At 01:06 AM 9/27/2009 +0000, Dave Godwin wrote:
>>"There are 5 of us MG Nuts planning to drive from NAC, Nanjing, China to MG Motors, Longbridge, UK starting out next April".

Woo-hoo! New MG land to old MG land.

>>"The route will take us over numerous mountain passes just over 3000m high. We all have 5 bearing 1800 MGB motors - as close to standard compression as possible. Have you had experience or do you have knowledge of high altitude driving",

Yup. I've been motoring around the USA Continental Divide in the Rocky Mountains considerably in the 6,000 to 9,000 ft altitude range. Also had my MGA up Pikes Peak a couple of times just over 14,000 ft.

>>"or is 3500m not high enough to worry"?

That's high enough to have significant affect on the MGs with about 1/3 less air than at sea level. Pikes Peak is high enough to cut air density in half, and you can feel light headed with any exercise at all

>>"What can we expect and what precautions should we take"?

Getting into these altitudes results in low density air. It's like having an engine 1/3 to 1/2 smaller and very low compression. The SU carburetors will also run rich. If this seems to be a problem you can lean them out as much as two full turns as required to get the mixture right to run smooth. Otherwise just catch a lower gear and keep your foot in it going up hills, and you will get there. As you get down to lower altitude you can rich up the mixture again.

Low density air also makes for less efficient radiator, so if you were pushing it hard up long hills you might find the coolant temperature rising some. Coolant also boils at slightly lower temperature at high altitude. The oil pressure gauge may read about 5 PSI higher than normal, and water temperature gauge may also read a little bit higher (about 10dF or 5dC) as both of these units are similar pressure gauges. The absolute pressure sources are independent of altitude, while the atmospheric pressure reference drops with altitude. Pressure differential being greater will drive the indicator needle a little higher.

Using 50/50 glycol and water coolant and a 7 PSI pressure cap you can still push it to 230dF (110dC) without boiling. High altitude usually goes hand in hand with cooler ambient temperature, something like 4dF drop for every 1000 ft gain in altitude. So high altitude driving is usually no problem, just running a little rich with lower engine torque.

>>"We expect to encounter low octane fuel along the way. We are not sure which countries nor what octane to expect. Would you have any advice in this area - we don't want to fit low compression pistons or double head gaskets unless it is seriously necessary"?

If the engine starts pinking enough to irritate you, just retard the spark timing a bit. Low octane fuel is more tolerable at high altitude where low atmospheric pressure effectively reduces combustion pressure (similar to very low compression engine).

The kicker is steep hills near sea level with low octane fuel. I ran into a little of that in 1989 along the west coast of the Baja Peninsula in Mexico (Tijuana to Ensenada). We intentionally mixed half a tank of the Mexican low octane stuff with half a tank of good California fuel. It ran okay and didn't ping much, but it did like to diesel on after ignition shut off. Easy cure for that is 3rd gear and clutch up to kill the rotations, which comes natural after the first couple of occurrences. I suppose the 10% alcohol fuel we have today is just as bad in that respect.

No special engine preparation required. I don't think you have anything to worry about from either high altitude or low octane fuel with a near stock MG engine. I've been running the lowest grade of pump fuel I can find for decades, commonly 87 RM/2 octane rating. If you run into a significant pinking situation with your engine one of the first things to go would be the ceramic insulator in the tip of the spark plug, but you will likely be carrying an spare set of plugs anyway.

At 07:07 AM 9/27/2009 +0000, Dave Godwin wrote:
>>"In summary, then, we need not fear - just lean out the mixture at altitude and use the gears to avoid overloading the motor"?

That's about it. Don't worry about problems you don't have. Just drive it.