Fuel Injection For Your C180 (Part 2)

Last new letter I discussed the installation of an IO-520D into a C180 that included the fuel injection system. In this article I’ll continue with a review of the performance and operational changes brought on by the modification. Unlike many members who have changed airplanes from a C180 to a C185 or vise-a-versa and the performance differences are noted about different airplanes this comparison is about the same airplane i.e. before and after the modification. The airframe is a 1966 H model long-range fuel and a float kit with all of the parasite drag devices found on these airframes. The following describes my airplane’s changes; the before was a 230 HP at 2600 RPM O-470R, an 88” two blade prop and an empty weight of 1790lbs. The after is a 300HP at 2850 RPM IO-520D and an 88” three blade prop weighing 1830lbs. empty.
Many of you have airplanes that will out perform these figures. This is simply what I observed on this airframe from this modification;
The first and most obvious change is from out side the airplane and is the appearance of the three-blade prop. Call it anything you want but it just looks different. I have always been a fan of two blade propellers and I had hoped to reuse my old two-blade prop on this conversion. When the installation began I approached the local FSDO with the two-blade request. My request was referred to engineering. This is FAA jargon for a rejection. The STC called for three-blade props only and any two-blade units would require noise test. Rather than fight it I caved in and bought Herb Chamber’s of CS Industries three-blade 88” McCauley C401 series prop and STC. I have never been outside of my airplane and watched it take-off but those that have tell me this combination is very loud. How Herb got it STC’d I don’t know but I doubt that it would pass any current noise standards. I like airplane noise but I know many others do not so I try to make some take-offs at 2500 rpm epically in noise sensitive areas. I see little if any takeoff distance changes using the lower rpm. I did the math and while this prop is turning 2850 rpm and at 100 mph as in the take off mode, the tips are supersonic. The actual tip speed is about 753mph, where mach 1 at 32 degrees F is 741mph. What this means in terms of performance I don’t know but I’ll say this, “The take-off and climb changes are stunning! Push the throttle in and you better be ready to fly it”. Pull test have shown the long three blades to beat the two blades in static (0 mph) test. At cruse speeds the differences are much smaller. In fact some of the fastest airplanes are two-bladders!
The combination is clearly much stronger in the take-off and climb phases. As an example there is a hill with a saddle in it off the north end of Johnson Creek, ID. My stock C180 would never get close to clearing the saddle after take-off. A turn to the left to over fly Yellow Pine and out the drainage was always required. With the new combination I can clear the saddle and the ridge next to it. The beast will maintain 1000 ft/min through 10,000 feet at 100 mph. I started the clock at the beginning of the take-off role from a 170 feet msl airport last week. Three minutes and 40 seconds latter I was at 5,000 feet, that’s at least one minute better than with the 230 HP engine from sea level.





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This propeller works like a speed brake also. At speeds over 100 kts a reduction of power to less than 15” puts the prop to low pitch as the aircraft drives the prop. This produces a braking action and as the aircraft slows down I can feel the pressure of the shoulder harness holding me back. Increase the power to 17” and the prop starts pulling again and I’m set back into the seat. All constant speed propellers do this but this new three blade is much more pronounced than the old two blade. I now have become a fan of this three-blade propeller.
There is also some true airspeed increase. I now see an average of 145 kts on the GPS where with the 0-470 I was seeing 135 to 140 kts. Somewhere I read that all things being the same, speed increases by the cube root of horsepower increase. For example 8 more HP will increase your speed about 2 kts, an increase of 27 HP increases the speed maybe 3 kts, etc. With this 70 HP increase, a speed increase of 4 kts is probably all that is expected. Any speed increase from horsepower increase comes with a higher fuel burn. At 23 squared the O-470 would run at 135 to 138 kts burning 11.8 to 12.2 gallons per hour. With the IO-520 at the same power setting it burns 13.4 to 14.0 gallons per hour and now does about 143 kts. At 5,500 feet and 25 squared burning 15 gallons I see a true airspeed of 150 kts where with the O470 it was more like 140 kts at “high cruse”. This is a very rough comparison as I’m comparing the before and after with both an engine and propeller change as well as different engine instrumentation.
The induction system appears to be more efficient. At 10,000 feet and full throttle before this conversion the best I would see was 19” of manifold pressure. With this conversion I now see 20.5 “of MP. My guess is there is some additional ram air available or maybe it’s just a different MP gauge or the tap point on the intake system is different. Don’t know but I appear to have more MP available at altitude.
This engine runs hotter than the original one did. The IO-520D has 8.5:1 compression ratio vs. 7.0:1 with the O-470R. Higher compression engines run hotter and with a six probe CHT system to monitor things in cruse I have to run with 1 notch of cowl flaps to see 380 degrees or lower. In climb I use all available cowl flap, higher airspeed and lots of gas to keep temps less than 400 degrees.
There are the usual hot start problems associated with fuel injection. I’m learning techniques to deal with this. There is no question however a carbureted big bore Continental starts when hot easier than ones with fuel injection. Other than the starting issues and perhaps the mixture being more sensitive the operation is the same. I of course have to run 100-octane fuel now where as before it would run on mo-gas or better. But then 100-octane fuel is all that I’m seeing available where I fly. I have consistent EGT readings through all altitudes now and I’m free from carburetor ice.
I would do it again. In general I like the conversion and the performance improvements. I have more utility available. The airplane is different now and as a comment, Canadian club member and friend Steve Cunningham says it best, “It is more difficult to fly a stock C180 with the lower horse power than one of these with 300 HP. Higher horse power makes any pilot look good”. Fly safe. Al Hewitt