ENGINE REVIEW HIRTENBERGER HP .61 R/C

The one engine review every R/Cer has been waiting for. The highly unorthodox Austrian engine has highest horsepower rating of all R/C .60's tested in our review series to date.

Seldom, in the space of such a short time, has a new make of model engine aroused so much curiosity as the Austrian-built HP. First seen as a prototype .15 cu. in. racing glow engine, in the hands of its designer Paul Bugl, in 1965, the HP engine soon attracted attention in the following year when a pre-production .15 glow established a new Austrian speed record in the FAI 2.5 c.c. class for Heinz Freundt, at the then very creditable speed of 225 km/hr. A year later, at the World Control-line Championships in England, the Austrian team of Guenther Hohenburg and Helmut Turk flew a diesel version into 2nd place only 1 second behind the Eta Special powered model of Herb Stockton and Don Jehlik in the closest-ever team race final. Not surprisingly, Don Jehlik quickly saw the potentialities of the challenge and subsequently acquired the sole U.S. agency for HP engines. The following year, he and Herb Stockton were back in Europe to win the Criterium of Aces Internationals and again in ]968 to retain the World T/R Championship, an event in which HP's (while still not yet strictly in quantity production) were more widely used than any other make and took four of the first six places.

Meanwhile, in 1967, prototypes of a new HP 61 R/C engine had appeared. Some promising speeds were recorded in Europe with these engines in model power boats and a demonstration model loaned to us at the time recorded the highest ever output figure for a throttle-equipped .60 cu. in. motor by developing approximately 1.50 bhp on test.

The production version of the HP. 61 R/C began reaching the market early in the 1968 season, together with an efficient muffler specially matched to it. Our test model was obtained from a distributor's stock and, picked at random, was an absolutely standard model. As the performance curves indicate, it falls somewhat short of the pre-production version handled earlier, but it still emerges as the most powerful R/C .60 featured in the MAN Engine Reviews to date. .

All the HP motors seen so far (in addition to the two .15's and the .61, there is a .40 in the development stage) are of the same basic design originated by Paul Bugl who, for a decade or more, had built model engines in small numbers for his own use (mainly for team-racing) and for the use of friends. The "HP" designation is derived from the initials of the Hirtenberger Pattronenfabrik of Hirtenberg, an old-established firm of cartridge manufacturers who have now undertaken the production of Bugl's designs.

Compared with other current R/C .60's, the HP 61 R/C is totally unorthodox. To some extent, this is evident even in the external appearance of the engine: its very long crankshaft housing, its deep but narrow exhaust and its strange carburetor design with intake steeply inclined upward through the backplate.

Essentially, of course, the HP is a single cylinder, twin ball-bearing, ringed piston glow-ignition engine with rotary-valve intake like most other motors in its class, but beyond this, it has little in common with its competitors.

The cylinder porting, for example, is a form of Schnuerle type with bypass passages fore and aft, supplemented by a small boost port opposite the exhaust and used in conjunction with a deflectorless piston. The bell valve intake is entirely different from the rotary-valves used by other model engines and offers definite theoretical advantages. Many minor innovations and structural differences are also to be found on stripping the engine down to its basic parts.

The HP .61 R/C has a larger bore and shorter stroke than other current R/C 60's. Its cylinder is of hardened steel with a .070 in. wall thickness. A necessary consequence of the Schnuerle porting system, the exhaust ports are smaller than usual and occupy only about 116 degrees of the cylinder circumference. Three in number, they are flanked on each side by two bypass ports which are cut at angles through the cylinder wall to direct gas toward the cylinder wall opposite the exhaust ports as well as upward, for maximum scavenging effect. The main bypass ports, therefore, are actually located fore and aft and are fed from two bypass passages formed in the front and back walls of the main casting. The fifth port, or "boost port", is positioned diametrically opposite the center exhaust port and is steeply inclined upward through the cylinder wall. The port is 0.275 in. wide and the bottom of the liner is slotted to a similar width below it to allows gas to enter a small additional bypass passage in the side of the casting. Entry to this passage take place through a suitable "window" port in the piston skirt. The bottom of the cylinder is also cut away front and rear to allow gas to flow from the crankcase to the main bypass passages.

The exhaust period, as measured on our test engine, is 136 degrees of crank angle while the bypass ports remain open for an average on 118 degrees. We say "average" because there was a variation of one or two degrees in the length of time that individual ports were open on our test sample. This is not an uncommon occurrence with engines having steeply inclined ports as, ensuring that all the "ports break through the wall at exactly the same height is rather critical. It is made the more so on the HP due to the fact that the ports are sloped both upward and sideways.

Above the level of the ports the cylinder has a narrow flange by which it is vertically located in the crankcase. Above the flange the cylinder is encased in a machined aluminum finned jacket, either press fitted or shrunk on. Another departure from normal practice, the head joint is made by metal-tometal contact between the cylinder-head and the top of the jacket-not the top edge of the cylinder itself, which is recessed slightly to prevent this. Four Allen cap screws pass through the head and fins to tie the complete cylinder assembly to the main casting.

The cylinder is offset relative to the crankshaft axis, a feature found on some other motors including the Merco and most Fox models but, whereas, in these engines, the offset is in the direction of rotation in order to reduce piston side thrust on the cylinder wall and to take advantage of slightly asymmetric port timing, the HP has its cylinder offset to the opposite side. This is an arrangement for which we find it difficult to provide any convincing argument but presumably the designer had a good reason for adopting it.

The aluminum piston is produced from a permanent mold casting and weighs only 0.35 oz. complete with its two rings. The latter, incidentally, are of a stepped section which reduces the actual surface in contact with the cylinder wall by about 40 percent, evidently with the object of cutting down the piston friction generally supposed to contribute the major part of the mechanical losses present in any engine.

Wrist-pin bosses are hung from the piston head and do not use bushes or oil holes. Instead, the wrist-pin holes are quite tightly fitted to the 6 mm. dia. pin so that wear movement is largely confined to the conrod bearing, which is bronze bushed to take care of this. The rod is also bushed at the lower end and both ends have long lubrication slits.

The piston skirt is cut away, front and rear, so as not to blanket gas transfer at the bottom of the stroke. The piston crown, as befits a Schnuerle port engine, is deflectorless. It is, in fact, perfectly flat and permits an efficient symmetrically shaped combustion chamber in the head. This features a 0.118 in. wide squish band with a deep chamber that is bell shaped rather than hemispherical. The plug hole is very deep and even 7/32 in. reach plugs (no plug is supplied with the engine) are quite deeply shrouded.

The crankshaft has a 0.472 in. dia. main journal, 1.90 in. long and a 0.374 in. dia. front journal. It has a full disc web, 1.308 in. dia. and 0.345 in. thick, internally counterbalanced, with aluminum sealing rim. The integral crankpin has a diameter of 0.275 in. and includes a 0.118 in. dia. spigot extension that drives the valve rotor. The shaft runs in a 12 x 28 mm. ball journal bearing at the rear and a 9.5 x 22 mm. bearing at the front. Beyond this, a machined dural prop driver is fitted on the shaft by means of a brass split taper collet, at which point the crankshaft terminates and is internally drilled and tapped for a special steel and aluminum prop retaining screw.

Of all the many unorthodox features of the HP, the rotary bell-valve induction system is,' perhaps, the most significant. Outwardly, there is not much to distinguish the engine from a normal disc valve rear-intake motor, except that the intake boss projects downward from the top of the backplate at a 40 degree angle to the vertical, instead of pointing upward or horizontally rearward. This, however, opens the actual inlet port through the top of the crankcase backplate, giving a very direct path for incoming gas flow and one which also conveys cool gas towards the interior of the piston.

The bell valve rotor itself is of case hardened steel, is counterbalanced and incorporates a short 0.138 in. dia. spindle running in a bronze bushing in the backplate. The valve is drum shaped, 1.270 in. dia. and 0.534 in. outer depth: The actual intake slot in the valve is extremely large, 0.350 in. wide and occupying some 140 degrees of the valve circumference. The big advantage here is that, when the valve begins to open at approximately 40 deg. ABDC, it takes only a further 40 deg. to open fully and then remains wide open for almost 100 degrees of crank angle before quickly closing again. This is achieved without going beyond a normal 180 degree total intake period. The bell valve would appear to offer less drag than more conventional rotary vi1ves.

The carburetor has a throat diameter of 6.5 mm. giving an unrestricted cross sectional area of just over 33 sq. mm. It has a venturi Shaped intake with a pressure cast aluminum throttle barrel installed in the convergent section of the venturi rather than in the throat. This means that when the throttle is rotated toward the idling position, it is the smaller "downstream" opening in the barrel that controls the amount of air admitted. Fuel is admitted at a point not within the barrel, as on most engines, but just downstream of it, so a means of restricting fuel flow as the throttle is closed and air reduced, is essential to balance the extra suction then exerted at the jet.

This is achieved by incorporating an adjustable fuel metering device in the needle valve body which is tubular and completely enclosed (but free to rotate) in a housing above the intake. The needle-valve releases fuel through a slit instead of a round jet hole and the effective size of this slit is controlled by rotating the needle valve body so that the slit is exposed, to a greater or lesser degree, by a drilled hole leading into the carburetor venturi. The principle, in fact, is similar to that of the Kavan carb but the design and construction are quite different. The metering device is linked to the throttle arm and an external throttle stop screw is used to adjust the idling mixture strength. Another, spring-loaded, screw couples this to the actual throttle barrel so that compensating adjustments to control the amount of air admitted at idling speed can be made.

Performance The manufacturer's recommended fuel for the HP.61 R/C is a mixture of 5 percent nitromethane, 25 percent castor-oil and 70 percent methanol which, in fact, corresponds exactly with our own standard blend used for all MAN tests on R/C engines. We installed a Swanson Fireball standard plug which was found to suit the engine very well.

The suggested break-in period for the HP.61 R/C is 30 minutes. Our engine needed rather longer than this before it became sufficiently freed off and we totaled 1-1/2 hours before starting torque tests. Typical prop revolutions recorded towards the end of this period included 11,200 on a Power Prop 12x6, 11,400 on a Rev-Up 11x7-1/2, 11,700 on a Rev-Up 11x7 and 12,200 on a Top-Flite "Super-M" 11x7.

Tested first without its muffler, the HP showed a maximum torque of 105 oz. in. at 9000 rpm and a peak power output of 1.28 bhp at 15,000 rpm. Although this is about 15 percent less than was obtained with the prototype engine under similar conditions, it is, nevertheless, the highest horsepower figure for a .60 class R/C engine obtained in tests to date. HP state that, with their muffler fitted, power output is "only slightly affected". The difference between this and most similar claims is that it happens to be true. With the muffler, the power loss was of the order of just over 10 percent only, dropping rpm by 300-500 according to prop size. This muffler/engine combination is certainly the best we have come across to date.

It took us about an hour of fiddling to get a satisfactory idle. After another hour, we came to the conclusion that the throttle really works but maybe ease of throttle adjustment is not one of the HP's strong points. For a start, it is a bit difficult, after years of using air bleeds, to get used to the idea of turning the mixture control (which looks like the throttle stop anyway) anticlockwise to lean it out and, maybe, to then have to turn another screw also anticlockwise to open up the idling speed because leaning the mixture has also closed the throttle barrel too far. It doesn't help either to have the intake pointing down somewhere into the innards of the model, thereby depriving one of a crafty look to see what's going on. However, procedures are clearly set out in a well-written English instruction leaflet accompanying the engine. A little patience is the only other thing you need.

In other respects handling qualities were very good. For all the potency of its performance, the HP.61 started easily and showed no tendency to bite, even when hand started on very small props. The needle-valve was not at all critical: in fact we felt that it might be better if a little more sensitive, as it could be moved most of a full turn, one way or the other, with little effect. Running qualities were also very good, particularly in regard to vibration level which was noticeably lower than average.

Obviously, the HP.6I R/C is not everyone's choice. It needs, for example, an engine bay two inches longer than most other current 60's and for that reason will not just drop' into any old model designed to take a .60. At $70.00 list, it is also pretty expensive. But, if it is power, particularly muffled power, that you are looking for, or something to slip unnoticed into a nice scale Spitfire or Mustang, this could be it.

Summary of Data
Type: Single cylinder Schnuerle-port two cycle with twin ball-bearings and rotary bell valve induction. Throttle type carburetor. (No exhaust restrictor.) Optional muffler.
Weight: 16 oz. (18.6 oz. with HP muffler)
Displacement: 9.900 C.c. = 0.6041 cu. in.
Bore: 24.5 mm. (0.9646 in.)
Stroke: 21.0 mm. (0.8268 in.)
Stroke/Bore Ratio: 0.857 : 1

Specific Output (as tested):
2.12 bhp/cu. in. (less muffler)
1.89 bhp/cu. in. (with muffler)
Power Weight Radio (as tested):
1.28/bhp lb. (less muffler)
0.98 bhp/lb. (with muffler)

Price in USA: $70.00 Manufacturer: Hirtenberger Patronen Zundhutchen and Metallwarenfabrik AG., A2552 Hirtenberg, Austria. U.S. Distributor: Performance Aero Products Inc., Box 6064 Shirlington Sta., Arlington, Va. 22206.