for a blow-off valve. The 8-branch exhaust system is on the off side.
The water pump is bolted to the base of the timing case; it faces forwards and is set between stays that bolt on to the front of the crankcase sides and extend forward to a circular front engine mounting. The pump delivers into a small-bore, four-branch pipe running along the off side of the block. Four threaded water outlets, two on the top off side of each head, carry a very “Henri-like” four-branch water return-pipe. Also at the base of the timing case is a gear-type external oil pump, which draws from a separate oil tank. The main bearing gallery pipe runs along the off side of the crankcase. The oil tank is with the engine. It apparently fitted in the car’s scuttle and has a heavily ribbed light-alloy plate in the bottom, from which runs the feed to the pump, via a gauze filter and a tap which the driver was apparently enabled to turn off when the engine wasn’t running, via remote linkage. The timing case also carries, higher up, an air pump, presumably for fuel feed. This engine is approximately 3 ft. 3 in. in length, not including the clutch, and is 24 in. high from the base of the crank-case to the highest part of the timing case, or 29 in. to the top of the blower casing; the crankshaft measures approximately 2 ft. 6 in. in length. It was mounted at three points, we believe in a chassis which was in one piece with the body. The clutch screws into the 12 1/2-in. dia. flywheel, and is contained inside a bellhousing at the rear of the crankcase. All parts, even the plates, are heavily drilled with lightening holes and there are twelve clutch springs. The gearbox is a very American-looking affair with squareish easing, giving 3 speeds and reverse. Its short central lever is bent back to lie horizontally, but is otherwise that of any American car and works in a ball gate. There appears to have been torque tube transmission, and there is an arrangement for disconnecting the layshaft when in top gear.