mike109

G'day Is diesel better? I would say it is different. It has some advantages over glow and it has some downsides.

Most of my engines now are four strokes. Like you I got sick of finicky, noisy glow two strokes and I find four strokes easier to live with - particularly the smaller Saitos. I have them inverted, sideways and upright and they all run well so long as the tank is in the right place and the engine has been run in. Initially they don't idle all that well and so I run them in in a trainer to get them running right before I use them in anything important.

I have a couple of diesel powered planes at the moment. One is powered by a PAW 1.5cc engine (a half size Dalaire Sportster oldtimer) and the other is powered by a Magnum 46 converted to diesel but with a carby from a GMS 32. The original carby was simply too large for operation as a diesel. The smaller carby tunes well although I probably loose a little top end performance but it has plenty anyway.

The diesels use fuel at about half the rate that a similar glow engine does but the fuel costs more. The big advantage is that once a diesel is running, it will rarely stop until it is out of fuel. When I was racing diesels years ago I used to curse them for not stopping when I had got the tune totally wrong. They just plodded on until the fuel ran out. The downside with fuel is it is more expensive and harder to get although in the US you can buy it far easier than I can here in Australia.

If you want to try diesels I would suggest you start with a smaller engine before trying to get your ST90 to run as a diesel. A small PAW or an OS 40 or 46 LA with a Davis head would be good starting points. The LA engines seem to make very good diesels. A bloke turns up at our Diesel Day every year with his LA46 conversion and it works really well. Then I would put it in a plane that does not matter too much and do some experimenting. If you like what you find, then go ahead with the big ST. Hobbsy seems very happy with his but he has a lot of diesel experience. A chat with Mr Davis would be a good idea too. I hear he is very helpful.

But if you simply want to spend less on fuel, your ST 90 will probably run quite happily and reliably on less nitro than you are using. Most European motors (the ST was designed there even if it was made in China) will run will very little nitro and low nitro fuel is much less expensive than high nitro stuff. Moki and Webra engines need little or no nitro. I have been making fuel here in Oz lately and FAI fuel (no nitro) costs me about AU$15 per four litres (about a gallon) where as 15% nitro costs about AU$27 or almost twice as much. But we have found that the FAI fuel lasts almost twice as long as 10% nitro fuel which makes it extremely cheap to use. 5% lasts almost as long and makes the engines far easier to tune.

So ... after long winded rant.

I would suggest that if you just want to save money, then using less nitro will probably save you heaps without any great loss of performance. If you want to try diesels, then they do have some real advantages - no glow plugs, rare flame outs, great torque, lower fuel use but the down sides are higher initial cost, higher fuel cost/gallon and a learning curve. If you still want to have a go (and I hope you do), then a smaller cheaper engine and a cheap plane would be a good place to start but your ST90 will run well as a diesel with a Davis head if you just want to jump straight into the deep end of the pool. If you do this, I still would suggest a cheap plane like a large trainer to test things out on.

The photo is my diesel converted Magnum 46 SL in a Great Planes Big Stick 40 with Bolly 11.5 x 5 prop. Even badly tuned it hauls the stick round really well and never looks like quitting.

All the best

Michael from Oz