Does altitude affect the performance of a 1940 car?

[CChinn]

What are the effects, if any of altitude (4400 ft) on the performance of a pre- WWII (1940s) car with carburetor or modified with EFI?  Any significant issues?  Ways to mitigate? Etc.  Thanks in advance for your replies. (This question does not relate to my recently purchased 1938 Caddy).  Asking for a friend who is thinking of purchasing a 40s car and moving it to their summer home in the NC mountains

Edited May 30 by CChinn (see edit history)

[Ben Bruce aka First Born]

CC, a carbureted car will most likely need re tuning [ different jets in the carburetor] for altitude to perform at it's best.  Not so with the EFI.  Pre war- post war makes no difference.

 

  Ben

[Pfeil]

My 76 Olds was jetted lean to meet CA. emissions. When I moved to Prescott AZ @ 5,000 feet it ran much better. My 69 H-O LeMans at 9.8 compression on its 462 needed 91 premium fuel at the beach until I moved to Prescott. Now it runs fine on 87. My two VW beetles' run fine at 5,000, but were jetted for sea level, spark plugs never load up and are gray tipped just like at sea level.

I do know this; at 5,000 feet I can add a lot more compression on my next build (426 cu in Pontiac, 428 Pontiac engine are really 426 and change) without fear of detonation especially with "E" heads.

Edited May 30 by Pfeil (see edit history)

[Bloo]

Thinner air at high altitude causes a richer mixture in many carbureted cars. This applies to all or nearly all prewar gasoline cars. My guess is there isn't anything high enough in NC to cause a problem. Driving though the rocky mountains might be different. Most cars offered "high altitude" jetting for places like Denver. Worst case, get a step leaner jets.

 

With most fuel injection, and some later carburetors small changes are handled automatically based on signals from an oxygen sensor. If the changes in altitude were huge, the computer would need a barometric pressure sensor to compensate for the changes in atmospheric pressure. When i was doing driveability work in the 80s and 90s, most cars did not have one. It didn't really matter because the computer would get and store a barometer reading from the MAP sensor when you turned the key on before you started the engine.

 

In theory, if you piled the car full of a bunch of your friends, and drove straight through from Seattle to Denver without shutting the engine off and restarting, it would probably be running like crap. That is the sort of thing that is nearly impossible to accomplish even if you are trying.

 

Edited May 30 by Bloo (see edit history)

[mechanician]

Carburetor adjustments aside engine performance is proportional to air density, which decreases with altitude. This effect is independent of how you meter the fuel, efi may correct mixture, but it can not offset the reduction in power. This is why high altitude piston engine aircraft were turbo/supercharged, and this is still common on general aviation aircraft in the mountain west.

[Crusty Trucker]

Many, many years ago I drove my factory stock '55 Ford six from the SF Bay Area over Tioga Pass in Yosemite Park (9,945 ft elevation). It stalled near the summit and would not restart. As I recall, we adjusted the idle screws down to a point where it stared and did not stall again. 

Years later, we drove over the Continental Divide in a '72 Ford F-100, 360 V-8, towing a 24'ft 5th wheel trailer at 12,000 ft +, and experienced no ill effects on the vehicle. 

I don't know if that's helpful, but with an older car, overall condition is important. There should be factory information available regarding the adjustments necessary for driving at different altitudes.

 

[7th Son]

1 hour ago, mechanician said:

Carburetor adjustments aside engine performance is proportional to air density, which decreases with altitude.

At higher altitudes are you saying that performance will increase because the air is less dense?

Am I correct in saying that if I took a drag race car from Los Angeles to Denver, with absolutely no changes, it would preform better in Denver because the air is less dense? I just need to clarify this in my own "dense" mind.    Thanks...

[Dave Wells]

Here is an article by the Holly Carburetor Co. discussing high altitude carburetor adjustments they recommend. It may shed even more light on this subject. The information should be generally applicable to most carbureted cars.

 

https://www.holley.com/blog/post/how_to_tune_your_holley_carburetor_for_driving_at_high_altitude/

[Bloo]

12 minutes ago, 7th Son said:

At higher altitudes are you saying that performance will increase because the air is less dense?

Probably the opposite. It gets worse. From a performance standpoint, getting more oxygen in the engine is the hardest part. Adjusting the fuel to match the oxygen you have is much easier by comparison.

 

Once you have put less fuel in because the less dense air at high altitude has less oxygen in it, you have less total air/fuel mixture in your engine (compared to what you would have at sea level) and less performance.

 

Edited May 30 by Bloo (see edit history)

[Lee H]

A rough, but easy way to look at it, is by air pressure. What “pushes” the air/fuel mixture into the cylinders in a normally aspirated engine. At sea level we are all living under 14.7 psi. At 4400 ft, the air pressure is ≈ 12.6 psi. 12.6/14.7 = 86%, and roughly the output of your engine, assuming correct jetting for that altitude.

[mechanician]

Drag force is also proportional to air density, but weight isn't.  In a prewar vehicle you are most likely to feel the reduced power when you are pulling hard, going slow, when air drag is not at its most significant anyway.  Also, the fraction of oxygen in air (21%) is uniform at all altitudes, even up where airliners fly.  It isn't exactly that there is less oxygen, there's less air (ie it's density is lower) so in a given volume of air (but not a given mass of air) there's less available to burn or for our own respiration.  A naturally aspirated engine can at best sweep only its own displacement, so a smaller charge of fuel will burn, less heat will be released by combustion, and power will be reduced.  Our lungs have the same problem, more or less.  Boosting intake pressure will address this problem, you can run a higher boost relative to ambient at higher altitudes (ie to achieve the same absolute intake pressure as intake gauge pressure increases is a result of lower ambient pressure).  Even at sea level, the benefit of supercharging comes from increasing air density.

[TAKerry]

I think it can have an effect at the right altitude but like was mentioned, I dont think anything in NC will make a diff. I drove a carb. motorcycle from Phoenix to the top of the Grand Canyon. At higher altitudes I was getting about half the mileage as I was on flat ground. That is my experience (other than not being able to breath in Breckenridge!).

[carbking]

The key in the question is 4400 feet.

 

In 1940, the major passenger carburetor companies in the USA (Carter, Holley, and Stromberg) specified different calibrations for most of their carburetors.

 

Carter 

   Sea level ~ 4000 feet - standard calibration

   4000 ~ 6000 feet - 1 size lean

   6000 ~ 7000 feet - 2 sizes lean

   above 7000 feet - 3 sizes lean

 

Holley - parameters were sea level to 5000, 5000 to 10000, 10000 to 15000, Stromberg the same as Carter

 

Holley and Stromberg effected leaner calibration by using smaller orifice jets.

Carter effected leaner calibration by using thicker metering rods.

 

Please note these are some fairly wide altitude ranges, and as mentioned in the article, susceptible to the daily weather.

 

So a racer that is attempting to maximize power on a given day at a given track, where horsepower increments may be critical if even 1 percent, the racer is going to calibrate his/her carburetor right before EACH RACE!

 

But for the average enthusiast driving a street car, breaking into a different range for a few miles, is NOT going to be a big deal.

 

So back to my first comment. If 4400 feet is the maximum that vehicle is going to see, find something else to worry about.  

 

But if one is going to live (not pass through) an area where the altitude is around 5000 feet, one might consider a leaner calibration.

 

What is MORE CRITICAL than too rich a calibration is one that is too lean. Take the same 5000 feet altitude from the previous statement. If the vehicle then takes its owner to an ocean beach, the engine is going to be too lean part of the time. Again, passing into a range where one is slightly lean is not going to be a huge problem, but if moving, then the carburetor should be recalibrated.

 

Not an advertisement, but a statement of fact: as we have the capability of fabricating metering rods, we have sold hundreds of metering rods for Carters as early as the early 1930's, and as late as the 1970's, to residents of Colorado. It slightly improves power (due to spark plugs not fouling), but it will improve fuel economy and reduced ignition maintenance. And since Carter used the metering rods for calibration, much easier than either the Strombergs or the Holleys, as the carb does not need to come apart.

 

Finally, one needs to consider the effects of ethanol. E10 is about 4 percent leaner than gasoline. So this change will enter into the range. We suggest to folks that live in Colorado that live at 5000 feet (1 size lean according to Carter) and using E10 to just keep the standard calibration.

 

Jon

[CChinn]

Thanks to all who have replied. You have very thoroughly answered the question and more!

[Rusty_OToole]

You might notice a slight reduction in power due to the thin air lowering compression.