Scott MH
Jan 24th, 09, 06:05 AM
I have to write an essay in my Automotive Technology class and I chose double pumpers. Now I know i have lost 90% of you already but if the few 10% of you could read over my essay and check it for content and accuracy that would be great. The grammer will be incorrect and the spelling may be off too. But, thats not what i am checking for just yet. Content and accuracy is what i want. Its kinda long, a little over 700 words.
Here it is...
You hear the phrase “double pumper” a lot among classic car guys. Mostly you hear it when someone says that you need one. But, what exactly is a double pumper carburetor? Hopefully, I will be able to shed some light on this for anyone that doesn’t know. A “double pumper” carburetor is a Holley design carburetor that uses two power valves instead of one; hence the term double pumper. The double pumper also uses mechanical secondary’s instead of the typical vacuum operated ones. Most carburetors only use one power valve for that extra shot of fuel needed when you slam your pedal to the floor.
So, what is a power valve and what does it do? A power valve is like a reserve. When you stomp on the gas pedal the power valve diaphragm is depressed by way of linkages on the side of the carb. This then creates a hydraulic force that pushes extra fuel into the engine.
Why is the extra fuel needed? When you floor a carbureted engine you are opening the butterfly flaps which allows more air to pass through the carburetor and into the combustion chamber. As this happens the vacuum that the engine creates drastically drops. If the carburetor only provided a set amount of fuel the engine would become lean and your car would simply fall on its face and stall. To counteract this lean condition the power valve shots and extra dose of fuel to compensate for the extra air. The goal is to keep the air/fuel mixture and a consistent rate no matter how you drive.
Now that we have the basics down let’s talk about double pumpers. Essentially the only difference is that the double pumper has two power valves instead of one and the use of linkage to open the secondaries. With the second valve in place more fuel would be injected into the intake manifold to mix with the air. In conjunction with the power valve is the secondaries, when the secondaries open they are allowing even more air into the engine by the way of mechanical linkages. You are forcing more fuel and air into the intake manifold. Whereas, a standard carburetor would only use the secondaries as they are needed because their operation is linked the engine vacuum. In a vacuum secondary carburetor the secondaries are held shut by the vacuum produced by the engine, as the RPMs rise the vacuum decreases which in turns allows the secondaries to open allowing more air and fuel to enter the intake manifold.
So, a double pumper would help my car perform better? Well, that is not necessarily true. Double pumpers were designed for high performance applications. If you look at Holleys website they have a few priorities for double pumper carburetors. They state that cars with automatic transmissions must be less than 3100lbs, have a 4000RPM stall converter, and at least a 3.73 rear gear. If you are using a manual gearbox then the 1st gear should be low such as a 3.08 and at least a 3.73 rear gears. If your 1st gear is high such a 2.20 then you should have at least a 4.56 rear gear. If you go by this then 90% of the cars with double pumper carburetors are not and cannot take advantage of the double pumper features. In fact when you use a double pumper carburetor on a car that is not setup for one you are actually causing a rich condition under hard acceleration which could foul plugs or even flood the engine. Have you ever been in a car floored it from a dead stop and have it sputter, cough, hiccup and then run like a bat out of hell? Bet there is a double pumper carburetor under the hood.
Ok, well what cars do benefit from double pumpers? Double pumper carburetors were designed for high performance cars. As I stated Holley has very specific guidelines. Engines that need a double pumper are high performance street engines and all out drag engines. Engines that have a “hot” cam could require them because of their lack of vacuum.
The double pumper is an extremely useful tool in the performance world, but as with any other tool it only works if you have the ability to harness its abilities. Not everyone needs a double pumper; most cars will run fine with a standard carburetor. But, they are always out there for that day you want to go to the next level with your car.
Here it is...
You hear the phrase “double pumper” a lot among classic car guys. Mostly you hear it when someone says that you need one. But, what exactly is a double pumper carburetor? Hopefully, I will be able to shed some light on this for anyone that doesn’t know. A “double pumper” carburetor is a Holley design carburetor that uses two power valves instead of one; hence the term double pumper. The double pumper also uses mechanical secondary’s instead of the typical vacuum operated ones. Most carburetors only use one power valve for that extra shot of fuel needed when you slam your pedal to the floor.
So, what is a power valve and what does it do? A power valve is like a reserve. When you stomp on the gas pedal the power valve diaphragm is depressed by way of linkages on the side of the carb. This then creates a hydraulic force that pushes extra fuel into the engine.
Why is the extra fuel needed? When you floor a carbureted engine you are opening the butterfly flaps which allows more air to pass through the carburetor and into the combustion chamber. As this happens the vacuum that the engine creates drastically drops. If the carburetor only provided a set amount of fuel the engine would become lean and your car would simply fall on its face and stall. To counteract this lean condition the power valve shots and extra dose of fuel to compensate for the extra air. The goal is to keep the air/fuel mixture and a consistent rate no matter how you drive.
Now that we have the basics down let’s talk about double pumpers. Essentially the only difference is that the double pumper has two power valves instead of one and the use of linkage to open the secondaries. With the second valve in place more fuel would be injected into the intake manifold to mix with the air. In conjunction with the power valve is the secondaries, when the secondaries open they are allowing even more air into the engine by the way of mechanical linkages. You are forcing more fuel and air into the intake manifold. Whereas, a standard carburetor would only use the secondaries as they are needed because their operation is linked the engine vacuum. In a vacuum secondary carburetor the secondaries are held shut by the vacuum produced by the engine, as the RPMs rise the vacuum decreases which in turns allows the secondaries to open allowing more air and fuel to enter the intake manifold.
So, a double pumper would help my car perform better? Well, that is not necessarily true. Double pumpers were designed for high performance applications. If you look at Holleys website they have a few priorities for double pumper carburetors. They state that cars with automatic transmissions must be less than 3100lbs, have a 4000RPM stall converter, and at least a 3.73 rear gear. If you are using a manual gearbox then the 1st gear should be low such as a 3.08 and at least a 3.73 rear gears. If your 1st gear is high such a 2.20 then you should have at least a 4.56 rear gear. If you go by this then 90% of the cars with double pumper carburetors are not and cannot take advantage of the double pumper features. In fact when you use a double pumper carburetor on a car that is not setup for one you are actually causing a rich condition under hard acceleration which could foul plugs or even flood the engine. Have you ever been in a car floored it from a dead stop and have it sputter, cough, hiccup and then run like a bat out of hell? Bet there is a double pumper carburetor under the hood.
Ok, well what cars do benefit from double pumpers? Double pumper carburetors were designed for high performance cars. As I stated Holley has very specific guidelines. Engines that need a double pumper are high performance street engines and all out drag engines. Engines that have a “hot” cam could require them because of their lack of vacuum.
The double pumper is an extremely useful tool in the performance world, but as with any other tool it only works if you have the ability to harness its abilities. Not everyone needs a double pumper; most cars will run fine with a standard carburetor. But, they are always out there for that day you want to go to the next level with your car.