Shootout at the Stromberg corral

Builders,

Most Experimental Aviation companies don’t do any more testing than they have to, its pure overhead, and lots of the stuff you see is rigged or slanted to persuade not to actually discover anything.  However, I look at my work in aviation predominantly as a learning and teaching exercise, to share with builders what they need to know to be the master of their creation, not just its owner.

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The next major testing will take place this coming weekend. With the help of other Corvair builders Like Paul Salter, Terry Hand, Jeff Moore and Dan and Tracy Sheridan, I have gathered a 1/2 dozen Stromberg NAS-3 carbs in a number of slightly different configurations to test in a wide range of power setting on a 3,000cc Corvair on my run stand I ‘m shooting to do them all the same day to minimize atmospheric variables. As always, I will just present the data and some thoughts on it, and suggestions to builders.

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Above, five of the six carbs on my workbench. There are small subtile variables between models and configuration. My Pietenpol always flew a Stromberg, and our second test mule the Skycoupe flew one, even when we turbocharged it. in 2004 Gus Warren and myself ran a series of comparative jetting flight tests on the Wagabond at the old hangar in Edgewater, and developed the configuration that is referenced in my conversion manual. We are just looking to expand that knowledge base a bit further.

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Would you like an example of how testing is better than “Hangar tales from local experts”? Try this: its always said that the mixture control on Strombergs isn’t effective, and you should just wire it shut. Here is what testing says: if the carb is assembled with worn parts, the mixture is less effective, but if properly overhauled, the mixture control can run the motor from so rich its smoking (9.5:1 air/fuel) to lean misfire/detonation (16:1) at any rpm above 1500.  It is true the mixture will not function as an idle cut off like a MA3’s will, but the Stromberg’s mixture can be much more effective than 95% of the people in your EAA chapter think.

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Above, my engine stand, with Stromberg #6 mounted. In the foreground is an integral part of the testing, it is a very sophisticated O2 sensor in the exhaust wired to a digital Air Fuel instrument that cost several hundred dollars. It logs the A/F ratio to a tenth of a point. Many pilots like to talk about using cheap one wire O2 sensors and $50 agues, and I have used these myself, and I wasn’t impressed with their accuracy nor reliability, and they have the ugly habit of defaulting to reading “Green” when they are in default.  Although this is also an O2 sensor, this one has a heating circut, run by the instrument. Look at the bonding jumpers which are installed to make sure there are no errors from grounding variations. This is critical, but ofter ignored.

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In the big picture, if your carb is set right, you can fly the plane without an O2 sensor or even an EGT.  If you think about it, Cessna 150s don’t even have a CHT gauge. They just have a specific engine configuration, including the carb and the cowl, and the plane is operated within parameters known to work. Our testing here is to expand the known configurations of Stromberg to support the same concept for Corvairs. Results to follow.

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Wewjr.

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