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I am thinking about a tour to switzerland this summer with my 69 L46 convertible. This means I will drive some mountain roads up to 7500 ft. altitude. Is there anyting I should observe doing this? Maybe changing the ignition timing or something related to the carb?
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Re: high altitude driving
I used to cross the Alps 2-4 times a year going from Austria to Italy return. For the few hours you will spend at the higher altitudes, it's not worth spending time rejetting the carb. You will notice remarkable power loss, but so will everyone else in a carbureted car. Slow down and enjoy the view!. -
Re: high altitude driving
Unless you're going to spend a long time in the mountains, it's not worth rejetting. Since carburetors meter on the basis of volume flow (not mass flow like modern engines) the mixture gets rich at altitute. Volume flow remains the same, but the loss of air density at altitude is what causes the rich condition.
Another thing you will notice is that the idle speed will drop - enough to possibly cause significant idle instability, especially with a SHP engine. You can merely adjust the idle speed up if you're going to be in the mountains for several days, and then readjust it back down when you arrive home. You might just want to increase the idle speed about 200 RPM at a convenient time before you begin the climb.
All engines - even turbos - lose power at altitude. Modern engine management systems compensate for ambient air denstiy loss and maintain a proper air fuel ratio for all operating conditions, but our old carbuerted dinosaurs just run rich at altitude. So does the Rochester FI, because it also meters on the basis of volume flow, but the FI is easy to lean out by just tweaking the economy and power stops.
DukeComment
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Re: high altitude driving
a trick to use if you have AFB carbs is to remove the spring under the power piston so the metering rod will stay on the lean (larger) diameter part of the rod. we used this when we were drag racing at higher alt. and did not have any extra jets with us.Comment
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That low???
Living at 8930 feet and periodically climbing to what we call the 'high country' outside Denver (many places you can drive to in the 10,000-11,000 ft range), we live with this all the time in our Corvettes. There's good and bad associated with it....
First, these cars were designed to go cross country (US) without re-adjust. So, as Duke says, unless you're going to STAY at altitude or do something SERIOUS at altitude (E.g. race/compete), do nothing. You'll run a bit rich and suffer the ususal HP loss due to O2 density change, but nothing 'bad' happens.
If you plan on making a stay for some period of time at altitude, then you can compensate by changing jets, re-adjusting carb idle mixture, tweaking idle RPM and jacking your ignition timing. You can get away with advance here that's virtually guaranteed to 'grenade' your engine at sea level! You can almost burn any low octane pump gas regardless of engine CR without pinging....
Have fun, ENJOY! Nothing like an open air HP car winding along 2-lane mountain roadways. Especially at dawn/dusk....Comment
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Re: high altitude driving
When I have taken my dual carb '61 to the mountains, I end up having to do as Duke suggested and set the idle faster so it won't die. It also pays to have a spare set of spark plugs handy in case you foul out the plugs due to running rich or in the case of the dual carbs, running extremely rich.Comment
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Re: high altitude driving
If you are using DOT-5 brake fluid and have not sufficiently bled the system, yoy could experience brake loss, as the air trapped in the system expands. Since air is compressable you can lose all pedal.Comment
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Re: high altitude driving
Mark,
I've done most of the Swiss Alpine passes in my '67 427 without rejetting and it does just fine. Wouldn't bother if I were you.
Overheatings likely to be a bigger problem...
BillComment
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