Moving from Central California (altitude 350') to N/W Arizona (altitude 3400') I find that my 462" powered roadster - with mild cam - had the idle vacuum go from 18.5" to `5.5 - 16." as noted on the in-car vacuum gauge. No changes were made and the engine ran ok except for a little slower idle which was probably caused by being too rich. Carb at the time was a race-o 750 cfm Carter set up one stage lean according to the Edelbrock tables. I installed a 750 cfm Edelbrock street model carb (original rods, jets etc.) and vacuum levels are about the same although it's not running as rich as the Carter carb did. The engine runs pretty good with this carb, no hesitation, stumbles etc. The engine pulls about 18" of vacuum at 60 mph with both carbs regardless of altitude. (Carters and Edelbrocks are the same carbs from what I can find out. They carry the same rod/jet combo's when new and both come off the Weber assembly line.) So my question is; has anyone else noticed the vacuum level dropping that much when going from low to moderate altitude?
What you are seeing is perfectly normal. I beleive that the rule of thumb is every 1000ft above sea level the average reading on a vac. gauge will be about 1 inch low. You gained about 3000ft of altitude and lost 3" of vacuum. Sounds like the rule of thumb works!
Yes I agree. For normally aspirated aircraft piston engines we measure engine power settings in inchs of manifold pressure. When climbing at a given throttle setting you will lose about 1 inch of manifold pressure per thousand feet of altitude. So say at a setting of 60% of max power of sea level we enter a climb. In order to maintain 60% we will have to keep adding throttle to maintain 60% until we hit an altitude where we have no more throttle to add in which case we start dropping back from 60%.
Thanks guys. (The original post should have said the vacuum went down to 15.5 - 16".) I've gone through some high mountain passes with the car, but never really noticed the vacuum level the few times we stopped. Guess I got spoiled living in the flatlands all those years. Next step, lean er out a bit. :bglasses:
When I lived in Colorado Springs (about 6500 ft), I had to jack my timing way up and lean out the carb way furthur that I thought I had to. The car ran much better too, no detonation and gas mileage improved considerably.
ArmyGuy, how far up did you take the timing? Did it give you problems if you ventured into the lowlands? I'm going to try two steps lean* to start with and see how it works out. Been thinking the engine could use a touch more advance as well, looking at 1-2 degrees because it runs so well where it's at, but I think it could use some more. *According to the Edelbrock tuning site - they have a lot of good info there. Only potential problem could be running down to the hill for some of the river runs at Lake Havasu, California and rod runs at the Laughlin, Nevada casinos. The river's elevation at those locations is about 350 - 400' I believe. Shouldn't be too much of a problem cuz the car only weighs 2400# so it doesn't take much throttle to climb grades. (11-12 miles of partially 6% grades fwiw.)
:TU: C9- I don not remember the exact numbers, but I do recall that initital timing was up near 15 BTDC. And I am running a Holley Street Avenger 770 cfm carb. The carb came with a #72 primary and #75 secondary jets installed and the car ran fine at low altitude. I had to install a #64 primary and a #67 secondary to get the car to act straight at higher altitudes. Now also keep in mind, my car car is not heavily modified...mostly stock 455 internals, headers, small cam, MSD ignition, stock 1970 heads and block. I hope any of this info can help you. There is a rule of thumb regarding how much timing per 1000 ft of altitude, but I can't recall it right now. And the car would have to most likely be re-tuned if you drove down the mountain so to speak. One of the many benefits EFI has over carbs.
