There are a lot of myths and misconceptions surrounding the subject of carburetion. Let’s first look at what a well-designed and calibrated carb can do, then at what a carb cannot do.
This Tech Tip is From the Full Book, DAVID VIZARD’S HOW TO SUPER TUNE AND MODIFY HOLLEY CARBURETORS. For a comprehensive guide on this entire subject you can visit this link:
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Functions of a Carburetor
What a carburetor is supposed to do can be described in a few simple sentences. Getting it to do just that, though, can be more difficult. Let’s start with a list of required functions.
- Mixture ratio
- Fuel atomization
On the face of it that short list does not sound like much of a challenge but a reality check reveals this is far from the real world. However, what we are trying to achieve is far from impossible. If you are of an older generation you may have heard a quote bandied about that sounds really neat but is in fact not true. The name of the professor responsible escapes my mind but the quote has stuck for 40 years and it goes like this: “A carburetor is a wonderfully ingenious device for giving the incorrect mixture at all engine speeds.”
It sounds pat, and coming from a learned person it tends to have all the makings of the truth. The truth of the matter is that a Holley (or any other good carb) can be calibrated to deliver a very precise mixture so as to produce excellent results under wideopen-throttle (WOT) conditions. In other words, let’s assume at some RPM your engine is capable of delivering 300, 400, and 500 hp at three points going up the RPM range. Get your carburetion right, on the circuits applicable to WOT, and your results will hit the mark at each point within one or two horsepower or better. That’s well into fuel injection territory but at a fraction of the cost. Let’s look at the three functions in a little more detail.
The carb must flow an adequate amount of air to meet the required needs for the engine’s intended purpose. For most humdrum applications, such as lawn mowers and common street machines, maximum output takes a distant second place to cost. If you are reading this book you probably want more power for greater speed and acceleration. That means knowing just how much airflow is enough, and, as it happens, not falling into the trap of supposing that a little too much is okay. With some carbs, such as an SU, a little too much is okay because the carb only opens as much as required to satisfy the engine’s airflow demand. This type of carb is called a constantvacuum carb. Most Holleys are a type of carb known as a fixed-jet/choke carb. Although there are many advantages to this type of design, as we shall see, there is a downside because too big a carb produces inferior results. Throughout this book it becomes apparent why this is so and how best to avoid the negative issues of too big a carb.
It is important for a carb to mix the appropriate amount of fuel for its current operating conditions. This means a full-power rich mixture when maximum output is called for and a suitably lean mixture when economy is required. I already stated that getting the full-power mixture right over a wide range is entirely possible. While getting reasonable economy is doable it is a lot more difficult to get super economy without losing drivability. Chapter 5 delves into the practicalities of super-lean operation for maximum economy; but remember that achieving it is not easy.
On the face of it, the subject of fuel atomization seems simple enough. The better the fuel is atomized, the better the engine performs. Well, once again, the real world is not quite that simple. In practice there is a best atomization process for economy and a best process for full power. In each case the goal is to maximize cylinder pressure for the amount of fuel used. Chapter 7 goes into much detail on carb, venturi, and booster design parameters in relation to intake manifold design and operating conditions.
Unless you are really into carb design, the inner workings of a carb look like a collection of parts for executing a little black magic. Don’t feel bad about that; most of today’s top carb designers were there once. The trick race Holley carb you see in Figure 1.1 is complex but not unfathomable. In reality, the function of any carb can be reduced to two simple functions that it must perform. The first of these functions is to deliver a certain ratio of fuel and air to the engine. The second is to atomize and distribute the fuel so that a significant portion is vaporized and the remaining wet fuel is sufficiently well atomized to turn to a vapor as the piston reaches top dead center (TDC) on the compression stroke. (See Chapter 3 for more detail.)
Economy Air/Fuel Ratios
Here I want to talk about air/ fuel ratios and the much-rumored 100-mpg carb that the fuel companies supposedly brought out and then buried to not lose profits. Let’s start with the fact that any respectable carb can be calibrated to deliver whatever air/fuel ratio you want to the extent that it can range from way too lean (insufficient fuel for the air) to way too rich (too much fuel). However, you must never loose sight of the fact that it’s not what you may want to see in the way of mixture ratio that counts; it’s what the engine wants.
At the lean end of the scale, the most important for fuel economy, most engines run into lean misfire at about 18:1 to 19:1 air/fuel ratio. However, with enough high-tech R&D and components put into the engine it is possible, as was found on my EconoMin project to run effectively at just over a 22:1 air/fuel ratio. I may have summed this up in a sentence but be assured there was more to it than just leaning out the mixture a lot more, in fact.
Now consider this: If you can get any decent carb to deliver an air/ fuel ratio significantly leaner than, say, 30:1, the problem of finding fuel mileage must not be constrained by that carb’s ability to deliver a lean enough mixture. In other words, any decent carb (and there are dozens out there) delivers an economy mixture far leaner than any current (2012) engine can burn.
Some carbs, such as the constantvacuum SU and Stromberg carbs, can atomize fuel better than a 45-psi fuel injection nozzle, so the question of whether or not fuel can be adequately atomized is answered: Yes, it can. Not all carbs are as good as these at atomization, but there is a fix.
If a carb fails to atomize the fuel well enough, a little manifold heat fixes the problem. Fuel-economy driving takes place at part throttle, so it also means that there is, if the right cam events are used, a lot of intake manifold vacuum present. This also has a strong influence on vaporization.
Any decently designed and correctly sized carb can cover a far wider range of fuel mixture conditions than the engine can actually deal with, so what is the rumored 100-mpg carb going to do that the ones we currently have available cannot already do? Answer: absolutely nothing! The engine is the limiting factor, not the carb. So you see the argument for the existence of a 100-mpg carb is starting to become somewhat tenuous. But I have not finished yet. We don’t need to consider the 100-mpg carb in terms of a 50-cc moped because that is not in context with what is so often referred to. When discussing the possibility of a 100-mpg carb, the context is a midsize sedan and speeds at which we could reasonably expect people to drive.
Let’s say such a vehicle is a typical 2,300-pound sedan (and that’s on the small side) and the speed is, say, 50 mph (and that’s on the slow side). By picking these conservative numbers I’m giving the benefit of the doubt to the 100-mpg carb. Powering such a car at a steady 50 mph typically takes about 15 hp. With brake specific fuel consumption (BSFC) figures of a modern engine being what they are at part throttle, such a car would turn in right around 50 mpg at that speed. Also it would be utilizing the fuel’s energy at close to 25 percent (that is, the engine’s thermal efficiency is 25 percent). If this hypothetical engine converted 100 percent of the fuel’s heat into mechanical energy the mileage would increase to about 200 mpg.
Those numbers demonstrate a lot of room for improvement in engine design. The problem is not with the carb’s design so much as the typical engine’s inability to convert the fuel’s total heat value into power. The carb has the ability to deliver any suitable mixture the engine may require. The carb is already nearly 100 percent effective, meaning the 100 mpg comes from engine development, not from fuel-delivery development (i.e., the carb). This means that long ago carburetors had already reached nearly maximum potential and there is little room left for improvement. From that, we can safely conclude that the 100-mpg carb never has existed and never will!
By now you should be convinced that as long as the carb is suitably calibrated, at least 98 percent of any mileage improvement comes mostly from engine development and maybe a little extra from fuels. If you want easy-to-get mileage you could drive more carefully, avoid passing, drive slower, and back into the driveway at night. But if you are anything approaching a type-A personality, like me, the very fact there is a car in front means that some overtaking is due!
I have to race something and the only way I have been able to consistently drive for economy is to race a trip computer that reads out in mileage to date and instantaneous mileage. The bottom line is: I need to get there now, not five minutes later. All this adds up to one thing for me: The vehicle I drive better be able to go fast on next to no fuel.
You can read my other CarTech books (How to Build Max-Performance Chevy Small-Blocks on a Budget, How to Build Max-Performance Chevy BigBlocks on a Budget, and David Vizard’s How to Build Horsepower) for more detail on how to increase the output of a typical Detroit-built street V-8 engine. The principles apply to all engines and explain how to increase output by as much as 100 percent. While focusing mostly on power a lot of the power-improving techniques bring about an increase in mileage due to better fuel usage and the reduction of losses by friction and exhaust system inefficiencies.
Yes, this book is about carbs, and Holley carbs in particular. To get mileage, select a carb that has features that favor part-throttle usage (see Chapter 6 for more detail). But to make the most of your econ-carb choice be aware of many engine modifications (some minor others not so minor) that complement what your carb can do. The questions you may want answered and the fuel-saving modifications are in alphabetical order on the following list:
- Big-bore short stroke or smallbore long stroke?
- Big valves or small?
- Camshafts for mileage
- Carb calibration
- Cold-air intake
- Compression ratio effect
- Exhaust system length and diameter
- Fuel • Gearing
- Ignition timing
- Intake port dimension and finish
- Intake-to-exhaust size ratio
- Internal friction
- Mixture preparation
- Oil and lube (how to go 750,000 miles before a rebuild)
- Optimum valve size
- Piston speed for optimum mileage
- Swirl and tumble
- Supercharger and turbocharger
- Thermal barrier
- Two-valve head versus four-valve head; which is more fuel efficient?
With a list this long you can see that getting better fuel mileage is just a little more than super-tuning your Holley. So, next time you hear about the 100-mpg carb you can rightly scoff at it.
Written by David Vizard and Posted with Permission of CarTechBooks