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Thread: OMC’s 4-Rotor Wankel Racing Engine - The Real Story

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    Default OMC’s 4-Rotor Wankel Racing Engine - The Real Story

    As told by John Sheldon
    Former OMC rotary engineer


    PREFACE
    After the successful introduction of the 35 HP and 45 HP air cool/charged cooled snowmobile engine, OMC started working on water cooled/charged cooled variants of the same engine. Basically all the parts were the same except the housings, which were water cooled. Contrary to previous OMC, Curtis Wright and Mazda water cooled engines which had the water flowing axially; parallel to the crankshaft; these engines were partially circumferentially cooled. Water entered before the spark plug, ran around the rotor housing and exited around the exhaust port. The side housings picked up water at the entrance point for the rotor housing, travel across the hot section of the housing and exited at a low pressure point after the exhaust port. Both single and dual rotor engines had been prototype and development work had begun. The single rotor produced 50/55 HP and the 2 rotor produced 110/120 HP. The increase in HP was due to the improved volumetric efficiency (increased air flow) due to the lower temperatures resulting from water-cooling. Both these engines were configured for outboard use and were coupled to current lower units. At one board of directors meeting the twin rotor engine was mounted to a boat for demo rides. The tachometer was disconnected and the engine was left running. Every one that got into the boat turned the key to start the engine. There was no noise, no motion, no vibration. It was impressive.

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    BIRTH OF THE 4 ROTOR RACE ENGINE

    Charlie Strang was the head of OMC at the time. At the world championships in Lake Havasu in 1972, Johnny Sanders won the race for Johnson, but only because Mercury broke. It was clearly apparent Mercury was faster than the OMC engines. Both companies were running 100 ci 2-cycle engines, but Mercury had an in line 6 cylinder and the OMC engines were V-4’s. The additional cylinders of the Mercury engines allow more air flow, resulting in additional power. The Mercury engine was producing approximately 200HP while the OMC V-4 was producing approximately 175 HP. There wasn’t much to do to get around this. After the race, in October, Charlie Strang came by to chat with George Miller. George was head of the rotary engine engineering group at the time. He confirmed with George that the 2 rotor engine was producing 115/120 HP. He speculated with George if we stacked 2 of the twins on top of one another, we would have an engine that produced 240 HP and we would finally have a chance to “beat those black bastards”. George reluctantly agreed, but pointed out to Charlie he didn’t have the manpower to assign to the project and it most likely would take a year or two. Charlie agreed, but said lets delay some of the other programs and put some one on it. Besides, he said, racing will find weaknesses and force development much faster than conventional engineering development. As he was leaving George’s office he said “oh, by the way, I want to race them at Parker, Az in March. So began what was referred to as HSXL; Havasu experimental limited and was assigned the project # D706.

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    INITIAL DESIGN WORK

    Reluctantly George assigned Mike Griffith to the program. The first issue addressed was the crankshaft. A stationary gear is required for each rotor and its pitch diameter is dictated by engine geometry and can’t be changed. Because this is smaller than the eccentric diameter, either the crankshaft or stationary gear has to be split. It was decided to keep the stock stationary gear and use 2 –two rotor crankshafts, some how magically held together in the middle. Mike remembered Mercedes Benz had used a coupling on their 3 and 4 rotor engines put into their C-111 experimental cars. After a little research, Mike found out Gleason Works had a curvic coupling* that resembles an involute gear standing on its end. OMC was one of Gleason Works biggest customers. After a few phone calls a curvic coupling design was developed by Gleason that would work for this application. Because of the separating forces generated by the curvic coupling and the fact we wanted the crankshaft to think it was one piece (more on this later) an enormous clamping load was required to hold the two cranks together. The solution was a thru bolt; a very special thru bolt. It was 7/8” in diameter and was made from EDT-180. A material having 180,000 psi minimum yield strength. The bolt was necked down along it center section to equalize the stress in the threaded ends, due to stress riser from the threads. The decision was also made to have 180 degree eccentric spacing instead of 90 degrees. This allowed the engine to be dynamically balanced without counter weights and use 2 of the current 2 rotor crankshafts. Rotors 1 and 4 along with 2/3 were in phase with each other. Because 2 rotors fired at the same time, the stock ignition system from the twin rotor could be used. Each plug had its own coil, but only 2 triggers were used. Charlie Strang came up with the idea on mounting the engine almost directly to the gear case eliminating the conventional mid-section and lowering the overall height of the package; i.e. lower center of gravity would me faster turns. Thus the so-called bucket came into being. It had to be tall enough to keep water from spilling over when at rest or idle and be waterproof. It was decided to use stock snowmobile carbs and all the internal hardware; rotors, seals, bearings, etc. This minimized the new parts that need to be designed and produced. Remember race in March. With the conceptual design complete, detail design started. All of the housings had to be new as long thru bolts outside of the conventional 2 rotor engine thru bolts were used to hold every thing together. Also, the bottom housing of the top engine and the top housing of the bottom engine had to be modified to allow mounting to the steering arm and the two engine to stand on each other. New snowmobile flywheels were cast without counter weights and using existing 2 rotor crankshaft castings allowed the modification for the coupling and thru bolt. The exhaust system for the 2 rotor was a steel casting with deflectors turning the exhaust flow 90 degrees downward. Two of these were used on the 4 rotor with the housings acting as the water cooled box around them. Designs were completed, parts were ordered and the first engine was assembled. The day of reckoning came and the engine was ready for its first dyno run. The engine produced 220 HP at 6500 RPM. Acceptable for the first try, but disappointing none the less. This was also crankshaft HP and the gear case absorbed approximately 15 HP. This meant the power to the prop wasn’t much more than the V-4.

    * footnote on Curvic couplings from Sam Cullis: They were developed by Gleason Works for early jet engines. The first outboard use of them was by Dieter Konig in the 1950's. I am certain Charlie Strang knew that Konig had used them. How Konig was able to acquire such advanced technology at that time is a mystery that would be a story worth telling if it could be learned.
    Last edited by Mark75H; 05-21-2009 at 05:53 AM.

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    INITIAL POWER DEVELOPMENT

    We knew from previous work that the exhaust restriction was hindering power and the engine was somewhat subjective to exhaust tuning. This is where I entered the program. It was the first of the year and we were running out of time. The bosses decided we would work 2 – 12 hr shifts, 7 days a week. Illinois labor laws say an hourly paid worker can only work 13 days in a row and then must have a day off. The law didn’t apply to salaried people so Mike and I were on 12/7. Unfortunately, Mike got sick the first week on this schedule and ended up in the hospital. This meant I had to cover both shifts. I would go home at 2 in the morning and return at 6. We started by removing the butterfly in the peripheral port, enlarged the bore and venturis of the carbs and started working on exhaust tuning. The dynos at OMC did not have remote controls at that time so the operator and others including myself were in the dyno rooms while the engine was running. If anyone ever heard these engines run at the races, multiply that time ten and that gives you an idea what it was like in the dyno room. After determining what exhaust length gave the best power, the challenge was package it in the space that already existed in the original design. Mike and his designer came up with the solution and had parts made. The system was made from 6 aluminum casting welded together. The fear was the enormous heat of the rotary exhaust would melt the castings. As a result, a second water pump was added that only cooled the exhaust system. With the additions of all these changes, power went up to 260 crankshaft HP. The next hurtle to cross, would it survive the 9 hrs of Parker. An engine was built and mounted on a old twin engine Molinari racing tunnel. We took it to our Florida test base in Stuart and ran up and down the Indian River above the locks. The engine performed well and was cruising in excess of 100 MPH even though we weren’t pushing things. The first day went fine without incident. About half way thru the second day, the driver got it too high out of corner and blew it over backwards. The water was only 10 ft deep so the motor end went to the bottom and the front stuck out of the water. We could not see the driver and feared he was stuck in the boat under water. Jack Leek and I were in the chase boat and started immediately to the crash sight. Unfortunately, we didn’t even get on plane and the engine quit. Franticly we tried to figure out what was wrong and restart the engine. We had run out of gas! After quickly changing tanks we rushed to the sight. On the way there I stripped down to my skivvies ready to dive in and rescue the driver. The day before we had seen a 12 ft alligator lying on the shore but today he wasn’t there. All I could think on the way to the crash site was diving into the water and having that alligator show up. As we got closer we saw Rich bobbing behind the boat. The front of the boat sticking out of the water had blocked our view of him. I dove in and grabbed Rick. He was mad as a hornet, but not hurt. Off to the hospital for him; standard procedure for any driver that goes into the water. My technician and I went with the chase boat and hauled the boat back on the trailer. After taking the spark plugs out, cranking it over to clean out the water and replacing the plugs the engine started back up and ran just fine. We let it run for a half hour squirting oil in the carbs every once in awhile. Now what? A call went into Jimbo McConnell and he flew down the next day and finished the 10 hrs of testing. We all felt pretty good about going to Parker. While I was in Fl. Mike built two engines for the Miami press demo and then onto Parker. The press demo went very well and the guys headed for Parker * note; of all the pictures you see of the boats running you can tell the Miami boat as they did not have the fuel fill caps installed yet.

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    THE PARKER RACE

    We had two boats at Parker. The Evinrude was driven by Jimbo McConnell and the Johnson by Tommy Posey. They were the talk of the prerace scene. The gear case used for the initial races was a twin pinion 15/17 ratio. This is used for max top speed. Parker was a “run what you brung” race, outboards, inboards, jet boats, hydros, single engine, dual engines, and even triples. It’s a river race with the boats going 6 ½ miles up the river, turn around and come back. The race went off as scheduled and everyone held their breath to see who would lead the first lap. Here they came with the two rotaries side by side; 1-2; with a 7 liter 3 point hydro chasing them. The hydro had been clocked at 140mph the week before. Consider the world speed record for outboards was 136 mph at the time. The driver of the hydro had bet $1000.00 he would lead the first lap and was doing everything but running upside down trying to catch the two rotaries. Jimbo and Tommy coolly drove by the crowd waving as they went. A big wave went to the Mercury camp as they flew by. Mike and I were all smiles and feeling pretty good about all the long hours and work we had put in. As the boats approached for the second lap, Tommy was still in first by a big margin, but Jimbo was nowhere to be seen. We found out shortly his engine had failed. Tommy continued to lead, but succumbed to engine failure before the first hour was complete. We went home dejected and waited for the guys to drive the boats back from Parker. As soon as they returned, we tore the engines apart and found failed rotor bearings. In reviewing the dyno sheets I found the engines were producing 265 HP on 18 gph of gas. That’s BS Fuel Consumption better then most diesels. In talking with Mike, he had adjusted each carb for max power and didn’t believe the fuel flow meters. He went out and bought 4 new Cox certified flow meters and repeated the run with the same result. He didn’t believe the data but had to button up the engines to leave for Miami.

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    THE GALVESTON RACE

    Even though Parker didn’t result the way we wanted, it scared the hell out of Mercury. Word went out from the Mercury factory that any driver who ran the Galveston race with a Mercury engine would cease to get any further factory help. We built and dyno tested 4 engines for Galveston. This time we set the carbs a rich as possible before losing power. All 4 engines were between 255/260 HP. We arrived in Galveston in the mitts of a tropical storm. Good sense would have called the race, but the rotaries were such a big deal, it went on. Most of the Mercury drivers didn’t show up and certainly the Mercury factory team wasn’t there. The race went on and the rotaries took 1-2-3-4. Contrary to what has been written before, Bobby Whitt did not crash during the race. He finished 4th and was milling around in the water waiting for his turn to get on the trailer. He got broadside to a wave and the boat rolled over. We went thru the dry out routine and the engine restarted just fine and was ready for the next race. For some reason Charlie Strang said “keep them on the trailers” for the second race. The second race did go on but the rotaries were spectators. Based on the results of Parker and hours of testing, we felt the drivers were picking props that allowed the engines to far exceed their 7000 RPM redline. They complained about slow acceleration out of the corners with the 15/17-gear case and they figured out 2 things. A smaller prop gave them better acceleration and that the HP curve was still going up at a 45 degree angle at 7000 RPM . To confirm our concerns, we put recorders on two of the Galveston engines. They recorded RPM vs time. Unfortunately, one of the recorders was on Bobby Whitt’s boat and the data was lost due to the unexpected swim. The other recorder did show above redline RPM but not terribly excessive. We took the engines back home, tore them apart and started getting ready for the next race.

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    PROVO UTAH RACE

    This was to be THE race. We found out Mercury was bringing everything they had including dual engine rigs. They flew in their European drivers and the latest Mollinari boats. Our team showed up in Provo several days early and we had to rejet all the carbs because of the 4300 ft elevation. We had 4 engines at Provo and were ready. This was a 3 hr race on a 3 mile oval 3 pin course. For the first time we used the bigger 14/23 gear case. It was slightly slower on top end, but greatly help acceleration. The bigger prop also gave better control of the boat. The rotaries lead 1-2-3-4 on the first lap. Within 15 minutes the rotaries had lapped the first Mercury boat. The OMC drivers got a huge kick out of pulling up next to their Mercury rivals, giving them a little wave and then powering away. This was also the start of a rotary racing driver team; Jimbo McConnell (191) and Barry Woods (35) for Evinrude and Tommy Posey (197) and Johnny Sanders (196) for Johnson. Two of the 4 engines broke before the end of the race, but they took 1-2. The Mercury brass was so pissed, their jet plane left before the race was half over, having to take off over the racecourse. A good number of Mercury execs had to find another way home.
    The two failures taught us a lesson. The cranks for these engines cost approx. $10,000 each and had a design life of 25 hrs. As a cost saving measure, I had several crank eccentrics reground and resurfaced with stellite. This worked great for bearing surfaces, but in grinding them down, we ground thru the original case hardening, losing the strength of the shaft. One of the failures was a broken crankshaft. The second failure was the thru bolt holding the crank together broke. This was the first of several of this type of bolt failure. One actually broke on the dyno sending the bolt thru the ceiling of the dyno room. The ignition for the engine was on the top 2-rotor, but the power take off was on the bottom 2-rotor. When the bolt breaks, the top engine goes to no-load and the bottom engine coupled to the dyno stops. Because the engine was balanced as a 4-rotor, when this happened the engine lost it’s dynamic balance and broke off the dyno. Here it was hanging from the throttle cables running no-load shaking all over the place. On of the technicians saw this happen and headed to the door post haste. Unfortunately, I was between him and the door. Ever have a 250# guy run over you? I got up and thru the throttles back to idle and fortunately the engine quit. No one got hurt, but there were several guys changing their underwear.
    After returning home from this race we tried to determine what was causing this failure. We instrumented a crank and found a natural frequency resonance at 7000 RPM; right at the operating speed of the race engines. We ended up using a special silver plated tapered washer under the bolt nut to lessen the stress on the first several threads of the bolt. This eliminated the bolt breaking at the top, but failures continued at the bottom thread. We tried various dampers on the bolt, but nothing eliminated it completely. We went so far as having SKF Research roll the threads after the bolt was hardened.

    After the Provo race Mercury, who ran APBA at the time, tried to ban the rotary from further competition.

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    Carol Springs Ill.

    We ran 4 engines in Carol Springs, more or less as an exhibition, as it wasn’t a major race on the circuit and Mercury didn’t care if we ran or not. The engines finished 1-2-3-4, but there was no real competition.

    The next major race was the 6 hrs of Paris. Mercury was successful in banning the rotary’s from this race. It was also the first race where both OMC and Mercury brought their V-6’s. OMC actually made two V-6’s just for this race. Cranks were made from V-4 cranks welded together. Sand-cast blocks and heads were made, but the rest of the parts were straight from V-4 racing engines. Johnny Sanders won this race with one of the home made V-6’s.

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    Windermere, England

    OMC brought 4 rotaries to this race and Mercury brought their V-6’s. This was the first head to head for these engines. Johnny Sanders engine broke during practice, broken crank bolt. The Mercury driver Johnny had just past said later he saw something fly 50 feet in the air just as Johnny broke. It was the bolt. Mercury asked OMC to go halves on a tanker truck of aviation gas, but because rotaries could run on 87 octane gas, Jack Leek said he wasn’t interested. The race progress with 2 of the remaining engines breaking before the finish, but while they were running they were ahead of the Merc’s. Mike Downard was the sole remaining rotary, and he took it easy trying to save the engine. Mercury took the lead for a short time, but succumbed to engine failure also. Mike won the race for Evinrude, but Charlie Strang got hold of me and said he wanted to see George Miller and me in his office the minute we returned to the States.

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    South Africa

    The Johnson distributor in South Africa requested we bring the rotaries to his race. It meant leaving the week before Christmas and returning after the 1st of the year. This was the only race I didn’t attend with rotaries running. Johnny Sanders went with one Johnson, winning the race easily. The only competition was old OMC V-4’s and Mercury inline 6’s.

    That ended the ’73 racing season. In February ’74, the Iranian gas crisis hit the country and gasoline went from $0.30/gal to $0.75-100/gal if you could get it. It was felt politically, racing would not project a good corporate image when people were waiting in line for hours to get gas; so the racing circuit was put on hold. Development continued in house. The compression ratio was increased from 8.5 to 1 too 10 to1. Transfer passages were enlarged and work started to design a true 4-rotor engine, not two 2’s stacked on top of each other. This meant 3 curvic couplings and 3 center housings. We also designed a turnbuckle (left and right threads) to hold each curvic together. This eliminated the long thru bolt, which still was breaking. By eliminating the large center section in the crank common to twin rotor engines, we were able to add a center main bearing, strengthening the entire assembly and reduce weight. Power increased to 265HP at the prop shaft. @ 7000RPM. We also installed ignition limiters, limiting max engine RPM to 7000. In testing you could hear the drivers running on the limiter most of the straight-aways. This didn’t last too long, as we were afraid of engine damage would result from the high speed missing and the drivers hated them.

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