William Wynne

Here's the O-200 on our dynamomemter, and your test crew from left to right, above: Gus Warren, Detroit Institute of Aeronautics, A&P 1990; Steve Upson, Northrop University, A&P 1976; yours truly, William Wynne, Embry-Riddle Aeronautical University, A&P 1991. While the way we dress may be slow to catch on in high fashion circles, we certainly know our stuff about all types of aircraft powerplants. This is 55 years of A&P experience working on engines in the field nearly every day. This experience, along with a good technical background, puts us in a good position to do real world testing.

On the left above is the Continental O-200 as removed from a 1959 Cessna 150. This engine is considered the standard against which all other 100hp class engines are measured. It is a direct drive 4-stroke, 4-cylinder engine of 200cid. It carries a horsepower rating of 100 at 2,750rpm. I have read that Continental produced about 50,000 O-200s. On the right is a 170cid Corvair engine. For size comparison, the O-200 is 32" wide without the baffling. The Corvair is 28" wide.

To adapt the O-200 to our dynamometer required making a new mount. Everything seen in gray, above, is part of that mount. The red cap at the center is a dust cap covering the bearing on which the mount pivots. This red cap is in exact alignment with the O-200's crankshaft. This way, the rotation against the torque is in line with the crankshaft. The mount was made from a Corvair wheel, a pirated VariEze motor mount, some spare tubing, and a Corvair blower bearing. This bearing is at the bottom, and rides on a steel U channel. This provides additional support to the mount, and restrains it from turning full circle. The vertical element is a 1.5" diameter steel tube. There is a pin on the back of this tube that engages the hydraulic cylinder. By comparing this mount to the blue Corvair bed mount seen in previous dyno tests, It is apparent how we can change the configuration in a few minutes. This is the charm of using the 5x4.75 bolt pattern wheel as the basic element.

The first photo on this page shows the engine with its stock McCauley prop. Above we see the engine fitted with our primary test prop, a 2-blade 60" Warp Drive. Since we normally use this prop on Corvairs, the blades here are turned around to work as standard rotation pusher blades. This will effectively load the engine for torque testing. The prop is ground adjustable, so we can fully load the engine at any rpm we desire. The carburetor is an overhauled MA3-SPA. The engine has new Slick mags. It is less than 500 SMOH. We first tested the timing at 24 degrees as per the AD, and then tested it at the pre-AD setting of 28. Differential compression showed all over 70. The engine turned its full rated static rpm with the certified McCauley prop, indicating it was a very healthy engine.

Above is an overview of the test rig. We used two different methods to measure the torque output. First, we used a hydraulic cylinder. This cylinder is located just above and in front of the battery. Second, we measured it with a digital scale. The scale is located just out of view, but it is driven by the 8-foot metal beam clamped onto the mount. We had 4 feet of it extending on each side, so that its weight would not affect the scale reading.

Above is a closeup of the hydraulic cylinder. The braided line runs to a remote gauge. You'll notice it's on the opposite side of the stand now. The O-200 has a different rotation than the Corvair, requiring the hydraulic unit to be moved to the other side of the stand. The gauge reading was calibrated by hanging weights on a 4-foot lever arm in 5 pound increments. The needle valve on the output of the cylinder is required because pulses on the line when cranking the starter motor are so fierce, they will damage the gauge. This is true with both the Corvair and the O-200.

In the photo above, you can see the electronic scale sitting off to the side. Pressure is put on the scale by the vertical stick clamped to the steel arm. We're going to refine this and make it a lot cleaner looking shortly, but for these tests, it worked flawlessly and provided repeatable accuracy. If you're wondering how all this stayed together in the prop blast, you're forgetting that the prop is functioning as a pusher. I was only concerned that some of the equipment would be inhaled. Although we got both methods of torque reading to agree, I feel in the future we'll probably use the electronic scale more often because it's subject to fewer variables. The dynamometer is also rigged for simultaneous thrust measurement, so we're going to put the hydraulic unit to that function for simultaneous readings.

Results

After a full day of testing, which included several dozen test runs, we came up with some surprising data. The engine performed substantially below its 100hp rating. I initially suspected that the engine was not performing at peak power, or that the test equipment was flawed. During the testing, we conducted all of the standard mechanics' tests to evaluate the condition of an engine, including differential compression, timing, and fuel flow. All of these showed the engine to be in good condition. The most telling test was that the engine turned its full static rpm with the certified propeller. It would not do this if it were down on power. Keep in mind that we use a digital optical tach to ensure that there is no error in rpm measurement.

We retested and calibrated the hydraulic cylinder system. It showed itself to be accurate. To doublecheck it, we came up with the digital scale system to corroborate the data. They both told the same story. As an A&P mechanic and a big fan of certified engines, I was very reluctant to conclude that the O-200's 100hp rating is probably a "gross" rating, as opposed to a "net" rating. If you're a fan of car engines, you probably know that in the 1960s, many car engines had gross hp ratings. These optimistic numbers had things like the fricitional drag of the engine and the accessories factored out. In the 1970s, net hp ratings became more popular. This is the power output you'd actually see at the prop flange. All of the numbers that we test are net. This is the only type of external measurement we can do. It is also the real world power output of the engine that you are going to use to go flying.

The torque peak of the O-200 occured at 2450rpm. The engine produced 160 foot pounds of torque. If you use the formula Torque x RPM / 5252 = HP, you'll see the engine was producing 74.6hp. We established the torque peak by running the prop at many different pitch settings until we homed in on the peak of 160.

The hp peak of the engine was very close to its rated peak of 2750rpm. We tested numbers slightly higher than this. However, I was reluctant to run the motor in the 3000s because it's above the engine's redline, it's a borrowed engine, and it's a certified piece of equipment which will very likely go back into another certified plane. So it behooves us to operate it accordingly. At this rpm, we measured the torque at 144 foot pounds. Using the formula, you'll see that the engine produced 75.9hp. Again, these are net horsepower numbers.

The temperature outside was 85F, and the RH was 60%. The pressure was almost standard, and we're only a few feet above sea level. A rudimentary calculation to account for the temperature difference above a standard 59F shows that the corrected hp output of the engine is in the neighborhood of 80-81hp. Again, keep in mind we worked all day in an attempt to raise this output. If you're reading this and thinking there's something we've missed, I can understand that. It's difficult to convey the work of three mechanics over a 12-hour period in a few paragraphs and photos, but I can assure you we left no stone unturned in our search. At the end of the day, I largely came to the conclusion that the 100 horsepower rating was a gross rating.

Keep in mind that I've been doing installations on planes for 10 years. In this time, we had numerous comparative studies which showed that the Corvair was a very powerful engine, and in many circumstances, could easily match the O-200. One which stands out in my mind was the break-in run of Mark Langford's Corvair engine at Corvair College #3. He had a pusher prop from an O-200 mounted on his Corvair. The manufacturer of the prop told him on the phone that the Corvair could never turn up the prop to any substantial rpm. When it did, the propmaker was something between impressed and stunned. Even though Mark's engine is a 3100, it was exceeding what the O-200 could do by a good margin. Over the years, a lot of circumstantial data like this makes more sense in light of finding that an O-200 has a far lower net output than previously suspected.

Does this mean that an O-200 is a bad engine? Does this mean that the VSI in every Cessna 150 isn't telling the truth? Of course not. The engine is and remains the standard measuring stick of 100hp engines. They have worked for nearly half a century, and will continue to do so. This said, I can assure you from our dynamometer testing that standard displacement Corvair engines will exceed the O-200's power output handily. This being true takes nothing away from the O-200's status. It just puts numbers on the success we've seen with the Corvair motor over the years.

As a coincidence, a few days after the testing we had a visit from Al Jonic. I worked with Al on the installation of the V-8 in Jim Rahm's Lancair IVP. Al won the EAA's highest honor for engineering, the August Raspet Award, for this work. He's a veteran of thousands of dynamometer runs. Although he's used much more sophisticated equipment, he was duly impressed with our setup and approach. He offered to send us sophisticated programs to use to correct the conditions for standard day performance. He also offered 40 years of insight on the value of dynamometer runs, correction factors, and gross vs. net ratings.

This dynamometer testing is an ongoing business. I didn't build it to run it a few times, and prove a few points. I regard it much more as an everyday tool. It takes all the talk out of engine building, and replaces it with hard testing. It is the perfect complement to our ability to rapidly flight test any modification. We're currently running an entire series of Corvair engine tests. Most of these will be done by Corvair College #8. The Corvair is already exceeding the power output of the O-200. We're just working to define by how much. When we have this data complete, we'll put it all on the Web page here.

Thank you.

William Wynne