How to Choose the Proper Carburetor and Fuel Pump

Sooner or later, everyone that runs a carbed car, and people that want to convert over from fuel injection, will have to choose a new carb. Maybe your old stocker is worn out, or maybe you just want to upgrade. Either way, the choices available can often seem very hard to narrow down. The purpose of this article is to give you a few pointers, and maybe some experienced advice on what brands and features to look for.

Brand
Holley
Holleys are by far the most popular aftermarket carb to be put on engines of all shapes and sizes for the past 50 or so years. People tend to either love or hate them, myself being one of the former. They do have some definite advantages, the biggest being commonality. They can be found everywhere for dirt cheap; swap meets, ebay, classifieds, you name it. Finding a fully functional used one for $20-40 is not at all uncommon. They have the largest aftermarket, parts are available at almost anyplace remotely automotive oriented. They are very simple to rebuild/repair. They are also extremely tunable. With the right combination of readily available jets and powervalves, almost any Holley can be made to operate at least fairly well on any engine. They are the undisputed horsepower king, coming out on top in the majority of dyno tests ever performed. However, the biggest down side to most people is they are not a bolt on and go type carb. Running one will require time spent learning how it works, some book reading, and some time/effort tuning it. They also have a reputation for being among the higher maintenance required carbs out there, and not the most fuel efficient.

Demon/Barry Grant
These carbs are basically a copy of Holley’s design, but made with better materials and a few minor improvements. They have a lot of interchangeable parts, which is a big plus, but they also have a much greater cost. They feel more "well made" to people and look a lot prettier. In the tale of the dyno however, they are pretty much exactly equal. It’s up to the individual consumer to decide if they want to ante up the few extra bucks.

Holley Street Avenger
These are basically a normal Holley carb that’s been cleaned up, given a shiny coating, have a few more bells and whistles bolted on, and sold at a much higher price. I don’t recommend them. I say you are better off getting a normal Holley, putting on whatever additional stuff you want on it, and getting the same thing for cheaper.

Edelbrock/Carter
These 2 carbs are very similar, so I will lump them together. Performance wise, they rank very low. They will show a loss compared to any similar Holley, Demon, or even a well prepped Q jet. They have maintained a very strong following however, because of a reputation for a few redeeming features. Foremost, the ability to bolt on out of the box and go. On most mild/average motors, they will require little to no tuning, little to no maintenance, and some people think they look better than other brands. They are simple and easy to work on. But, if you do need to tune or rebuild it, it is not as simple or cheap a matter as a Holley, nor are parts as readily available. Generally speaking, they are for the person who it willing to sacrifice performance in the name of reliability and not having to mess with it.

Rochester Quadrajet
Countless Q jets have been put out by GM on everything from V6’s to big blocks in both computer controlled and non-CC forms. Therefore, there are a lot of people out there very familiar and very fond of them. They still have a strong following, even though none have been made since the mid 80’s. Not entirely for sentimental reasons either; they’re not a bad carb at all in the hands of someone who knows how to work with them. They can be made to work with a huge variety of motors, they are reliable, fuel efficient, are more likely to be emissions legal, and can make decent performance (albeit not up to the level of a Demon or Holley). The downsides are that they are much more complex and hard to work on than the rest, and there is a limited aftermarket or parts availability. Getting a Qjet or any parts for it usually means a trip the junkyard or swap meet to scrounge. Obviously, the computer controlled versions have that additional huge downside built in.

Other
There are many other brands of carb out there that I haven’t discussed. Webers, Predators, and Strombergs just to name a few. Suffice it to say, if you have to read this article, such exotics are not for you. Stick to more proven and common brands.

Vacuum Secondaries vs. Mechanical
When looking at carbs, you will note that some, particularly Holley-style ones, will be advertised as either vacuum secondaries or mechanical (double pumpers). Choosing this is a very important part of the selection process. As you should be aware by now, the type of carbs typically talked about here are 4 barrels. The majority of the time, they operate on the front 2 only, moving to the rear 2 as RPM increases. Vacuum secondaries, as the name implies, opens the secondaries according to a spring resisted draw from a ported vacuum source. Mechanicals open progressively by method of some linkage to the primaries.

Opinions vary on exactly where you should draw the line between the two. I think the most people would agree that you should stick to a VS carb you have an automatic car with less than 3.73 gears and/or a less than 3000 stall converter. With more than that, or with a manual car, you should strongly consider a double pumper.

CFM Rating
As many of us know, the consumption of an engine can be described with the equation {(CIDxRPM)/3456} * VE. CID means the cubic inch displacement of your motor, RPM is the max RPM reached, and VE means the volumetric efficiency. VE on a stockish motor is usually 60-70%, where as professional full race motors can reach close to and even a little above 100%.

Anyway, that equation spits out some number in CFM, and often sends people to buying a carb as close to that size as they can get for their motor. Well it doesn’t really work quite like that. I read an article awhile back on a small block build up and dyno test. It was a pretty healthy engine (dynoed in the mid 400s), but it was still just 350cid and streetable. They ended up going with an 850cfm carb because the dyno showed a significant gain over a 750, both Holley DP’s. Even at 100% VE that only means about 600-700 CFM, yet it needs an 850cfm carb.

Well the reason is that equation represents the AVERAGE intake of a motor. But motors don’t take in air in a nice smooth average flow. They take in air in pulses as each cylinder fills. An engine may only average 500cfm, but it takes in at a 1000cfm rate, then 0, then 1000, then 0 in a matter of milliseconds. So even though it may only consume 500cfm according to that equation, you still need a 1000cfm carb to feed it properly. Note, those numbers are just for example to illustrate a point, and are not at all realistic.

In reality the intake manifold (specifically the plenum) exists in-between the cylinders and carb, and acts as a sort of buffer. The greater the volume of the plenum, the less draw there is directly on the carb. Smaller manifolds, like say, your basic street dual plane, place more draw on the carb.

That is why TPI-based EFI engines, say like the Ramjet502, live well with such a relatively small throttle body (48mm, even on the 502). They have that huge honk’n plenum that makes sure the TB only sees the average consumption rather than the instantaneous. It’s also why big single planes have such a bad reputation for being boggy. People decide to build a hot street/strip motor, get a huge single plane and way over carb it. It’s also partly why an 800 whatever cfm Qjet does so well with a crappy stock manifold on a small motor.

No, I’m not saying you should go bolt a Dominator onto your stock 305 with a stock manifold for best results. Depending on the manifold, you should multiply the number that standard equation gives you by 1.2-1.5 roughly to give you a better idea of what your engine really needs. For any 305, 600-650cfm should be plenty. For any healthy 350 CID or larger motor, you shouldn’t run any less than a 750cfm, and perhaps up depending on your particular combo.

Selecting a Fuel Pump
When upgrading, it’s common to jump for whatever super duper 500,000gph electric pump catches your eye, or just stick a regulator on your stock pump for the EFI guys. The fact is, for any NA street car, a normal Carter or Holley mechanical pump will work just fine, be much simpler, quieter, and lower maintenance. If you’re running a poweradder, or if your OEM cam doesn’t have a fuel pump lobe, only then should you really consider an electric. Also, if you are swapping from EFI, I would like to highly recommend not just putting a regulator on your stock pump. In the long run, you are just costing yourself more. Your stock pump will fail eventually, and you’ll have to do it right anyway. Drop your fuel tank and install a carb pickup or sump from the beginning.