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Carburetor Tuning for Density Altitude

By: Clint Turner, jayhawkclint

Have you ever spent hours tuning your carb, getting it to purr at idle, make lots of horsepower, and maximize fuel efficiency to the point where you almost think you've got fuel injection under the hood? Then, on a frigid cold morning you go to start 'er up and much to your dismay it is stumbling a little at idle, stuttering at WOT, possibly even stalling. What happened? What you may have experienced is an extreme change in density altitude.

For years, I've been making a rookie's mistake that I should've learned to fix long ago: I've been tuning my carburetors based on altitude. More specifically, MSL altitude. Basically, I would just check a map or ask at the airport for the elevation, and I thought I would be good to go. Makes sense, right? If the instruction booklet that came with my Holley says that it was bench flowed at sea-level conditions, and I am to drop one jet size for every 2000' increase in elevation, then I figured all I should have to do is find my elevation and perform some simple mathematics. If only I had been paying attention during 10th grade science.

In order to understand the concept of density altitude, it is first important to know how a carburetor meters fuel. This will be an abbreviated version in order to provide a rudimentary knowledge; if you want an ME's explanation, there are plenty to choose from on the web. Basically, for the purposes of understanding density altitude, know that.

A piston engine with its intake valve(s) open and piston(s) moving downward creates vacuum. This vacuum, since it is contained primarily within the intake manifold, is aptly named "manifold vacuum".
That vacuum creates a pressure differential between the air outside the manifold and the air inside the manifold, the outside air being of much higher pressure. Assuming no leaks, when the throttle plate is opened (i.e. you step on the gas pedal), the outside pressure pushes air through the opening of the carburetor.
Within this opening is a venturi. The venturi also creates vacuum, but on a much smaller scale than manifold vacuum. This venturi vacuum is what draws fuel through some type of metering device within the carburetor. The size of the discharge nozzle (commonly called "jets") on this device will determine how much fuel the venturi vacuum draws into the air that is moving past it, i.e. on its way down to the intake manifold. The venturi also atomizes the fuel to create a somewhat uniform mixture.

Now that we understand the carburetor is a device that meters and mixes fuel with air, and that mixing process is based on vacuum, we can start to understand what density altitude has to do with why your engine is running like crap. Remember the four cycles of your engine? "Suck, squeeze, bang, blow." Oxygen and fuel in, compress it, ignite it, carbon monoxide out. So let's go back to that Holley instruction manual for a moment with respect to the 4 cycles. Do you really think your engine knows what elevation it's at? Honestly, does it understand elevation in the terms that people refer to it? In short, no.

It doesn't care whether it's cruising Daytona Beach at sea level, or plowing snow in Denver. Those types of elevation numbers are only useful to people. Think about the 4 cycles again. The only thing your engine cares about is the amount and ratio of oxygen and fuel it is getting fed, the ratio of compression that it is going to exert on that mixture, how complete your ignition can light it off, and how fast your exhaust can let it escape so that the next cycle can start. You can change jet sizes to increase or decrease the amount of fuel in the mixture, change pistons to increase compression, machine bigger valves to allow the passage of gasses in and out with less effort, install a more powerful ignition to burn every molecule of atomized fuel available, but you can't do anything about the oxygen available. Why is this?

Oxygen is a gas at atmospheric temperatures. But it is only one of many gasses that make up the air that surrounds us, the same air that enters into the throat of your carb when manifold vacuum is created. Remember the engine only cares about oxygen, but beggars can't be choosers, and so we take what we get, which includes nitrogen, argon, and a host of other stuff we really don't need to be worried about. When air is compressed, all these gasses get closer together, so you end up with more oxygen per cubic foot, along with more of all the other stuff, but again, we really only care about oxygen. This pressure, although it is fairly consistent throughout the world, actually varies by small amounts due to weather. Now, you might not really care if one cubic foot of air has a few more molecules of oxygen in, say, Lawrence, KS compared to another cubic foot of air down at sea level in Pensacola, FL, and you'd be right not to. But my Holley can flow 470 of those cubes every minute! Suddenly you are multiplying small amounts of molecules and they become large amounts very quickly! Examples of this include weather fronts such as cold fronts or warm air cells that skew the barometer. Lower barometer readings = less pressure = less available oxygen per cubic foot.

Other factors contribute to the amount of available oxygen as well, like temperature. Back to that 10th grade science again that I slept through. Did you know that air is more dense at a lower temperature than it is as a higher temperature? As it turns out, when you add heat to something it expands, and when you remove heat from something it contracts. Air does this, and so (we're finally getting to the useful stuff) when you go out on that cold morning to start your Heep, there are more oxygen molecules available for your engine to breathe than on that warm summer day when you first got 'er tagged and rolling and tuned the carb. Lower temperature = contracted gasses = more available oxygen per cubic foot.

To a lesser extent, humidity also plays a factor because hydrogen has already bonded to some (but not all) of the available oxygen molecules in the air leaving less for your engine to burn. Less humidity = less bonded molecules = more available oxygen molecules per cubic foot.

And, of course, let's not forget about our earlier talk of elevation. It does affect our performance, but perhaps not in the way you have been accustomed to thinking about it. Have you ever dove into some really deep water? Do you remember what it felt like the deeper you went? The closer you got to the bottom, the more pressure you felt being exerted on your body. The atmosphere works the same way. The more altitude you gain, the less atmosphere exists above you, therefore the less pressure is exerted on our cubic foot of air that is below it. Lower altitude = more pressure = more available oxygen molecules per cubic foot.

Now taking into account all these factors, we can calculate a number that is useful for your engine to know just how much oxygen is available for it to breathe. By comparing it to a reference sea level value, like the one provided us in our aforementioned Holley instruction book, we can even determine just how far above or below those measurements our carb currently sits for tuning purposes! We call this measure of available oxygen and it's comparison to a reference sea level amount "Density Altitude."

I know what you're thinking, "Good grief Jayhawkclint, you made me read through all that crap just for you to tell me that there is more oxygen available in some parts of the world than in others? Why I oughtta! This had better be good, or else!" Well, before you string me up and lynch me, let me tell you the good news: If you can read this post on the internet, then you have all the tools necessary to calculate the density altitude of the exact spot where your Heep currently sits. Let's get to it. First, you need to gather some information. Remember all the things I told you that contributed to the amount of free oxygen available? Get a little scratch pad and pencil handy, then go to:
http://www.weather.gov/

After you type in your ZIP, gather up the following:

You need your current elevation in feet. Usually located in the upper left corner of the NWS page in parenthesis. If it doesn't show up, just Google it.
You need your air temperature in degrees Fahrenheit.
You need the barometric reading in inches of Mercury(Hg).
You need the dew point in degrees Fahrenheit.

After you have jotted this information down on your scratch paper, go here:
http://wahiduddin.net/calc/calc_hp_dp.htm

Enter the data into the appropriate blocks. Note that "altimeter" is your barometer reading. Don't forget your decimal point and two numbers to the right of the decimal or else it will give you erroneous data. Click on "calculate." There will be several numbers spit out. You can do what you want with them, some are just FYI, but the most important number is "density altitude." With this number, you can now refer back to your carb literature to make educated changes to your jet sizing.

So, finally, let's talk about why your Heep runs like crap on a cold day. Let's use our Lawrence, KS example mentioned earlier. Suppose you finish up a resto, fresh motor, new carb and intake, hot cam, and you want to tune it. Are you going to stand out in the winter cold and change out jets all day long (hard to do with gloves on!) until we get an acceptable level of performance? No. Of course not. We'd rather do it in the summer time when Jeepin' with the top down is cool and we're making frequent trips to the ORVs. This would be our first mistake. After playing with jet sizes all day, we settle on a 52 primary and 54 secondary. The temperature is 92° F, the barometer is 30.32", the dew point is 65° F, and our field elevation is 810'. Plugging in the numbers, we come up with a density altitude of 2901'. We drive it all summer, the engine runs great, awesome fuel economy, and that thing just climbs up most stuff with the skinny pedal resting at idle. Life is good.

Then winter sets in. You move the Heep to the garage. Your reliable daily driver, its life a miserable existence out in the freezing temperatures of the uncovered driveway so that other "more promising projects" can fill valuable garage real estate, finally decides it's had enough and refuses to start one morning. What to do? You're going to be late for work if you don't find transportation quick. You decide to brave the cold in the CJ. Now the conditions are much different than when you tuned the carb on that sunny day. The temperature is 20° F, the barometer is at 30.01", the dew point at 15° F, and elevation is still the same 810'. Plug in the numbers and you will now find that density altitude is -1741' (read seventeen hundred feet below standard sea level).

Would you believe that we haven't even left home and yet we've somehow dropped a whopping 4642' in air density? Or, more specifically as it relates to our engine, we're breathing air that seems 4600' lower than when we first tuned it. That's over two jet sizes! What happens as a result? Well, according to our Holley literature, for every 2000' decrease in elevation, we increase one jet size. We decreased over 4000' in elevation, but we haven't changed the jets yet. We should've increased the size, which means the ones that are in there are two sizes too small. We're running way lean! On a cold day, you're going to have a hard time starting, once running it will stumble at idle, you'll run out of gas at WOT, and possibly stall when letting out the clutch. You better call a friend for a ride today or else you might end up walking to work.

How can we prevent this? There are two ways on carbs that have fixed jet sizes. One is to record your winter and summer settings, then just change the jet sizes with the change of the seasons. That means you've got to take your carb apart every fall and spring, but if you're driving a CJ, you probably take it apart this often anyway. It's a little more work, but will run well. The second method is to average your barometer and temperature readings for a given area. If you're in an area like Cali or Florida where the temperature stays fairly constant year round, you're in luck. In the Midwest, though, you probably won't be able to get away with it due to the temperature differentials. You'll essentially be running one jet size too rich in the summer, and one jet size too lean in the winter. If one jet size is an acceptable performance loss compared to not having to take your carb apart twice a year, then this method would work well for you.

Aviators have used this measure for decades to predict aircraft performance and tune accordingly when their life (and their passengers') depends on it. Snipers use it in the field to select the proper round for a particular set of conditions. Drag racers use this at the track to maximize their air to fuel ratios and edge out opponents. Modern fuel injection systems essentially make a similar comparison. Thanks to the internet, now you've got the same tools necessary right from the comfort of your home computer. So now that you've calculated it, get out to your garage; the carb ain't gonna tune itself!

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