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For most gear heads, the attraction to cars is all about horsepower. Move more air through the engine and the power will follow. But what is rarely addressed is the quality of the air the engine is ingesting. This is important because this air is constantly changing – sometimes it’s really good but more often it’s far from ideal.
Everybody knows that carburetors need to be tuned for changes in air quality. The beauty of electronic fuel injection is that the manifold absolute pressure (MAP) sensor and inlet air temperature sensor (IAS) measure changes in the air and automatically compensate as the vehicle is driven. Carburetors, unfortunately, don’t enjoy that luxury, so the tuner has to make these changes.
But before we climb that mountain of little restrictors and tuning adjustments, we need to establish some physical concepts that directly affect carburetor tuning. Atmospheric pressure is a big contributor and changes often occur from natural variations in the pressure exerted by the atmosphere. Pressure is also directly affected by elevation or height above sea level. Let’s start with why this happens.
The time tested method for testing air pressure is a simple barometer. Even a simple one like this barometer can be used as long as it is calibrated.
Imagine a 12-inch x 12-inch x 12-inch cube sitting on top of your carburetor. Inside this one cubic foot of air is a mixture of 81 percent nitrogen, 18 percent oxygen and one percent of a mixture of other gasses. While air is light, it still has mass. If we vertically extend the top of that cube from sea level all the way up to the edge of the earth’s atmosphere, that column of air will generate enough weight to produce a given pressure. At sea level, this pressure is 14.7 psi(a). The small (a) indicates absolute pressure which is different from gauge pressure which would be expressed as psi(g). Pressure can also be expressed in other forms, such as in inches of mercury (“Hg) which compared to 14.7 psi(a) equals 29.97 “ Hg or as 101 kilopascals (kPa).
Now if we dump the clutch and spin the tires in the beach sand and accelerate into the mountains to 10,000 feet and measure the air pressure, that standing column of air is now shorter which reduces the atmospheric pressure. At 10,000 feet the pressure changes to barely 10.1 psi(a) or 20.6 “Hg. This reduced pressure also does not squeeze the air molecules nearly as close together within a given cubic foot of air so there are fewer oxygen molecules within that same space.
This is important because at altitude there is both less pressure pushing air into our engine while at the same time there are fewer molecules of oxygen to enhance combustion. Gasoline mixes with air at the ratio of 14.7 parts of air for one part fuel to create a chemically correct complete combustion process. This is known as the stoichiometric air fuel ratio. This is generally considered acceptable for lean cruising and where factory EFI engines are tuned to operate. Carbureted engines often run slightly richer at part throttle – closer to 13.5 to 13.8:1. For maximum power, the air fuel ratio (AFR) generally will be closer to 12.5:1 to 12.8:1. This is richer in an attempt to ensure all the oxygen molecules contribute to combustion.
So far, we’ve discussed air pressure as dictated by altitude. But air pressure also changes based on atmospheric conditions. Low or high pressure weather conditions affect the quality of the air at any given altitude. The combination of temperature and humidity mixed with the existing air pressure combine to create what is commonly called air density.
There is a rather complex formula for determining this density if you are inclined to do the research. We’ll focus instead on a somewhat simpler definition of air density. While density is most often expressed in pounds per cubic feet, a more easily understood way to express this is in relation to air density at a give altitude – commonly referred to as density altitude (DA). This is a combination of pressure, temperature, and humidity expressed in feet above sea level.
Stated another way, on a perfect day at sea level with 14.7 psi(a) and 60 degrees F with zero humidity, the density altitude would be zero. But a combination of higher elevation, warmer temperature, and more humidity (vapor pressure) the density altitude would be at a much higher altitude. For example, with an 80 degree day with a pressure of 29.29 and some humidity the density altitude would be equal to the above stated perfect day at an altitude of 2,400 feet. This is in essence a shorthand expression of all the above variables condensed into a simple altitude figure that is easily understood.
This density altitude can then be used as a tuning tool. With some previous experience and good record keeping, most racers know the ideal jetting for their engine at a given density altitude.
The old school way to determine humidity was to read a sling psychrometer that displays dry and wet bulb temperature readings to indicate the relative humidity. There are far easier ways to determine humidity now but this is how your dad probably did it.
Today, you can get all the important data from a digital weather station like this one from Altronics. This weather station displays not only the density altitude (2,980 feet) but also the barometric pressure (29.15 “Hg), temperature (79.5 degrees F), and the humidity (53.7 percent).
As part of this story we will focus on the tuning required for a particular HP 750 cfm mechanical secondary Holley carburetor at altitude. We measured all the restrictions and jets in this carburetor as shipped from Holley and have listed them in an accompanying chart. The next step was to talk with Holley engineers to get an idea of what changes would be required to tune this carburetor for operation at 7,000 feet of altitude with 70 degree F temperature and 45 percent relative humidity.
These conditions at sea level would place the density altitude at roughly 950 feet. At 7,000 feet of elevation, this would put the density altitude at around 9,600 feet. By using our accompanying chart that lists atmospheric pressure at varying altitudes, we calculated an altitude of 7,000 feet reduces the pressure by 23 percent. This is a substantial reduction in atmospheric pressure which means we will need to lean the carburetor jetting and metering orifices in order to compensate for the drop in atmospheric pressure.
We will get into main jetting for maximum power in a moment but for street engines, an equally important issue is idle and light throttle air-fuel mixture since that is where street engines spend most of their time. The idle circuit for most performance Holley carburetors is designed slightly rich to allow these carburetors to function over a wide variety of displacements, manifolds, and camshaft timing. As a result most Holley HP and other carburetors will tend to run rich with milder engine combinations even at near sea level conditions. So an HP carburetor at 7,000 feet of altitude will certainly need addressing.
Here is a small wire placed into the idle feed restrictor on a primary metering block. With these brass inserts be sure to place the wire in the small hole at the bottom of the cup as that is the actual restrictor. Leave enough wire protruding to ensure the wire remains in place.
The main issue here is to reduce the size of the idle feed restrictor to decrease the amount of fuel delivered at idle. Think of this restrictor as the main jet for the idle circuit. A typical idle feed restrictor size for a 750 cfm Holley will be 0.032-inch in diameter combined with an idle air bleed on top of the carburetor measuring 0.075-inch. These specs are also the dimensions for our Holley 750 HP.
Most Holleys using cast style metering blocks use a pressed-in idle feed restrictor. This is a small brass cup with a tiny orifice drilled in the bottom of the cup. While you could replace this cup with a screw-in restrictor, for initial tuning it might be easier to merely use a ½-inch length of electrical wire roughly 0.017-inch in diameter. Place this length of wire into the small hole and bend the remainder over the top of the metering block to allow the gasket to retain the wire. This way, the wire is easy to change should the first attempt not be sufficient.
Most Holley four barrel carburetors mount the idle fuel restrictor in the position shown in the previous photo. Many modern aluminum metering blocks move the idle restrictor to an upper position as shown by the screwdriver using a tiny, 6-32 screw-in bleed that is much easier to change.
The wire is used to reduce the area of the restrictor and therefore the volume of fuel delivered to the idle circuit. This will also give the idle mixture screws greater authority over the final adjustments. You know you will have the idle feed restrictor close to the proper size when it requires about one full turn counterclockwise from fully seated of the idle mixture screws to have the idle mixture achieve the highest idle manifold vacuum.
It’s also a good idea to adjust the idle air bleed size as well. This is a general relationship where the size of the idle air bleed is proportional to the size of the idle feed restrictor. Our stock 750 Holley used a 0.032-inch idle feed restrictor combined with a 0.075-inch idle air bleed. We’re going to reduce the idle feed restrictor flow area roughly 20 percent, which would be the same as changing the restrictor to 0.029-inch. While the idle air bleed on the top could also be reduced, it might be a good idea to leave it for now and evaluate the change to the idle feed restrictor first. This will allow you to better judge changes to the idle air bleed separately.
Air bleeds are located on the top of the carburetor most often with two bleeds per venturi. These are called two-circuit carburetors. The high speed bleeds are located inboard (arrow 1) while the idle bleeds are outboard (arrow 2).
Switching now to the main metering circuit, Holley says that each main jet number references a flow number rather than an area. Each jet size represents a 3.5 percent change in flow, so changing from a 70 to 71 primary jet for example would increase fuel flow by 3.5 percent. Holley also suggests decreasing one jet size for every 2,000 foot increase in altitude. So if we were going to re-jet a carburetor for 7,000 feet, this would mean reducing both the primary and secondary jets by three numbers – or from the 73 down to 70.
If, for example, your carburetor was factory jetted at 72 on all four corners but through tuning your engine preferred a sea level jetting of slightly richer – say a 74 jet size, then moving to 7,000 feet would mean reducing the jet size from your preferred number of 74 rather than the factory 72.
Most Holley four-barrel carbs do not use a power valve in the secondary. Ours, however, does use one which is why the primary and secondary jetting is the same. If the carb does not use a secondary power valve, the secondary jetting will be roughly 6 to 8 jet sizes larger.
Rather than make a drastic change to the power valve channel restrictors (PVCR), you might consider leaning the primary jets at least one additional number and leaving the stock PVCR in place. This leaner main jet will compensate for leaving the power valve channel restrictors in the stock setting. Since our carburetor also had a power valve in the secondary side, this is something to consider for the secondary as well.
Among the best ways to evaluate changes to the carburetor would be with a wide-band air-fuel ratio (AFR) meter. There are several on the market that will deliver accurate readings of the engine’s average air-fuel ratio. Keep in mind that you should tune using the engine as a gauge as to what it prefers – using either drivability clues at part throttle or drag strip mph trap speeds as your indicator. Tune this way rather than to a predetermined number like 12.5:1 AFR. If you pay attention, the engine will tell you where it prefers to run.
Since every engine combination is different regarding displacement, compression, and cam timing, these recommendations can only be a starting point for the final tuning. Keep in mind that with reduced cylinder pressure at higher altitudes, many engines will want more ignition timing as well. The wide-band AFR meter is a good way to get the engine close to a decent operating mixture.
There are many more techniques and minor tuning tricks that can be used to dial in a carburetor beyond what we’ve covered here, but this generic approach should get the air-fuel ratio closer to ideal which will allow the engine to run smoothly and closer to an ideal air-fuel ratio. It will require some hands-on carburetor play, but most gear heads live for that anyway, right?
When setting idle mixture, make very small changes to get the idle vacuum as high as possible. If after adjusting the idle mixture, if the idle speed is higher than desired then slow the speed down with the curb idle adjustment screw. As altitude increases, engines will idle with less manifold vacuum. This vacuum gauge reads 10 inches of mercury (“Hg) as indicated on the inboard scale.
If you don’t have access to an air-fuel ratio meter, you can still tune by looking at the spark plugs. If the insulator is bone white (left), the mixture is too lean. Black soot indicates the ratio is way too rich (right). When the air-fuel ratio is closer to ideal, you will see a light tan coloring on the center insulator. This can work for both part-throttle and max power tuning.
We’ve devoted much of this story to idle circuit tuning because street cars spend a majority of their time in this area. Trimming the idle circuit to achieve a lean-ideal air-fuel ratio will result in improved throttle response and a happy engine that will make driving even in higher altitudes much more fun.
The following are specs for a mechanical secondary Holley 750 cfm Classic HP, PN 0-80528-2. The second column lists suggested changes for use at 7,000 feet altitude.
There is a 23% density change from sea level to 7,000 ft. altitude.