Feeling Good About Your Mods
| 31 January 2008
Are you thinking about modifying your early V-8? There are a lot of things that you can do to improve its performance Or maybe you just want to dress up the engine and the engine compartment so that you feel good when you pop the hood.
If you’re planning to go down this road, decide right up front what you want the outcome to be. That way you can buy the parts that will work well together to achieve the results you want. Part’s synergy is important because everything you change will affect something else and your end result will depend on how well all of the parts compliment each other.
Maybe you’re just looking to replace your 2-barrel carburetor with a 4-barrel. If that’s all you want, you might, at the same time, consider a better exhaust system for better breathing on the outlet side, complimenting improved breathing from the carburetor on the intake side. But if you’re also thinking about a “bigger” cam and different heads, for example, you need to make that decision before you buy the carburetor. Why? Because it’s very likely that the engine will need a different size carburetor when you put the other parts on it. The point being, you don’t want to buy a carburetor for a stock engine and then later decide to do the other mods. That approach could easily result in the need to buy another new carburetor to take advantage of the added performance inherent in the other parts.
But let’s assume that the 4-barrel is all you want. First, don’t be sucked in by “bigger is better.” A “big” carburetor will probably be more expensive and if it’s too big (flows too much air), it will almost certainly reduce the performance of your engine and adversely affect drivability. Not exactly what you want!
Does that suggest that you should keep the 2-barrel and forget about the 4-barrel? No, not at all. A properly sized 4-barrel in and of itself, and especially if coupled with a better exhaust system, has the potential to help performance. That’s particularly true when the engine is operating in the upper end of its RPM range. You should also enjoy better drivability at low RPMs. And contrary to some lines of thinking, the right 4-barrel can be slightly more economical, but only if you keep your foot out of it most of the time.
The displacement of the engine, its maximum operating RPM, and its breathing efficiency let you calculate the cubic feet of air per minute (CFM) that it can handle. A carburetor that flows that amount of air or slightly more is what you’re looking for. Carburetors that flow fewer CFM are too “small” and they’ll tend to “strangle” the engine when it tries to run at high RPMs. On the other hand, carburetors that are too large won’t properly control the air column entering the engine because they won’t have enough restriction to build the required vacuum. In turn, that will result in poor atomization of the fuel.
Manufacturers of aftermarket carbs can help you choose the right one based on whether the engine will remain stock or have modifications. But to get yourself into the ballpark, the following formula is a good place to start. It gives you a simple means of calculating the cubic feet of air per minute that your engine requires at its maximum RPM:
(CID / 2) × Max. RPM × (1 / 1728) = CFM
Which reduces to:
(CID × Max RPM) / 3456 = CFM
The formula is based on a 4-stroke engine where each cylinder takes a breath once every second revolution (every intake stroke), hence the cubic inch displacement (CID) divided by two. And the 1728 factor converts the cubic inches of engine displacement to cubic feet so that we can talk in the CFM terminology used for carburetors (1728 cubic inches equals one cubic foot).
Using a 289 HiPo as our example, the maximum RPM is 8000. Plugging the numbers into the formula, here’s what we get:
(289 x 8000) ÷ 3456 = CFM
2312000 ÷ 3456 = CFM
668.98 = CFM
The result tells us that we need a 670 CFM carb at the theoretical (100%) capacity of the engine at maximum RPMs.
But the engine doesn’t operate at its theoretical capacity because it has restrictions that interfere with the amount of air it can take in. The level it operates at is called its volumetric efficiency; i.e., the percent of air it can actually take in compared to what it would like to take in if there were no restrictions. Each level of tune on an engine changes the volumetric efficiency within a range from about 80% in a stock low performance street engine to 100%, or even slightly more than 100%, for an all-out race engine. Exact values would have to be measured but it’s reasonable to say that the K-code in our example would be somewhere near 90%. And an A-code version of the 289 would perhaps be about 85%.
Taking the 670 CFM that we calculated for the HiPo and assuming the engine can only breath-in 90% of its theoretical maximum, we calculate a carburetor requirement of 603 CFM (670 x 90%). So if we use a 600 CFM carburetor, we should be about right. Ford agreed, by the way; they used a 600 CFM for the HiPo.
Using the same formula to look at a stock A-code with a maximum RPM of 6000, the theoretical capacity would be 501.18 CFM. At its assumed 85% volumetric efficiency, the correct carburetor would need to flow 426 CFM. But Ford was a little more generous when they specified the A-code carburetor. They used a 470 CFM unit, suggesting that the volumetric efficiency of the A-code was 94%. We know that 94% isn’t reasonable if the K-code is 90% so we can infer that going slightly oversize, as Ford did, won’t cause vacuum to drop off far enough to interfere with decent carburetor operation.
The calculations are all well and good but in the real world, there’s a downer. The 4-barrel that we spec’d-out may not flow at its rated capacity. That’s because all of the calculations assume that your engine is in top notch condition and that it will pull exactly 1.5 inches of (mercury) vacuum at WOT (wide open throttle), after you bolt your 4-barrel in place. Four-barrel ratings are determined at 1.5” so any departure from that level of vacuum will affect what actually flows onto your engine. An engine that pulls less than 1.5” will flow fewer CFM than required even though the carburetor has the correct rating. But on the other side of the coin, a really strong engine, one that pulls more than 1.5”, will force the carburetor to flow more than its rated capacity.
How do you know what the WOT vacuum is? Ideally, it should be measured on a test bench but a close approximation can be measured on the street with an in-car vacuum gauge. To test with an in-car gauge, briefly drive the car in high gear from slow speed at WOT and read the gauge. Then calculate the impact on CFM using the following formula:
Rated CFM / (Rated Vacuum ÷ Actual Vacuum) 0. 5 = Actual CFM
The formula tells you to divide the rated CFM by the square root of the rated vacuum (which is 1.5”) divided by the actual (measured) vacuum. If you want to check a 2-barrel carb, substitute 3.0” for 1.5”; i.e., the 2-barrel needs exactly 3.0 inches of (mercury) vacuum at WOT to flow its rated capacity.
If your engine can’t develop the WOT rated vacuum, what are the options? The obvious first consideration would be a rebuild but if that’s not in the cards, a bigger carb may seem like a way to recover the CFM lost to low vacuum. But in reality, a bigger carb will cause the vacuum to fall even lower, exacerbating an already marginal situation. A slightly smaller carb will cause the vacuum to increase but if it doesn’t go above 1.5” and thereby force the carb to flow above its rating, that’s not the answer either. On balance, it’s probably best to use the carb with a capacity equal-to (or slightly-larger-than) indicated by the original calculations and then recognize that an engine rebuild is needed to get full benefit from it.
Now that you’ve completed your calculations and you’ve decided on the carburetor size, you need to consider at least one other important feature. And that would be drivability.
Drivability can be influenced to a significant degree by choosing between mechanical secondaries and vacuum secondaries? The difference is that the mechanical secondaries open and close depending on how much throttle your right foot is calling for. Vacuum secondaries, on the other hand, “know” when your right foot says it’s time to go but they only open when the engine can support the extra air flow. It’s an important difference because too much throttle, when the engine isn’t turning fast enough to handle it, can cause stumbling or bogging.
Mechanical secondaries are desirable on race engines and suitable for highly modified street engines. But the progressive nature of vacuum secondaries make that design more streetable, in many cases. Envision the engine using the vacuum signal that it generates to “talk” to the secondaries. An increasing vacuum signal tells the secondaries that the engine can handle more air and in turn causes the secondaries to open in proportion to the signal; i.e., progressively.
If you prefer to control the secondaries directly with your right foot (mechanical secondaries), you should at least consider a carb with restrictor plates at the top of the secondary throats. The restrictor plates are closed like choke butterflies when the secondary throttle plates are closed and even if you snap all four throttle plates wide open, the secondary restrictors won’t open at the same time. In effect, they stifle air flow through the secondaries until the engine can support the extra flow; then they open progressively. The arrangement gives you the best of both worlds: mechanical control of the secondaries with reasonable control over drivability. The restrictor plates are typically spring loaded and adjustable. Adjustability gives you the opportunity to decide when you want them to open; i.e., when should the air column entering the engine be allowed to push them open. It’s a matter of trial and error to find the adjustment that gives you the proper balance between smooth performance and stumbling or bogging.
If you plan to go beyond a simple carburetor change or a carb coupled with an exhaust upgrade, then you’re getting into serious engine modifications. If that’s the case, there are several engine builders that can give you a good piece or you could buy a crate engine. Both approaches are worth considering because you could be pretty well assured of well developed parts synergy. On the other hand, if a do-it-yourself project is what you have in mind, then a performance parts supplier like an Edelbrock, for example, can help point you in the right direction with complete kits designed and tested to work together.
By way of a footnote, when it comes to replacing a 2-barrel carburetor with a 4-barrel unit, the intake manifold must be changed at the same time. Intake manifold design is important. Race design is different from street design. Be sure to get the right one. At some time in the future, there may be a Pony Tricks article on intake manifolds.
The bottom line is that it’s best to plan ahead so that in the end, you’re happy with the outcome of your project.