Saturday, April 05, 2008


Heater, carb heat and pressure sensors.

(As always, double click the pictures if you want more detail.)

Heater - I decided a long while ago to take the cabin heat off the oil cooler rather than an exhaust muff. I am in the UK which is a temperate climate, so I do not need Alaskan quantities of heat, to keep warm. (As I write this it is snowing!) Those that do it this way usually report plenty of heat, though there are plenty of folk out there who will argue it is a really bad idea. To me the pluses are a much reduced problem of carbon monoxide, fewer parts/less weight, and those that do it this way report a much nicer, less 'scorched ', quality of air in the cabin. What is the downside? Very little as far as I can see. Worst case I can always buy an exhaust muff and hook it up. I had to make up a shroud to gather the air after it has been through the cooler which exercised my fiberglass skills. It's not pretty but it is reasonable on the inside, where it matters. People will reasonably comment that the efficiency of the cooler will drop dramatically. That is certainly true, but while I don't live in Alaska I don't live in Arizona either. My Supercub with an O-320 and the same cooler struggles to keep warm, and that only travels at 100mph. For break in, I will disconnect the cabin heat system, and seal it up, and remove the collector, since clearly at that stage the engine will be generating more than normal heat. If all else fails I will click the switch which turns on the Mercedes Benz seat heater that I will be sitting on.

Carb Heat - This is now all connected up. Its a tight fit in a -4.

As each additional system is installed, despite my utmost efforts to think ahead and plan what else has to be added, the systems maximise their efforts to claim the same space. A good example was the screw on the heat box in the picture above. Its nut was claiming the same space as the tail of the bolt on the gear leg. If I had known this when I was putting the gear leg on I could have packed the head out with two more washers. I contemplated pulling it out in situ, but ended up by filing some of the tail off. To bolt the heat valve on, 8 #6 nuts and bolts still took 4 hours!

Pressure Sensors - FP, OP, MP have all found a home in front of the battery on top of the step. They are accessible if they ever need replacement and the hose runs are quite reasonable. I took advice from VANS as to which hoses to buy for a -4 and they certainly mostly erred on the long side. Better than short I guess.

There has been a little discussion about the oil tube/ dipstick for -4 on Vansairforce recently. Many people end up with one that is too long. Aerosport Power knew of the issue. This is what they supplied to me. If you cross their palm with silver I am sure they will sell you one.

Postscript - 30/7/08 - Carb Heat

It turns out that the carb heat muff works well as you can see from the graph below.

After an initial 2000'climb from 300'ASL, the engine settles down at about 23 square, and the engine leaned. The OAT is around 58F, and the carb temperature due to the venturi and vapourisation of the fuel, running at about 51F. On pulling full carb heat the temperature rises to around 63F. It looks as though if I had left it on a little longer, I would have had a slightly higher gain since the gradient of the curve is not yet flat. I do this 3 times. At around 13:05, I reduce power and again pull the carb heat. This results in a much larger gain, which is in fact when you really want it. This time with the engine developing little power for the descent, it goes more than 15F above the ambient temperature.

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