A LIGHTWEIGHT TORQUE MACHINE

Having successfully "stretched" their .61ci 2-stroke to a .75, SuperTigre has now boldly used the same crankcase for this .90-size middle-of-the-range sports engine the G90.

What are the ramifications of "boring and stroking" a given engine size? Commercially, it makes sense: this is a "new" engine at minimal cost; it fits existing structures; and spares are readily available.

TORQUE TALK

Looking at torque figures is just one way of comparing engines. The G90's torque at lower rpm is good—so good that this apparently "normal sports engine" is number two in my "Comparative Torque" list of the top 12 engines I've tested. These illustrious engines are all of medium to large capacity; clearly, there's a scale effect at work. But I use two differently directed torque parameters (oz.-in./lb. and oz.-in./cc), so many engines don't make list—none of the top R/C car or marine engines, for example. The list highlights only reasonably light aircraft engines that have higher than average cylinder pressures (good bmep figures). It includes at least two International Class F3A engines (R/C aerobatics) and two ducted fans. The surprise is that it includes three sports engines, and a greater surprise is that the G90 fares so well in the comparison! Maybe there's a new F3A engine here?!

Technically, the G90 appears to benefit more than usual from its narrow transfer passage and high gas transfer velocity (which usually follows from boring out a given crankcase); this favors low-rpm operation. Those who worry about airplane noise and the associated problem of flying-field retention might be pleased with the figures in the torque chart. The higher these figures are for any given engine, the more amenable it will be to stringent silencing measures without an undue loss of power.

Engine testers are often criticized because they use horsepower figures instead of just torque—and these are open-exhaust figures anyway. I find torque the most interesting aspect of engine performance. This fundamental force is the effort imparted, for example, to the crankshaft of an internal-combustion engine by the piston's reciprocating motion. At any time, it's really the only thing the engine is producing. Horsepower merely includes the dimension of time: the closer together those individual torque impulses occur, the more horsepower the engine is making available.

It's interesting to observe the way in which torque "unfolds" as rpm ranges from minimum to maximum, and is, quite naturally, a fascinating outcome of complete testing. The fact then: horsepower reporting itself is solely a mathematical summation of that varied torque release against time. Maybe it would help if the torque curve were placed more prominently at the top of the normal results graph and the rather more misleading hp curve made to skulk at the bottom for a change! (Any comments?)

Admittedly, reporting open-exhaust figures is more contentious because of the wide range of available mufflers. And manufacturers might be inclined to supply reviewers with less restrictive mufflers to keep their engines' hp figures looking more favorable. Like their customers, they still see horsepower figures as more meaningful.

Open-exhaust performance figures allow us to compare engines scientifically. Noise-restriction rules are for national and international bodies to agree on and enforce rather than for individual (maybe biased) engine testers to influence. I'm happy to test engines and report any significant moves in this area and other technical advances (even those that aren't noise friendly).

The results of the FAI's , imposing sound-level regulations on the competition in the F3A class have been interesting. Using the appropriate tuned-pipe technique, the new Webra 120 operates at a very low 7,000rpm to meet the FAI requirements (test imminent). This type of organization-led development will also affect normal sports-model setups, but much more needs to be done to ensure a problem-free future.

G90 MECHANICALS

Crankcase. Over the years, SuperTigre engines have developed steadily but almost inconspicuously. Very sound engineering and design have led to high-quality engines that also represent very good value. Their crankcase structures, and surface finishes are sound and visually appealing, and the G90's one-piece design is very robust; in fact, it's one of the finest crankcase structures around.

Bore and stroke. Bore and stroke are 15 percent and 13 percent greater (respectively) than those of the standard .61 engine; overall weight is only 0.25 ounce greater, and height has been increased by only 4 mm.

Crankshaft. The heat-treated nickel-chrome crankshaft is the other essential that allows power flow to continue unaffected by any distortions or wear.

Crankpin and crankweb. These have been forged to ensure that the metal "grain" flows around the various angles without interruption-another example of quietly improving engineering without a fare. I'm not sure why SuperTigre doesn't boast about this improvement—time to go public maybe? SuperTigre is one of the rare manufacturers that opt to fine-finish-grind the crank-nose threads (as seen recently in the MDS 46).

Piston/liner combination. This is a single-ring medium, high-silicon piston running with 0.002-inch clearance in a hardened-steel liner. The piston has been externally honed (this is well worth the time it takes because it's essential to true roundness), and the liner has been internally ground to its finished size.

Porting. The liner has the standard Schnuerle transfer and boost ports with restricted port timings that take advantage of the expected lowish rpm. Externally, the very wide boost passage is of the usual SuperTigre style, yet internally, it's about only half as wide a strong case!

Cylinder head. This is a one-piece unit with a wide squish band angled at 3 degrees, and the chamber is of the usual "bowler-hat" shape. The brass threaded glow-plug insert gives long-term reliability to glow-plug fitting.

Compression ratio. This is set at a relatively gentle 7.6: I-a feamm that effectively retards ignition points (again indicating the expected low-rpm operation). An operational effect of this lowish ratio is that the glow plug acts colder than it would on other high-compression engines and would thus not easily sustain correct part-throttle running if the fuel setting was too rich. I had to keep the plug lit until the engine reached a leaner fuel setting.

PERFORMANCE

To help readers make their own comparisons, I ran the G90 with a wide range of props.

The piston/liner combo led me to expect a lengthy running-in period, but SuperTigre's precise workmanship allowed a fairly easy transition to full-bore operation after approximately 30 minutes. I used a variety of the listed props—using the short-run technique—and gradually increased load and throttle openings.

Test 1. Open exhaust; fuel—5 percent nitro, 10 percent castor, 10 percent ML70 synthetic oil, 1.5 percent ether, and the remainder methanol; glow plug—SuperTigre. Following the manufacturer's fuel recommendations, I obtained rpm ranging from a high 16,867 to a sensibly lowest feasible 4,400rpm. The lowish rpm bias in torque (maximizing at 7,300rpm) soon became apparent, though hp was still rising well at 13,000rpm showing that all design features narrow transfer passages, low port timings and relatively small-bore (8.0 mm) carburetor worked well together in the final analysis.

Test 2. SuperTigre "Quiet" muffler, same fuel and plug as in Test 1. Subjectively (indoors), I found the effect of this "new-generation" backpressure muffler quite marked. Torque and hp were affected of course , but there was significant gain in fuel economy.

Using the fuel-efficiency parameter of brake-specific fuel consumption in cubic centimeters used in 1 hour if developing lhp (bsfc), the G90's 755 cc and hp are rarely achieved by 2-stroke engines run on methanol. The O.S. 35 BGX and, more recently, the Irvine 150 are equally creditable in this respect.

The dip in torque production as rpm rose was probably caused by the use of this new muffler, but being mild has little practical effect on which rpm to use between the limits 6,000 to 12,000.

Sound levels. Using the ST "Quiet" muffler, this engine certainly meets official requirements. But, as my figures show, this can only be done by restricting the rpm of this large 15 cc engine down to 7,000.

Test 3. Genesis 60/90 tuned pipe; same fuel and plug as previous tests. Provided by Weston Products and recommended by the U.K.'s SuperTigre distributor Mike Wilshere, this "quiet" tuned pipe was fitted uncut at a 500nim length from piston face to first maximum diameter using a standard SuperTigre manifold. I expected it to enhance lowish rpm, so I was not surprised to see that torque was much raised in the 8,000rpm area—way above the muffler levels and somewhat over open-exhaust levels. The quite wide and flexible rpm bandwidth was also shown to favor "sports" users.

I didn't use a range of props wide enough to do the Genesis pipe full justice; of those used, only the 14xl4 APC and the 16x6 Airflow would be of use where low rpm are required.

Test 4. Bolly EQ63 (square) tuned pipe; same fuel and plug. The volumes of both the Bolly and the Genesis pipes are nominally too small for a .90ci engine. This relationship has to take into account the expected rpm levels as well as the simple ratio of cylinder capacity to pipe volume. Clearly, at low rpm, the gas throughput is much reduced and so does not see a small volume pipe as much of a restriction.

To obtain high-rpm data, I deliberately set the Bofly pipe at a shorter effective resonating length than the Genesis (640 mm, piston face to internal baffle). A fair comparison of the two would only be possible if both were tested at a range of lengthsand perhaps on a variety of engines! But this would then be a pipe test instead of an engine test! Basically, both pipes should yield the same results. The differences between them shown on the graph result mostly from their different resonant lengths. With the Bolly pipe, the 16x6 Airflow, 15x7 -olly and, maybe, 12xl2 APC props look useful.

TORQUE FOR SPORT

On the test bench, it has always been apparent that relative engine performances with a variety of exhaust systems and rpm levels are better illustrated by the torque curves rather than the hp curves. Except for real out-and-out racing engines, torque curves allow us to quickly see where in the rpm band a given system should operate; but be careful to steer clear of the declining torque areas toward the left (at lower rpm) side of the graph. If you don't, you'll suffer a rapid decline in power levels whenever the engine load is increased.

This concentration on the beneficial torque areas has a downside; the use of heavier props that increase the load on the engine obviously means higher inertia for the engine to cope with. It's better to increase rpm and decrease load either by using a lighter prop (maybe wooden) or increasing its pitch. That's why we're seeing more "strange" prop sizes: 14xl4, 12x10, etc.

Remember that my prop rpm data are "static" ground-based figures. In flight, we'll see varied increases (if any) -up to around 15 percent. This will depend on precise placement of ground rpm on the particular section of torque curve.

IDLING

Using a 15x8 Graupner prop and the Quiet muffler (providing pressureassist to the fuel tank) led to an easily obtained 1,600rpm. The SuperTigre "Mag" twin-needle carb (now on all their engines) continues to give easily controllable operation with the usual quick pickup to full throttle.

SUMMARY

The G90 test session produced some surprisingly good figures. Generally, SuperTigre engines are giving great value to users in a quiet, undemonstrative way. We're besieged by engines from all over the globe, so this was an appropriate time to take another look at a product from one of the most longstanding model engine manufacturers.

WEIGHTS AND DIMENSIONS
Capacity: 0.900458ci (14.756 cc)
Bore: 1.083 in. (27.52 mm)
Stroke: 0.9775 in. (24.828 mm)
Stroke/bore Ratio: 0.9026:1
Timing Periods:
 Exhaust 142 degrees
 Transfer 118 degrees
 Boost 113 degrees (Angled up 50 degrees)
 Front Induction opens 41 degrees ABDC
 closes 51 degrees ATDC
 total period 190 degrees
 blowdown 12 degrees
Combustion Volume: 1.6 cc
Compression Ratios: Geometric—10.22:1
 Effective—7.59:1
Exhaust-port Height: 0.279 in. (7.09 mm)
Cylinder-head Squish: 0.040 in. (1.016 mm)
Cylinder-head Squish Angle: 3 degrees
Squish-band Width: 0.185 in. (4.7 mm)
Carburetor Bore: 0.348 in. (8.85 mm)
Crankshaft Diameter: 0.669 in. (17 mm)
Crankshaft Bore: 0.348 in. (8.85 mm)
Crankpin Diameter: 0.275 in. (7 mm)
Crankshaft Nose Thread: 0.310 in. X 24 TP{I (5/16 UNF)
Wristpin Diameter: 0.275 in. (7 mm)
Connecting-rod Centers: 1.73 in. (44 mm)
Engine Height: 4.153 in. (105.5 mm)
Width: 2.397 in. (60.9 mm)
Length (backplate to prop driver): 3.69 in. (93.74 mm)
Width Between Bearers: 1.692 in. (43 mm)
Mounting-hole Dimensions: 1.968 x 0.787 x 0.167 in. (50x20x4.24 mm)
Exhaust-manifold Bolt Spacing: 1.85 in. (47 mm)
Frontal Area (bare): 7.23 sq.in.
 with muffler: 11.33 sq.in.
 with tuned pipe: 13 sq.in.
Weight (bare): 20 oz. (566 gm)
 with quiet muffler: 25.9 oz. (734 gm)
 with Genesis 60x90 or Bolly EQ63: 28 oz. (794 gm)
 (Tuned and Manifold)
Crankshaft Weight: 3.15 oz. (90 gm)
Piston Weight: 0.50 oz. (14 gm)
PERFORMANCE
Max. b.hp 2.35 @ 14,418rpm (open exhaust/5% nitro)
 2.34 @ 11,670rpm (Bolly pipe @ 480 mm/5% nitro)
 2.03 @ 8,891rpm (Genesis pipe @ 510 mm/5% nitro)
 1.97 @ 12,768rpm (SuperTigre Quiet Muffler/5% nitro)
Max. Torque 231 @ 7,813rpm (Genesis pipe @ 510 mm)
 229 @ 9,000rpm (Bolly pipe @ 480 mm)
 220 @ 7,300rpm (open exhaust)
 186 @ 5,923rpm (SuperTigre quiet muffler)

Reprinted with permission.
December, 1996 Model Airplane News
Editor: Gerry Yarrish