Is a 600cfm carb too big for a street engine?

This is a trick question because it depends on what kind of a carb it is. Is it a vacuum secondary or a mechanical secondary? Is it a stock engine or a performance engine. Do you have a manual transmission or an automatic? Do you have a stall converter if you have an automatic transmission?

These are all factors when choosing the right carb. Before I start, let me remind everyone of this; NASCAR engines have over 750 horsepower and spin between 8,000 and 9,000 RPM for hours-on-end, and in some races where restrictor plates are required they run 390cfm. That's right, 390cfm!

First of all, lets take a 600 double pumper, like the Holley's #4776 which is a mechanical secondary carb. It's a great little carb for many performance engines. because it's a mechanical secondary design, when the throttle is pulled back, the primaries begin to open and a shot of fuel gets squirted down the throat from the primary accelerator pump. This is typical of any carburetor, BUT when the primaries get about halfway open, the secondaries begin to open (mechanically) along with another shot of fuel through the squirters from the second accelerator pump, hence the name "double pumper" because it has two accelerator pumps.

This is all fine and dandy unless you've got an automatic transmission without enough stall RPM in the converter, a heavy car or high rear-end gears and you stab the throttle off the line, the engine will probably choke, cough and spit before it sluggishly starts to go. I call this "flushing the toilet". The key here is air speed or "velocity", or rather lack of it!

Picture this, take a shop vac and feel the suction at the end of the hose. Now make the hole smaller. The air being sucked in speeds up! This is an increase in velocity. If you remove the tip and make the hole bigger, (like increasing the size of the carb), the air speed slows down and it won't pick-up any dirt off the floor. Double pumper carburetors act similar in that when the throttle is stabbed, all 4 barrels open-up at once and the air speed slows way down because the engine lost its velocity. An engine can't go unless air (and fuel) are going into it! The faster the speed the air and fuel goes into the engine, the quicker the pick-up and acceleration it will have! Without much air speed the carb can't deliver its fuel. You have to remember; fuel is "siphoned" through the boosters. It isn't pressure fed like fuel injection is, so a fast air speed makes the carb more responsive and delivers fuel better, plus a faster air speed makes the engine react quicker and have better throttle response when driving.

Now lets take another 600cfm carb, Holley's #1850 like what you see in this next picture. This is a vacuum secondary carb which has only one accelerator pump, and secondaries that open via air speed passing through the carb. When the throttle is hammered, only the primaries open. This allows a faster air speed to feed the engine and no "toilet flushing". As the air passing through the primaries speeds-up, it passes over a small tube that acts like a siphon. This siphon is attached to the secondary diaphragm which opens the secondaries "on demand". They won't open-up until your engine says "Hey, I need more!". When they open, the air passing through the secondary side of the carb draws the fuel it needs through the rear jets, or in some cases a metering plate. No bogging or dogging off the line!

I have seen a number of times over the years, cars coming into my shop with vacuum secondary carburetors with a small screw installed in the secondary linkage so when the gas pedal gets mashed, the secondaries are mechanically forced open instead of being pulled with vacuum as the engine needs, or wants it. I guess they think it turns their vacuum secondary carb into a mechanical secondary one or something. Do not do this! You'll just make a turd out of a good carb and defeat the purpose of vacuum secondaries. Guys who do this are not somehow "outsmarting" the Holley engineers. Manually forcing the secondaries open without having a second accelerator pump to even-out that big gulp of air is only going to lean-out your engine and cause it to bog down. If you want mechanical secondaries, then install a mechanical secondary carb. Cletus and Bubba need to step away from the carburetors, LOL.

Here's the deal on carbs: you can take a general V-8 engine and put a 650 double pumper (Holley #4777) carb on it. It will drive OK but will probably be a bit of a turd out of the hole and get horrible gas mileage. You'd be much better off with a 750 vacuum secondary carb (Holley #3310) instead like the one in this picture.

Yes, CFM wise it's bigger, and technically it is overall BUT, because it's a vacuum secondary design, you are only driving around on the primaries, even if you nail it to the floor off the line, you are only opening the primaries. This means you are actually driving around with a 375 cfm 2 barrel until the engine pulls enough air volume and speed through it to open-up the secondaries by that diaphragm dashpot that you see in the 12 o'clock position of that image.

If you look at the carb in the very first image of this tech tip, you'll notice that it doesn't have one of those because that's a 600 cfm mechanical secondary or "double pumper" carb. The picture next to this paragraph is a vacuum secondary design which has the dashpot on the side that opens the secondaries instead of mechanical linkage.

All vacuum secondary carbs have a dashpot, but mechanical secondary carbs don't because the secondaries are pulled open "mechanically" BUT they also have a 2nd accelerator pump that squirts additional fuel into the secondary barrels so it doesn't go lean before it starts pulling off the secondary main jets. A vacuum secondary carb only has one accelerator pump on the front of the carb. There isn't one on the back for the secondaries because it doesn't need one because they pull open slowly in order to keep the velocity up so it can pull fuel through the secondary boosters better. If they slammed open, like you can do with mechanical secondaries, then they would need a rear accelerator pump. 

So, is driving around with a 375 cfm 2 barrel too big of a carb for your engine? Hardly! Always remember this; when you put a carb that is WAY too big on your engine, it will actually run LEANER than it will with a smaller carb. This is because it takes AIR SPEED (velocity) to PULL fuel through the boosters. Remember that shop vac analogy I used earlier? Putting too big of a carb on is like removing the tip from your shop vac hose. The engine takes a big gulp of air, but at a slow velocity which can't siphon fuel through the main jets and past the boosters because there is no air speed to pull it through. Carbs don't GIVE the engine fuel under pressure. They aren't EFI systems. The engine pulls fuel through the carb via a "venturi effect" and velocity. No velocity = a lazy carb, no fuel delivery, and a sluggish engine, especially at lower RPM's.

The secondary opening rate of a vacuum secondary carb can be adjusted by changing the spring inside the diaphragm housing. It can be made to open sooner (with a lighter spring), or later (with a heavier spring) depending on how much air it requires to overcome the spring tension on your particular engine.

A larger cubic inch engine will need the secondaries to open sooner, and a smaller cubic inch engine will need them to open later. Again, all engines are driving styles, whether on the street or at the track, are different, so each and every carb needs to be tailored differently to the specific engine it's on, and this is one of the ways of doing it.

It's a hell of a lot better than your foot trying to do it for you by "flushing the toilet" with a double pumper carb that's a bit too big, unless you have really low gears, a high stall converter and/or launch and drive at higher RPMs. In reality, vacuum secondary carbs work much better on the street and at the track in most street/strip cases. This is especially true for cars that are heavy or are mostly stock or mild performance. The engine will rev faster, the car will launch and pull a lot harder, and it'll get better gas mileage!

In most mild street performance engines a double pumper is NOT the way to go and the cfm rating is NOT the same as the cfm rating of a vacuum secondary carb because of the way they work. If you have to "feather the throttle", your carb is too big. You don't want to be a dog off the line... always remember... 80% of the race is the first 80 feet, so you want to be able to mash the gas and GO with no bogging, hesitation or having to feather the throttle. That's why there are vacuum secondary carbs. 

Now... if you really think your 600 cfm carb isn't big enough, or your 500, or 650 cfm, or whatever size you have, and you want to find out if you need a bigger one, there is an easy way to find out.

If your carb is too big, you'll get the performance results that I explain in our "How To Choose The Right Size Carb" tech tip, which is similar to this one but different. 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.

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