DIESEL VS. GLOW WHO HAS THE BEEF?
By Larry Ranger

Just how good are those Davis Diesel Heads, anyway? This is a question which has nagged me since I first had the opportunity to run one while I worked for Cox Hobbies several years ago. Bob Davis and various writers (including myself) have given qualitative judgments that the Diesel converted engines produce less top end peak power and speed, but more static thrust and climbing ability. The purpose of the investigation presented in this article was to answer the question with some good, hard data.

Test Technique
The engine I chose for testing is the O.S. Max 1OFSR. The conversion head was by Davis Diesel Development

For the glow runs, I used a plug without an idle bar and K & B 1000 fuel (25% nitro) to give me the maximum performance in that configuration. The Diesel runs were done on Davis Diesel 1/2A fuel. To avoid contamination as much as possible, I used separate fuel tanks for the two types of fuel.
Rpms were measured with a factory calibrated Royal ProTach, rated at a precision of better than 5%. All testing was done by hand starting.

Test Results
Now, on to the good stuff! The results of my tests surprised the heck out of me. The Diesel conversion out-ran the glow head on every propeller. That certainly made the basic answer to the performance question easy, but there is even more interesting information to be gained from thorough examination of the data (Figure 2). As you look at the various propellers, you can see that the rpms fall off quicker on the glow head than the Diesel for heavy loads.

This is exactly the sort of performance which Bob Davis has been claiming all along. Incidentally, the fuel consumption of the Diesel was much less than half that of the glow version (7.5 minutes per ounce!), and the fuels used were comparably priced (save money and reduce both the room needed in the front of your model and the C.G. shift at the same time).