Let me make one thing clear: If you make a poor initial carb selection for your application, it will certainly impede and perhaps prevent you from achieving top results. In this chapter I discuss a rarely covered subject, namely brake specific air consumption (BSAC), and how it directly affects just how much horsepower a given carb’s CFM supports. In simple terms, the subject here is how to get a smaller carb than you think the engine needs to support more horsepower than you thought it would allow. From that you should be able to see that it is an important part of the quest for more from less!

 


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In Chapter 2, I mentioned the advantages of going to Holley’s website to use the online carb selector. This is a really good way to start your carb selection.

Before you begin the selection process, you need to know two things: The first is the total CFM the engine is likely to require and the second is whether you should use a vacuum secondary.

 

Fig. 6.1. Shown here with a Dominator, this Chevy big-block 572-ci engine proved to be capable of more than 750 hp in street trim with a 4150 carb (850 cfm, PN 0-80531). This was a case of knowing how to utilize the CFM capacity of the carb to the fullest extent.

 

I start with a process for determining the best CFM for maximum output. Next I deal with all the possible advantages of a vacuum secondary. Finally I delve into the ways and means of utilizing every possible CFM that passes through the carb and into the engine in the fullest manner possible.

 

How Much Carb CFM is Needed?

The first step in installing the best carb for the job is to make a preliminary selection based on the engine’s displacement. Next, modify this result by factoring in relevant engine spec details such as the heads and cam used. For the initial calculation step, determine the amount of CFM the engine is likely to inhale if it were able to breath at 100-percent efficiency.

To do this, multiply the cubic inches (ci) of displacement by the anticipated RPM the engine is likely to turn to. Let me stress that it is very important to be realistic and therefore as accurate as possible when making the RPM estimation.

 

Fig. 6.2. This is the intake from my 2002 Chrysler Cup Car engine. The points to note are the generously rounded leading edges of the runners and the as-cast runner floors. Polishing the floor of an intake is a no-no!

 

Fig. 6.3. This is a NASCAR Cup Car intake for a pair of Chevy small-block 18-degree heads. Nothing aids flow into the head like a straight shot to the port. The more efficient the intake, the more carburetion capacity the engine can use.

 

Estimate where peak power is likely to occur and then add 200 rpm to allow for over-speed. At this juncture, you may feel your application needs more over-speed that just 200 rpm. Even if that is the case, the ability to effectively run past peak power is a function of the cylinder heads and cam rather than choosing a larger carb. Producing a good over-speed capability for, say, a short track engine that runs in one gear only can make a winning difference.

Equally important is how the engine responds coming off the turns. This is an important factor not just to a circle track racer but also to the performance street enthusiast.

To find the amount of air a 100-percent-efficient engine inhales per minute (CFM), you multiply the displacement (cubic inches) by the engine speed (RPM). And because this is a four-cycle engine, which has an induction stroke every other revolution, you divide by 2. Then, to convert to cubic feet, you divide by 1,728. Here’s the formula:

CFM =ci x RPM / 2 x 1,728

 

Volumetric Efficiency

The calculation above assumes the engine has a 100-percent breathing efficiency. For a race engine, where the exhaust scavenging is a factor, the volumetric efficiency can exceed 100 percent by quite a big margin. For instance, a well-built race 350, with no regulatory race restrictions placed on it, can reach about 115-percent volumetric efficiency. This means that such an engine, as far as the carb is concerned, seems to displace 400 ci, not the 350 it actually displaces.

At the other end of the range an absolutely stock street engine may have a volumetric efficiency of only about 75 percent. This means that an engine of the same 350-ci displacement appears, from the carb’s point of reference, to be displacing only about 290 ci. Carburetor selection needs to take this into account.

An engine’s required airflow depends primarily on the cam and the breathing capability of the heads. Assuming that the compression ratio and exhaust system are appropriate for the engine, the heads and cam are the most influential components in carb size selection. As cams get longer, the engine’s volumetric efficiency improves. The volumetric efficiency also improves as the cylinder head’s flow capability improves.

 

Fig. 6.4. Assuming the compression ratio and the exhaust are appropriate for the engine, the heads and cam duration are the most influential factors in selecting the right size carb. To obtain the correction factor for a particular application, first choose a curve for the cylinder head spec from the list below. Next, locate your 0.050 cam duration figure along the bottom scale. Then go straight up the graph until you intersect the previously chosen curve. Now go to the left for the correction factor on the vertical scale.

Red = super race heads such as ProStock and NASAR Cup Car heads

Orange = top-of-the-line race-ported heads such as used by pro racers

Green = race-ported conventional heads

Blue = street-ported heads

Magenta = pocket-ported pre-1990 heads or stock Vortec or aftermarket heads

Black = stock OE heads of pre-1990 design

 

Figure 6.4 gives a correction factor (CF), which takes into account cam duration and cylinder head flow capability. Using this correction factor, here is the formula for predicting the required carb CFM:

CFM =ci x RPM x CF / 2 x 1,728

 As an example, let’s use one of my 482-ci Chevy big-block engines. This street/strip build, which was mostly in the low-buck category, targeted peak power at 6,800 rpm so the maximum RPM figure (at 200 over that) would be 7,000. The CF for a Comp Cams street roller (248 degrees at 0.050) with the basic race-ported Dart Iron Eagle heads (using the green curve in Figure 6.4) came out to 1.065. Putting this data into the equation, you get:

CFM = 482 x 7,000 x 1.065 / 2 x 1,728=1,039.7

The answer rounds up to 1,040. The carb selected was a 1050 Holley Dominator, which worked out very well.

Here’s another example: a Ford small-block 5.0 built for my road race Mustang. This 306-ci engine featured race-ported Dart heads, a Comp Cams solid street roller cam with 258 degrees duration at 0.050, and peak power at 7,600 rpm. The correction factor (using the green curve in Figure 6.4) came out to 1.07. Putting the numbers into the equation you get:

CFM = 306 x 7,800 x 1.07 / 2 x 1,728 = 738.96875

The answer rounds up to 740 cfm. The carb used was a 750 Street HP and this pump-gas 306 turned out 525 hp and 396 ft-lbs of torque.

This example targets just about the biggest carb you should use. However, it makes no allowance for the fact that a tricked-out carb with high-gain boosters can successfully use greater CFM.

Let’s say you took a stock 750 and spent time streamlining the throttle shafts and butterflies. This can increase the airflow by about 35 cfm if you do a halfway respectable job. This allows a little more power to be developed without sacrifice in the lower RPM range. Going this route means you have to know your carbs or work with a carb specialist.

 

  MuscleCarB

   

 

Factoring in Dual-Plane Intakes

So, you should be able to calculate, to within relatively precise limits, what is needed in the way of carb CFM for any given application. But the previous examples assume the engine is equipped with an effectively flowing single-plane intake. When a true dual-plane is used, in which one plenum is completely separated from the other, the carb CFM seen by any one cylinder is almost halved. Unless this is allowed for, the engine could be very short of its true potential.

However, dual-plane intake design must, in many cases, take into account issues that are relatively unimportant for a high-performance single-plane. Such things as exhaust gas recirculation (EGR), hood clearance, installation compatibility with A/C, etc. need to be factored in. All these and many more issues influence, to a greater or lesser degree, just how flow efficient the intake can be.

To demonstrate let’s consider the dyno figures from a couple different intake manifolds. The first is a Wei-and dual-plane used on a Ford 351 Windsor equipped with a Scat Enterprises 1/2-inch stroker crank, which produces 408 ci. I tested this engine with a 750 HP Street carb and a black 950 Ultra Race carb.

 

Fig. 6.5. A well-designed single-plane manifold, such as this Parker Funnel Web for the Ford small-block, can have great airflow capability compared to a dual-plane intake.

 

Fig. 6.6. This is a port runner schematic for a production Chevy small-block with dual-plane intake. As you can see, there is no such thing as a “straight shot” route for port runners from the carb to the  head’s intake ports. The stock flow numbers are the ones posted in the port openings and then modified on the outside of the runners.

 

Fig. 6.7. I used this stock-port factory intake on my dirt race car. Although rules mandated no porting, I found an extra 20 ft-lbs and 20 hp. (See Chapter 12 for more details.)

 

Fig. 6.8. There are two things to note on this intake manifold. First, it is a stock-height, or “low-rise,” manifold. Second, it has an exhaust heat crossover (arrow). Neither factor is good for output.

 

Fig. 6.9. Many manifolds, such as this Weiand Street Warrior for a Ford 351 Windsor V-8, are compatible with all stock factory equipment so they are a direct replacement. The downside is that they give up flow potential over a high-rise intake design.

 

Fig. 6.10. If your intended carb is too small for the engine (as this 950 is for the 572-inch big-block), a street single-plane makes better results than a dual-plane intake.

 

The Weiand intake was designed to be compatible with all OE installations. The manifold carb pad was virtually at stock height so making the manifold taller to get a more favorable runner shape was not incorporated in this design. This manifold’s strong point is that its design produced very good cylinder-to-cylinder mixture ratios to the extent that no stagger jetting was needed. Its air-flow, though, compared to some of the taller, raised-pad manifolds, was down by a measurable margin.

The result was that the 408 cubes this engine had could be satisfied in the low and mid ranges but not at the top end. Because the manifold became the prime restriction, the 750 HP Street carb produced as good an output as a 950 all the way to about 4,800 rpm. It was only between 4,800 and 6,000 that the 950 showed any benefit. Even then it only improved by about 5 hp! The point to note here is that if the manifold is not really strong on flow the need for a higher flowing carb is largely negated.

 

High-Performance  Dual-Planes

By the time the new millennia started some serious steps had been taken among intake manifold manufacturers to design and produce a new class of high-performance dual-plane intake manifold. This was a category that bridged the gap between the typical dual-plane and the one that retained a dual-plane layout but featured a raised carb pad and runner shapes that maximized airflow to the cylinders.

In effect these types of dual-plane intakes bridged the gap between the conventional “stock replacement” intakes and high-performance single-plane intakes. They were also probably the first volume-production manifolds to be designed using computational fluid dynamics (CFD).

 

Fig. 6.11. This illustration shows how dual-plane intake runners evolved from a very inefficient shape to the current high-efficiency designs.

 

Fig. 6.12. Forms, such as these, are typically used to manufacture modern high-rise dual-plane intakes. Given the right carburetion, this type of intake can show extremely good performance increases throughout the entire RPM range. The performance potential and effectiveness of this style of intake is proven by the Chevy 383/408 small-block build that produced streetable outputs between 530 and 560 hp (featured in my book How to Build Max-Performance Chevy Small-Blocks on a Budget).

 

Fig. 6.13. This graph shows why a modern, high-tech, high-flow, dual-plane needs much more carb CFM than an older and significantly less efficient design. Look at the average flow loss (Columns 1 in yellow) of the three sample intakes. You see that the current Performer-style intake is far more efficient, so it reduces head flow by much less than the stock intake. Shown in Columns 3 (red) is what happens to the flow when a 750 carb is installed on the intake: The intake flow is reduced by a smaller amount on the inefficient stock intake.

However, because the more efficient Performer intake can convey a greater demand to the carb via a more efficient manifold, the 750 carb itself becomes the “cork” in the system. This is why current high-efficiency dual-plane intakes run best with more carburetion than might be expected.

 

Fig. 6.14. A high- performance, dual-plane intake cannot deliver much increase on an otherwise stock smog- gear-laden engine such as this 1980 Chevy 350. The red lines represent the hampered Chevy 350, and it doesn’t produce much power unless the highly restrictive exhaust is uncorked first. When that’s done, installing a good intake results in an entirely different story. The blue lines represent the stock factory manifold.

 

In terms of carb CFM, these intakes require a serious amount of consideration when it comes to choosing a carb. Because the runners are far more efficient than a typical dual-plane intake, they are able to communicate the engine’s airdemand to the carburetor far more effectively. In turn, this means that an engine so equipped is far more sensitive to carb capacity.

On a single-plane intake, all cylinders see all four barrels of the carb to draw on. But consider this: Given a dual-plane intake with efficiently flowing runners, the carb flow seen by any one cylinder of the engine is half of what it is on a single-plane intake. This means the 750-cfm carb that worked so well with a good single-plane manifold looks more like a 375- to 400-cfm carb. With such intake manifolds the required carb capacity can go way over what you might ordinarily expect (see Figure 6.15). A good dual-plane, air-gap-style intake for a small-block Chevy or Ford that is physically capable of about 550 hp given all the induction it wants, stops showing output increase at about 1,100 cfm of carb capacity because the limit is now the manifold’s runner flow capacity.

 

Dual-Plane Cutout

Some high-performance dual-plane intakes have the divider between the plenums cut away to form a communicating passage between them (see Figures 6.16, 6.17, 6.18). The purpose of the cutout is to allow any one cylinder to see more  than just the two barrels of the carb immediately over the plenum. This  has the effect of improving output at the top end. The drawback is usually reduced low-speed torque and idle to low-speed cruise vacuum.

This cutout brings about various consequences. In effect, it turns a dual-plane manifold into a single-plane manifold with much longer but more tortuous ports. In other words, the cutout turns a potentially good dual-plane into a substandard single-plane. That factor may not be good as, in part, it indicates that if the cutout were necessary then maybe you should have chosen a streetable single-plane intake. Also, if the cut-out helped top-end output, it is a sure sign that the carb is too small for the application.

There is most certainly a delicate balance here. My thoughts are that it is better to use a slightly bigger CFM carb without a cutout in the intake than a slightly smaller carb with a plenum cutout in the manifold.

 

Fig. 6.15. Dyno tests support my “think bigger” philosophy when using a flow-efficient, dual–plane intake. The relatively basic 383 test engine has a set of Gil Mink–ported World Products Sportsman iron heads with a 10.5:1 CR. The cam is one of my hot street-spec hydraulic flat-tappet grinds. As you can see, it produces 536 hp and that’s pretty respectable output for an engine like this. Most engines with this sort of spec don’t make that high an output with a single-plane race-style intake.

The point, however, is that if the typically recommended 750 carb had been used, the output peaks would have been 476 ft-lbs of torque and 511 hp. Although that’s hardly an output that anyone would complain about, it is not the 487 ft-lbs and 536 hp seen with the big carb. A couple of points to note to validate the results here are that the intake had no plenum cutout and the torque curves of all three carbs were virtually identical up to 4,000 rpm.

 

Fig. 6.16. This Weiand high-rise air-gap-style intake does not utilize a plenum cutout, which is commonly at the point indicated by the upper arrow. The bulge indicated by the lower arrow is an attempt to equalize the plenum volume seen by each pair of carb barrels. With no cutout, this manifold is far more sensitive to carb CFM. Given sufficient CFM this style of manifold can produce excellent results at both ends of the RPM range.

 

Fig. 6.17. This particular high-performance intake features an inter-plenum cutout. This slightly compromises idle quality and reduces torque at the bottom of the RPM range. However, it does make the intake a little less sensitive to carb CFM.

 

 Fig. 6.18. If you need to keep overall costs down without compromising quality, a high-flow dual-plane intake with a plenum cutout (shown) used with a 750 vacuum secondary Holley works well. In such instances, a bigger carb on, say, a 383 shows less of a gain over the 750.

 

 Fig. 6.19. Dual-plane, air-gap intakes, such as this one for the Ford 302 small-block, really work well. They ramp up the low-speed torque of these engines, which provides an important performance improvement because low-speed output is not a 302’s strong point.

 

Fig. 6.20. I had a Holley 950 body, a big-butterfly base plate, and the goal to build as high an airflow carb as possible without getting into anything too drastic. I dressed up the boosters and venturis and then reworked the butterflies and shafts. These modifications delivered 990 cfm. This worked well on one of my 468-inch, dual-plane intake, big-block Chevys.

 

Fig. 6.21. TWPE built this Chrysler Wedge 500-ci big-block. To be able to make any kind of top-end output on a 4150 carb called for some serious attention to the size of the inter-plenum cutout. By progressively enlarging the cutout, the engine produced 35 hp more than having no cutout.

 

There are times when a cutout for carb barrel sharing between the cylinders becomes essential, but again, it is because the carb for the application is just too small. A good example is the use of a 4150 carb on a dual-plane intake that has to feed a 500-inch (or more) engine. Some excellent big-block (Chevy, Chrysler, Ford) dual-plane intakes have good runner design, and that’s despite a runner design requirement that impedes flow. However, at the end of the day, the 4150 carb so often used is woefully short of adequate CFM capacity. For these intakes to make a decent top-end output, their plenum cutouts extend to the very limits of what the wall between the two plenums allow.

Let me remind you that if the plenum is cut out, it is progressively turning a dual-plane intake into a single-plane intake, but without the flow advantages of a single-plane. If you are in a position to perform tests, my advice concerning dual-plane intakes is to use as large a carb as possible first. If that does not satisfy the top-end output needs, start slotting the divider. It helps if you have access to a dyno.

 

Multiple Carbs

Much of the popularity of the single-plane, single 4-barrel carb is the fact that it works well for the money spent. However, it might just leave you to wonder if and by how much a pair of Holleys on a tunnel ram would better a single 4-barrel setup.

It is very often claimed by many who should know better that a tunnel-ram-style intake is for the race track only. If you consider nothing other than installation hassles and the inevitably large hood scope, that viewpoint is largely correct. However, if you are attempting to make the best torque over the widest RPM range possible, the “race only” label is totally wrong.

I have built a few street/tunnel-ram setups and they have delivered great drivability and output. Also, mileage was better than might have been expected considering the strong bias toward performance. A typical power advantage by using “two fours” is shown in Figure 6.24. For this test, the carbs used a four- corner idle setup. This proved to be an asset because the amount of power that can be developed while still in the idle/transition mode is considerably higher, as the engine has eight barrels to draw on. This also means that if you have a street cruiser, a meticulous setup of the idle/transition circuits can bring about some respectable fuel mileage.

 

Fig. 6.22. If your intent is to use a 4150 carb big-block, consider the use of a single-plane street intake as this makes more use of the carb’s full airflow potential. (This Chevy big-block intake manifold is PN 88961161.)

 

Fig. 6.23. This Chevy 505-ci big-block sported two tricked-out 750 Holley carbs. It had extremely good manners for an engine with 300/320-degree (at lash) nitrous-oriented cam and made, in its final form, about 835 hp on the engine and 1,540 on a relatively conservative amount of nitrous. Idle was a smooth 780 rpm.

 

Fig. 6.24. This graph shows what you can expect in the way of additional output by utilizing a pair of mildly modified 600-cfm 4-barrel Holleys versus a single 1,020-cfm unit. Note that in spite of often being labeled as “race-only,” a tunnel ram setup can produce a more streetable output curve than even the best single-plane 1x4 installations.

 

Fig. 6.25. There seems to be a renewed interest in the three 2-barrel performance street intakes that were popular during the 1960s and 1970s. This is Holley’s latest for small-block Chevys.

 

Another fuel mileage move is to use vacuum secondary carbs. As far as carb capacity goes, if you purchase a set of carbs specifically calibrated for 2 x 4 use with a CFM as indicated by the calculations given on page 58 you will be in good shape. Going this route on the carb CFM selection may cause you to wonder where the extra power is going to come from if the CFM is about the same as with a 1 x 4 single-plane manifold setup. The tunnel ram has several assets that allow it to produce a better output.

First, the port runners are a straight shot to the cylinder head ports, so the manifold is more flow efficient. Second, the carb barrels are directly over the manifold’s port runners so fuel distribution problems are minimized. Third, most wet flow problems arise due to a runner’s change of direction but because a tunnel ram’s runners are almost straight it has fewer issues with wet fuel flow. Finally, the pressure-wave tuning brought about by the interaction of the plenum and the port runners is far better than with any single-plane intake. All this adds up to a better result for a given amount of carb CFM.

Let’s look at the possible hitches you may run into and how to avoid them. The first and largest pitfall is buying a pair of carbs not explicitly calibrated and otherwise set up for running in a 2 x 4 configuration. Sure, given time you can get any pair of similar Holley carbs calibrated to produce positive results, but it’s very time consuming and complex. If you feel you can afford a 2 x 4 setup your absolute best plan is to call Holley and buy what they recommend. This is the shortest and easiest way to achieve some really worthwhile results.

Once you have a set of carbs with the correct general calibration, it generally becomes a straightforward procedure to fine tune the calibration.

 

  MuscleCarB

   

 

Three Twos

From the 1950s into the 1970s, three 2-barrel carbs were offered by several Detroit manufacturers as a V-8 street-performance package. The idea was that for sedate street driving, the engine ran on the center carb, which had calibrations biased toward fuel economy. When power was called for, the throttle linkage or vacuum actuation opened the other two carbs and fed the system, with the front and back carb feeding a full-power mixture charge.

Be aware that designing an intake manifold to satisfy this carburetion configuration is not without its flow-efficiency problems. However, even with the greater difficulty of designing high-flow runners this concept can actually work well. As for carb capacity, the sum total of the three carbs should be about 10 percent more than if it were a single-plane intake. However, the 3 x 2 configuration has a far more limited selection of carb sizes.

On a typical small-block the usual configuration seems to favor  325 cfm for the center carb and 350 cfm for the outer carbs. Those carbs are rated at 3 inches of depression, so you have to divide by 1.4 to get the equivalent 4-barrel rating.

The potential for better fuel economy, with careful calibrations, is available. As for outright power, a good air-gap dual-plane with the right carb still beats a 3 x 2. That said, a well set up 3 x 2 can have great drivability on the street while turning in excellent drag strip times on an otherwise street-orientated carb setup.

Last point: They also look pretty cool on the engine.

 

Spacers

Spacers are seen as anything from the expert carb tuner’s black magic to a simple parts change on the dyno to explore what the engine might need. As simple as a spacer is, its mode of operation is often not understood. The reality is that spacers work because they have increased something that the engine likes. That increase may take the form of extra flow, more velocity, greater anti-reversion properties, or additional plenum volume.

 

Fig. 6.26. This variety of spacers should just about cover the ones you are likely to come across. Each has its virtues. The dyno and the drag strip are likely to establish which one performs best for your application.

 

Fig. 6.27. A spacer has the ability to make the plenum volume larger and, usually, helps the air flow through the carb by up to 20 cfm. Because a stretched big-block is always hurting for air it is worthwhile finding out whether or not a spacer helps. In most cases they do.

 

Fig. 6.28. Most intake manifolds are designed with the minimum height possible for the application. This means that the plenum is often too small, so the use of an open spacer is an easy fix and it reaps the benefits of additional output.

 

Fig. 6.29. This type of exit-blended spacer not only increases plenum volume but also aerodynamically tidies up the air/fuel charge’s exit from the carb barrels. The result is that the engine sees greater flow from the carb. If the carb is a little on the small side, this type of spacer almost always pays dividends in output.

 

It takes little more than lifting the carb and installing the spacer and longer studs to find out what the engine likes. This means it is a good idea to test a spacer any time the opportunity presents itself. But be aware that installing a spacer always increases the plenum volume, often making a small but relevant reduction in the sharpness of the signal at the booster. Consequently, if the jetting was on the money before the spacer was installed, the carb may need to have a size or two larger main jet to compensate.

 

Vacuum Secondaries

Because successful racers use mechanical secondaries many street performance enthusiasts tend to regard vacuum secondaries as something of a necessary performance downgrade dictated solely by the need to have a streetable induction system. In reality nothing could be further from the truth!

The correct way to view a vacuum secondary carb is as a high-flow performance carb fitted with a device that allows you to use that carb in a far more effective manner on the  street. In fact, a well set up vacuum secondary carb can provide better  performance and faster times on the track than a mechanical secondary carb. The reason is that, in effect, a vacuum secondary carb is like two carbs rolled into one.

A small-CFM 2-barrel (due to a sufficiently active venturi and booster combination) can supply a well-atomized mixture to the engine at part throttle and low-speed WOT.

 

Fig. 6.30. This spacer is a four-hole/open-hybrid type. On about 1 in 10 engines, they seem to deliver what is called for. This spacer provides the same effect as using both the four-hole and open spacers.

 

Fig. 6.31. This spacer has tubular, sharp-edge extensions, which protrude into the plenum of the intake manifold. This provides a measure of anti-reversion properties to the flow exiting the carb.

 

Fig. 6.32. This spacer not only has anti-reversion lips on the four exits but also fuel-shear ridges on the wall of the open part of the spacer.

 

 Fig. 6.33. This type of spacer acts as a means of altering fuel distribution to a more favorable pattern and as an anti-reversion devise. The slot allows the fuel to enrichen a weak area within the plenum. This spacer seems to work best when used in conjunction with a 1- to 2-inch open spacer.

 

When the engine’s demand for air outpaces the primary barrels, the secondaries open up and provide the mid- and top-end airflow and fuel requirements. In practice, this means that the user of a vacuum secondary carb can ultimately select a slightly bigger carb CFM without any penalty at the low end.

A vacuum secondary is of little or no advantage when the stall speed of the converter is above the RPM at which a vacuum secondary comes in. The vehicle’s gearing and its weight are contributing factors as well. If the car transitions through the first gear to RPM somewhere at or above peak torque RPM very quickly then, once again, a vacuum secondary might not be of any advantage.

So here is my advice on the subject of vacuum secondaries: If the torque output of the engine (below 4,000 rpm for smaller engines around 300 inches or 3,000 rpm for larger engines of more than 380 inches) constitutes part of the engine’s operating range, you should be looking at a vacuum secondary carb. If you select a vacuum secondary carb for an engine that does not really need one, there is no real downside. If you select a mechanical secondary for an engine that could really use a vacuum secondary the downside is a possible reduction in output everywhere.

 

Sizing with Alcohol-Based Fuels or Nitrous

So far the subject of sizing the carb CFM to the engine has been discussed assuming that gasoline was the fuel being used. Let’s consider the changes that may be needed with methanol and ethanol-based E85 fuels and nitrous oxide.

 

Alcohols

With methanol and ethanol-based E85 fuels the vaporization curve is far less favorable for good combustion initiation than with a good gasoline blend. On top of that, the amount of fuel for an optimal-output air/fuel ratio is far greater. This means that any potential mixture quality or wet flow problems that might be imminent with gasoline can be greatly magnified to the point that any power gains that may have been possible with the alcohol fuels are nullified. To combat this, make sure that the fuel is well atomized.

Rule number one here is: Do not use a carb with venturis too big for the application. Rule number two is: Make sure that the booster is of high enough gain to generate good atomization. Rule number three is: It is often better to err on the smaller side for an alky carb.

Because these fuels cool the carb so much more than gasoline does, the mass flow (lbs-min) can increase. However, countering this is that the fuel takes up a lot more room in the intake so carb CFM is reduced. The bottom line here is that you want as high a speed in the venturi as possible along with the strongest booster signal possible. For example, if the carb is a 4150-style unit, consider a main venturi minor diameter of 1.45 inches maximum but preferably about 1.4. This, along with a big throttle bore, seems to work well.

 

Nitrous

If you’re using nitrous oxide, there are two avenues to consider. The first is a street/strip situation where having good manners and decent fuel mileage are prime considerations. Under such circumstances, choose a carb that errs on the smaller side by about 50 cfm. The rationale here is that the nitrous produces all the extra power required within the realm of streetable mechanical reliability. This being the case you may as well have the benefits of good street manners from your engine; selecting a carb a little smaller favors that aspect.

For a race-only situation, things change a little. Here you have three goals. First is to go as fast as possible, second is to use as little nitrous as possible, and third is to have your engine survive the rigors of a very substantial power output.

When the nitrous comes into action the temperature of the charge in the intake drops considerably. This causes the air that passes through the carb and into the manifold to shrink. At first, this looks as if it should increase the airflow into the engine, but the reality is actually the reverse. A portion of the liquid nitrous entering the induction system turns to a gas, and consequently it takes up room that would have otherwise been occupied by the air from the carb. This usually more than counters the potential increase in flow from the charge’s temperature reduction.

All this might be leading you to think that using a smaller carb for the street is the best route, but in many cases, the reverse applies. The use of a slightly larger carb usually pays off, especially if the nitrous is port injected.

 

Brake Specific Air  Consumption

Some advocate rating a carburetor by the horsepower it can readily support. In my opinion, it’s far better to obtain ultimate performance by matching the carb’s CFM to the engine volumetric capacity. At first sizing a carb according to the horsepower it can support seems like a better method, but it makes an assumption that can throw a wrench into the works. For example, a NAS-CAR Cup Car engine makes the point. Before NASCAR’s top series made the change to fuel injection (2012) the carb called for was a version of the 830 that Holley originally brought out for such use. This carb, when fully prepped, was good for about 960 cfm (although some teams were running radically modified carbs with well over 1,000 cfm) and, for a Cup Car application, supported the needs of a 900-hp 355-ci engine.

Another example is the use of a 4150-platform carb on a modified 572-ci big-block Chevy. Although way too small according to the CFM calculations presented earlier, a 950 Ultra HP can pass enough air to support more than 800 hp from a street/strip 572.

If you do the math in terms of the required CFM, these carbs look far too small to be able to allow the production of such big horsepower numbers. But there is one factor that you should be aware of: An engine may well draw in a certain amount of air, but it is very important how efficiently it uses that air.

To demonstrate, let’s use two big-block Chevys as examples. Each engine made virtually the same 1,100 hp. The first had very well sorted combustion characteristics. In fact, the induction system produced a well-prepared charge and delivered a BSFC of 0.39 pound of fuel per horsepower per hour at peak power. This engine also ran at leaner-than-normal air/fuel ratio because manifold distribution was nearly perfect. As a result it produced 1,100 hp for the 96 pounds of air it consumed each minute. This works out to be a BSAC of 5.2 lbs/hp/hr. This translates into a carb flow demand of 1,260 cfm.

The other big-block ingested 110 pounds of air for 1,100 hp. That was good but not as good as the first example. The same output was delivered with a BSFC of 0.46 lb/hp/hr while the best air/fuel ratio was 13:1. These figures indicated a more than reasonably well sorted induction system. However, this engine had a BSAC figure of 6 lb/hp/hr and, therefore, it had an air demand of 1,450 cfm.

So at the same pressure drop across the carb, the second engine needed a carb of about 200 cfm more.

Say you are installing a carb that passes a certain amount of air at a certain pressure drop into the engine. Whatever volume of air the carb flows, it is now up to you, as the engine builder, to utilize that air as efficiently as possible. My engines make a lot of power on a relatively small carb because I have 50 years of experience and sufficient proficiency for spec’ing engines to make the most of the air passing through the carb.

As an example, a 750-cfm carb on one of my street/strip Chevy 383 small-blocks can make more than 600 streetable hp, whereas an engine less well spec’d may only make 540 to 550. For what it is worth, it is possible to get the BSAC figure below 5 lbs/hp/hr. Couple this with good head and induction system flow and you can expect output numbers that trailer your competition (see my best-selling CarTech book How to Build Horsepower).

 

Written by Tony Candela and Posted with Permission of CarTechBooks

 

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