Strange as it may seem, the main jet on a Holley carb is not what you should be using to calibrate the WOT mixture delivered to the engine. That job is the function of the power valve and the PVRC. The main jet, however, is the most often changed jet; the one changed in an effort to optimize the mixture for maximum power. While a main jet change for a raceonly vehicle may be suitable, it isn’t necessarily the case for a true street vehicle because in addition to power, both emissions and mileage are concerns. It is time to examine in detail the calibration of the cruise and WOT power mixture.
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A lot of care and attention has gone into the building of this Chevy 505-ci big-block. Now is the time to put an equal amount of care and attention into the calibration of the main jet/power valve circuits, so this monster delivers all the horses it is capable of.
Main Jet
If you are setting up a Holleycarbureted street-performance vehicle, optimizing the cruise mixture is a major priority. When calibrating for cruise, only the primary barrels are opening so all cruise jetting is on that end of the carb. A word of caution: Do not shortchange your efforts to optimize cruise calibrations. They directly affect drivability, mileage, and emissions. As a racer of more than 50 years, I appreciate that the real fun is going fast. But you should not have a rough-running fuel-hungry smogspewing drive when you are not driving flat out.
Remember, a performance street machine does not run flat out all the time, so if your vehicle is smoking, the fun can wear really thin and do so quickly. If the mixture is too rich during cruise, the engine also experiences much greater bore wear than normal, and this reduces power. Spend the time to get things right here, and you not only reap the obvious fuel saving benefits but also make fine tuning at the track both less fussy and more productive. Let me be clear about the jets I’m talking about when optimizing the cruise and WOT settings. We are dealing with the main jets, the highspeed air bleeds (air correctors), the PVRC jets, and possibly the emulsion well jets/bleeds.
Other than highspeed bleeds, which are in the carb’s main body, here are the components for calibrating the cruise and WOT mixture. Arrow A points to the main jets, which screw into the fuel bowl side of the metering block (not shown). B indicates the replaceable emulsion well jets in a race metering block. C (compare it to B) shows the emulsion calibration on a regular metering block. Note that there are only two holes here. D indicates the replaceable PVRC jetting. E shows a secondary metering block with no power valve usage. F is the power valve that locates in position D.
This engine produces 710 hp and 665 ft-lbs with drivability right off a 700-rpm idle, and it runs on service-station pump fuel. To get these results, the 1050 Dominator calibrations were given a great deal of attention at WOT and at part throttle/idle. I went down three sizes of main jet and up two sizes of idle circuit air corrector (as it came from Holley). This fine tuning resulted in the 700-hp mark being surpassed.
In about 80 percent of cases, a carb calibrated by Holley needs only minor main jet recalibration. To accommodate this rejetting, just purchase jets up to four sizes above and below what is originally installed.
Until the main jets come into play, the engine relies on the transition slot for calibration. Here, the base plate has been removed from the carb body to better show the butterfly position for the slot’s best operation. The amount of slot remaining above the butterfly is shown by the red arrow. The amount of slot as viewed from the underside is shown by the yellow arrow. The slot is open about 11 ⁄2 turns of the idle speed adjusting screw.
The fuel is drawn from the bottom of the fuel bowl to a level just above the fuel level in the bowl. The passage goes back down to the idle screw. Somewhere near the top of the passage is a small jet. This is the idle feed restriction (IFR), or idle jet. The idle jet’s up-and-down feed path prevents fuel from siphoning off into the engine from the fuel bowl through the idle circuit. The idle jet can be anywhere in this passage. In the top metering block, the idle jet is at the bottom of the main jet well (red arrow). In the lower plate, it is at the top of the down leg passage (yellow arrow) to the idle mixture adjustment screw. Optimizing the size of this jet is the first step in achieving good cruise fuel economy.
By now you should be familiar with the main jet; its function and its location. Essentially, the main jet should start to pull into operation when the edges of the butterflies are at, or nearing the end of, the transition slot. If the carb’s main venturi is the result of the correct carb selection, the primary barrel’s boosters should just start to dribble fuel, which has been drawn from the fuel bowl via the main jet. Atomization of the fuel at this point is poor, but there is one saving grace. As the fuel droplets hit the butterfly, they run down the slope of the butterfly and get sheared off by the typically high-speed air passing around the edge of the butterfly and into the manifold.
When the butterflies are about 1/4 inch open and viewed from a point directly above the carb, the booster is reasonably well into its operational mode and is delivering most of the engine’s fuel requirement. But, in most cases, this amount of throttle opening is still such that an appreciable amount of manifold vacuum is seen. In other words the engine is still far from being called upon to deliver its full power. At this amount of throttle opening, the vehicle would almost certainly be running at mid- to high-speed cruise, which is typically 45 to 75 or 80 mph. The amount of manifold vacuum should not activate the power valve, which brings in the additional fuel that the power valve restriction channel (jet) delivers if the throttle was moved to or near WOT. With the engine running reasonably well, manifold vacuum is at about 70-mph cruise. The vacuum switching point of the power valve selected must be at least 2 inches less than the cruise vacuum or the idle vacuum. For a highperformance street machine, that may call for the use of a 41 ⁄2-inch switching power valve.
Cruise Calibrations
Under typical cruise conditions, you should be targeting calibrations for low emissions and good mileage. If you took my earlier advice, you have at least equipped your vehicle with a good oxygen mixture analyzer. Also (and I consider this a must), you should have a vacuum gauge installed on the manifold to read intake manifold vacuum. Remember, it must be manifold vacuum not ported vacuum. Once again, you need to know the intake manifold vacuum at idle. To do cruise calibration, the power valve must be closed. Therefore, under both idle and cruise conditions, the vacuum indicated on the vacuum gauge must be higher than the switching vacuum stamped on the power valve. In other words, at idle or cruise no additional fuel is fed via the PVRC to the engine.
When, and only when, you are sure this situation has been met and no additional fuel is being fed, it’s time to size the main jet to give the best mileage while still maintaining drivability. Also do not forget that cruise is almost always done only on the primary barrels. This means that the main jets and, to a slightly lesser extent, the high-speed air bleeds are your concern here.
To accurately size the primary main jets, check the current air/fuel ratio. In the absence of an oxygen mixture analyzer, you need to do a plug check. If your intent is to do the job properly, you won’t be too far into a plug reading session before you realize that getting a good oxygen mixture analyzer is one heck of a good idea. After all, correctly reading a plug is an art and a science. But if you choose to read the plugs, here are some tips. Although driving 5 miles along a flat piece of highway colors the plugs somewhat, a 20-mile drive is much better. Read the plugs to see if the mixture needs to go leaner. If it is already lean and the drivability is acceptable, you may want to experiment with an even leaner jet calibration. The best mileage is most likely to happen just shy of any perceptible drivability degradation.
The 1050 Dominator on this low-buck street Chevy bigblock 482 was ordered from Holley based on the engine spec. It required minimal jetting to make 650 hp and 636 ft-lbs.
Carefully choose a head and manifold that have cross-sectional areas to deliver the best port velocity over the RPM range applicable. Ideal cross-sectional areas pay dividends, especially when used together with a carb that is sized just right. I built this 347 using a mildly modified 750 HP, and it flowed about 815 cfm but still with near stock size main venturis. On pump fuel, this 347 made 472 ft-lbs and 562 hp. Idle was a smooth 750 rpm.
A street stroker rebuild of a Chevy smallblock 350 can produce amazing performance. This is a prime example of a budget-oriented engine built for real street performance. Other than a couple of size changes in the main jets and main circuit air correctors, the 950 Holley was right out of the box. The result was 604 hp and 539 ft-lbs of torque. And all that came with great street drivability
If the plugs indicate that the mixture is rich, lean out the mixture by reducing the main jet until it is as lean as possible, short of producing a drivability issue. It takes time to drive a while, stop, pull plugs, read them, and jet as required. But if you took my earlier advice, you are using an oxygen mixture analyzer to determine the mixture ratio under these cruise conditions. In most cases, a good starting ratio is 15:1, as almost every engine runs just fine at that ratio during cruise. If the ratio is richer than 15:1 when making an initial run to test the cruise mixture, take steps to lean it out to about 15:1 by reducing the main jet size. Once you consistently see a 15:1 ratio under cruise conditions, use it as a baseline setting to see if leaner mixtures can be successfully used short of drivability issues.
Keep in mind that almost any Holley-equipped V-8 with a good ignition system can run a 16:1 air/ fuel ratio at cruise. If the mixture ratio changes from one cruise speed to a higher speed, you should consider changing the slope by changing air bleeds (see Chapter 2). The air bleed affects the mixture as the engine RPM increases so the mixture slope becomes progressively richer or leaner due to the air corrector changes. At low speed, when the transition slot is the primary source of fuel and air, it is the idle air bleeds that need attention. At higher speeds, when the booster has pulled over, the high-speed air bleeds require resizing.
If the mixture is richer at the higher speed than at the lower speed, you need to increase the size of the air bleed. On the other hand, if a leaner mixture is seen at higher speeds, you need to reduce the size of the air bleed. At WOT, the air bleeds tend to be more influential on the mixture for a given size change than the main jets. However, at part-throttle openings, their influence is markedly reduced. Regardless of the specific situation, make your jetting changes a couple of thousandths at a time. Also, do not lose sight of the fact that what you do here affects any subsequent WOT mixture settings.
Track-Only Settings
We have looked at the cruise calibration and its importance for a vehicle that’s primarily driven on the street. But what about cruise calibrations for a race engine? My thoughts are that it is just as important, as the very essence of a race engine is the pursuit of optimal results. For a circle track application, fuel consumption can be a factor in winning. For a drag racer, having an engine that runs strong at part throttle is an indicator that you have a desire to do things right. I always spend the time on both idle and partthrottle calibrations to get the very best results.
Max-Power
Calibrations Once the cruise mixture has been set, it’s time to find the best output calibrations. Again, an oxygen mixture analyzer is the route to quick settings. But here you change the size of the PVRC (not the main jet) to achieve the mixture ratios needed. This is where replaceable PVRC jets are useful.
If you have a set of metering blocks with fixed PVRC jets, it is a case of replacing the metering blocks or using drills to progressively enlarge the PVRC jets. Without an oxygen sensor to measure the WOT mixture you can be in danger of opening the PVRC jets too far. If that happens you have to install replaceable PVRC jets. A factor to note here is that most carbs have a power valve installed in the primary metering blocks only. In such cases, you need to calibrate the WOT mixture of the primary barrels by changing the PVRC jets and by changing the main jets in the secondary metering blocks.
Be sure to use a power valve that switches at a lower vacuum than the idle or cruise vacuum. A 6.5 (as shown here) is typical for a performance street engine. If it is a really hot street/strip unit, a 4.5 is more appropriate.
Although contrary to what you may have been told about calibrating for max output, you should change the PVRC jet to optimize performance. If the carb does not have a replaceable jet metering block, resize the current PVRC holes. Once close, fine tune using the main jets. This is okay for a race-only engine, but if the mixture had previously been optimized for mileage, any subsequent main jet changes affect the cruise mixture.
Here you see changes made to various jets. The blue lines represent the main jet. If the mixture is too rich, reduce the size of the main or PVRC jet, which moves the mixture curve from the lower blue line toward the upper blue line. The curve/slope remains more or less constant. Next, let us assume that the mixture gets richer as RPM increases, represented by the lower green line. Increasing the size of the high-speed bleed (or air corrector) leans out the top end more than the bottom end thus bringing the curve nearer to the 13:1 horizontal line. Assuming that the mixture follows the curve represented by the top purple line, trimming the hole pattern of the emulsion tube/well can fix things. Here, the mixture is leanest in the higher mid range. To fix this, reduce the size of the holes in the lower mid part of the length of the emulsion tube/well. (For a review of how the emulsion calibration works, check out Figure 2.9 on page 16.)
At this juncture, the mixture curve is actually a straight line. By that I mean it is the same at the bottom of the RPM range as it is at the top and every point in between. In practice, that is rarely the case, which means you have to change the slope (and possibly the shape) of the curve. As with the cruise calibration, changing the size of the main circuit high-speed bleed or air corrector jet changes the slope, so if the mixture is leaner at the top end than at the lower end, reducing the size of the air bleed richens the mixture. To lean the top end compared to the bottom end, increase the size of the air bleed. Do not be surprised to find that the cruise-optimized highspeed bleeds are not exactly what is needed for WOT. If that is the case, a fix must be done later, so for now let us assume that the slope of the mixFig. 9.10. Be sure to use a power valve ture curve is good.
One thing often overlooked is that because a race vehicle only uses the top 20 to 40 percent of the engine’s RPM range, the air-bleed changes affect the lower part of the RPM range as well. For example, let us assume that a typical drag race engine’s RPM range is from 4,500 to 7,500 rpm. Increasing the size of the high-speed air bleeds leans out the mixture at 4,500 rpm but does so to a greater extent at 7,500 rpm. What this means is that when an air bleed change is made to alter the fuel curve slope, an accompanying PVRC/main jet change may also be needed. To make the proper calibrations in this situation often requires a great deal of experience. Let me repeat: Without an oxygen system you are hard pressed to discern these small but vital factors.
Target Air/Fuel Ratios
According to my test data and experience, your air/fuel target is about 13.2:1. However, the engine may run at its best with a different ratio, which is dependent on the mixture quality, distribution, and possible unwanted wet-flow issues. For this first round of tests, 13.2:1 is our goal and we adopt it as a baseline. Once achieved, it is the trap speed (not ET) that determines what the jetting and, consequently, the mixture ratio need to be. So let us assume that the mixture is at about the 13.2:1 mark. Remember, you should have achieved this reading by increasing or decreasing the main jet in the secondary and the PVRC jet in the primary. You need to keep reading the oxygen mixture gauge until you take an average of the readings to arrive at 13.2:1, which requires some interpretation and subjectivity.
It may look trick, but unless you know otherwise from dyno testing, the floor of this intake manifold should be as rough as the cast finish you see on the outside. When reading the plugs excess wet flow can hide the true mixture.
In 2008, I sat in on the testing of this 500-ci ProStock engine with Dick Maskin of Dart Machine. The task for the day was to see if there was any additional power to be had from some fine tuning of an already finely tuned carburetion system. We started with an output at the low 1,400-hp mark. By the end of the day (some 50 pulls later) an additional 15 hp had been found. It is this attention to detail that wins races.
After calibrating Terry Walter’s dyno, we tested my budget-build Chevy 383 race engine with $7,200 in new parts and a stock remachined block. The oxygen readings ranged from 13.1 to 13.3 for best power. When tested on other dynos, this 12.5:1 engine returned readings as high as 660 hp and 549 ft-lbs. However I believe our dyno calibrations were accurate. This engine employed an outof-the-box Holley 950 Ultra carb. The readings shown here were produced with only a two-size main jet and air corrector change all around. This engine was installed in a 2-speed Powerglide car that weighed 3,050 pounds and went mid-9s at just under 140 mph.
Once the required reading is achieved, you can use trap speed as a jetting reference. It is now a question of increasing or decreasing jet and air bleed size as dictated by the trap speed. Also check the plugs for fuel distribution and stagger jet. As for the mixture ratio that produces the highest speed for your vehicle, just remember that the more effectively everything works, the nearer to the 13.5:1 mark the best results are. At the other end of the scale if everything (combustion efficiency, atomization, fuel distribution, etc.) is just a shade off the mark, the fastest air/fuel ratio can be 12.8:1. Your job is to find what your engine needs to do its best.
Fuel Curve and Emulsion Wells
We have already looked at the main jets and PVRC jetting to move the air/fuel ratio up and down as a whole. We have also looked at how the high-speed bleed (or air corrector) jets have to change the slope of the fuel curve to optimize it throughout the RPM range. But you may encounter a mixture that does not react in a linear manner. For instance, the mixture could start off at, say, 13:1, go to 14:1, and back to 13:1 at the top end. Obviously, any previous jetting changes do not fix the fact that the mixture is going lean in the mid range only. In such a case, you need to make calibration changes on the main jet emulsion well. (See Chapter 2 and, especially, Figure 2.9 on page 16.) With the lean mid-range situation in this example, you reduce the size of the holes in the emulsion well at about the mid point in the length of the tube. Here, a race metering block comes in handy because jet size can be manipulated to trim the air/fuel curve as required. To make a measurable difference, a relatively large change in emulsion well jetting is required. To make a whole ratio change in, say, the midrange RPM, the jetting may have to decrease or increase in area by as much as 50 percent.
Some Holleys, particularly Dominators, have an intermediate transition/ cruise circuit and carbs. These circuits are typically used on carbs that have big venturis in relation to the butterfly diameter.
Weather and Altitude Changes
Getting the carburetion right on the calibration/test day is important because it gives a working baseline to make corrections due to changes in weather conditions and altitude.
Jetting
The effects of elevation changes and weather conditions cannot be ignored. However, sometimes the need to compensate can be over emphasized. If you are racing in one local event or on a single track, the effects of weather are relatively limited in most cases. However, air density can change dramatically, and that’s the time to recognize the value of calibrating to suit the weather. For example, when I lived in Tucson, Arizona, jetting for the day was worthwhile. During July and August, Tucson went through its wet season. Days could have really high humidity (well in excess of 65 percent) with temperatures over 100 degrees F and the barometer an inch lower than typical. The following week we could be racing on a clear evening when temperatures had dropped to 60 degrees F or less, humidity to less than 5 percent, and the barometer high.
The optimal ignition timing can change from that required on a cool, dry day to that required on a hot, humid day. You can use a thumb wheel to change the timing by one degree without having to loosen the distributor, which is a definite asset.
Dyno testing could be a nightmare. One day the correction factor might be as high as 25 percent and a couple days later, as low as 5 percent. These correction factors relate directly to the track and jetting. Such a swing in atmospheric conditions resulted in a jetting change of three to four sizes for maximum performance.
Timing
As much as the weather affects optimal jetting, it also affects optimal ignition timing. Most racers completely overlook this factor. That is not good, as with some engines it can have as much effect as getting the jetting right for the day. As a rule, you can expect the optimum timing to need advancing when the barometer drops or humidity rises. In this respect, it is a good idea to have an ignition system that can be advanced or retarded by a precise increment, rather than having to loosen the distributor and retime it with a timing light.
There are many ways to adjust ignition retard and advance. The simplest, if you are racing a popular Detroit V-8, is to use a Performance Distributors or Professional Products distributor with a manual advance/ retard thumb wheel. If you have a high-end race ignition system, such as that produced by MSD, it is easy enough to incorporate an advance/ retard control into the system. There are reasonably well defined trends as to how the optimal ignition timing changes with atmospheric variations, but be aware that these are not set in concrete. Idiosyncrasies of the engine can play a part in countering what might typically be expected.
For a dyno operator this comprehensive Holley main jet kit is a must-have item. For the typical racer a set of jets up to four sizes bigger and smaller is often all that is needed to fine tune a Holley
For a high-performance engine, make sure “picture window” power valves are used, as they flow more fuel than the drilled-hole ones. Note the use of the replaceable PVRC jets where the power valve locates. These are the jets you should be using to tune the WOT mixture.
With that warning in mind you can make certain assumptions with a fair degree of certainty. As air pressure/density rises, the rate of burn increases, which calls for a reduction in advance. If the temperature drops substantially, the timing may need to be advanced slightly. If the humidity rises substantially, the timing needs to be advanced over that required for dry air. Over a season of racing, you can expect to see as much as 3 degrees difference in optimal timing. Assuming the jetting was right on the money when initially calibrated, the main jets need to be reduced by one size for every 2,000 feet increase in density altitude.
To figure out the density altitude, you need a weather station to measure the barometric pressure, temperature, and humidity. You also need a calculator to establish, from the readings taken, what the density altitude is. You compare this density altitude number to the one that existed when the carb’s jetting was initially calibrated. Change the jets as needed to compensate for the variation. If you are in the habit of taking your laptop computer to the track, you can purchase and use Performance Trends’ Weather Wiz program. It not only shows the atmospheric figures you need to know but also computes the jet changes required.
Benefits
Jet and ignition timing changes beg the question, What is going to all this trouble worth? Here’s an example: Proper jetting on a North Carolina mid- to high- 9-second car going from a cold April setting to a hot-and-humid August setting is worth about 0.1 second. With timing changes that bumps to almost 0.2 second. That’s more than enough for the difference to go from eighth to first place. So the next question is, How badly do you want to win?
Vacuum Secondary Systems
In about 1982 I finally got a stateof-the-art dyno that would do just about any tests that General Motors could do at that time including a drag strip simulation with all the gear changes. I had a stout dyno mule, a 355-ci small-block Chevy (well, what did you expect?) that was representative of a street/strip engine of the day. It utilized a set of factory 186 head castings that I had extensively ported and milled to give (with the pistons used) a 10.5:1 CR. The cam was a flattappet hydraulic that had about 278 degrees seat-to-seat duration. My intent was to test carbs, intakes, and exhausts. Part of the carb testing was to establish the value and the optimal point of opening of the secondary barrels of a 4-barrel carb on a single- and a dual-plane intake. The carb had a much-modified 750 vacuum secondary that ultimately flowed about 920 cfm. The project involved a lot of tests over a period of about a week and several hundred pulls.
The point I want to make, though, can be summarized by looking at the results of the tests done on an Edelbrock Victor Jr singleplane intake in Figure 9.20. This shows a test of primary barrels only versus all 4-barrels.
The numbers show that below 3,500 rpm, running with the primaries only, the carb produced more torque and drivability as well as better BSFC. Up to 3,500 there was no downside. Starting at 3,500 rpm, the primary barrel’s ability to meet the engine’s air requirement fell off considerably. From 3,500 up, the use of all 4-barrels paid off. Peak power was up by no less than 106 hp and peak torque by 31 ft-lbs. With these numbers in mind, let me ask a question. Why would you want to disable the vacuum secondaries? Opening the secondaries too late reduces power and opening it too soon hinders torque, so this leads to one obvious conclusion. If we open it at the right time and the right rate for the vacuum secondaries, the carb delivers a better power curve as well as considerably enhanced lowspeed drivability.
The red curves on this graph show the output on just the primary barrels of a 4-barrel carb. The blue curves show the output with all 4-barrels in operation. Note how much better the output is with just the primaries in use at RPM below 3,500.
When the primary throttles are opened, the airflow through the venturi causes a drop in pressure at the primary venturi tapping. The secondary venturi tapping bleeds off part of this signal at this stage. As airflow increases, the partial vacuum at the primary tapping overcomes the secondary tapping bleed and so draws air from the vacuum chamber. This causes the diaphragm to be sucked up and, via the actuating rod, pulls the secondary throttles open. At this point, the bleed at the secondary venturi tapping becomes an additional source of vacuum and serves to augment the rate at which the secondary is opened.
The red arrow indicates the primary venturi tapping for the operation of the vacuum secondary butterflies. The vacuum signal generated at that tapping is communicated to the vacuum diaphragm housing, indicated by the blue arrow.
Let’s consider drivability. In Figure 9.3, you see torque numbers from just the primaries in operation, and torque is increased by about 43 ft-lbs at 2,200 rpm. What does not show on the graph is the very rough running at that RPM when all 4-barrels are open. The engine ran very smoothly with just the primaries in operation. Why? Because these 2-barrels were well able to supply the engines air demand and provided a strong booster signal for good fuel atomization. On the other hand, the fuel atomization at these low RPM with all 4-barrels in operation was very poor. In addition, using the primaries only resulted in a much better BSFC.
Using a vacuum secondary also means that you can size a carb better for both ends of the RPM range. This means the ability to use a larger carb before low-speed drivability becomes an issue. Note that this carb flowed 920 cfm. Ask yourself how many 476-hp 350s could use a mechanical secondary carb with that much CFM without giving away any low-speed output potential. Also because of the better low-speed torque an engine equipped with a vacuum secondary carb can be paired with a tighter converter, giving the same performance but with better fuel economy. So, have I convinced you that there is much merit to the use of a vacuum secondary carb? If torque output and fuel efficiency below about 3,000 to 3,500 rpm is important, then a vacuum secondary is the way to go. But consider this: Even if your engine did not really require a vacuum secondary, there is no real downside to a vacuum secondary carb if optimally calibrated.
On the other hand, if you used a mechanical secondary, you could be losing a great deal of low-speed torque unless your carb is equipped with very active boosters.
Vacuum Secondary Function
Take a look down the carb’s barrels. There is a hole in both primary and secondary barrels on the side that has the vacuum diaphragm, situated right at the venturi minor diameter. The one located in the primary barrel senses the pressure drop at the venturi, which increases as the engine’s demand for air increases. This primary venturi passage is connected to the space above the vacuum diaphragm and provides the vacuum to lift the diaphragm to open the secondaries. Control of when and how fast it opens is achieved by several means.
A major control factor is a passage smaller than in the primary, which leads to the secondary venturi and is also connected to the primary passage. This passage has two functions. First, it initially bleeds off signal generated by the primary venturi (see Figure 9.10). Just how much it bleeds off is controlled by the relative size of this hole and the one in the primary. At some point the vacuum signal generated by the primary overcomes the influence of both the secondary barrel bleed and the spring in the diaphragm housing. The secondaries then open. As a result airflow through the secondary barrels causes a vacuum to be generated at the bleed hole.
Make no mistake, this Holley 750 Ultra Street is a no-holds-barred high-performance carb. Note that it has vacuum secondary barrels! I have run several of these on the dyno and at the track, and they perform excellently.
Holley has a wide range of vacuum secondary springs to suit just about every application. The set you see here is available in Holley’s kit (PN 20-13).
If the stock top cover is replaced with this quick-change modification available from Holley (PN 20-59), the vacuum diaphragm spring can be replaced in seconds instead of minutes.
This graph shows the delivered load and stiffness of the springs for tuning in the secondary action on a vacuum secondary carb. The easiest way to get the best spring rate is to start with one that is too stiff and gives clean results, then progressively move to a lighter, weaker spring. When you feel the engine “bog,” go back to the last spring that gave a totally smooth transition onto the secondaries.
Here is a typical out-of-thebox Holley vacuum diaphragm housing. Changing the spring contained within this housing is a little more time consuming than you might expect unless the stock cover is replaced.
Second, the bleed hole reverses its function and assists in the delivery of the vacuum signal produced by the primary. Once the secondaries start to open, the bleed hole accelerates the opening process. Also, according to some Holley engineering friends of mine, this secondary action stops the system from hunting at or close to the transition point. If you follow the vacuum passage toward the diaphragm housing you come to a ball check valve. (In some newer, smaller CFM carbs this is replaced with a calibrated air bleed but the function is similar.) The seat where the check valve is located is grooved, producing a controlled leak.
The principle purpose of this check valve is to allow the vacuum in the diaphragm housing to build at a controlled rate consistent with a smooth, progressive opening. If the ball is removed, the secondaries open much quicker and, as a result, the manifold vacuum may change fast enough to warrant some accelerator pump action. But there is no accelerator pump on the secondaries unless you build it with one. The second function of this check valve is tied to rapid throttle closure. When the throttle is closed the vacuum source ceases to produce vacuum. The vacuum existing in the diaphragm housing then causes the ball to lift from its seat, thus rapidly releasing any vacuum in the diaphragm housing. This, in turn, quickly closes the secondary butterflies.
A popular hop-up modification is to remove the ball. Doing so increases the rate of opening but it also reduces control over that opening rate. The only time this works is when the carb is way too small for the job at hand. In practice, spring selection gives you 90 percent of all control. Holley has a number of color-coded springs ranging in length and stiffness. For the most part you need only concern yourself with five springs: one is white (PN 38R-1195, not included in the PN 20-13 spring kit), two are yellow (one short and one long), one is purple, and one is naturally colored steel. See Figure 9.26 for spring characteristics.
Vacuum Secondary Tuning
Considerable mystery surrounds the tuning of the secondaries on a Holley carb. In fact, it is not at all complex, but it can be time consuming. As mentioned earlier, by selecting the correct spring for the vacuum chamber, you take care of 90 percent of the tuning of the secondaries. The entire point of a vacuum secondary carb is to eliminate a stumble or hesitation by not presenting the engine with more carb CFM than it needs. This very issue dictates the way to a good secondary tune. In essence, you should start with a spring that is too stiff, so it delays the opening longer than necessary. Then test a progressively lighter/shorter spring until you find the lightest one that just causes a stumble. You simply install the previous spring and most of the tuning is done.
Shown here is the quickchange top cover installed.
To get an idea of the order in which you should try the springs, refer to Figure 9.26. When selecting the lightest spring do not be fooled into thinking that if you can feel the secondaries coming in, it must be better. If you think it is, go to the drag strip and verify that it is happening. The sensation of a secondary surge is usually because, just before the surge, the engine is starting to lay down, giving the impression of more power when the engine recovers. A trip to the strip to test this and a spring that is slightly stiffer and brings the secondaries in smoothly is almost always faster.
Spring Changes
Changing the spring in the diaphragm housing can be difficult. It takes a few minutes to remove the housing and you usually have to take off the electric choke to get at all the screws. In addition, you also need three hands to relocate the diaphragm. The way around this is to use Holley’s “quick change” kit (PN 20-59) that allows access to the spring through the top cover without having to touch the diaphragm or its securing screws.
If you want easier adjustment, consider Quick Fuel Technology’s externally adjustable vacuum diaphragm housing. It has a vacuum orifice adjusting screw in the cap. This provides the means to make adjustments to the size of the bleed communicating from the carb’s barrels to the diaphragm housing. My daughter, who incidentally builds her own race engines, uses a Quick Fuel Technology adjustable vacuum secondary setup as you see in Figure 9.28 on her 525-hp 302, which she runs in a 1969 Boss 302 Mustang bracket car. The car is good for a consistent 6.7 seconds on the 1/8-mile.
The upgraded vacuum secondary housing is reinstalled on the original 390 carb. To change the spring, you only need to remove the two retaining screws holding the bridge part of the cap for access directly to the spring housed inside.
This adjustable secondary diaphragm vacuum housing from Quick Fuel Technology can be the frosting on the cake for serious bracket racers and those wanting the secondary operation as close to optimal as possible.
The technique to set up one of these secondaries is to adjust the screw right in against the seat. Then turn the screw about 11 ⁄4 turns out. Next, select the spring. (Note that for most street and street/ strip engines the purple spring in Fig. 9.26 works just fine for probably about 90 percent of applications.)
Once you have chosen a bogfree spring, fine tune the setup at the track by either turning the screw in (slows the secondary opening rate) or back the screw out (speed the opening rate). The ability to adjust the rate of secondary opening is about equal to half the difference between one spring and another so don’t expect it to be the solution to an otherwise problematic secondary. It’s a finetuning tool; no more, no less.
Written by David Vizard and Posted with Permission of CarTechBooks
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