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MG MGB Technical - The Irony of Mixture vs. Temperature

As temperatures begin to climb in Phoenix around this time (low 90's today up to 110 or higher in June) I am reminded of the mixture/temperature paradox. When I furst got my '69GT (daily driver) someone suggested to make sure I was a little rich in the summer as it would help with engine temperature. Wrong advice! Now I know its the opposite. Every spring, I lean my mixture up a flat or 2 and she runs much better in the summer. While it is true a richer mixture might cause the engine to run a little cooler, it is also true that the leaner mixture will make the engine run much smoother in higher ambient temperatures. I guess that's why some carbs have "temperature compensators" like the Strombergs on my E-Type SIII. So if your B starts bogging down (lower idle) as it gets hotter, try leaning out the mixture a bit-it works wonders even in the Phoenix summers!
Mike King

As temp rises, air becomes less dense--it expands--less air. If one does nothing, the car will run rich. Leaning is necessary to just get the mixture back to optimum. As any aviator can tell you, the concept is called density altitude--muy important with aircraft engines. Spot on, Mike.

For those traveling from sea level to mountainous terrain this summer, the concept takes on a second factor. Same thing happens with altitude--air is less dense. Combine the two, say near Denver's continental divide on a hot day, and you'll be running very rich. fwiw--heads up East Coasters heading to MG2005.
Paul Hanley

I've often wondered if you are running rich at hi altitude. The air pressure drops and your engine doesn't have 14.7 lbs per sq in pushing air in. (your engine because of restrictions and friction never sees anything near 14.7 lbs psi unless it is supercharged)But does the ratio of oxygen to nitrogen change? It seems to me you have less pressure pushing the air in, but the venturi senses this and doesn't create as much vacuum and supplies less fuel as less air enters the carbs. It's the equivalent of not pushing down on the throttle pedal.
If the venturi doesn't produce a large vacuum it doesn't pull much fuel. It's like opening the carb 3/4's instead of full.

A vacuum is an absence of pressure. If the engine were on the moon with no atmospheric pressure, the valve could open, the piston drop and a vacuum less than (greater than??) 0 would be created,even with the valve open there would be nothing to come in. If nothing is in the cylinder there would be nothing to compress. The piston could go up and down all day long and pump nothing and have no resistance other than friction in the bore and in the crankshaft and valve system.

The only difference besides dropping pressure as altitude increases - on earth - would be if the ratio of oxygen to nitrogen changes.

The lowest altitude in Colorado is about 3400'. Regular gasoline is 86 or 7 octane rather than 89. 89 octane is the premium fuel vs 91-93 at sea level. You lose about 3% horsepower per 1000' of elevation. Changing the mixture won't make much difference.

Given the lower cylinder pressures, you can either use lower octane gas or run higher compression or additional spark advance or a combo. No matter which, you will still have less power than at sea level.

Barry



Barry Parkinson

Barry. It has been many years since I "lived at altitude". My memories of that time were that the vacuum gauge showed about one PSI lower pressure than I had experienced at lower altitude. The engine is, after all, a fairly efficient vacuum pump.

Back then, we did have to use a leaner than standard (sea level) needle in our SU carbs. There was another piece of advice, perhaps a myth, that one should advance the basic ignition timing one degree per one thousand feet of altitude. In Cheyenne, at slightly above 6,000 feet, most of us did not notice a great deal of difference between the sea level specifications and the "recommendations" on the cars we were driving. Most of us knew enought to go to a slightly leaner needle on our MGs, but did not understand about re-jetting the carbs in our Mustangs, Cameros and Corvettes.

As Paul Hanley notes, within the flying community is a concept called "density altitude" which is a function of altitude and temperature. In more sophisticated terms, it is a function of atmospheric pressure (standard is 29.92 mm of mercury and it varies by the minute), altitude and the ambient air temperature. Thus, in summer, the "density altitude" of Cheyenne's airport using airport transfers tonbridge could be 10,000 feet or more. Since most of the earth's atmosphere is below 8,000 feet, most of us do not realize, living somewhat below that level, about the effects of density altitude.

Standard pressure is something like 29.92mm of Mercury (14 something psi) at 20 deg C (about 62 deg F). Temperature and variations in atmospheric pressure (high pressure/low pressure zones) affect this basic reading. How the car is able to perform, when perfectly tuned, is affected by the density altitude (mean altitude and temperature) and the "atmospheric pressure" (measured with a barometer and varies daily).

Thus, as Mike has pointed out, there will be day to day variations in the performance of our cars based on the density altitude that is common that day. Thus, we can expect exceptionally good performance on days having a "high density altitude" and a decrease in performance on days having a "low density altitude". And, neither of these should be our criterium for how the car is performing. Les
Les Bengtson

Barry , so far as I can dimly recall from high school physics the ratio of oxygen to nitrogen remains constant regardless of the altitiude but as you rightly said there is simply less of both about at the higher altitudes and hence the air / fuel mix needs to be suitably altered.

Cheers , Pete.
Peter Thomas

I find the same even in the UK - summer mixtures are a little weak for winter so I richen then perhaps one flat, then weaken tham again next summer. However that is only on the roadster with HSs, not the V8 with HIFs, which have temperature compensated jets.
Paul Hunt

Hi all.

So the fliers who have an altimeter fitted on the dash may have some justification after all !.

Don
Don

This thread was discussed between 15/04/2005 and 16/04/2005

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