Carb Confusion

Right up there with "What cam should I run?" is one of the most asked questions to ever come up at any bench racing session. "Which carburetor should I use?" Today there are way too many choices to make this an easy decision. Carbs come in all sorts of shapes, sizes, designs, and even colors! And the good ones are not cheap, so buying more than one just to try them out is kinda tough. The easy thing about choosing a carb is that all engines, regardless of who's name is on the valve covers, will react the same to either the right or wrong fuel mixer. To find out which carb might be the best, we took a pretty generic V-8 (see sidebar) and ran three new Demon carbs on it. To make sure that they all had a fair chance of showing their best results, we tuned each carb to perfection on the dyno. We've printed those results for you to peruse. Hopefully, it can help you make the right choice.

Even though carbs mix both fuel and air, a long time ago someone decided that carburetors should be rated only by how much air they flow. That's probably because trying to "wet-flow" a carburetor with gasoline is not only difficult, it's downright dangerous! Instead a system was developed to pull dry air through the carburetor creating a "depression" or negative pressure, which is just the scientific way of saying "vacuum," at the bottom of the carb. The industry standard depression is 28 inches of water, which means that it's enough negative pressure to lift a column of water in glass tube 28 inches high: just like sucking soda through a straw, but it's a very large straw.

Now that you've mastered AirFlow 101, we'll explain "cfm." Cfm is just an acronym for cubic feet per minute (they left out the "p"). Since air has a given volume at a certain moisture density, it can be used as a way to measure the flow potential of an orifice. But this is getting way too technical, so just try to keep in mind that a 600-cfm carb is rated to flow 600 cubic feet of "dry" air at wide-open throttle for one minute. Therefore, the larger the cfm rating, the more air the carb can flow.

Here are the reasons why it's important to know all this. The cfm rating has actually very little do with how much fuel any carburetor will flow. One might make the wrong assumption that a larger cfm carb also flows more fuel, but that's not always true. See, an engine only needs a certain amount of both fuel and air--mixed in the proper proportions--to make max power. If you replaced a small carb that didn't flow enough air for the engine to make max power with a larger carb, the larger carb would probably make more power. But, it might use only slightly more or sometimes just the same amount of fuel to do it. (Check out our fuel consumption chart to see this in action.) All this has to do with the engine's brake specific fuel consumption, or "bsfc" as it's called in the dyno world. It is simply a mathematical calculation of how much fuel the engine used to make its power. Measured in pounds of fuel flowed per horsepower per hour, this can get very confusing. So just try to remember that a lower bsfc number means an engine is making good power for the amount of fuel it's burning. A higher bsfc means the engine is not very efficient with its fuel, unless it's got a supercharger on it--then everything changes!

This has nothing to do with mpg or fuel consumption in a car, however. That's because on the dyno, we measure everything at wide-open throttle, and in trucks, mpgs are calculated over many miles at all throttle positions. There's also factors like aerodynamics, hills and valleys, and stop-and-go driving that all play a part in the mpg you'll get. So don't think for a second that putting smaller carb on your engine will automatically give you better gas mileage. All it might do is make less power instead.

There's really no easier way...        read full caption There's really no easier way to find power differences than on the dyno. We used this 355-cid small-block Chevy and three different Demon carbs to do our testing.

The Holley Street Dominator...        read full caption The Holley Street Dominator intake manifold was a great power maker. It pulled strong up to 6,200 when equipped with the 750-cfm Speed Demon carb.

Both of the Road Demon carbs...        read full caption Both of the Road Demon carbs we tested came equipped with electric chokes, which were adjusted to stay wide-open during all tests.

Due to the way a dyno loads...        read full caption Due to the way a dyno loads an engine versus how a car loads an engine, vacuum secondary carbs rarely work well on dynos. We usually have to wire the secondaries wide open for the tests, as we did here. This does not mean the carbs won't work well on the street, it's just that dynos don't allow enough time during an acceleration test for the secondaries to fully open.

We played with jetting on...        read full caption We played with jetting on each carb to maximize power and minimize fuel consumption. Remember that an engine needs fuel to make power, and if you remove fuel in an effort to save gas, you'll be also removing power.

To keep track of how well...        read full caption To keep track of how well each carb worked, we monitored intake manifold vacuum and cfm going through all of them.

The engine's builder and original...        read full caption The engine's builder and original owner, our good friend Carl Casanova, came by to help with our tests. He's pretty proud of how the motor he calls, "The Butcher," worked out.

Check out the primary venturi...        read full caption Check out the primary venturi inside the choke housing. The top carb is the 525, and note how it has large annular boosters. This might restrict airflow, but responds very well on smaller-displacement engines. The bottom carb is the 625 with standard down-leg boosters on the primary side that lets more air pass.

Only minor work was needed...        read full caption Only minor work was needed on this stout 355 prior to dyno testing. The Comp Cams 1.5:1 roller rockers needed adjustment. Other than the rockers and the new carbs, the rest of the motor was used (it had only recently been freshened with rings and bearings).

To explain all this information on carb cfms versus horsepower without some real-world test results backing it up would be lame. So we strapped an engine to Westech's Superflow 901 engine dyno and let 'er rip!

In addition to the torque and horsepower figures, we've also listed intake manifold vacuum and actual cfm through the carb as a way of comparing carb sizes. If a carb is sized correctly for the engine it's on, there will be very little vacuum at wide-open throttle, regardless of how much air is flowing into the carb. If there is high vacuum at wide-open throttle--more than about 1.5 inches--that means the carb is not flowing enough and the engine is starving for air. But, there's a point of diminishing returns in regards to vacuum and wide-open throttle. Note that even the 750 Speed Demon mechanical secondary carb we tested still measured over 1 inch of vacuum at peak power. Yet, it made way more power than either smaller carb and used only a little more fuel than the smallest 525-cfm carb we tested. But, dynos load engines differently than trucks, and if we were to put an even bigger cfm carb on the dyno, it might continue to make even more power, but at a price. Put that big carb on the road and feel your throttle response go away and your low-speed power drop off. That's because big carbs (800 cfm+) on small engines (under 350 cid) have trouble metering fuel correctly and will subsequently make less power down low, which is where you spend most of your time driving.

With almost 2.5 inches of wide-open throttle vacuum and the lowest power numbers of any test, it was obvious that the 525-cfm carb was too small for this hungry motor. Yet, note that the engine didn't actually pull a full 525 cfm out of this carb.

Even though it was rated 100 cfm larger, the 625-cfm carb still did not flow enough air, judging by the 2 inches of vacuum in the intake manifold. Also note on the fuel consumption chart below that the 625 used the most fuel to produce only mediocre power. The engine only pulled 513 max cfm out of it too.

The mechanical secondary Speed Demon 750-cfm carb produced the best power and used only a little more fuel than the much smaller 525-cfm carb to get there. But, things like off-idle throttle response and power below 2,000 rpm may be slightly lower with this larger carb on the street, especially with an automatic transmission behind it.

Note: Even though the 750-cfm carb made considerably more power, the engine still didn't pull more than 540 total cfm to do it. All that means is that regardless of carb size, if it's metering air and fuel correctly, the right carb will flow all the engine needs.

Check out how all three carbs started out making just about the same torque at 2,600 rpm, but the larger cabs began to take off by 3,000 rpm. Interestingly, the 525RD caught back up to the 625RD at the very end, but neither one could touch the 750SD by that time.

Also note that we have no idea why there's a big dip in torque from 3,600 to 4,200 rpm. But we know that it wasn't caused by the carbs because all three of them had it.

See how the 750 and 525's fuel curves are closer to each other. The 750 used an average seven percent more fuel than the 525 to make the most power of all carbs, while the 625 used twice that (average 14 percent more fuel than the 525) to make only middle-of-pack power. That means the 625 had the highest bsfc of all the carbs we tested. But, if we took fuel out of it by leaning out the jets, power also dropped off, meaning that this engine did not like the 625RD as much as the 750SD.

Many of you might like what you see and want to know how to build one for yourself. Well, most of this engine could be easily duplicated, except the heads. They're old cast-iron Bow Tie Chevy heads that were hand ported by Air Flow Research (AFR) years ago. Although AFR is still in business, they've moved into the aluminum cylinder head manufacturing arena and don't do custom hand porting anymore. However, the port job was just a simple bowl clean up and gasket match, with polished exhaust runners and chambers that most competent cylinder head shops could do. The rest of the engine is a pretty ordinary 0.030-over 350 with a GM crank and stock rods swinging Speed Pro forged flat-tops inside a factory two-bolt block with around 9.3:1 compression. The cam is a Comp Cams Xtreme Energy XE268H hydraulic flat tappet with 1.5:1 ratio Comp Cams roller rockers. Combined with the Holley Street Avenger (PN 300-36) dual-plane intake manifold and the 750 Speed Demon carb, it managed to crank out some very impressive power figures for a pump gas street engine.