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.
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.