Engine Review – Norvel AX-40

by Brian Hampton

I’ve owned a Norvel A15BB (control line version) for some time now and I’ve been quite impressed by it and it’s rather unusual construction method so when a brand-new-in-box 40 came up on my favourite supermarket (Ebay) with a “Buy it Now” price I grabbed it.

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.

The next thing on the “nice to see” is the prop mounting method. No bent crankshafts here because a removable stud screws into the crankshaft itself. But it’s a double ended thread with a machined centre section to properly centre the propellor. The machined section is 1/4″ (actually 6.25mm or about .004″ under 1/4″) and the  6mm threaded ends have different lengths to accomodate props with thick or thin hubs. The nice bits don’t stop there because the nut has a captive floating washer. The prop driver is a bit unusual in that it’s got a sand blasted finish instead of the more normal machined finish because it appears to be a casting incorporating the D drive to mate with the flat on the crankshaft.

One of the things I’ve begun doing is checking the pinch on a new engine before it’s run just to get some idea of any change that might take place after running in. Even after oiling with castor I’ll never slowly turn an ABx (or in this case, AAO) engine through the pinch so my method is to fit a degree wheel, turn the engine by the crankshaft until I feel the beginning of the pinch and take a reading. Then turn it backwards away from the pinch until it pinches again and take another reading. The difference in the readings is the total pinch and half that reading is the beginning of the pinch from TDC. In this case the pinch covered a total of 46 degrees so it was beginning at 23 degrees BTDC. Later when I had the head off I measured the pinch as starting 1mm before TDC. It seems surprising that the last 23 degrees only moves the piston 1mm but that’s all because of the rod angles.

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.

The cylinder head arrangement on the 40 is more complex than on the 15 which uses a one piece screw in head which is tightened by a C spanner (a Cox spanner fits perfectly). The 40 though uses a head button which is clamped down by a seperate screw in ring. This ring has two ways to tighten/loosen it. The first needs a special spanner to fit the 3 holes but this wasn’t included with the engine. Then there are 3x 4mm screw holes in the clamp ring which apparently are designed to give extra locking (and clamping) to the head button. The photo shows the method I used to turn the clamp ring. OK, so two screws and a stick would have done the same thing :).

Here are the two pieces looking from the top. The head button takes a full long reach plug, medium length like OS and Enya plugs are slightly recessed.

And now from underneath. The squish band is flat with a conical combustion chamber, easy to machine up a new one.

This is how they look when fitted together.

While the head was off I fitted my trusty degree wheel again to check the timings as follows.
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

Other measurements:
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 :).