Tuning Team Racing Engines

By E. C. MARTIN: From Model Airplane News, March 1954

The advanced flier can improve his engine for ideal results in this highly demanding category. The qualities particularly desirable in a special team racing engine may be summed up as follows:

High bhp output at moderate rpm and high torque throughout the speed range, in order to obtain high flying speeds; brisk take-off acceleration; and swing coarse pitch props to produce high airspeed at the lowest compatible rpm.
Instant starting, both hot and cold, without needle valve alteration from the optimum performance setting, when mounted inverted.
Exceptional fuel economy.
Low rate of wear, and ability to withstand severe abuse.
Ducted exhaust system for cowling convenience, and general configuration and mounting method suited to a racing application.
Air intake easily accessible for choking, clear of hot cylinder and exhaust, and located to promote efficiency fuel feed from a conveniently placed fuel tank.

While various .29 engines can be used in team racing, the McCoy Redhead 29 most closely fulfils these requirements. To satisfy Point 1, a little modification is necessary. Point 2 is met satisfactorily. Point 3 requires modification. Points 4, 5 and 6 are "natural and the Mac lends itself easily to the required alterations. The writer can recommend the following modifications from first hand experience.

Let us first deal with the problem of torque. It is reasonably true to say that if the number of power strokes in a given time is reduced, then fuel consumption will also be reduced, but it also follows that the amount of energy produced in a given time will likewise decrease, with a consequent loss of power. However, if it is possible to increase the pressure acting on the piston during each power stroke, and the engine is capable of conveying this pressure to the crankshaft in the form of torque, then there will be an increase in available power for the given number of strokes. Taking it a stage farther, if the increased pressure can be derived from the same quantity of fuel, which it can, then we will get more power from the quantity. Accordingly will be able, with a suitable prop, to move a given load a greater distance with the same fuel consumption. Hence the value of high torque.

The McCoy .29 is a racing engine and consequently has fairly advanced port timing to permit the high rpm necessary to very high bhp. As a result, the still expanding exhaust gases arc allowed to escape rather early in the cycle in order to make room for the incoming low pressure charge from the bypass ports. As we require increased torque at somewhat lower rpm, we can tolerate a slightly lower volumetric efficiency and may therefore retard the port timing. As a result, the exhaust port will open slightly later in the cycle and we shall extract a little more useful work from each power stroke.

The timing may be easily retarded in the McCoy .29 by simply lowering the location of the cylinder liner in the block as shown in Fig. 1.(seeTHE FIGURES ) As a result of experiment with three different engines, it was found that between .020 in. and .025 in. gave the desired results, and, since the effective stroke increased (that is the stroke of the piston the point where all ports are closed), the compression ratio is also increased slightly. A further increase in compression ratio accrues from facing the top of the block to enable the head to clamp the liner-flange in part

The result of raising compression, explained earlier in this series, is increased maximum cylinder pressure. The Mac is able, owing to ball bearings and high structural strength, to convert this pressure torque, but suffers an rpm and bhp loss on nitro methane and nitro propane fuels to detonation as a result. However suppose we accept this for the moment and use a prop of coarse pitch that holds the static rpm down to about 12,000. It will be found that this figure is appreciably higher than is obtained on the same prop before the modifications were carried out. You have, in fact, more power in the lower speed range and, with suitable props, will therefore get more miles per gallon.

The execution of this alteration is not difficult, and a lathe not essential. However is necessary to make a simple boring bar that is a good slip fit in the bore of the block, thus providing a pilot. The cylinder must, of course, first be removed by the block above a gas flame or in the electric oven, and withdrawing the liner with a wire hook.

The toolbit is first set in the boring bar as shown in Fig. 2 and the flange recess increased in depth by .020-.025 in. This may be checked with a steel rule, .015 in equivalent to 1/64 in. Finally, the tool bit is set so that the cylinder head seating is faced off by the same amount, and the flange therefore restored to its original depth. This cut may be executed progressively by increasing the toolbit cutting radius in steps in order to reduce the cutting effort.

It is, of course, vital that the faces remain at right angles to the axis of the bore, and piloting is the only sure way of guaranteeing they do.

One undesirable result of lowering the liner is that the effective port area is reduced, and another is that the piston skirt uncovers larger area of the exhaust ports at top dead center. The latter occurrence does not have any noticeable effect on carburetion, although it must inevitably reduce suction. The former snag may be offset by increasing the lateral dimension of the liner ports by carefully the port bars down to a width of .030-.025 in. Care must be taken to insure that this dimension is maintained on the inside edges. The outer edges may be chamfered slightly all round the ports to assist gas flow. The port areas will be slightly larger than stock as a result of this measure.

The piston crown should now be reworked to improve its efficiency, and a little careful filing in conjunction with Fig. 3 will make he exhaust ports open simultaneously. Only he edges of the baffle should be touched as the interior of the piston is cored out to conform with the external contours.

Because we propose operating at lower rpm, the stock rotary inlet timing will be too advanced for maximum efficiency. The result is that blowback through the carburetor will occur with consequent wastage of valuable fuel It is therefore advisable to reduce the angular dwell of the port opening after TDC from 44' to 30*.

There is no straightforward way of modifying the existing rotor and the simplest way is the construction of a new one. A glance at Fig. 4A will indicate how this can be done without too much trouble. If a piece t 3/16 in. duralumin sheet with a smooth at surface is used and care taken not to damage it while filing, it will be found that good valve seal can be obtained by lapping on a flat surface with metal polish. The balancing holes can be supplemented by smaller holes in the appropriate places until perfect static balance is achieved.

The preferred, though rather tedious, an alternative is carefully to make and fit an aluminum plug that will block off a 14 deg. segment of the stock rotor, and pin it in position as illustrated in Fig. 4B. This plug Should not come in contact with the backplate face but should clear it by .033-.055 in. Again, small holes should be drilled 1/8 in. deep in the appropriate places to restore balance.

An obvious advantage of the above is that the electro-chromed stock rotor may be used a conjunction with the steel backplate facing, with less wear and friction than likely with the hand-made version.

A last refinement concerns carburetion. It is important that only fresh air go through he carb, and to this end, it is worth making long intake which is bent at right angles to project slightly from the fuselage. This also facilitates choking and creates the opportunity to relocate the jet assembly in a position most conductive to efficient fuel feed.

The jet should not be more than 1 1/2 in. from the backplate, or starting will become uncertain A 1/4 in. bore x 3/8 in. outside dia. soft aluminum tube may be used and threaded 3/8-32 to fit the backplate. A 9/64 in. dia. hole drilled through the tube will take a McCoy .19 type spraybar assembly and give improved fuel suction to offset the suction loss occasioned by the liner modification. Volumetric efficiency will not be impaired, as more air than normal will be induced under the piston.

The final product is an engine which is somewhat detuned by speed model standards. It is capable of flying 20 per cent farther on similar fuel, and in the writer's models only dropped 5 mph in comparison with a stock engine, but stayed invariably right on the bit through the entire tank. This frequently meant an unaltered average speed and enabled the elimination of one, and occasionally two, pit stops in a 10 mile race.

The high compression ratio built into the engine calls for a little fuel tailoring, and detonation will occur on nitropropane fuels. However, the addition of 5 per cent of nitro benzine to the normal gasoline/nitropropane/SAE 50 mixture will usually cure it. If not, a .005 in. aluminum decompression gasket must be fitted under the head. The addition of a further amount of nitrobenzine is inadvisable as it reduces the potential range per tank of fuel. Experiments should be carried out with compression and fuel for optimum performance along the lines suggested in an earlier article in this series, and checked in flight rather than on the bench. A medium heat long life plug should be used to reduce the risk of plug failure.

Finally, to get the greatest efficiency from your engine, the mechanical features should receive detailed attention and this aspect will be covered in a subsequent article dealing with speed tuning.