MOKI .61 Long Stroke By Clarence LeeThe Moki engines, produced in Budapest, Hungary, have gained recognition throughout Eastern Europe during their 30 year existence for their quality, performance and excellent longevity However, due to being manufactured by a government controlled facility behind the "Iron Curtain" until three or four years ago, few of the Moki engines made their way into the U.S. marketplace. With the collapse of the Communist regime, control of the Moki manufacturing facility was turned over to a private modeling organization and the ban on exporting products lifted. The Moki line of engines were first imported into the U.S. by Bob Davis of Davis Model Products. However, due to a dispute over production schedules, Bob's contract was terminated upon expiration and a new, exclusive North American importer appointed - Jim Gerard, who heads Gerard Enterprises, Inc. Our first review ol' a Moki engine was the Moki 150 in the March '92 issue and this was followed by the conventional bore/stroke Moki 61 in the July '92 issue.
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ModificationsThis past year the original Moki 61 was discontinued and replaced by a new "long stroke" engine. The new long stroke 61 is not just a modified version of the older engine, but a completely new design utilizing a different crankcase, internal parts, etc. The new engine is also considerably more powerful, placing it right up there with the stronger running 60/61 displacement size engines.
The Moki 61 Long Stroke is available in four versions consisting of either side or rear exhaust configuration, with each of these available with either straight ABC piston/sleeve metalurgy or ABC with ring. Our review engine is the side exhaust, ABC with ring model which should be the most appealing model for the general sport flier. The rear exhaust models intended for tuned pipe use would be more appropriate for the competition pattern flyer.
Let's take a look at the long stroke Moki 61 and see what changes have been made that contribute to the engine's increased power level.
The original "overbore" Moki 61 had a bore of 24mm and stroke of 22mm. The new long stroke Moki 61 has a bore of 23mm and stroke of 24mm making for a 1 mm decrease in bore size and 2mm increase in the stroke. Needless to say, this is a significant change in the bore/stroke ratio. As such, one would expect this change to increase the engine's low end torque and ability to turn the larger prop sizes at higher rpm which is exactly what the long stroke 61 does, as will be seen in the performance section,
Crankcase:
Like the earlier Moki 61, the new engine utilizes what appears to be an investment casting composed of a one piece center block with removable front housing and backcover. The new case has been beefed up with heavier mounting lugs and due to the longer stroke, increased in height 3mm (.118") and also in width 3mm. The original case had an exhaust outlet measuring .820" wide by .280" high. The new case has an exhaust outlet measuring .785" wide by .360" high. Although the new case's exhaust is slightly narrower, the height has been increased considerably, in turn, increasing the outlet area by 25%. Other changes include extending the cooling fins that previously ended at the top of the exhaust stack all the way to the bottom of the stack and widening the boost port passage both externally and internally. All machine work appears to have been performed with conventional rather than CNC machining equipment, but even so, is top quality and some of the best I have seen.
Crankshaft and Bearings:
The very stout crankshaft has been machined in one piece from bar stock steel, hardened, and finish ground on all bearing surfaces, including the crank pin which is always a desirable feature. The crankshaft has been finish ground "on centers" meaning that the crankshaft is supported at the front and back ends during the grinding operation resulting in both of the bearing seats and the main journal being dead true with each other. Something that cannot be said of "centerless" grinding used by the majority of the engine manufacturers due to being both faster and less expensive. Although slower and more costly, grinding on centers results in a more precise part of higher quality.
A constant thickness counterbalance design is utilized, cut away on either side of the crank pin for counterbalance action. Due to the increase in the length of the stroke, the diameter of the counterbalance has been increased from 1.315" to 1.395" or .080". The thickness of the counterbalance has also been increased from .335" to .350" or .015". This resulted in the counterbalance balancing out the full weight of the piston, wrist pin and approximately 2.5 grams of the piston's 10 gram weight or 25%. As a comparison, the earlier engine's counterbalance would balance out the full weight of the rod and wrist pin only and no part of the piston weight. The additional counterbalance weight obviously paid off as the long stroke engine's vibration level was no higher than the earlier overbore engine we tested and long stroke engines usually exhibit higher vibration levels. Although not vibration free, the engine was considerably smoother than some of the long stroke engines we have run in the past few years.
Following the trend in recent years to larger diameter main journals and rear bearings which, in turn, allow a larger intake port and gas passage through the crank, a 30mm (1, 181 ") o.d. x 17mm i.d. steel caged rear bearing is used. The front of the crank is supported by a more conventional size 22mm (.866") o.d. x 8mm (.315") i.d. brass caged, unshielded bearing. Both bearing sizes being the same as the earlier engine. The gas passage through the crank has been increased from .461 " to .471 " and the length of the intake port increased from .590" to .630". The intake port was timed to open 38' after BDC and close 53' after TDC for a total induction period of 195'. Although having essentially the same intake duration as the earlier engine, the opening and closing timing of the port has been delayed, i.e., the earlier engine's intake port opens 34' before BDC and closes 48' after TDC. The larger intake port, gas passage through the crank, and later closing being contributing factors to the engine's increased power level.
Piston, Sleeve, and Rod:
The earlier Moki 61 we tested utilized a ringed aluminum piston running in a hard chrome plated, steel sleeve. The new engine also uses a ringed aluminum piston but running in a hard chrome plated, brass sleeve. Normally, even when running a ringed aluminum piston in a brass sleeve the piston is fit on the loose side, depending on the ring for compression seal. The Moki differs here in that the high silicon content piston has been fit tight at the top, the same as an ABC set-up without the ring. Needless to say, this results in a terrific compression seal, and one that will last under less than desirable flying conditions, i,e., dirt and foreign matter ingestion. When the piston loses its fit, the ring still makes the compression seal, increasing the longevity of the engine considerably over the ABC set-up without a ring. The piston has been machined from a high silicon content aluminum casting and the .042" wide expansion type ring has been pinned to prevent rotation and interference with the sleeve ports. In order to help hold down reciprocating weight, the 6.5mm (.256") diameter, hardened steel, tubular wrist pin features a rather thin ]mm (.0394") wall thickness with the wrist pin, in turn, being retained in the piston by wire snap rings at either end.
The sleeve utilizes conventional Schnuerle plus boost port scavenging with a single transfer port on either side of the divided exhaust port and a single boost port directly opposite the exhaust. The exhaust was timed to open 74' before BDC and close 74' after B DC for an open duration of 148' and the transfer and boost ports timed to open 64' before BDC and close 64' after BDC for a total duration of 128'. Although the exhaust timing remains the same as the earlier engine, the duration of the transfer and boost ports has been lengthened by 12'; another contributing factor to the engine's increase in power.
The connection rod has been machined from bar stock aluminum and bronze bushed at both the crank pin and wrist pin ends, another desirable feature. Two .050" diameter holes, offset from each other, have been drilled in the bottom of the crank pin end to provide lubrication, and a single .065" diameter hole in the top of the rod provides lubrication for the wrist pin.
Although the stroke of the engine has been increased approximately .080" over the earlier engine, the overall length of the rod has been increased .120". The purpose of this being to decrease rod angularity and piston side loading. Rod angularity being the angle of the rod in relation to the cylinder centerline with the crankshaft (crank pin) at the 90' rotation point. The lower the rod angularity, the more direct the force or push on the crank pin with the crank pin in the 90' position. The end result being more power transmitted to the crankshaft and less ffictional loss due to piston side loading. It is evident that those responsible for the design of the Long Stroke Moki 61 are knowledgeable in the field of engine design and theory.
Cylinder Head:
The head has been machined from bar stock aluminum, but unlike the earlier Moki 61 that utilized a one piece head, the Long Stroke Moki 61 now uses a two piece head with a separate combustion chamber button. The combustion chamber shape has also been changed from a flat bottomed bowl to a deeper bowl, .2 1 0 " deep, surrounded by a .165" wide squish band. The measured combustion chamber volume was 1.1cc which computes to a compression ratio of 7.46:1 figured with the closing of the .272" high exhaust port and a full stroke compression ratio of 10.07:1. The long stroke engine's 7.46:1 compression ratio being slightly lower than the earlier engine's 8.02:1. Evidently, the compression ratio was lowered to lessen the chance of any preignition tendency when the engine is loaded down with the higher pitch props now in common usage.
Carburetor:
Rather than the 2-needle, rotating barrel type of carburetor utilized on the earlier Moki 61, the Long Stroke Moki 61 uses a more sophisticated design of fuel metering carburetor like that used on the Moki 150 we reviewed in the March '92 issue. For those interested in a detailed explanation of the carburetor's operating principle, we will refer you to the Moki 150 review. The photographs that accompany this article will give you a general idea of the carburetor's workings. The throat diameter of the new carburetor has been reduced from the earlier carburetor's .335" to .315". However, with the elimination of the idle mixture needle the actual intake area remains almost identical. A nice feature of the new carburetor is the positioning of the needle valve on the back side of the carburetor body facing rearward at a 30' angle. A nice move to help keep knuckles out of the prop when adjusting the high speed mixture. Another nice feature is the use of a positive idle speed stop. Many of the model engine manufacturers are eliminating the positive idle speed stop due to many fellows closing the carburetor to kill the engine at the end of a flight and using the transmitter throttle trim to set the idle speed which can often be a hit or miss proposition. In operation, I found the new carburetor to perform beautifully, being very easy to set up and adjust.
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PerformanceAt the time of this writing, Jim Gerard supplied an O.S. 61 size muffler with adapter with the engine but will be introducing a new muffler of Canadian manufacture that is exhibiting promising performance. This new muffler will be offered as optional by the time this review appears in print. For direct comparison purposes we opted to test the engine with the same fuel, props, and muffler used for testing the earlier engine.
Normally, our 2-stroke test fuel consists of 10% nitromethane, 18% Klotz KL-200 synthetic, 2% castor oil, and balance methanol. However, as the instructions that accompanied the earlier engine recommended only 5% nitromethane and cautioned against the use of straight synthetic oil, it was tested with a fuel mix consisting of 5% nitromethane, 10% BakerAA castor oil, 10% Klotz KL-200, and balance methanol. This same fuel mix was used for testing this month's long stroke Moki 61. It should also be noted that the engine came equipped with a glow plug, something the majority of the foreign engine manufacturers omit for cost reasons. Following our standard 30 minute break-in period, the following power figures were recorded. For comparison purposes we are also running the power figures for the earlier engine. The Temperature of the day was a warm 80'F, the Relative humidity 44%, and the Barometric pressure a standard 29.92 inches of mercury. All propellers were Zingers.
| | Moki 61 Long Stroke | | Moki 61 Standard Stroke | | 11 x 7 | 12,800 | 11 x 7 | 12,900 | | 11 x 8 | 12,100 | 11 x 8 | 11,450 | | 11 x 10 | 10,300 | 11 x 10 | 9,900 | | 12 x 6 | 11,600 | 12 x 6 | 11,300 | | 12 x 8 | 10,300 | 12 x 8 | 9,950 | | 13 x 6 | 11,100 | 13 x 6 | 10,500 | | 13 x 8 | 8,200 | 13 x 8 | 7,750 | | 14 x 6 | 9,000 | 14 x 6 | 8,400 |
As will be noted, the standard stroke Moki 61 had a very slight 100 rpm edge with the 11 x 7 prop. However, when loaded down to 12,000 rpm or lower, the long stroke Moki 61's "long legged" horses came into action with a minimum of 300 rpm gain (12 x 6) and maximum rpm gain of 650 rpm (11 x 8) for all of the prop sizes.
The overall handling was exceptionally nice with easy one or two flip restarts hot or cold following prop changes and idle checking. The engine would hold a steady, reliable 2,000 rpm idle with the 11 " prop sizes, 1,850 rpm with the 12" sizes, 1,750 rpm with the 13" sizes, and a remarkable 1,600 rpm with the 14 x 6. Pretty impressive idle figures, especially when the engine's top end performance is also taken into consideration. Acceleration following a prolonged idle period was almost instantaneous with the Moki fuel metering carburetor design doing a commendable job.
The Moki 61 Long Stroke carries a suggested list price of $255. 00 at the time of this writing. However I am sure the mail order houses, etc., will be selling the engine for considerably less. For the money you are getting an engine of excellent quality and performance that should give many years of dependable operation if given halfway decent treatment.
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