Holley Carburetors: Analyzing Mixture Measurement

Early in my career, people asked me, “How come your car goes so quickly with so little done to it?” The fact of the matter was that I was only one step short of TV and radio broadcasting my “secret” to the world. For some reason, however, it was really hard to convince the mainstream enthusiast that my secret was dyno tuning. No one wanted to believe it was a legitimate reason. For the typical enthusiast, this is where the chassis dyno really stands out as a tool for winning. And clearly, it is not a tool to tell you how many horses your vehicle has but one to allow you to find horsepower that until then has been eluding you.

 


This Tech Tip is From the Full Book, DAVID VIZARD’S HOW TO SUPER TUNE AND MODIFY HOLLEY CARBURETORS. For a comprehensive guide on this entire subject you can visit this link:
LEARN MORE ABOUT THIS BOOK HERE

 

SHARE THIS ARTICLE: Please feel free to share this article on Facebook, in Forums, or with any Clubs you participate in. You can copy and paste this link to share: http://www.musclecardiy.com/performance/holley-carburetors-analyzing-mixture-measurement/


 

Here’s an example. In the late 1960s and early 1970s I did quite well selling what I called my dyno pack for the twin-cam Lotus Ford Cortina. These cars had a quoted 105 flywheel horsepower figure from the 1.5-liter engine. This typically translated into about 75 or so at the wheels, which in my mind left the claimed flywheel horsepower somewhat in question. With a few lengthy chassis dyno sessions, I got to know the idiosyncrasies of this engine quite well, and so I was in a position to sell my expertise for top dollar. I investigated and found the best air filter of the day. I optimized carb calibrations, and that meant more than just a jet change or two for the Weber carbs concerned. I had found the best spark plugs and plug cables for the ignition. For the distributor, I set up the advance curve and equipped it with a contact breaker set that worked better and to higher RPM than the stock equipment. I even swapped out the fan belt for one that showed the least amount of loss and set it to the most mechanically efficient tension.

 

Other than the use of a set of fully ported and assembled Dart 170-cc intake runner heads, Jacque Vizard built this engine entirely on her own and from mostly used dyno mule parts. It was to power her race car for the following year. This 306-inch 10.5:1 engine was equipped with a Comp Cams Extreme 280 hydraulic roller and an Edelbrock Air Gap Performer intake. After a day’s dyno testing at Doug Herbert’s shop, it cranked out 380 ft-lbs and 447 hp on a slightly tweaked vacuum secondary 650. Mods included replaceable air bleeds and secondary metering block, four-corner idle. Result? A glass-smooth 600-rpm idle, great throttle response, and strong torque output from 1,200 rpm up. Jacque completed this engine two days past her sixteenth birthday.

Other than the use of a set of fully ported and assembled Dart 170-cc intake runner heads, Jacque Vizard built this engine entirely on her own and from mostly used dyno mule parts. It was to power her race car for the following year. This 306-inch 10.5:1 engine was equipped with a Comp Cams Extreme 280 hydraulic roller and an Edelbrock Air Gap Performer intake. After a day’s dyno testing at Doug Herbert’s shop, it cranked out 380 ft-lbs and 447 hp on a slightly tweaked vacuum secondary 650. Mods included replaceable air bleeds and secondary metering block, four-corner idle. Result? A glass-smooth 600-rpm idle, great throttle response, and strong torque output from 1,200 rpm up. Jacque completed this engine two days past her sixteenth birthday.

 

This 1990 5.0 Mustang with a 5-speed was inexpensive to build. The 220,000-mile bottom end was freshened up with new rings and bearings. A 274-degree Comp Cams hydraulic flat-tappet cam took care of the valve motions in a set of ported, iron factory heads. We installed a 750 vacuum secondary Holley on a dual-plane intake and spent four hours dialing in on the chassis dyno. The result was 302 rear wheel horsepower. As seen here that was enough to easily burn those drag radials.

This 1990 5.0 Mustang with a 5-speed was inexpensive to build. The 220,000-mile bottom end was freshened up with new rings and bearings. A 274-degree Comp Cams hydraulic flat-tappet cam took care of the valve motions in a set of ported, iron factory heads. We installed a 750 vacuum secondary Holley on a dual-plane intake and spent four hours dialing in on the chassis dyno. The result was 302 rear wheel horsepower. As seen here that was enough to easily burn those drag radials.

 

All this resulted in an increase in rear wheel horsepower to about 95. That’s 20 percent more power, and this increase was mostly in the form of additional torque. Did my customers notice this increase? You bet. They were over the moon with their car’s performance; but when looking at the engine, they saw nothing different from stock. So hopefully I have made my point here. If you want to get the best from your efforts, a chassis dyno is the way to go. That said, though, it is a good idea to get the engine tuned in as close as possible beforehand so as to minimize what could be expensive dyno time.

There are, however, instances where a chassis dyno session to sort out your engine is simply impractical. From either cost or location difficulties, it may be necessary to adopt the next most viable means of determining a gain or loss from a change. Here is where knowledgeable use of a drag strip can be a huge benefit. Because the subject of this book is carburetors, let’s focus on the two most important aspects of an engine setup: mixture and ignition timing. Getting the best from your engine means understanding how the carb settings affect what the engine needs in the way of ignition timing, so at some stage of the procedure, you have to look at both factors. Having established the importance of calibrations, let’s discuss the simplest means of analyzing the mixture being delivered by the carb.

 

The First Circuits to Calibrate

To make your first mixture ratio assessments, get your Holleyequipped engine running. Although it is a little out of sequence, I detail initial idle setup here, so you can make your first mixture assessments. The starting point is to establish whether the mixture is in the ballpark. When tackling carb calibrations, the very first move is always to set up the idle and transition circuits so the engine runs without stalling every time you attempt to make an adjustment. The idle setup then needs to be followed by the accelerator pump system, putting you in a position to blip the throttle and get instant response. Only after this is done should you start to look at any WOT calibrations.

Even before contemplating a dyno run or a pass down the strip get the idle, transition, and accelerator pump system functioning reasonably well. You cannot make the assumption that these factors are going to be close enough. In most instances, they may be, but the calibrations could be off by a significant amount. If you buy new, the initial setting of the idle and transition could, as often as not, be close to optimum. The bottom line here is that you do not know how close the idle and transition settings are until you start the engine. This means making some basic determination of the mixture ratios that are being delivered by the carb’s initial calibrations. Achieving good ballpark settings for the circuits is actually simply recognizing visual cues.

 

First-Time Startup

First, be sure the carb has been installed correctly, it is mechanically functional, and its operation isn’t hampered. Check that the throttle pedal fully opens the butterflies, the return spring fully closes them, all vacuum hoses are attached, etc. After checking all this, the next step is to start the engine.

 

MuscleCarB

   

 

If you want to avoid an induction/carb fire, your very first move is to make sure that the timing is not too far retarded. By whatever means, make sure that the initial timing is at least 8-degrees advance; it doesn’t hurt to have as much as 20 degrees of initial advance so long as you promptly reset it with a timing light to about 10 to 12 degrees.

 

Idle Mixture

Here I cover the basics of getting the engine started for the first time. See Chapter 8 for a more in-depth breakdown of what you need to know about the idle circuits. Before starting the engine, get the idle mixture in the ballpark by turning all the idle mixture screws fully in, then back them out two turns.

Note that some carbs, usually street applications with short-cam engines, have an idle system on the primaries only. However, when a hot cam and less intake manifold vacuum is involved, you should use a carb with a four-corner idle system. Although fuel calibration is present, a four-corner idle setup may not have a regular idle-speed adjustment screw on it.

If this is the case there is an idlespeed stop adjustment screw accessible from the underside of the base plate. Not having a secondary idle adjustment screw is usually limited to carbs for no more than a hot street application. If you buy a carb for use on a big-cam engine, you should get one with an idle-speed adjustment screw in the secondaries. Keep in mind, there is no downside other than initial cost to having a full fourcorner idle system.

The next step is to open the idlespeed adjustment screw sufficiently to produce a fast idle. Remember, some Holleys have idle speed adjustment on the primary barrels only; some for racier applications also have it on the secondary barrels, for an initial idle speed of about 1,000 rpm and about 1,200 for a race engine. Just how far open you turn the idle adjustment screw depends on several factors. If the engine is a short-cammed unit and the butterflies have no additional air holes, open the butterflies until about 0.050 inch of the transition slot is uncovered. You can see this from the manifold side of the base plate. (See Chapter 8, page 80 for a good shot of the extent of the opening; the function/need for idle air holes in the butterfly is discussed in Chapter 8.) Uncovering the transition slot by the requisite amount usually entails about two turns of the idle-speed adjustment screw. If the engine has a big street cam, open the secondary butterflies a like amount. If the engine has a race cam that generates minimal idle vacuum, you should have a carb with holes for additional air drilled into at least the primary butterfly, if not both primary and secondary butterflies. Further, note that many of the latest Holly performance/race carbs (from about 2010 on) have an idle air bypass adjusting screw under the centrally located air filter stud hole. This does away with the need to drill holes in the butterflies.

Assuming the float level is correct, you are ready to fire up the engine. If the float bowl level is not correct, set it as explained in Chapter 12. If it is a “dry” engine, switch on the fuel pump for about three seconds then switch it off, wait a few seconds, and hit it again for about three seconds then back off. Switch it on again and check for fuel leaks. If you have a fuel-pressure gauge in the system (highly recommended) verify that the fuel pressure is stable at around 6 psi. If it exceeds 9 psi, take steps to reduce it. If you are starting an engine with a new flat-tappet cam, be sure you perform the correct break in for the cam, as that takes priority over all else at this stage. If it is a newly installed engine, be sure you have filled the cooling system. If everything checks out okay, give the throttle three good stabs to the wide-open point. This puts fuel from the pump jets into the still dry intake manifold.

If you have everything just as I have detailed, the engine should fire right up. If it does not, check for a good spark. Assuming the engine does as it should and starts the first time around, to keep it going in the absence of a cold-start system, you may have to put in some fuel from the pump jet system by giving the throttle some short stabs. After about a minute or so, the engine should idle on its own. If it does not, adjust the idle-speed screw for faster idle. If that does not fix the idle, more fuel is needed, so adjust the idle-mixture screws to a richer setting.

 

For those carbs not equipped with a secondary idle-speed adjustment screw on the linkage, the idle-speed stop, situated on the underside of the base plate, adjusts idle speed.

For those carbs not equipped with a secondary idle-speed adjustment screw on the linkage, the idle-speed stop, situated on the underside of the base plate, adjusts idle speed.

 
A serious high-performance engine should use this form of plug tip. It reduces the heat in the electrode and provides a better spark path for cleaner ignition.

A serious high-performance engine should use this form of plug tip. It reduces the heat in the electrode and provides a better spark path for cleaner ignition.

 
I use this type of plug for my race engines. It doesn’t last as long, but it usually contributes to a better charge light-off.

I use this type of plug for my race engines. It doesn’t last as long, but it usually contributes to a better charge light-off.

 

The spark plug should run at a temperature to ensure deposit burn-off and no hotter. Here are the factors that affect it.

The spark plug should run at a temperature to ensure deposit burn-off and no hotter. Here are the factors that affect it.

 

If the engine still doesn’t run, the idle system may be delivering an idle mixture that’s too lean. This can be established by actuating the accelerator pump lever arm to see if the fuel injected from the accelerator pump temporarily fixes the situation. If it does, it’s a fair bet that a bigger idle jet is needed. (See Chapter 8 for changing the idle jet). At this point you should have the engine running.

As soon as it is hot enough to run without your attention on the throttle, take a look at the exhaust. If there is any trace of black smoke, the mixture is too rich. If this is the case you need to make an idle mixture screw adjustment. Turn in each idle screw progressively until the exhaust is free of black smoke. As the mixture is leaned out, it becomes progressively harder to see any black smoke. Under artificial lighting, it’s particularly difficult to see black smoke. (If you are indoors, make sure there is plenty of ventilation because a rich mixture produces a lot of potentially fatal carbon monoxide.) To help see the last remnants of black smoke, hold a sheet of white paper in close proximity of the exhaust pipe for about a minute. This gives you a guide to the mixture status from two aspects. If there is still black smoke, it shows much more against a white background. And if the mixture is still too rich, it discolors the white paper and the paper has a smell tinged with a gasoline aroma. Be aware that excessively rich mixtures considerably accelerate bore wear, so avoid running at an overly rich mixture as much as possible.

At this point, you have now carried out the simplest means of analyzing the mixture. And all it has cost is the price of a piece of white paper! The engine should be warmed up sufficiently to transmit heat to the intake so that the intake is hot to touch but not so hot that you cannot hold your hand on it. Typical operating temperature is about 120 degrees F for a race engine and about 140 to 150 degrees F for a street engine, as measured about an inch below the carb mounting pad just below the base of the carb. Reset the idle to about 100 rpm more than the likely stall RPM of the engine. For instance, if you wind down the idle speed, and it gets to 800 rpm and sounds ready to stall, you then wind it back up to 900 rpm.

After you have established a steady idle speed you can start to calibrate idle parameters a little closer. Your first step is to turn in the idle mixture screws. Do this progressively for each one. If the idle jet is about right, you should find the idle mixture adjustment that gives the maximum RPM or vacuum measured on a vacuum gauge, which occurs at 11 ⁄2 to 2 turns out. Reset the idle speed to about 100 rpm more than the perceived or determined stall speed. Now reset the ignition timing with a timing light to whatever is appropriate for the application in hand. (See sidebar “Idle Speed Ignition Advance” on page 38 as a guide as to what the ignition timing is likely to be.)

Now the mixture should be close but one more reset doesn’t hurt. Again use the tach for RPM or a vacuum gauge (or both) to adjust the idle screws in or out until the best RPM/vacuum is achieved. Reset the idle RPM to a little more than the lowest speed that won’t stall when, if applicable, the vehicle is put into gear (automatic transmission) and/or the A/C is turned on.

 

Accelerator Pump

To see where you are, initially blip the throttle and have someone check the exhaust to see if any black smoke came out. If some was seen, the pump system is adding too much fuel. The best pump shot is one that supplies just enough fuel to cover the effect of fuel dropout with rapidly diminishing intake manifold vacuum. Two things determine what is needed: the pump squirter nozzle size and the stroke of the pump as delivered by the cam on the throttle spindle. If the throttle opening causes the engine to produce black exhaust smoke, you need to reduce the squirter size. If dropping the squirter size a few numbers does not fix the problem, go to a slower-acting accelerator pump cam. Keep making adjustments between the squirter size and the cam until (no matter how quickly it is opened) a clean RPM pickup is achieved.

 

Plug Reading

Many experienced carb tuners may be asking why it took so long to get to the subject of plug reading. It has its place in the grand scheme of carb tuning, but it is all too often assumed to be far more accurate and easy to do than is actually the case. Although plugs can be read to good effect and provide a reasonable picture of carb calibration, doing so takes a lot of experience. Any time plug reading can be replaced with something a little more precise (such as an oxygen sensor mixture analyzer), you should do so if your budget allows.

Here’s a comment I have heard many times: “If plug reading is good enough for Smokey, it’s good enough for me.” Smokey Yunick once told me that, even after 30 years of regularly reading plugs, he was still refining the art. As I write this, my experience doing so exceeds 50 years. I am well aware of its potential limitations and pitfalls. The guys who are really good at plug reading are also packing better than a quarter century of experience. If you are new to the sport of motor racing, ask yourself if you can afford to spend 25 years acquiring a skill that can be done faster, more accurately, and right now with some appropriate equipment, such as an oxygen mixture analyzer. Your answer should be “No, I can’t.” But plug reading will always have a place in the carb tuner’s tool box.

 

Coloration Factors

Before going any further, let’s look at some factors that can throw off a mixture interpretation. First, changing the plug heat range can change the plug color for a given mixture ratio. The location of the plug in the chamber and the style of combustion chamber can also affect how the plug looks. Though rarely the case, this factor can dramatically throw off plug reading accuracy.

One race engine I built for Chrysler many years ago showed a plug that indicated a lean mixture (about 14:1 or 15:1) when all the mixture measuring equipment showed it was actually right on 13:1, and one jet size larger in each carb barrel would reduce output. Unless this discrepancy was a known factor, using plug reading to calibrate the engine’s carburetion would have been nothing short of a disaster. It would not have shown a commonly acceptable plug coloration until the mixture was in the 11:1 range. At this point the engine was over 5 percent down on its best output. This demonstrates that what you think you see in terms of plug-indicated mixture ratio is not always what your carburetion is delivering.

 

I pulled this plug from one of my Chevy big-block 496-ci engines while dyno testing. The jetting had just made the best power curve and the oxygen mixture readouts showed 13 to 13.2:1 throughout the pull. Get the mixture right on, and this is about what you can expect to see on 93-octane pump fuel.

I pulled this plug from one of my Chevy big-block 496-ci engines while dyno testing. The jetting had just made the best power curve and the oxygen mixture readouts showed 13 to 13.2:1 throughout the pull. Get the mixture right on, and this is about what you can expect to see on 93-octane pump fuel.

 

You can learn a lot from a close inspection of the plug. This Moroso illuminated magnifier allows a far better inspection to be made.

You can learn a lot from a close inspection of the plug. This Moroso illuminated magnifier allows a far better inspection to be made.

 

Even though Terry Walters Precision Engines’ (TWPE) Jack Sein has all the high-tech equipment you need to test engines, he still relies on an experienced eye. At the end of the day, understanding what is being shown can reveal or confirm anomalies that even a sophisticated oxygen system or in-cylinder pressure measurement may only allude to.

Even though Terry Walters Precision Engines’ (TWPE) Jack Sein has all the high-tech equipment you need to test engines, he still relies on an experienced eye. At the end of the day, understanding what is being shown can reveal or confirm anomalies that even a sophisticated oxygen system or in-cylinder pressure measurement may only allude to.

 

This chart is for a gasolinefueled engine. Left to right is rich to lean with the middle third of the chart being your target color. Top to bottom is the change in color that’s seen between leaded and unleaded fuels.

This chart is for a gasolinefueled engine. Left to right is rich to lean with the middle third of the chart being your target color. Top to bottom is the change in color that’s seen between leaded and unleaded fuels.

 

Another factor that affects plug coloration is the type of fuel. Alcohol and alcohol-based fuels color differently than gasoline and leaded colors differently than unleaded fuel. Also consider that a plug reading only gives an average indication of the mixture ratio over the time span and RPM range involved. If you are looking at plugs that experience slightly rich to slightly lean and back to slightly rich over the test RPM range, the plug coloration only reveals the average mixture ratio. You do not get the best unless an instrument such as an oxygen mixture analyzer is used on the engine to read the instantaneous mixture ratio, which can see these mixture fluctuations. The lesson here is: Don’t assume that plug reading is absolutely accurate.

 

Determine the Mixture

So, with the carb’s limitations in mind let’s look at what can be done in the way of optimizing power from its main-circuit WOT calibrations. In essence almost all plug reading is based on the presence (or lack) of soot from the combustion process. If there is insufficient oxygen for the fuel present (the mixture is too rich), an appreciable amount of soot forms. The soot, incidentally, is nothing more than the carbon left from the heat that the unburned fuel has experienced. It has decomposed from a hydrocarbon to a nearly pure carbon. Remember, maximum power typically occurs at 13:1 and the chemically correct mixture (stoichiometric ratio) is right around 14.7:1 for a typical gasoline. This being the case, the presence of a little soot on the plug is typically just what you want to see. The question is just how much soot you should expect if the mixture is right. To answer that question, see the coloration chart in Figure 4.10. You can determine more than just the overall mixture with a reasonable degree of functionality and accuracy. Here is how to make the most of plug reading to set the mixture.

 

I have run high-performance street engines with the street Avengers from 570 to 870 and a dual-plane intake on an appropriately spec’d small-block. They have all delivered stellar results. WOT jetting was within two or three sizes of optimal while pump and idle circuits were close enough to need no further jetting.

I have run high-performance street engines with the street Avengers from 570 to 870 and a dual-plane intake on an appropriately spec’d small-block. They have all delivered stellar results. WOT jetting was within two or three sizes of optimal while pump and idle circuits were close enough to need no further jetting.

 

Set the idle mixture so that it is about as lean as it can be while still having a decent idle. If the engine is on the dyno, this is not so important because you can do a “plug cut” at the end of the test. (A plug cut, or chop, is closing the throttle quickly and killing ignition at the end of a test, so you have colored the plugs at that particular RPM.) You can also do this at the track at the end of a pass. The usual practice is to close the throttle, kill ignition, and hit neutral. Next you have two options. After cutting the engine at the end of the pass, you can coast to somewhere convenient to do the plug check. Or, you can tow the car back to the paddock. Both of these options are a little inconvenient, but there is a little-appreciated but easily accomplished alternative. You set the idle mixture right around the stoichiometric ratio, and set the idle speed at just enough for the car to return to the pits under its own power. It takes a long time for such running conditions to recolor the plugs. This is because there is neither free oxygen nor fuel present under these conditions, so unless you lean on the gas pedal and blow/burn off whatever evidence of mixture the plugs might reveal, the plug coloration doesn’t change fast enough to be significant.

And this is as good a time as any to make sure the accelerator pump action produced no black smoke; if it does, it skews the results. By adopting this technique plug reading is only very minimally affected by your return drive from the end of the strip to your spot in the paddock.

 

Track Practice

Should you use a 1/8- or 1/4-mile drag strip? I have successfully used both, but a 1/4-mile track has the advantage of coloring the plugs to a greater extent. It’s harder to read a plug after only a 1/8-mile run, and on occasion you may have to run two or three passes to get any sort of worthwhile indication of the mixture. If it proves necessary to make additional passes to color the plugs sufficiently make subsequent passes as close to the first as possible. Also do not do a burnout for the subsequent passes. Drive around the wet burnout box and go directly to the start line.

Set up your launch parameters so as to make the most consistent starts possible. This means making the most consistent but not the fastest 60-foot times. Now take note here: You must judge the results based on the trap speed, not the ET. If the start is consistent, your average power over the RPM used is completely represented by the trap speed. With this in mind, don’t try fine tuning on a day with wind gusts; a 5-mph head wind on one run versus a 5-mph tail wind on another affects your assessment of the carb’s maximum power calibration.

Before actually starting any power calibration tests, set the total advance ignition timing to about 2 to 3 degrees less than you expect it to be in its optimal position. It is relatively important that you do not overestimate the amount of total advance needed. Start with a slightly retarded ignition timing, set up the carb for the fastest trap speed, and then do the timing. In most cases, this procedure is quicker, easier, and safer to get optimum results.

If the timing is just a few degrees too far advanced and the initial mixture from the carb is too lean, the engine can run into detonation and, on an open exhaust, you don’t hear it. Persist in traveling this road and the first indication that something is wrong could come in the form of a melted piston or two! The next step is to install new plugs after the engine is near or at operating temperature and right before you start testing. If you want to make the most of plug reading, you can perform a couple preliminary steps that help determine factors other than just the air/fuel ratio. The first is to prep the spark plugs. This gap spec ensures the best spark possible. (Autolite race plugs come with an electrode length that requires minimal reworking to get what you are looking for. I like using Autolites because the dyno tells me they work because torque and horsepower are higher, and the cost is low for a quality plug.)

Use as much spark plug gap as possible for strong running but not to the point of a misfire. With most race ignition systems the plug gap tends to be in the 0.050- to 0.060- inch range. Once gapped install all the plugs. The next move is to get a rough idea of the flow extent and the flow pattern (in relation to the plug) of any wet flow that may exist during full-throttle operation. To do this, you put a dab of paint on the side of the plug adjacent to the intake valve. (For a small-block Ford, that means all the paint marks are on the front of the plug on one bank and on the rear on the other bank. On a Chevy, the marks face each other in pairs down each bank.) Marking the plugs this way allows a quick reference as to which side of the plug is wettest/coolest and this is largely affected by which side of the plug the intake valve is on.

Now you are ready to make a pass or two down the drag strip. After that, it’s time to inspect and assess what is going on in the cylinders.

 

Analyzing Run Results

Assuming the fuel is unleaded or low-lead (an important factor to remember) let us consider mixture ratio extremes. If the plugs are well blackened and sooty after just one or two passes down the strip, the average mixture is way too rich, so lean it out. The simplest solution is to use smaller main jets all around. However, there is more to fixing an overly rich mixture than just putting in smaller main jets, but for now, this is the easy option. (See page 83 where I detail the correct procedure to get the main circuit doing just what it is supposed to: deliver a maximum power mixture only while the throttle is near or at wide open.)

If the plugs show little discoloration and they look tinged gray or nearly white, the mixture average is too lean. If this is the case, install larger main jets. If all the plugs look okay when compared to the coloration chart in Figure 4.10, you are in good shape. But chances are that (due to uneven mixture distribution) only some of the plugs look like they should. So now you need to check for uniformity, one plug to another.

Now I think it’s worth discussing air and fuel distribution issues within an intake manifold because it greatly influences subsequent jetting decisions. First (and this may come as a surprise too many), mass flow of air between the cylinders and the mass flow of fuel can be and usually is different. On a V-8 with perfect induction, you expect 12.5 percent of the fuel as well as the air to go to each cylinder. In practice a poorer flowing manifold runner may only deliver 11.5 percent of the air to the cylinder it’s feeding, and that’s less than it should. However, it may also deliver 13.5 percent of the fuel (more than its share).

 

MuscleCarB

 

That initially looks as if there is a 2-percent difference from what it should be, but it’s actually a lot more as you have to look at the ratio between 13.5 and 11.5 percent. To do that you divide the smaller number into the larger, which reveals a whopping 17.4-percent increase in air/fuel ratio to that cylinder. That is significant power detraction and warrants a real effort to bring about a correction. The air and fuel flow idiosyncrasies of each manifold runner cause variations in the air/fuel ratio arriving at each cylinder. Usually the worst offenders here are dual-plane intake manifolds. Some dual-plane intakes deliver a very consistent cylinder-tocylinder ratio; these are usually stock replacement emissions-legal manifolds. Of the performance air-gapstyle manifolds, even the best of them can suffer a significant cylinderto-cylinder variation.

 

Stagger Jetting

Power is lost when some cylinders are too rich and others are too lean. So how do you fix this? The first step is to jet each corner of the carb in an attempt to compensate. I once drag raced a car where the rules called for an out-of-the-box dual-plane intake where port matching only 1 inch from the manifold face was allowed. This meant the only recourse to evening out the mixture between cylinders was to “stagger jet” the carb. Differences for a good race spec single-plane intake, such as an Edelbrock Super Victor (and others from Professional Products, Dart, and Weiand) usually require minimal stagger jetting.

 

Single-Plane Intake

Do not let the simple-looking runner design of a single 4-barrel, single-plane intake lull you into believing jetting anomalies are minimal or unlikely. Assuming that could cost a sizable chunk of power. Let me give you an example. It was about 1984 and I had one of my 355 small-block Chevy mule engines on my Super Flow dyno. This 530-hp unit was essentially a race intake test engine. It was equipped with an Edelbrock Victor Jr. which, at the time, was one of the best intakes available. I had this mule on the dyno a while and was confident I had the jetting just about optimum. The dyno printout showed a 13:1 air/fuel ratio from the start of a pull to the end.

Then a friend of mine brought in the prototype oxygen mixture sensor setup he was building for me. This was intended to look at the mixture delivered to each cylinder individually. Up to this point, we had been identifying the mixture ratio by means of the measured air mass going into the engine along with the fuel mass. One divided into the other gives the overall air/fuel ratio. But note that it is “overall,” not the mixture delivered to each cylinder. After installing the new oxygen setup we found that at the back of the engine, cylinders 5 and 7 (90-degrees phase separation) as a pair pulled less air than the other back pair of cylinders, 6 and 8 (270 degrees phase separation). With a stock firing order there is an interaction between cylinders 5 and 7.

 

With any single-carb setup on even the best single-plane intakes, an uneven venturi draw is seen due to a pair of adjacent cylinders having an intake event within 90 degrees. On Chevy engines with a 1-8-4-3-6-5-7-2 firing order, cylinders 5 and 7 draw 90 degrees apart so cylinder 5 is only halfway through its intake event when cylinder 7 starts its intake event.

With any single-carb setup on even the best single-plane intakes, an uneven venturi draw is seen due to a pair of adjacent cylinders having an intake event within 90 degrees. On Chevy engines with a 1-8-4-3-6-5-7-2 firing order, cylinders 5 and 7 draw 90 degrees apart so cylinder 5 is only halfway through its intake event when cylinder 7 starts its intake event. The beginning of an intake event is the most severe because the exhaust produces a substantial scavenge effect to drive the intake event. The result is that the venturi directly over runners is slightly overloaded, and consequently runners 5 and 7 rob the two adjacent venturis of some of the fuel. To compensate the 2-barrels indicated are richened. (This effect moves around the jetting pattern when alternate firing orders are used.) This is not the only jetting scenario you may find with a Chevy firing order, but it is a good one to start with. With a dual-plane intake, stagger jetting is more about compensating for the uneven flow of the runners and is far more difficult to sort out. Good plug reading skills are key here.

 

Cylinder 5 gets the air going, and as 7 starts to draw air in 90 degrees later it takes advantage of the fact the air in the plenum is already moving in nearly the right direction. Also, and this is the real influence, during the overlap, a good exhaust system produces an exhaust-driven induction pulse from cylinder 7 that is far stronger than the piston induction pulse from cylinder 5. This results in cylinder 7 robbing 5 of some of its charge.

In terms of overall airflow, cylinder 7’s greed does not fully compensate for 5’s loss. However, the longer pull from that corner of the intake affects how the two adjacent barrels deliver. To compensate jetting has to be increased as shown in Figure 4.12. Now that you understand what is going on within the intake, let’s get back to the 355 test engine. By dropping two jet sizes in the number-5 and number-7 corner and increasing the jet two sizes in the number-6 and number-8 corner, horsepower increased to 546. That’s a 16-hp gain just by putting fuel into the intake to compensate for a distribution problem.

Granted, for a single-plane intake, that was an extreme example. But conscientious stagger jetting can produce 6- to 10-hp increases.

 

Dual-Plane Intake

With a dual-plane intake and the bigger runner-to-runner flow differences, stagger jetting can pay off. In a race engine, this jetting produces an 8- to 10-hp increase almost every time. However, getting the distribution to the best possible state is far more complex than it is with a singleplane intake. You have to take into account that one corner of the carb feeds one cylinder on each bank. You almost certainly find instances in which one cylinder sharing a carb corner has a distinctly different jetting requirement from the other.

As an example, consider the cylinders fed by the front right-hand barrel of the carb. On a small-block Chevy this feeds cylinders 1 (righthand bank) and 4 (left-hand bank). If jetted to get cylinder 1 correct, then cylinder 4 runs lean. If you correctly jet cylinder 4, then 1 runs rich.

In practice, I have been able to get at least six of the cylinders to run right with the other two averaged at the best possible mixture. With some intakes, this has called for jetting, ranging from smallest to largest, having a span of about eight numbers. Was the effort worth it? Judge for yourself: On an 1/8-mile track, with the particular race car in question, I went from a 6.49 to a 6.43 ET with about a 1-mph increase in speed.

 

Fuel Wash

A fuel wash signature is not hard to spot because it directly affects plug coloration. Most obvious is that the plug is wet. It is a relatively consistent pattern from cylinder to cylinder; so to spot it, lay out all the plugs and make sure you have correctly identified each particular plug with its respective cylinder. Use the paint dots I mentioned earlier. If the black, sooty part and the wet part of the plug are in a similar position relative to the paint dot, there is, for some reason, excessive wet flow entering the cylinder.

As for the uniformity of this test, take into account the effect manifold runners have on the flow pattern arriving at the intake valve. With a single-plane, single 4-barrel intake, the runners on one end of the engine can and usually do enhance mixture swirl and suppress it at the other end. This factor can change the wetflow wash pattern on the plug. With a dual-plane intake any uniformity you might see is very much modified from cylinder to cylinder by an intake manifold that can have major effects on the wet-flow pattern arriving at the cylinders.

If the engine is equipped with a tunnel ram intake where the carb barrels are situated almost exactly over the runners, the fuel distribution is usually good. However, and this depends on the manifold design, there is still an influence between any adjacent cylinders that draw only 90 degrees apart. For example, cylinders that draw 90 degrees apart on small- and big-block Chevys are 1 and 2, and 5 and 7. And any of the influences I discussed here need to be taken into account.

 

Wet Source

A wet look can come from one or more of three sources. One, water is getting into the cylinder, and that means a head gasket problem or, worse yet, a cracked head. The fix for that is obvious. Two, oil is getting into the combustion chamber. This indicates the intake oil stem seals are not doing their job properly or the piston rings are not working as they should. Three, you have a new engine; it could be that the rings have not yet seated in. No matter the reason, the problem has to be fixed.

When an oil film is deposited on the plug body, it takes quite a lot of running with no further oil deposited to burn off the oil that is already there. If the plug shows any sign of oil deposit, wash the plug in a strong solvent. My number-one preference is lacquer thinner but spray-can brake cleaners also work. Be sure to blow the plugs off afterward. Also, just to be sure, I also heat the plugs with a propane torch until they are good and hot. Before installing the plugs (you should have all of them out), crank the engine to blow out any residual fuel in the cylinders that might rewet the plugs. At this point make some passes down the strip to reestablish the cylinder’s burn properties.

 

This is a head chamber from a Ford 289 Falcon vintage racer. You can see the chamber wash from wet flow. Fixes lie in many areas including booster selection, manifold finish, and general port shaping.

This is a head chamber from a Ford 289 Falcon vintage racer. You can see the chamber wash from wet flow. Fixes lie in many areas including booster selection, manifold finish, and general port shaping.

 

Fuel Wash Fixes

If the plugs indicate fuel wash, it could stem from two issues working together. It could be that the cylinder heads have poor wet-flow characteristics. Although it may fundamentally be a cylinder head and intake manifold problem, carb calibrations can minimize any negative impact. You must not lose site of the fact that the better the fuel is atomized at the carb, the better the mixture quality upon its arrival at the cylinder. Be aware that an overly rich mixture has more wet-flow problems than a correct mixture. This makes your first step in fixing a fuel wash problem one of leaning out the mixture until the plugs indicate a slightly lean condition and then going back up a jet size or two. The principal challenge in fixing the fuel wash almost always means fixing a lack of fuel atomization.

 

This set of plugs was used in a stagger-jetting exercise that produced the best performance. The engine is a Chevy big-block (the front is to the right). Only plug 6 is showing different by a readable amount. The arrow points to a slight amount of soot that cylinder was producing and it was slightly richer than the other seven plugs.

This set of plugs was used in a stagger-jetting exercise that produced the best performance. The engine is a Chevy big-block (the front is to the right). Only plug 6 is showing different by a readable amount. The arrow points to a slight amount of soot that cylinder was producing and it was slightly richer than the other seven plugs.

 

Here is a list of possible issues that can lead to wet flow problems:

  • Carb is too big for the engine and has insufficient venturi air speed in conjunction with limited booster signal.
  • With a mechanical secondary throttle system, too much venturi area is presented to the engine too soon. In such a case, a vacuum secondary would have probably been better.
  • Booster style and venturi size are not generating enough signal.
  • Booster design has insufficient fuel-shearing properties.
  • Fuel temperature and vaporizing characteristics are not right for the engine. More intake temperature or more light-front-end hydrocarbons helps fix the problem (see Chapter 12 and Chapter 13 for more details on fuel).

As you can see, all of these problems are beyond the scope of fixes at the race track. However, if the budget allows for a dyno session, a possible fix with the right parts and tools is more likely and convenient. As for the fixes themselves, acquaint yourself with the ins and outs of booster design and how they interact with the air correction jets and the emulsion well (see Chapter 9).

 

Ignition Timing

When you have arrived at a point where plug checks and time slips or dyno readouts show no useful gains then, and only then, can you turn your attention to optimizing ignition timing. The reason for doing this last is that the speed of the flame travel is affected by the mixture ratio. For instance, best output with an overly rich 11:1 air/fuel ratio might be as much as 42 degrees. With the same fuel but a 13.2:1 air/fuel ratio, 36 degrees could well be best. Usually a mixture slightly richer than stoichiometric needs the least timing for most power while both richer and leaner mixtures require progressively more timing.

Back when every experienced guy I worked with was older and usually more experienced than I was, I often heard it said that more or less ignition timing can also affect the mixture. While this has some truth, the degree to which it takes place is very minimal, particularly if the engine is already in the ballpark. The lesson here is: If you are working with a somewhat older generation of racers, and they make this point, nod politely and ignore them. The procedure for putting the final touches on the timing requirements is best done in small increments. Usually from an assumed retarded setting, I advance the timing by 2 degrees and make a pass. If the car goes a lot faster (in my judgment), I move it 2 more degrees for the next run. If the gain is small I only move it 1 degree at a time. The idea is to establish where the gains level out as further advance is dialed in. You need the least amount of advance that gets the job done.

Also be aware that highly leaded fuels have a longer ignition delay time than unleaded fuels. The ignition timing you come up with on the test day suits what you have in the tank on that day. Changing fuels, however, means a reevaluation of the timing required.

 

Written by David Vizard and Posted with Permission of CarTechBooks

GET A DEAL ON THIS BOOK!

If you liked this article you will LOVE the full book. Click the button below and we will send you an exclusive deal on this book. Deal-Button