I will be driving my 1974 MGB to Breckenridge, CO in a few weeks. Does it make any sense to change to higher heat range spark plugs at higher altitudes (6,000’+) to prevent plug fouling?
It might be advisable to advance the timing until it is just short of detonation and/or kicking back on the starter. If course, that’s just the proper way to time any engine capable of being timed.
Is it better to use lower octane fuel or higher? I normally run on 89 octane.
You don’t need to change the plugs, and you will probably be able to get by with lower octane fuel (in fact you will only be able to get 85, 87 and 91 octane in most of CO). Here the midgrade is 87 and regular is 85 due to the altitude.
Why, by the way, are you using 89 octane anyway? Is this a high compression engine?
Not sure on the compression ratio, the engine was rebuilt by a previous owner. It pings on the 87 but is happy with the 89, so the compression must be more than 8.5:1. It has the original distributor and the advance is set to factory spec.
When you get there, you can bump the timing up 5 degrees over specs. The SU CV type carbs do a good job of compensating for altitude and the stock plugs should be fine.
As I recall, Breckenridge is way up in the air. The principle problem with 1960 cars at high altitude used to be getting them to start when cold. I think that 1970s and later cars are much better. A place I worked at back in the 1960s used to do an annual camping weekend at Mammoth Lakes, CA. Elevation around 8000 feet. It often took a couple of hours to get everyone’s sea-level configured car running come Sunday morning.
You might want to throw a spray can of hardware store starter fluid in your trunk. You might need it.
You might also make sure that your air cleaner hold down hardware is not rusted and that you have a screwdriver along. If you manage to flood the engine – easy to do with a 1960s engine at 10000 feet, you may need to remove the air cleaner and wedge the choke open with a screwdriver in order to get the car started.
If you do manage to immobilize the vehicle, any local guy with some gray hairs will probably be able to help you.