HOLLEY CARB TUNING TIPS
Part 1: Carb CFM Sizing
One of the first things to determine is what size carburetor you need. A number of factors come into play. What cubic inch size is your engine? What do you want to do with it? Race, street use, towing or street use with occasional trips to the track? What type of intake manifold do you have, split plenum, open plenum, tunnel ram, individual runner? How fast do you spin the motor? What is the volumetric efficiency of your engine? Do you have a manual or automatic transmission? What is your rear gear ratio? Do you want to get the best gas mileage possible or do you want to develop the most power possible? Keep in mind that a carburetor is just one part of the equation. All of the parts need to work together. Putting a larger carburetor on is NOT going to immediately put 100 more horsepower at your disposal. The carb needs to work with the other parts you have chosen and your intentions on how the vehicle is going to be mostly used for.
The general rule of thumb uses a formula to determine the CFM requirements of your engine. It goes like
In a controlled situation on an engine dyno, the amount of air actually consumed by the engine can be measured. Since most folks don't have access to a dyno, the above formula will get you in the ballpark. There are some exceptions of course.
Using a split plenum type street manifold allows the use of a larger than "normal" carb CFM. This is because the plenum volume is cut in half by the divider, so each cylinder only has half of the total plenum volume and carb CFM to draw thru. Likewise a carburetor with vacuum secondaries will only open the secondaries enough to feed the engine what it needs. Consequently, on a street driven vehicle, a split plenum intake with a vacuum secondary carb is the way to go. The vacuum signal stays high for good throttle response at low and mid range rpms. Fuel mileage is good.
An "open" plenum intake, generally speaking, has opposite effects. Low rpm throttle response is decreased, but high rpm breathing is improved because of an increase in available manifold plenum volume to each engine cylinder. Open plenum intakes are a little more sensitive to the CFM size of the carb. If you are drag racing, most folks are willing to sacrifice some low end power for higher rpm horsepower. Fuel mileage isn't a concern and the engine spends most of its time at full throttle. Open plenum race intakes and "Double Pumper" carburetors are the norm.
Part 2: JET CHANGES AND ALTITUDE AND TEMPERATURE
Holley carburetors are calibrated for sea level operation and an inlet air temperature of 70 degrees Fahrenheit. Once you know the correct stock jetting for your particular Holley carb, you can determine whether you live or race at an altitude above sea level. For every 2000 feet increase in altitude, you can reduce the jet size by one size. If you had a carb which has a stock jet size of 80 and you live or race at 2000 feet above sea level, then you would use a #79 Holley jet in the carb. Similarly, a change in the carburetor's inlet air temperature may require a change in the jet size from the stock calibration. Many racers go a step further by combining all of the weather variables, temperature, barometric pressure, dew point and humidity with the altitude of the track they are racing at to determine the "density altitude". This is a "corrected" altitude above sea level. From there they can determine whether a jet change is necessary to maintain performance or whether to change their "dial in", if they are bracket drag racers.
Part 3: DRAG RACE JET CHANGES and MPH
Drag racers should try to optimize jetting by looking for the jet size that gives the best MPH, rather than best elapsed time (ET).
Part 4: ACCELERATOR PUMP CAMS AND SHOOTERS
Accelerator pump cams come in various sizes and are color coded and number coded by Holley. The cams have different shaped ramps that the arm of the accelerator pump rides on. By changing the size and shape of the arc on the cam, the pump shot can be tailored to start early or later as you go from off idle to full throttle. Changing the cams can have an effect on the way a vehicle leaves the start line in a drag race. If you leave the line off idle or at a higher RPM (while foot braking or when using a trans brake or when using a clutch with a manual transmission) experimenting with the pump cams can help. There is no set rule for this use. You just have to experiment with the different cams and the different cam positioning holes in the throttle linkage of the carb. Holley sells individual cams or a kit that includes an assortment of cams.
Pump shooters are another area of experimentation. Holley carburetors come with a standard shooter size, which varies by the carb list #. If you experience a bog or hesitation off idle, you can try a higher # (larger) shooter size. The bog or hesitation may be caused by a momentary lean condition when the carb goes from the idle throttle position to the main metering system. The shooters help richen this momentary condition and eliminate the stumble. Keep going up in shooter size until a puff of black smoke comes out the exhaust, then go back one or two sizes. Playing with shooter sizes is helpful when you have an intake with a large plenum area, such as the Torker/Victor series or Tunnel Ram. When shooter sizes go over .037, Holley recommends that you use the "hollow" screw (PN-26-12) which allows more fuel to flow to the shooters.
Part 5: POWER VALVES and ENGINE VACUUM
There is a lot of misunderstanding concerning power valves in Holley carburetors. Many 4-barrels come with a particular power valve depending on the carb list # and application. Some carburetors have two power valves, while others only have one. The power valves are numbered by the amount of engine vacuum in inches at which they will open and add additional fuel to the power circuit. In other words a 6.5 power valve will open when the vacuum signal on the engine drops below 6.5" of vacuum and will remain closed above. One of the misconceptions is that they can't be trusted to work because an engine backfire can "blow out" the power valve. Many of the newer Holley performance carburetors now come with a built in power valve "blow out" protection which eliminates this problem. If you have an older model carb you can purchase a small, inexpensive, easy to install Holley kit #PN - 125-500 that will protect the power valves in case of an engine backfire in the carb. I use the CENTEK, "Power Valve Shield". See their website at www.powervalveshield.com , which takes about two minutes to install and does not require any drilling.
Many tuners will automatically remove the power valves and use a "plug" thinking this is the "hot" ticket. However, if the power valve is removed and plugged, the main jet size must be increased 6-10 jet sizes to make up the required fuel amount lost by the removal of the power valve. When the power valve circuit is plugged, part throttle fuel economy is lost and may become overly rich. Plug fouling may become a problem at part throttle.
Stock engines have high vacuum readings (10-18 inches at idle) and the Holley power valves with higher readings like 6.5 to 10.5 will work. Longer duration non-stock camshafts and other performance related parts can cause a problem, because engine manifold vacuum may be lower. The power valve, if incorrect, will always be open, even at part throttle, leading to an overly rich air/fuel mixture. The solution is to choose the correct power valve. To determine this, you need a vacuum gauge. On a manual transmission vehicle, hook up the vacuum gauge and take the reading with the engine at idle. Then use a power valve that is rated 1-2 inches below that amount. For example, a motor that shows 7" of vacuum at idle should use a 6.5 or 5.5 rated power valve. If you have an automatic transmission, take the vacuum reading at idle in "DRIVE" (with the emergency brake on and the wheels blocked) and chose the power valve 1-2 inches below that figure. You can get a little more detailed information by driving the car with a vacuum gauge hooked up with a longer hose so you can read it while driving. Drive the car at medium loads and while cruising and note the various vacuum readings. Then chose the appropriate power valve rating.
Holley makes a performance style "standard" flow or the "high" flow power valve. The latter has larger openings for more fuel flow with bigger engines. "Single stage" power valves are available in 1" increment sizes from 2.5" thru 10.5". Holley also makes a "two stage" power valve that is more for "economy" minded users rather than "performance" enthusiasts.
Part 6: ADJUSTING THE OPENING POINT OF VACUUM SECONDARIES
There are a number of ways to tune the moment when the vacuum secondaries open on a Holley 4 barrel carb. The vacuum secondaries are controlled by a diaphragm and a color coded spring. Holley makes a number of different springs with different tension on these springs. You can change the springs and change the opening point. The color coded springs run from a light to heavy tension:
If there is a bog or hesitation when the secondaries open, the spring tension is too light, so go to the next heavier spring. Holley offers a kit (PN-20-13) which contains one each of the above color springs. When you change springs you'll note that the stock cover over the spring and diaphragm is not all that easy to get to. Holley makes a special open style cover (PN-20-59) that makes spring changes quick. Finally, Holley also makes a completely adjustable thumbscrew operated diaphragm cover (PN-20-99). This cover limits the travel of the diaphragm and therefore limits how far the secondary throttle plate can open. It makes secondary throttle opening adjustable. Very nice to have if you are a bracket racer and are using a carb with vacuum secondaries. You can adjust the throttle for changes in weather and track conditions or for changes in your "dial in".