07-01-2011, 05:40 AM
The following is a partial list of Holley Carburetors and their specific information. This list covers some of the more widely used Holleys for Ford engines. 4150 carb types use a rear metering block with jets similar to the front metering blocks. 4160 carb types use a rear metering plate with passages for fuel flow instead of a metering block, thus making the overall length of the 4160 carbs shorter than 4150 carbs and requiring shorter fuel feed tubes between bowls. One reason often given for this change in design was to lower the cost of manufacture as well as eliminate the use of rear jets and metering block. The larger sized float bowls, sometimes called "Cathedral" bowls, have the float suspended from the center for better fuel levels under hard cornering. The smaller sized float bowls have the float suspended from the side. Most 600 CFM Holleys use the smaller bowls while the 725, 780, and highter CFM carbs often use the Cathedral bowls. The CFM ratings refer to the cubic feet per minute of air flow.
Without going into specific carburetor dynamics, just keep in mind that many engine variables determine what size of carburetor is best for your application. A regular Ford 302 will run just fine with a Holley 600 CFM while the Boss 302 uses a 780 Holley because of much different engine dynamics, design, and use. The stock Ford Autolite 4 barrel carburetor is rated at 470 CFM so you be the judge. Most stock 302's of the day were 2 barrel engines anyway so just going to a Ford 4 barrel carb will show significant HP increases. The rear two "barrels" of the carburetor are opened during operation in two ways, vacuum secondary and mechanical secondary. Vacuum secondary carburetors use a vacuum diagphragm to open using engine vacuum signals to let the engine "tell" the carburetor it needs more fuel under acceleration. Mechanical secondary carburetors use a mechanical linkage that opens the rear barrels as you move the throttle for more fuel. For most street use, it's recommended that you use vacuum secondary carburetors.
The power valve is a vacuum operated valve that allows fuel into the delivery circuit off idle. The rating of the power valve, such as 8.5 or 10.5, is the engine vacuum where, when it drops below that, engine vacuum is not enough to hold the power valve closed and fuel will be allowed to mix with the idle circuit. When camshafts have lower than normal engine vacuum at idle, this can cause an overly rich condition thus the reason for different power valve ratings. Stock camshafts have higher engine vacuum at idle than a racing camshaft would. Now everyone has their favorite brand of oil, gas, car, and carburetor. I myself have used and worked on Holleys since 1971. They are easy to work on and calibrate. Their worst trait is that they don't like sitting for too long. The metering block gaskets will sometimes dry out and shrink causing idle problems. Just make sure that if you have someone work on your Holley, that they know what they are doing. At any rate, here is just a basic look at some different Holley carburetors and their specifications should you decide to go this route with your engine.
Special Note: DO NOT USE the Holley rebuild kits that use the big thick nut with the needle and seat. They can strip the threads in your float bowl. Use only rebuild kits that use the Holley style needle and seat adjusting nut and screw. Just a personal warning.
[attachment=4970]
[attachment=4971]
Without going into specific carburetor dynamics, just keep in mind that many engine variables determine what size of carburetor is best for your application. A regular Ford 302 will run just fine with a Holley 600 CFM while the Boss 302 uses a 780 Holley because of much different engine dynamics, design, and use. The stock Ford Autolite 4 barrel carburetor is rated at 470 CFM so you be the judge. Most stock 302's of the day were 2 barrel engines anyway so just going to a Ford 4 barrel carb will show significant HP increases. The rear two "barrels" of the carburetor are opened during operation in two ways, vacuum secondary and mechanical secondary. Vacuum secondary carburetors use a vacuum diagphragm to open using engine vacuum signals to let the engine "tell" the carburetor it needs more fuel under acceleration. Mechanical secondary carburetors use a mechanical linkage that opens the rear barrels as you move the throttle for more fuel. For most street use, it's recommended that you use vacuum secondary carburetors.
The power valve is a vacuum operated valve that allows fuel into the delivery circuit off idle. The rating of the power valve, such as 8.5 or 10.5, is the engine vacuum where, when it drops below that, engine vacuum is not enough to hold the power valve closed and fuel will be allowed to mix with the idle circuit. When camshafts have lower than normal engine vacuum at idle, this can cause an overly rich condition thus the reason for different power valve ratings. Stock camshafts have higher engine vacuum at idle than a racing camshaft would. Now everyone has their favorite brand of oil, gas, car, and carburetor. I myself have used and worked on Holleys since 1971. They are easy to work on and calibrate. Their worst trait is that they don't like sitting for too long. The metering block gaskets will sometimes dry out and shrink causing idle problems. Just make sure that if you have someone work on your Holley, that they know what they are doing. At any rate, here is just a basic look at some different Holley carburetors and their specifications should you decide to go this route with your engine.
Special Note: DO NOT USE the Holley rebuild kits that use the big thick nut with the needle and seat. They can strip the threads in your float bowl. Use only rebuild kits that use the Holley style needle and seat adjusting nut and screw. Just a personal warning.
[attachment=4970]
[attachment=4971]