On this page you will find articles on different aspects of engine care and performance. At the bottom, you will find a form for submitting questions to us pertaining to engine problems. If you do have an engine problem you would like help with, and submit it, please fill out the form as thoroughly as possible.Also, please be aware that the procedures, techniques, and other material on this page, as well as answers to your engine questions, may not work for your particular application, and in some instances, may be hazardous. It is up to you to use your own judgment as to whether information you find here will work for you. With that said, lets get to this months article.
Induction 101
Every so often, we get an e-mail wanting to know what carburetor will work best on a certain engine application. Or, what carburetor is the best, or would fuel injection be better than a carburator? There is no simple answer for all the applications out there, only advise, so that our readers can determine what is best for them.
To get started, lets look at how carburetors, and fuel injection systems work. First, we'll cover the basics of the venerable carburator. The carburator has a few basic components that must work together for delivering the proper fuel air ratio. Lets look closer at how they work together. Also, there is a basic law of physics that should be remembered, since it affects all aspects of engine performance, as well as carburator performance. That rule is called "Boyles Law", and it states, "for a given volume, there is a given pressure.".
The part you see most often, is the venturies, and the boosters. They are the round holes going through the body of the carburator, and the rings nested inside the holes. You have probably noticed that the venturies get smaller in the upper part of the opening, and widen as they go down. The taper is what makes the venturi do its job. Air must accelerate to get past the taper, and this creates a low pressure area at this point. The booster helps in the acceleration of the air, and provides the point where the fuel is mixed with the air, during off idle operation. So often we think of the air and fuel being sucked through the venturies, when the opposite is true. Its actually pushed through due to the greater outside air pressure (Boyles Law).
Next is the main metering system, which consists of the main jet, and air bleeds. We won't go into the air bleeds, since there is very little that can be done to them to improve performance, unless you have some expensive equipment. The main jet is at the bottom of the float bowl, and determines how much fuel can be delivered to the boosters at a given air flow. Remember, its the air flowing through the venturi causing low pressure, and the outside air pressure pushing the fuel through the main metering system. If there is a big pressure drop, say wide open throttle, there will be a larger amount of fuel pushed through the main jet, and up to the booster. A larger main jet will allow a greater volume of fuel to reach the booster, and a smaller jet will allow less fuel through. Remember, its the volume of fuel, and not the speed of the fuel (Boyles Law again).
The float bowl, or bowls, is our next stop. The float bowls provide a reservoir of fuel for the main metering system to draw fuel from. The fuel level in the bowls is kept constant through the float acting on the needle valve, and slowing, or stopping fuel from entering the bowl when it reaches a predetermined height. The fuel level is most often kept at a nearly full height, to insure the main jets never become uncovered. In some instances, the main jets will be uncovered, unless slosh tubes are installed in the float bowls.
Finally, there are the throttle plates at the base of the carburator. They regulate the amount of air through the venturies. As the plates open, more air is allowed to pass through, and vice versa. Also, the idle circuit fuel discharge is located below the throttle plates, to deliver fuel when there is not enough air flow past the venturies to deliver fuel properly.
When all these parts work properly, fuel and air is delivered to the cylinders at the proper ratio, about 15 to 1. If they don't work properly, or if a lean or rich condition develops, the carburator will need to be adjusted.
As far as what carburator is best, that's not really the proper question. What carburator is best for you and your application is the question that should be asked. For a day cruiser, or a boat that is used mostly for pleasure, A Carter AFB, or a Holley will work fine. They are both relatively inexpensive, and will give good service and performance. If your into all out performance and racing, a Barry Grant, or Predator is the choice to go with. No matter what carburator you choose, don't make the mistake of thinking bigger is better, IT'S NOT. To determine what size carburator you need, use this formula. Engine displacement multiplied by the Redline RPM, divided by 2, and then divided by 1728. This will give you the amount of air your engine can draw in at Redline RPM in cubic feet per minute (CFM). An example would be a 468 cubic inch engine with a redline of 6000 RPM. We multiply 468 times 6000 and get 2,808,000 cubic inches or air per minute. We take half of this figure, since a four stroke engine only intakes on every other revolution, and get 1,404,000. We then convert this figure from cubic inches to cubic feet by dividing it by 1728, and get 812.5 CFM. So for this engine, an 850 CFM Holley or other carburator would work fine. Also, heads and intake manifolds don't flow at 100 percent efficiency. Most flow in the high 80 to mid 90 percent ranges. A single dominator would be too much for this engine, because the air velocity through the venturies would not give good fuel atomization, or maximum fuel flow through the main jet.
Now lets look at fuel injection. Fuel injection comes in several types: throttle body, port, constant flow, and sequential. The basic components of all fuel injections is the same. They consist of a high pressure fuel pump, a bypass valve or jet, throttle plate or plates, and the injectors. To understand how they work, we'll look closer.
I'll cover mainly constant flow injection, since its used most at the moment. However, electronic sequential is gaining ground. First is the fuel pump. It can be driven off the cam, or it can be belt driven. It doesn't matter really how its driven, as long as its driven at the same speed as the cam. Remember, the engine only intakes at every other revolution, and the pump is designed to provide proper volume and pressure in relation to the engine's needs. The pump must be matched to the engine in order to get the correct volume of fuel delivered, at the proper pressure. If the pump is too small, a lean out at high RPM would occur, and vice versa.
We then need the main bypass jet. The main bypass jet allows fuel that's not needed at the barrel valve to be returned to the fuel system, and determines the final mixture at full throttle. Now here's where things get some people confused. To richen a fuel injection system, a smaller jet is installed, to reduce the amount of fuel being bypassed.
Next is the barrel valve, and it regulates the amount of fuel going to the individual injectors, by allowing more or less fuel to be bypassed. The barrel valve is connected to the throttle plate linkage in a 1 to 1 ratio to deliver fuel accurately, depending on the throttle plate position. The barrel valve has a small notch machined into it, that lets fuel flow to the injectors, and as it rotates in its housing, more and more of the notch is uncovered.
For part throttle operation, a secondary bypass is used. The pump can deliver the full fuel requirement at partial RPM and full throttle, but this is more than what is needed at part throttle operation, and needs to be bypassed to prevent an over rich condition at part throttle. The secondary bypass is usually a valve that is controlled by spring pressure. A spring of a given stiffness, and shims are used to adjust it. It's function is the same as the main bypass, but it performs the same job in a different way.
The throttle plates regulate the air flow into the engine, and are linked to the barrel valve. Electronic fuel injection uses a single throttle plate feeding a plenum to regulate air flow, while a mechanical usually has one plate per cylinder for regulating the air flow. The electronic has a single plate to keep things simpler, since the computer reads the throttle position to help adjust the fuel mixture.
Finally, there's the injectors. The injectors are sized to meet the fuel requirements of the engine. On a mechanical system, the injectors are delivering fuel constantly, where as on an electronic system, the injectors are pulsed to deliver the correct amount of fuel.
Both mechanical and electronic fuel injection systems can be sequential, however, the mechanical version is quite complex, and needs some very expensive equipment to set it up properly. The electronic sequential systems come in several varieties, and range widely in expense. The most common sequential system pulses four injectors, on a V8 engine, at a time. They are designed this way to cut down on the expense of having to have a lot of extra wiring, and better computer. The true sequential system, one that pulses the injector only when the intake stroke on that cylinder is occurring, can run $15,000 or more.
One other difference between mechanical and most electronic systems, is the intake Ram Tubes. As we said earlier, most of the electronic fuel injections have a single throttle plate to regulate the air flow. The mechanical systems have one plate per cylinder, and can therefore use a Ram Tube on each cylinder. The Ram Tube is made to a certain length in order to take advantage of harmonics in the intake pulses. This can actually compress the incoming air, and give an efficiency of greater than 100%. The Ram Tube has to be made to the proper length for the RPM the engine will produce the most power at.
Whether its a carburator, or fuel injection, if your buying it used, check it out thoroughly. For carbs, look for signs of water in the float bowl, and check the base plate for warps. Make sure all the linkage is still tight, and not bent or damaged. For fuel injection, the barrel valve and pump need to be checked. You'll probably have to send them off for this test, unless there is some one in your area that can do it properly. Also, for fuel injection, unless you have a great deal of experience with a mechanical system, don't try to piece one together. The results could be disappointing.