This chapter is all about calibration of a Holley-style carb. It is not so much how to do it but more about what is required in order to do it. Without this knowledge you absolutely cannot “super tune” a carb. As simple as it may initially sound, only two factors need to be right for perfect carburetion: the mixture ratio and the mixture quality (how evenly the fuel is mixed with the air that contains it and how well it is atomized). Each must be exactly what the engine requires for the particular circumstances.
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Max-Power Mixture Ratio
Before continuing, it is a good idea if you reacquaint yourself with Figure 2.13 on page 17. You can see that dyno tests have shown that there needs to be more fuel than with the chemically correct, or stoichiometric, ratio. For a typical non-oxygenated gasoline that ratio is about 14.6 to 14.8:11. (It is somewhat different for alcohols and alcohol blends, covered in Chapter 13.) This ratio is the amount of air (in pounds) required to completely burn 1 pound of fuel and, at the end of the combustion event, leave no unused fuel or oxygen in the spent charge. In the process, all the available oxygen in the charge combines with all the available fuel. At first this seems to be an ideal situation for maximum output but the dyno tests in Figure 2.13 on page 17 indicate otherwise.
There are a number of reasons why a stoichiometric mixture ratio is not where things need to be for maximum power. Some are big, some are small, and some almost indeterminate. There are lots of theories flying around about the electrical or magnetic charge of the fuel. Several would-be inventors have tried to convince me that there is something significant in terms of power and economy that is brought about by the charge characteristics of the fuel. As of 2013, I have yet to see any real proof of this on the dyno, on the road, or on the track. Although there may be fringe factors that have a small influence on output, there are three principal factors that affect exactly how much richer than stoichiometric the maximum power mixture can be.
First, there is oxygen-to-fuel proximity at the point/time of ignition. If the atomization of the fuel is less than optimal, the larger droplets do not have sufficient oxygen immediately adjacent to them to ignite as quickly or as completely as smaller droplets. The way to fix that is to put in a little more fuel than required for complete usage of the oxygen available. The second reason for a slight excess of fuel is related to charge temperature. The extra fuel cools the charge more and thus allows a greater amount of air to be drawn in. It’s a small effect but still significant in terms of output. The third factor is fuel distribution within the intake manifold. If a manifold has poor fuel distribution between the cylinders, the best power is made on an overall richer mixture than would otherwise be the case.
The fuel’s component blend can be considered the major influence on just how rich the mixture needs to be for maximum output. Remember, we are not dealing with a single chemical entity in a typical gasoline. (See Chapter 13 for more on the subject of fuels and their specific calibration needs.) For now, let us consider the consequences of those three factors and their effect on the optimum air-tofuel ratio. The easiest way to do that is to consider typical “best-case” and typical “worst-case” scenarios.
Positive Consequences: For naturally aspirated engines, I have seen air/fuel ratios for maximum output range from 13.4 to 12.5:1. The most recent engine that impressed me in this department was a Steve Schmidt big-block Chevy drag race unit of about 612 ci. It produced BSFC figures of 0.32 lb/hp/hr at the lower end of the race RPM range (about 4,000) and a shade under 0.40 at peak power (about 7,000 rpm). These are excellent figures, as was the overall 1,245 hp produced on the single 4-barrel Holley-style BLP carb.
The factors that produced the excellent results on a mixture ratio less rich than usual were:
- Good fuel atomization; not too coarse and not too fine
- Good wet flow characteristics from the manifold plenum right through to the combustion chamber
- Good combustion characteristics from the combustion chamber itself
Negative Consequences: Engines have come into my shop that needed sorting out because they were not making competitive power. (Engines like this are far more numerous than the “good” engines, such as the Steve Schmidt big-block discussed above.) An initial setup for such an engine typically yields the best results when the air/fuel ratio is in the 12.5:1 range. Even after diligent jetting for maximum output, power is still off the mark and the BSFC figures are poor. They are typically in the high 0.4s to low 0.5s at the beginning RPM of the dyno pull to about 0.55 at peak power. Making several dyno pulls typically causes the engine to develop a very black exhaust and plugs that are black and sooty. In that situation, the engine is down on power and also consumes much more fuel, fouls plugs easier, and wears out bores significantly faster.
If you are testing an engine that looks as if it falls into this category (in terms of air/fuel ratio and mixture quality), the fixes are to investigate the atomization at the carb’s venturi. It could be that the venturi is just too big for the application and fuel atomization is suffering as a result. It could also be a case of having a booster that is just not boosting the main venturi signal enough, resulting in insufficient signal to atomize the fuel. Yet another problem could be that the particular combination of the booster and the air correction jets are not getting the job done.
If the mixture quality coming out of the carb is poor, the trend most likely continues downward. In most cases, the mixture quality becomes worse as the air and fuel travel through the induction tract. If the mixture quality coming from the carb is questionable, things such as the shiny ports with lazy flow areas compound the problems. (Refer to my book David Vizard’s How to Port & Flow Test Cylinder Heads, which details this and the fixes, as well as those for a host of wet flow problems.) Adding a combustion chamber that is a little off the pace in terms of its ability to deal with excess wet flow can cause the engine to fail to live up to expectations.
As unexpected as it may seem, many Holley-equipped engines, as they came from the factory, have (due to less-than-stellar carb spec’ing) poor mixture atomization and consequently poor mixture quality from the carb. But, back in the day, factories used a couple fixes that you should never use if you are seeking engine performance: manifold heat and high vacuum, which are easy to achieve with a short street cam but not with a big-performance cam. This should tell you that, to an extent, the odds start stacking against you as soon as you step outside the “stock” arena.
Now let’s take a serious look at idle mixtures with a view toward using the least amount of fuel to get the job done. At idle, several significant factors come into play. Some are a great help and some are a hindrance. What we have going for us, in terms of aiding an effective combustion process, is the fact that at idle a typical shortcammed street engine has quite a lot of manifold vacuum. When the intake pressure drops significantly below typical atmospheric pressure, the fuel has a far greater propensity to vaporize. This, and the temperature of the induction system as a whole, means that most, if not all, of the fuel is vaporized and transformed into a gaseous state. As such, it is in the most ignitable form for the engine to deal with. Not only is it easy to light off, but any cylinder-to-cylinder mixture variations are minimized.
All the foregoing favors an idle mixture that could theoretically run very lean. What prevents this from happening is the dilution of the incoming charge by exhaust residue left in the combustion chamber at the end of the exhaust stroke. In addition, you can also have exhaust dilution due to exhaust-flow reversion at the end of the exhaust stroke. This is where the overlap generated by the cam comes into play. Also, exhaust pollution of the intake charge within the intake manifold can occur if the manifold is a single-plane type where all the cylinders are connected to a common plenum. A dual-plane intake with induction pulses separated by 180 degrees instead of 90 does not have this inter-cylinder pollution. (The effects of different manifold styles on intake vacuum are covered in Chapter 6.)
The charge remains in the combustion chamber at TDC on the overlap, which is the end of the exhaust cycle and the beginning of intake. The cam overlap itself and manifold configurations play a great part in diluting the charge emanating from the carb with exhaust. This makes it more difficult to ignite, and the fix is more fuel for any given amount of air. In the case of a big cam and singleplane intake, this setup requires more idle air with a lot more fuel. If not for exhaust dilution/ pollution you might be able to run an idle mixture as lean as 18:1 or more, given a stout ignition system. For a street engine, an important part of getting a good idle is having the highest compression ratio (CR) possible, which achieves two things. First, it reduces the residual exhaust because the chamber volume is smaller. Second, the higher the CR, the less overlap the engine needs to achieve a target top end. The reduced overlap is typically seen as a wider camshaft lobe centerline angle (LCA).
Any effort put into reducing exhaust contamination of the intake charge can pay worthwhile dividends. Unfortunately, no matter what the effort may be to counteract this contamination of the idle mixture, it is always going to occur. The final result is that it is necessary to run a richer mixture than would otherwise be the case. Also, the greater the contamination, the richer the mixture needed to get a decent idle. As if all that were not enough, there is yet another factor coming into play: the bigger the cam, the lower the manifold vacuum. This means the fuel is less likely to vaporize. Additionally, with a highperformance engine, which typically has a much cooler intake manifold, one major asset (fuel vaporization) for a smooth, low-RPM idle is being seriously eroded.
All the idle mixture issues discussed so far add up to the fact that as the manifold vacuum decreases, the mixture ratio needed gets richer and the total amount of air required for best idle increases. Work at it and you can get a street engine equipped with a short-cammed 180-degree intake manifold to idle at 500 to 600 rpm on a 14.5 to 15:1 air/fuel ratio. At the other end of the scale a typical race engine needs 12.5 to 13:1 for its best chance at a steady idle. Even so, that idle can well be in the 1,000- to 1,100-rpm range.
Before concluding this discussion on idle mixtures, I have a few words on emissions. If you have a street machine you should make every effort to have it run as clean as possible. At idle, the ignition timing is crucial if the minimum amount of fuel is to be consumed. Unfortunately, at minimum fuel consumption, the parts/million of unburned hydrocarbons and the carbon monoxide emissions are highest. Minimum fuel consumption at idle usually entails manifold vacuum pulling in about 25 degrees or more of additional timing. The optimum idle advance is typically about 35 to 40 degrees for a short-cammed street engine and (though not commonly realized) as much as 50 degrees for a street/strip engine. For OE situations with strict emission standards, the targeting of minimum idle fuel consumption was, for emissions reasons, unacceptable. This led to the introduction of “ported vacuum.” Here is how it works.
First, the vacuum port connecting the carb to the distributor is above the butterfly. This means no vacuum signal is seen by the distributor until the throttle is opened just enough for a low-speed cruise. Idle ignition timing is whatever the initial timing is set at. That’s typically 8 to 12 degrees BTDC. To get the engine to idle with less timing than it actually wants, the throttle must be opened quite a bit more. This reduces the amount of idle vacuum so less exhaust is pulled back into the intake during the overlap period. This, in turn, reduces the exhaust dilution. The extra air pulled in for the idle requirement is now potentially better for combustibility so it can run with a leaner mixture. The result is less idle emissions but a higher idle fuel consumption because the throttle is open significantly wider. As soon as the butterfly passes the vacuum port the timing is pulled in to whatever is appropriate for the current part-throttle conditions.
When it comes to fuel economy, getting the cruise mixture to the optimal setting is where the most mileage is found. Two words are crucial here: lean burn. Chapter 1 discusses the mythical 100-mpg carb, but that does not mean that your carburetor choice and its calibration are not significant. To effectively burn a super-lean ratio (less than about 15:1), focus on the engine spec. If the engine is built with power output as the main priority, a typical Holley-style carb spec’d more toward power output can still deliver a burnable 15:1 ratio. This gets reasonable economy and is easy to achieve.
Things get more demanding when greater emphasis is put on fuel economy and power takes a back seat. This does not mean losing power of any real consequence. Calculations based on fuel economy results indicate that it is entirely possible to build a 300-hp, 375-ft-lbs 302 in a 5-speed 5.0 Mustang returning somewhere close to 30 mpg on the freeway at 65 mph. A car with that output and weighing 3,200 pounds would run the quarter-mile in about 13.35 seconds with a trap speed of around 102 mph. Increase the power by 30 to 40 horses and the mileage may only drop about 2 mpg while performance increases to a high- 12-second ET and a trap speed of about 104 to 105 mph.
When performance was a substantial priority but the engine had to remain totally streetable, I have built 420-hp, 390 ft-lbs 302s that returned better than 25 mpg on the freeway and ran 11.85 and 115 in the quarter. Such an engine still has a fully functional cold-start system and a vacuum advance distributor. With that much power, though, slicks are a vital part of the equation. These mileage figures rely on the ability of the carb tuner to select an appropriate carb spec and also to tune it to deliver as lean a mixture as the engine can tolerate. Assuming the ignition system is really hot, a cam that has appropriate timing for economy, a good exhaust, and a decent compression ratio (10:1 plus), air/fuel ratios of more than 17:1 are realistic. Given development time, the engine spec could handle mixtures as lean as 19:1.
When an engine gets to this stage, the calibration of a Holleystyle carb becomes hypercritical. The biggest obstacle is the transition from a steady-state lean burn at one RPM to another as the throttle is opened wider. The amount of manifold vacuum is generally much higher than average (on this mileagespec engine), and even a small change in throttle position causes fuel to momentarily drop out of the air/fuel mixture. Because the mixture is already near the lean limit, any further leaning out, even a very small amount, runs the engine into a momentarily lean misfire.
This, as you may imagine, causes drivability problems. These are the biggest obstacles to overcome for fuel economy.
Written by David Vizard and Posted with Permission of CarTechBooks