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How to choose the right size carburetor for your engine



How to choose the right size carburetor for your engine

Choosing the right size carburetor is key for getting the performance you're looking for out of your engine. There are so many choices! 500, 600, 650, 750, 850, dual 4s, Dominators, and more, so which one is the right one for you?

Well, for starters, did you know that the vast majority of performance car owners have carburetors on their engines that are too big and they're killing performance at low to mid range RPMs? This tech tip will tell you why, and how, to not make the same mistake most hot rodders do when it comes to choosing the right size, and type of carb. Watch the video and/or read the written tech tip below to learn how to choose the right size carburetor for your car.

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The type and size (cfm) carb you should be running on your engine will vary depending on numerous factors. Let me begin by saying this first; about 90% of the so called "hot rods" out there have carbs that are WAY too big. More carburetion does NOT mean more power. In fact, what it really means is less torque, less get-up and go at lower RPMs, and less acceleration off the line, which is where most races are won... and lost.

First off; you have to consider how a carb works. It's a simple design with several transition "circuits", from the idle circuit, the transition slot, the accelerator pump circuit, the power valve circuit, to the main jet circuit. Then there are other "helper" circuits such as the low speed air bleeds and the the high speed air bleeds, the emulsifiers, etc.

When an engine is idling, you are on the idle circuit. As soon as you mash the gas pedal, the throttle plates open and the engine (being nothing more than an air pump) takes a gulp of air. Well, your engine doesn't run on air because air doesn't burn, so you have to add a quick shot of fuel along with that air so the engine will respond and not fall on its face.

The accelerator pump squirts a metered shot of fuel along with that initial gulp of air to get that engine to respond and begin to accelerate. In literally about 1 second of time, the accelerator pump shot stops and the increasing air flow going through the barrels / over the boosters begins to "siphon" fuel through the main jets - maintaining a metered amount of fuel along with that steady incoming air flow.

The "size of the air pump" (your engine cubic inches) determines the velocity and volume of air going through a given size of carb. A smaller carb will respond quicker because the air speed is going through it faster than what a larger carb would have going through it.

Think of it like this; your lungs are your body's air pump. Grab a typical soda straw and blow through it hard. The air coming out the straw is fast and powerful. Now grab a piece of 3/4" heater hose and cut it to the same length as the straw and blow through it as hard as you can. The air speed coming out the other end isn't nearly as fast or as "feelable" with your hand as it was with the smaller straw. Now imaging doing this with a 2" piece of radiator hose. You would probably feel no air velocity coming out the other side because that hose is so big compared to your air pump (lungs) that your air pump (lungs) can't push enough air fast enough through it to feel it. You would need a bigger set of lungs with much more capacity to get better air flow and speed through that 3/4" heater hose, and even more yet to get any kind of velocity going through that 2" radiator hose. Well, engines are no different. They are nothing but an air pump like your lungs.

That air pump (your engine) pulling through a small carb is going to have a MUCH higher velocity going through it than one that is too big, which means it reacts quick. Low velocity means the carb can't react and "give" fuel to the engine very quick to meet the amount of air it is trying to take-in at any given time. This is why guys with too big of carbs need to "feather the throttle" at low RPMs to get their car going and then say: "Yeah, but once I get going, it REALLY goes!". Yeah, because at lower RPMs your engine isn't pumping much air, so that big carb can't react UNTIL you get going and your RPMs start coming up! You nail the throttle and the engine takes a gulp of air but can't get the fuel to maintain the air/fuel ratio to make it accelerate, so what do you have to do? Feather the throttle (open and close it) to help "create" more velocity to make it react and to get that accelerator pump to squirt fuel to meet all of that air the engine wants to take-in but can't get the fuel with it. Once you get the RPMs up, the engine is pumping enough air volume to create enough velocity to finally make the boosters react and feed fuel to the engine. In the mean time; a guy with a properly sized carb can nail his throttle and accelerate off the line leaving you and your sluggish carb 3, 4 or 5 car lengths behind. Well, in a stop light to stop light race... you just got your doors blown off! It's also why a lot of guys can either not do, or maintain a good burnout, because the carb is simply way too big to get the engine to react to break the tires loose and maintain that burnout all the way down the block. As soon as the engine starts to see that load, the air speed (velocity) slows down and the carb fails to deliver an ample amount of fuel to make any power, so it bogs down and you look like a fool with a turd of a car in front of all of your friends. This gets even worse with multiple carbs! Two 4's are bad enough, but an 8 stack Weber system is the worst! Now add things like your distributor not being curved right to deliver the timing (advance) your engine needs at low RPM to generate power and you REALLY end-up with a turd. I'm here to help you learn and understand these things so you aren't driving around in a big, giant turd. 

How To Choose The Right Size Carb

Now, obviously "race cars" use big carbs (even two big carbs sometimes) and accelerate great, but you have to remember a few things; Real race cars run really low gears, such as 4.56's, 4.88s or lower to get that engine TO it's peak power making RPM as quickly as possible. They also run high stall converters - again to get that engine INTO it's best torque making RPM to launch as hard as possible, which is sometimes at upwards of 5,000 RPM or more. How many times are you running at, or above 5,000 RPM or more on the street under normal driving conditions? How many stop lights do you, (or even CAN you) launch your car at 5,000 RPM? Without a 4,500 - 5,000 RPM stall converter, you won't be able to, where a real "race engine" launches in that RPM rane every single time. You certainly don't see any race cars at the track taking-off at just above an idle, or running down the track at less than their peak power making RPM, which is usually between 6,000 and 8,000 RPM, so why would anyone consider running the same size carbs that those types of engines and cars run on their street car? It's dumb is what it is, yet a LOT of guys do it and then wonder why a mom with her 4 kids in a V8 Mercedes or BMW SUV can blow their doors off and not even know you were trying to "race" them.

You have to consider that even NASCARs with their 356 cubic inch engines run 750 carbs and sustain 9,000 RPMs making 800+ HP (before they went to EFI), so are all of those guys who are over carbbed trying to tell me that their engine needs to, or is even capable of running at 9,000+ RPM and make 800+ HP? Hardly! So if a 750 carb is "big enough" to support an 800+ HP on an engine that sustains 8,000 - 9,000 RPM, then why is that same size carb on engines making HALF of that power and running at half that RPM? Again, it's because most guys over carb their engines. And to compound the problem and make it even worse, they run rear-end gearing that is WAY too tall, have stall converters that are way too tight, and cams that are way too big and lumpy, because they want that "race car sound", and then wonder why mom and her 4 kids in that stock V8 (a SMALL V8 at that) can out run them on any given day. It's because that SUV was DESIGNED so everything works TOGETHER, where most guys with "hot rods" don't "design" anything into their car, and instead, just run what either everyone else that runs slow is running, or they run what they see guys at the track running without taking into consideration a couple of VERY important factors; 1) Race cars at the track don't drive like you do on the street, and 2) Race cars at the track have other things besides the engine to help them accelerate and go, such as gearing, a high stall converter, and run at what RPM they NEED to run at compared to what your street car runs at 90% of the time.

How To Choose The Right Size Carb

So, what size carb SHOULD you be running on your engine? This chart will enlarge when you click on it and will show you, by physics & mathematics, what size carb is required for an engine running at the PEAK RPM shown at the top.

Keep in mind; that is PEAK RPM, not "sustained" or normal running RPM, and it doesn't take into consideration things like gearing and stall converters. Obviously if you have a hot little 350 engine in your car and that car is lightened-up a bit and has something like 4.11 gears in the back and a 3,000 RPM stall converter, you'll be able to get away with running a larger carb than someone with a heavy car, tall gearing and a stock torque converter to get that engine moving, so even with physics and math, nothing is absolute or static, so use the chart as a "guide" and again, keep in mind you are using a PEAK RPM number to base this carb size by, so in MOST CASES, a slightly smaller carb that what your peak RPM is will be a better, all around performing carb than one that is on the big side.

Just remember; a smaller carb = quicker throttle response and lower RPM acceleration. A bigger carb means nothing more than better TOP-END acceleration. On a carb that is too big, everything below that peak operating RPM WILL suffer, meaning; your so called "Jack Rabbit" starts will be more like slow turtle starts. Your throttle response will be sluggish. Your gas mileage will suffer because at lower RPMs you'll be feathering the throttle more to meet that sluggish velocity with fuel needed from the accelerator pump rather than from the main jets where it's supposed to be coming from. If you have to choose between two sizes of carbs, in MOST cases on the street (or even at the track sometimes), you'll be happier with the smaller carb.

So looking at the chart and taking something simple as a base line engine, such as a 350, you can see that a 600 cfm carb would be best for an engine who sees a PEAK RPM of 6,000, and that a 550 or even a 500 cfm would be even better yet for something that isn't seeing that 6,000 RPM range much.

Keep in mind, MOST flat tappet cams and even most hydraulic roller cams peak out at about 5,500 - 6,000 RPM, including what many would consider being fairly "healthy" cams that have between 225 and 240 degrees of duration @ .050", so even a so called "healthy" cam in "most" typical 350 sized engines will peter-out at, or around, 5,500 to 6,000 RPM, so why would you want to stuff a 750, 800 or an 850 carb on something that simply cannot utilize it and doesn't NEED it? Unless of course you LIKE having to feather the throttle and have a turd of a car with mom and her 4 kids waiving at you from their SUV as they pull away from you at the stop light or getting on the freeway.

How To Choose The Right Size Carb

Checking to see if your carb is too big isn't as straight forward as it is to check if it's too small. If your carb is too big, you'll get the performance results that I explained earlier in this tech tip, such as sluggish launches, slow acceleration, having to feather the throttle in order to accelerate from a dead stop, etc. But checking to see if your carb (or intake manifold) is too small is actually really easy by using a vacuum gauge. 

When you mash the gas, the engine vacuum will go down to zero. This is true for pretty much any engine, whether it's a bone stock 4 banger or a big performance engine. Vacuum only happens as a result of restriction, and when you're idling or are cruising under part throttle, your carburetor's throttle plates are closed, or almost closed, which is restricting the engine's air intake, thus causing vacuum. Boost works the same way, but it's pressure trying to get shoved through the intake system that "backs-up" and is seen as boost, so basically both boost and vacuum are measurements of restriction in one form or another.

Now, if your carb (or intake manifold) is big enough, there should be no vacuum at wide open throttle, so a way to check to see if you have a "restriction" going-on from your carb not being big enough, or your intake manifold runners being too small for the size of your engine, a simple way to test for this is to use a vacuum gauge. 

You can either use a dash mounted one, like one of the popular name brand gauges, or you can even use one of the hand held ones from an engine tuning kit and simply run a long vacuum line from either a port on your intake manifold, or from a manifold vacuum port on your carburetor to the inside of your car and see what it reads at your peak RPM under wide open throttle. Keep in mind, it must be a "manifold vacuum port", not a "ported vacuum" port on your carb. 

So from a dead-stop, if you mash the gas, the gauge will go to zero because there is no vacuum at WOT. As you are accelerating and the engine is coming up in RPM as you go through the gears, it should always be reading zero. If, however, your RPM comes up and you start to see the gauge reading a couple of inches of vacuum up towards your max shift points, it means the engine is trying to take-in air that it can't get and a restriction is taking place, thus causing some vacuum to show up on the gauge. That's telling you that either the carb is too small, or that your intake manifold isn't flowing enough to meet the demands of the engine. Like you trying to run but breathing through your nose. Your body can't get enough air because your nose is more restrictive than breathing through your mouth. So at WOT under a load, you should see no vacuum if your engine is able to take-in the amount of air it needs. Using a vacuum gauge is a quick and easy way to find out.

Hey, if you liked this tech tip then please be sure to share it with your fellow gear head friends and on your favorite car forums by copying the URL at the top of the page and sharing it with them. Thanks! 

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