MVVS 21 Diesel Review

By Dave Ridgway
(Model Flyer September 2000) Deep Heat

It may seem rather strange that these diesel conversions originate from the USA – the "home" of the glow plug ignition engine. Apart from a few isolated examples the vast majority of American engines have been glow plug ignition. In the UK and the rest of Europe the diesel was the most popular power unit, both for free flight and control line, with a few exceptions these were generally of capacities under .21’s. In the late sixties/early seventies with the rapid rise in popularity of radio control flying the need for larger capacity throttled engines grew and without exception these were glow plug units. In more recent times PAW and Irvine have produced larger capacity diesel engines, they have never achieved popularity with R/C modelers in general. However, Davis Diesel Developments have been producing conversion heads with success in the US for a very long time. Over the years the range has expanded to include many popular engines now, thanks to Just Engines, they are available in the UK. Unlike a conventional diesel engines the contra-piston is in the replacement cylinder head, rather than the cylinder bore. The contra-piston is sealed with what the makers say is a "space age" material and advises against removing the contra-piston; as special tools are required to replace it. The adjustment of the compression is by a socket head screw, this is prevented from moving by a small headless Allen drag screw pressing on the internal Helicoil thread margin built into the head. This tightening of the tread margin holds the compression adjusting screw from moving until adjusted. The amount of friction (drag) applied is easily adjusted by tightening this small drag screw. The heads are a direct replacement for the standard glow head and as can be seen in the photographs are machined from aluminum alloy bar stock to a very high standard.

Paul Landels, of Just Engines, supplied an MVVS 3.5 (21) Standard R/C glow plug engine along with a Davis Diesel head for evaluation.

The instruction sheet supplied with the Davis head advises fully running in the engine as a glow before converting it to diesel operation. This gave me an opportunity to get some performance figures. As expected, the engine performed extremely well, running very smoothly and starting by hand was usually first time-hot or cold. After an hour or so running in, the MVVS felt nice and free with a perfect compression seal from the ABC piston/liner assembly. The conversion to diesel takes but a few minutes to change the head over. The standard head gasket is carefully removed from the original head and transferred to the diesel head. The glow needle settings are retained as a starting point for diesel operation load stays similar the compression screw can be left alone once set, the engine warming up after about a minute to full speed. This is by far the best throttling diesel I have witnessed. The special seals used for the contra-piston obviously work well, as does the compression adjustment work smoothly – none of the sticking contra – pistons encountered on many conventional diesel set ups.

The Davis conversion of the excellent MVVS works very well indeed, it could be used by the most inept "Diesel Dummy!" Starting is easy the engine being very docile with no tendency to bite at all. If you fancy throwing away all the paraphernalia associated with glow motors and going flying with just a can of fuel, or are simply just a diesel nut, try one of these excellent conversions.

Prop.	Irvine 20 5mm carb	Irvine 20 6.5mm carb	MVVS 21 6.5mm carb
11x6	6300	6800	8000
10x6	7400	8400	9600
9x6	8000	11000	11500
8x6			13000