by Brian Hampton
Norvel’s are made in Russia by ex-aerospace engineers who, as the story goes, when their aero-space industry took a dive, were asked to come up with ideas on how to make a profit from their expertise and the equipment available. One of the ideas was making model engines. Hello Norvel! Not content with just copying the normal type of engine, they developed what is variously known as ceramic, anodising or AAO for the liner. AAO stands for Aluminium piston with Aluminium Oxide coated aluminium liner. Al Oxide is what sapphires are made from and although not quite as hard as diamond it’s harder than chrome and doesn’t have the toxic problems chrome has. So it’s a good thing and I believe will become common practise in all engines.
But to me, the way Norvel have used it by making the liner one piece with the cooling fins, much like the old steel finned OS Max’s from way back, is perfect. This gives a direct heat path from the interior surface to the fins. A normal slip in liner has a heat barrier between the liner and the crankcase which slows down heat transference so the Norvel set-up should run somewhat cooler and react faster to an over lean condition. Whatever, I like the idea.
So, open the box and have a read of the instructions like we all do when we get a new engine. Yeah, right! Throw them aside for now and take out the engine for a look. Construction-wise it’s near identical to the 15 but a little heavier than I expected at 306 grams (10.8 ounces) without muffler and 376 grams (13.26 ounces) with muffler. Actually for an RC engine this is quite good and puts it close to an OS LA40.
Back to the instructions and as usual they’re a bit vague, especially on the running in side as far as correct mixture goes. However the instructions appear to have been written by the American importer so………………………………..
OK, now for the engine itself.
The carb is fitted with the needle valve turned right back for protection in transit but uses the usual pinch bolt. Loosening the pinch bolt and straightening the carb still leaves the needle valve angled back (and slightly upward) by about 15 degrees. Angling it a little more forward would give some safety margin to adjust the idle mix if you’re that way inclined. The carb barrel is aluminium but with the same hard ceramic coating as the liner so wear won’t be an issue. One feature I really liked is that the main needle has a notch machined in the end as a marker to accurately count the number of turns it’s opened. That saves me making a file mark as I normally do. Factory “setting” was 5/8 turns open so I’ve opened it to 2.5 as a first try before running it. The venturi size is 6mm so it’s not excessively large like so many engines are today. This means it should be far better suited to the revs normally flown at and give good fuel draw without relying so much on muffler pressure to push the fuel through. The carb body has been nicely machined from solid.
Looking through the exhaust port gave a surprise because two boost ports can be seen. This is a 5 port engine and both boost ports have their own individual transfer channels running up from the crankcase. The passages are quite small and my first impression was that someone (but not me) will be getting out the Dremel. Unfortunately I wasn’t able to get the cylinder out for photos because it seemed to be rather firmly attached to its gasket and I didn’t want to go tearing up the gasket but this shows the 4 inlet ports. The ports all have rounded top openings so they won’t open quite as abruptly as the usual square topped ports would do. Notice the screw thread above the 0.2mm shim, more on that shortly.
Exhaust opens 77 degrees BBDC (154 degrees total for those who like to say it that way)
Transfers open 60 BBDC
Boost ports open 53 BBDC
Inlet opens 40 ABDC
Inlet closes 44 ATDC
Total squish clearance with .2mm shim….0.45mm (.018″)
Measured volume of the combustion chamber by itself was .487cc
Compression ratio with shim 11.1:1
Compression without shim 12.3:1 (giving squish clearance of .25mm or .010″)
Either of these compressions make it ideal to use zero nitro fuel which is what I use anyway :).