I am considering buying a used Davis head for ST.40-.51's. Does the "kit" only include the head? How do these engines cold start? I have considerable knowledge of Diesels but I realize that these conversions must still not compare to larger engines in compression, fuel used, and method of ignition timing. Obviously a ST .40 is not capable of the 22 or 25:1 compression ratios I am used to. By memory I think the minimum I have ever seen in a "full size" application was 18:1. Are the model Diesel fuels of a high enough Cetane to start and run by "low compression" alone? If I did make a successful conversion of a ST .40 what kind of performance should I expect at about 4500' above sea level?

Thanks in advance. Take care, Cliff

Model diesels are not really diesel engines, but rather compression detonation engines. Model diesel fuel runs aroud 30-45% ether. Compression ratios on 40 size diesels are about the same as 40-size glow engines. In fact a 40-size glow engine will run fine on diesel fuel once it warms up. I would think an ST 40 dieselized would swing an 12 x 6 prop better than 10,000 RPM.

Jim

Cliff, one of the great things about the Davis heads is that normally you don't have to change the compression setting unless there is a huge change in ambien temp. My ST .51 conversion turns a 12x6 at 10,500. The Diesels are most effecient if you prop them for 8,000 to 8,500 rpm. Your .40 should turn a 12x6 three blade in that range.

my st40 if memory serves me 8900 -9000 12x6 wood zinger apc plastic props
will turn a little faster at least 300rpm. also use an electric starter when warm
will hand start.any overcompression on a hand start is a finger wack.