Choosing a carburetor
Choosing the right carburetor is critical for your engine combination. It’s one of the most critical choices deciding the way your engine makes power and how easy it is to live with on a daily basis. So take your time to consider this carefully, and make sure you don’t jump out and buy what you heard some guy recommend, or what you saw mounted on top of a race engine last weekend.
There are several factors to consider.
-The first is the physical layout, will it fit your engine?
-The second is the brand. For some parts the actual brand is almost irrelevant, because the physical part beneath the brand sticker is in fact the same, this goes for some ignition systems and for some camshafts. But when it comes to carburetors, the brands are quite different.
-The third and maybe most important choice is the size of the carburetor. This is without doubt the area where most people go overboard, with the best intentions (trying to be faster than the other guy is a good intention, right?), but with disappointing results. Fortunately there is a formula to calculate the needed size, so it should be straight forward.
-The fourth choice is when you’ve chosen the brand and size, some brands still give you further choices to make, depending on whether you want a daily driver with decent fuel economy or a weekend racer trailered to the track to race only.
All carburetors try to obtain the same thing, delivering the perfect volume and air/fuel ratio regardless of engine rpm, regardless of engine vacuum and regardless of external factors like temperature, atmospheric pressure (affected primarily by height above sea level) and humidity. Not a small task. And that task has to be performed sitting on an engine that shakes, that see g-forces when accelerating, cornering or braking, and that sometimes gets so hot there’s risk of vapor lock etc. This has to be obtained to perfection regardless of whether you’re idling a winter morning or flooring the pedal on the freeway. And it has to do all this and use as little fuel as possible. A lot of engineering and testing has gone into all the carburetors you can buy today.
Some words about Electronic Fuel Injection. On modern engines the carburetors have been replaced by computer controlled fuel injection. Why is that? It’s not because fuel injection can do some magic that a carburetor can’t, they both try to meter the fuel volume and air/fuel ratios as perfectly as they can, the laws of physics didn’t change when fuel injection came along. But the advantage of fuel injection is an ability to meter the fuel volume and air/fuel ratio more precisely. Initially fuel injection systems had two major disadvantages seen from the enthusiast’s point of view. Firstly it was very expensive, and secondly it was non adjustable unless you were almost a NASA engineer. Today it’s still at least double the price of a good carburetor, but the good news is that modern fuel injection systems are fully adjustable if you have or can borrow a laptop. If you decide to go for that extra expense and get fuel injection instead of a carburetor, do read on anyway. The Tuning section of these pages deals primarily with how you adjust a carburetor, but all the information on what kind of fuel volume and air/fuel ratio the engine needs in any given situation, and the information on how to e.g. read your spark plugs and your vacuum gauge to decide on whether to go more rich or more lean, all that information applies regardless of whether you adjust your carburetor bent over the engine with a screwdriver of whether you sit in the driver’s seat with a laptop connected to your fuel injection system.
Physical Layout:
In the performance world of American V-8s, carburetors are 2-bore or 4-bore. Unless you are restricted by class rules, always take a 4-bore. And if you want to buy a new carburetor for your engine and that engine has a 2-bore manifold, don’t buy a carburetor. Wait, save up the money to buy a manifold and carburetor with 4-bore layout. So, the main decision is mostly whether you have a spread bore or a square bore manifold to attach the carburetor to. There are other layouts, like Ford’s Autolite, but nobody seems to build aftermarket parts for that layout in any volume.
Some words about adapters. You might think adapters can save you some money, they help you in some situations, so you can avoid buying both a carburetor and a manifold, right? Well, maybe. But adapters do what you’d call “compensating” instead of “correcting” the issue. Or to use a more popular phrase, it’s a band aid solution. The engineers spend a lot of time and money optimizing the air flow on the products they sell, always assuming that there are no adapters involved. So when you add an adapter you never get the power you were intended to get. The engine will of course work and produce power, but you’re missing out on some potential. And that’s the aim of these pages, to ensure you get full potential of your investment and time. So again: don’t buy an adapter, save up and buy both a manifold and a carburetor. See Choosing an intake manifold.
Brand:
We’re looking for a carburetor that’ll give us the performance and drivability we want, and not use too much fuel. No brand out there is better than all the other brands on everything. Some brands give power, some provide good fuel economy, some provide long trouble- and adjustment free service, some provide almost indefinite number of adjustment possibilities. And finally, some provide a brand name that most people like to be associated with. After all, if the real fast guys have x-brand sticker on their cars, why should we settle for less?
First let’s discuss some of the factory carburetors. Brands like the Rochester Quadrajet don’t have much prestige attached to it these days. But they are excellent carburetors in many ways. We guess we’ve all at some point in time driven GM cars with a Rochester, and never had to worry about what was going on under the air cleaner. They just keep on working and don’t need much adjustment. They provide excellent fuel economy, and almost (if well tuned) the same performance as any performance carburetor. A strong claim? Many drag racers have used Quadrajets with success. But the Quadrajet has some things working against it also. It’s not a known performance brand name; have you ever seen a Rochester sticker on a race car? It doesn’t look to sexy either, being tall and round. And while the two previous reasons are somewhat psychological, the real draw back is that the Quadrajet is not that easy to modify and tune. Changing primary jets on the Quadrajet between races is not an easy task. On the other hand, richening the secondary metering can be done in 10 seconds, because it can be done without taking the carburetor apart.
Quadrajet also has an excellent bore configuration, with small primary and large secondary bores, good for economy and performance. And another clever design detail is the power system, where it has gradual amount of extra fuel added as opposed to e.g. a Holley where the power system (power valve) is more an on/off thing, good for all out performance but not for part throttle economy.
Now on to Performance Carburetors. One brand name pops up immediately: Holley. Holley is without doubt the most used and most winning performance carburetor out there. Some are sold under the Holley name, but some specialist shops like Willy’s and Quick Fuel take Holley carburetors and make them even better. Other brands include the Edelbrock Performer and Thunder Series, which has a very good design. It’s fully adjustable like a Holley, it has metering rods that can be changed with little disassembly and it has a leak free design. Edelbrock claims that their carburetors are more ready to use out of the box than a Holley: That might be true, but should be irrelevant to readers of these pages, because we’re not satisfied with out-of-the-box performance, we want to tune it to perfection, right? Lastly, if you like a like shine, the Edelbrock carburetor looks like chrome with its EnduraShine finish.
So which is the best? Nobody can really say, but each has their own preference. You can’t go totally wrong buying a Holley, because they’re so widely used. Everybody knows about them, most of your friends will have parts lying around, when you need a new power valve on a Sunday. And you can buy very nice tuner-kits that contain all you need. So if you want to make life easy for yourself, buy a Holley. It’s like starting with a small-block Chevy, parts are a plenty and cheap. But if you’re the kind of person who takes a personal pleasure in going against the trend, if you try to build power with a Ford Cleveland just because everybody else is building small-block Chevys, why not see if you can get a competitive advantage by buying something else?
Our personal experience has primarily been with Quadrajets and Holleys. We’ve raced several cars with Rochesters, they were quite heavily modified with kits from places like The Carburetor Shop, and we’re probably some of the few people that can take the top of a Quadrajet with our eyes closed. Our fully stock 68 Camaro has the original Quadrajet and that’ll never change. That said, in the later years we’ve used Holleys whenever we want to go faster. Mostly because we understand them, and we have many jets, power valves etc. on the shelf. Holleys have their problems though, and for years we've wondered why their R&D department didn't fix it. They could have made tons of money if they updated to a better carburetor and if it was not fully compatible with the parts of the old design, people would have to buy new parts for it. Holley never did that, and went bankrupt.
Today vendors like Summit Racing have their own carburetor. It looks like the carburetor that Holley should have built but never did. More adjustable where it matters, easier to adjust e.g. fuel level, better venturis etc.. And it shares many Holley parts. We've so far bought two of these Summit carbs, both worked extremely well out of the box. Especially throttle response and drivability was the best we've ever felt from a carburetor right out of the box, without a single adjustment. It looks good as well and is cheap, so if it comes in the size and vacuum/mechanical configuration that you need, you might want to try it.
Size:
We all know size matters. But as opposed to many things in life, here bigger is definitely not better. The most common error we've seen on street cars and a few race cars is too big a carburetor. But if the engine is an air pump, why not get the biggest carburetor, to avoid having a restriction there? Without getting too technical, big bores mean less velocity and less acceleration of air/fuel mixture. That may be a little difficult to accept, one would think that the less restriction, the more velocity and acceleration. Though it’s not exactly the same principle, think of garden hose where the water is flowing at a certain rate. If you then squeeze the end of the hose, then you can get it to spray much further. At the top end of the rpm band, too small a carburetor will be a restriction and limit power. Small but adequate and non-restrictive passages builds air/fuel speed and thereby throttle response. Too small a carburetor may lose some power in the high rpm band, but will work excellent in the 0-5000 rpm band. But while too big a carburetor will work well in the 5-7000 rpm range, it’ll be sluggish everywhere else, and especially the throttle response will suffer. Do you want a car like that?
So what‘s the ideal size? Fortunately there is a simple formula. But with such a formula available and accepted by everybody, how come so many people still buy the wrong size carburetor? Unfortunately the result of the formula is usually much lower than what self claimed experts tell you. So if your friends or competition runs 750 or 850 cfm carburetors, do you dare buying a 600 cfm? More must be better, right? No. This, like camshaft duration, is an area where you’ll be thankful that you trusted the formula and not the hype. The formula works. If the thermometer shows 30 degrees and your friend tells you it feels more like 40 degrees to him, which do you trust?
The formula is:
Engine size x Engine rpm x Engine VE
3456
-Engine size is in cubic inches (cid). This is the easy one.
-Engine rpm is the max rpm that the carburetor should be able so support. Don’t go overboard on this one. You may wish to run a 7500 rpm screamer, but unless you strengthen the engine internals like crank, rods and pistons to be able to withstand it, you’ll only try it once. And honestly, when we accelerate full throttle, do we wait until max rpm to shift into next gear? Rarely, because it doesn’t make the car faster (see How to win, "4"), and most of us have so much money and time invested in that engine we want to keep it alive. So if you add up the total amount of time your engine has been above 5500 rpm, it’ll be a few seconds only.
-Engine VE is Volumetric Efficiency, a number telling how effective an air pump the engine is. It takes a dyno to measure your engine’s Volumetric Efficiency, but here are some ground rules. For stock smog engines VE would be around 0.8, for a well built performance engine it would be around 0.9. An engine with forced induction could be just above 1.0.
Example: A well tuned 350 cid engine that will see max 6000 rpm would need the following size carburetor:
350 x 6000 x 0.9
3456
equal to 547 cfm(!) So a Holley 600 would be a great and fully adequate choice. Still many engines have 750 cfm or 850 cfm carburetors on them, enabling them to theoretically go into rpms that they never do. So it’s a waste of money buying the big carburetor. But even worse, the oversized carburetor makes the car less fun to drive below 5000 rpm because the carburetor is ruining drivability and throttle response.
Type:
Besides the Physical Layout, Brand and Size we’ve just looked at, if you end up choosing an Edelbrock or a Holley carburetor, you need to choose the right series.
Edelbrock has the Performer Series and Thunder Series. We’d go with the Thunder Series simply because the secondary air valve can be easily adjusted, a feature the Performer Series doesn’t have. And if you buy and Edelbrock, remember to buy the Calibration Kit, you’re going to need it.
On Holleys the important choice is whether to go with vacuum secondaries or a double pumper. The difference is that on the vacuum secondary carburetor the second stage only opens based on demand from the engine. This is obtained by using springs to keep the secondaries closed until the engine demands more flow. But the double pumper opens the secondaries via a direct link to the throttle, and has an extra accelerator pump to allow that to happen without a creating a bog. No doubt that the double pumper is the most expensive and uses the most gas. And maybe looks most exotic? But a well tuned vacuum secondary carburetor will often work just as well. Rarely does an engine really need a double pumper, but of course they do make tuning the secondaries less critical. But again, a well tuned vacuum secondaries carburetor will perform as well as a double pumper and use a lot less gas. If you buy a Holley, remember to buy tuning kits as well: Jets, Power Valves, Accelerator Pump, and Secondary Springs etc. And unless you enjoy spending 30 minutes scraping off old gasket material, buy reusable gaskets.