For our budget-oriented purposes we find that conceptually at least, GM’s HEI distributor is a near ideal choice in terms of practicality. However in terms of high-RPM function, it falls well short of ideal. But before looking at the possible fixes lets consider why we might find it best to stick with an HEI-style distributor instead of just swapping out the entire ignition system.
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First just ask yourself where else you can find a distributor and ignition system packaged into one compact, easy-to-use assembly. Installation involves little more than just dropping the unit into the motor and plugging in one wire. It’s hard to imagine anything much simpler, and on that basis, the HEI deserves high marks.
Because the HEI is such a simple ignition system to use and has the potential to service a 9,000-rpm race engine, I’m not going to delve into endless other possibilities here. If you want to stray from my recommendations here, feel free to do so but consider this: if you are going to do your own R&D, it’s just that and you are on your own!
Before going into a lot of details about what you can do and what you should do let me set the playing field here. There are a number of companies who are in the HEI business and I pretty much know all the bosses of all these companies. I say this so you understand the fact that I am in a strong position to test all of the units involved. These companies are MSD, Accel, Pertronix, and lastly, Performance Distributors. All of these companies make good stuff, but Performance Distributors has, by far, the best and most personal customer service. Performance Distributors can and will deal with every customer on a one-on-one basis and that makes it very convenient for you to precisely get what you and your engine need. You will pay a little more when dealing with Performance Distributors but when you come up against a problem—especially ones related to ignition curves—you will be glad you did. With that said, you have the option to deal with any of the other companies mentioned for whatever parts you are looking for, but from here on out, I am going to be referring to Performance Distributors almost exclusively. Just an aside here: I have been dealing with Performance Distributors for much of my HEI ignition needs and have used its units in well over 60 magazine project engines. Never, in more than 30 years, have I had one single problem with its products. Every one has functioned just like it should!
Since we are dealing with budget builds, there are two factors that must be taken into account. First, your intent might be to install a used unit and it may or may not be in need of a rebuild. Second, an HEI in stock trim falls short of what we need for performance. Although outstanding in concept, the high-RPM spark capability it delivers most certainly is not. Depending on the coil and module characteristics, a stock HEI’s spark energy starts to fall off at about 3,300 rpm. Although still capable of jumping a typical plug gap, the spark intensity begins to drop below what’s needed for a higher-compression performance motor. At about 5,250 to 5,750 rpm, a stock HEI will drop sparks. Although a Band-Aid fix is to close the plug gaps, this isn’t what’s needed to make good power.
As a first step, let’s look at ways and means of sourcing an HEI with which to work. If you bought a complete motor or already have an HEI because your project engine is already so equipped, you’re in a good position. HEIs don’t wear their bearings as much as points distributors because the loading at the top is uniform, and at the bottom is well lubricated. This means there’s a fair chance you can hop up your existing unit to get the job done.
If you don’t already have a distributor, you have two options. First, pick up a used unit at the salvage yard and rebuild it. This will take a couple of hours and will require, as a minimum, the replacement of the rotor and cap. You also need to deter- mine if the bearings in the body are up to further service. If there is more than just a hint of side-to-side movement it’s a fair chance that the bearings need replacement. The best bet here is to start with another used unit because at least 50 per- cent of any pile of used HEI still have acceptable bearing clearance. Also check the advance weights. These wear at the pivot point and as a result produce an inconsistent advance curve. Check the pivot points here. If the actual posts are worn then you might consider dumping the whole unit because it could need a new shaft and that is a little pricey. How- ever there is a point you might want to consider before tossing whatever unit you have. Performance Distributors can furnish you with a shaft with weights and springs with a custom curve already done to compliment the engines specs you have. If the bearings in the body are decent you may want to consider that route. If only the weights are worn then you can get a set of replacement ones along with springs to build the advance curve needed from Performance Distributors.
Although it will cost a little more, your second option is to buy an Accel Blue Print rebuilt unit. These go for about $150 through any of the big parts houses. At first, this may seem just a shade more costly than doing the job yourself. The Accel unit, though, is more distributor than just a stock rebuild. My own spin tests show it to be capable of at about 750-rpm more than a typical stock unit because of a superior module. In addition, it comes with an adjustable vacuum advance and springs for the mechanical advance that will allow a dozen different mechanical advance curves. In all, this unit is a good deal.
More Spark, More RPM
If the application requires more than 5,500 rpm, you’ll need to replace performance-related parts such as the coil or module. That is what we will deal with now.
At this point I’ll cover ways to make a basic HEI unit run to higher RPM. If you’re expecting dyno test results in this chapter, forget it. It’s already a proven fact that bigger, fatter, hotter sparks are unlikely to hurt power. For this reason I’ll rely on bench tests to show what can be done to improve the stock HEI to full race capability.
With an HEI distributor, the critical components for a big, high-RPM spark capability are the coil and the module. To perform the tests shown in this chapter, the HEI’s spark output was dumped into a chamber pressurized with pure nitrogen at some 142 psi. Although this can be considered a valid A-to-B comparative test procedure, there are some points to note so you can more precisely relate results to actual use in an engine. Pure nitrogen is a far better insulator than air, which basically consists of 20-percent oxygen and almost 80-percent nitrogen. Also, cylinder heat (which was not simulated in our tests) makes it easier for the spark to jump the gap. This means it will be harder for an ignition system to fire the plugs in the test rig than in an engine. As a result, an ignition system starts drop- ping sparks in the test rig at about 15- to 20-percent sooner than it does in a running engine. The results you see here have been corrected for this.
Although the coil’s turn ratio for high- output voltage is a prime concern, there are two other factors to consider such as the total current draw and the rate at which charging and discharging take place. With- out a quick enough charging time, a coil won’t fully charge between sparks at high RPM. A more rapid charging rate can be brought about by reducing coil inductance, which usually means fewer turns on the primary side. Making such a change proves to be a double-edged sword. Although reducing the primary turns increases the primary-to-secondary turns ratio, thus producing a higher secondary voltage, it also increases the current draw. The coil design must be the result of a workable compromise. After testing, it was found that the rate of current built up in the stock coil was 2.5 amps per millisecond. The same test on a good aftermarket coil showed it to be faster at 3 amps per millisecond.
To see how this and any other coil design differences affected the spark capability, an otherwise-stock HEI was spin- tested first with the stock and then the MSD, Accel, and Performance Distributors coils. The stock HEI ran to a spark-drop- ping limit of 4,800 rpm. Replacing the stock coil with the MSD coil raised this to 6,100 rpm. The Accel and Performance Distributors coils, within 1 percent, duplicated the MSD coils performance. This test then shows that all these coils are capable of delivering a spark-limited RPM increase of 25 percent or more over stock (Fig 10-1).
The function of the module in an HEI is much the same as contact breaker points in an older-style distributor. Its first job is to supply some 200 to 300 volts to the coil primary and switch it on and off at the appropriate time. The best module is one that reliably handles the high- est current possible, delivers the highest voltage to the coil, switches as fast as possible, and has the longest dwell time (switched-on time) possible.
I tested all these factors on a high- performance module, but the most significant differences between this and a stock module showed up in the current and voltage delivered to the coil. A stock GM module typically delivers a maximum of some 5.5 amps. By comparison, the high-performance module delivered 7.5 amps, so its prospects for delivering a strong spark at significantly higher RPM obviously looked good.
The next tests we’re going to look at relate to the module’s ability to deliver voltage to the coil. The winner in this event will be the module that produces the highest voltage up to the highest RPM. For the results, check out Fig 10-2. As you can see, the high-performance aftermarket module wins by a healthy margin once again. The test results shown in Fig 10-2 show what’s required to produce more spark at higher RPM. Modules from Performance distributors, Accel and MSD all seek to do this.
The spark-producing components for an HEI from the companies I am mentioning here certainly prove their worth individually, but the real question is what they’re worth collectively. A 4,800-rpm limit on the test rig was equal to 5,500 on a real engine, so I will use this ratio for some real-world RPM capabilities. But there is a factor I need to throw in here. The stock HEI numbers were with a stock-size plug gap of 0.040. A high-performance engine responds to an increase in plug gap because the wider gap produces a more energetic and powerful spark. The tests for the upgraded HEI were with 0.055 plug gaps. The Performance Distributors coil and module equipped unit was good for a solid 9,200 rpm (Performance Distributors claim 9,000). The Accel components drove the HEI to a little over 8,900, and the MSD components drove our test HEI to the limit of our spin machine, which was 9,600 rpm.
As tempting as it may seem to use the system that gave the highest RPM, you should be aware that on the way to the rev limit there was no measurable difference in the sparks. This means if you intend to build an engine that turns only, say, 7,000 rpm, a system that goes to 7,500 will get the job done in exactly the same fashion as one that goes to 9,500. The point I am trying to make here is you don’t need to buy any more RPM than about 500 above what you perceive is needed. It’s OK if the system you choose goes more, but there are no benefits to be had other than you know the system is more than up to the job.
Assuming you have prepped the distributor as suggested with at least a high- performance module, then your plug cable requirements for an affordable alternative start to look good. The worst situation is to use old carbon string-type leads. These tend to decay in capability and cheap ones are often the source of a sizable power loss when they’ve seen 40,000 miles or two years. However, such cables, when used with the improved capability of a modified HEI, will deliver results as good as the high-quality, wire- wound inductive suppressed cables from Accel, MSD, Moroso, etc. The only difference is they won’t last, but they will get the job done for a year at least. For a reliable name brand at a low price, check out Accel’s Super Stock cables.
Once a high-energy spark can be generated at sufficiently high RPM, the next issue to address is ignition timing and advance curve characteristics. Knowing what controls the advance curve and how to change it is almost common knowledge. The big questions are: what to change it to, and why did it need changing?
The grassroots answers are: cylinder pressure prior to ignition, and, to a lesser extent, heat. Contrary to popular belief, a charge doesn’t explode, but burns progressively. The best output is usually achieved when peak pressure occurs at about 15 degrees after TDC. To achieve this, it’s necessary to ignite the charge well before. The compression pressure significantly affects the speed of the burn. The higher the pressure, the faster the burn rate. This means that as the CR is increased, the ignition advance required is reduced. When longer period cams are used, the low-speed compression pressure is reduced, so more advance is required until the dynamic effects take over and start filling the cylinder to significantly higher levels.
Let’s go through some examples and see how this works. For a starter, let’s assume we have a short cammed 9:1 truck motor. The early closing of the intake valve will mean a lot of charge is trapped at low RPM; this, in conjunction with the higher-than-stock CR, will produce a cranking pressure of around 180 or more psi. At low RPM, the charge will burn faster than in a stock unit. Result: this engine now needs less initial mechanical advance, and the total required will probably be less, so the mechanical advance will only need to come in slowly. Initial timing may range from 5-degrees BTDC to zero and total around 28 to 32 at the most degrees. When the engine is cruising at part throttle, the pressure just prior to ignition is substantially reduced due to the fact that the engine is throttled. To get fuel efficiency under such circumstances, it’s necessary to advance the timing considerably. At 60 mph in high gear, a typical small-block Chevy needs between 45 and 55 degrees of advance. Because of the short cam, the vacuum advance in our current example will not need to come in until about 7 to maybe as much as 10 inches of mercury, and the amount required will be at most about 18-degrees distributor (36 crank). This, added to the slow mechanical advance occurring at 2,000 to 2,500 RPM, should result in about 45 degrees under these conditions.
If a relatively large cam is used, then the advance needs to come on faster at first, although little or no additional total may be required. If the cam was used with the CR’s recommended, the cranking pressure won’t be significantly different. This means the quicker rate of advance needed by the longer cam can, to a large extent, be offset by the increased CR. Mechanical advance won’t change significantly until race-category cams are used. The vacuum advance for a longer cam, though, will need rethinking. The longer the cam, the sooner the vacuum canister needs to start pulling in advance. A 285-degree cam may need the “can” to start applying vacuum at 3 to 5 inches with full vacuum advance in at around 10 to 14 inches. At cruise, such a cam will, even if the CR is well matched, require about 50 degrees.
Now if all these interrelated factors sound a little confusing, just remember that Performance Distributors can build what your engine needs at a very reason- able price.
To achieve the timing required, we have three system controls. These are: 1) the mechanical advance, 2) the vacuum advance, and, 3) the total advance stop. Ideally, all the required advance characteristics should be determined by testing on a chassis dyno. But dyno time costs money, and though I strongly recommend a dyno setting up session even for the budget racer, it’s worthwhile getting as many adjustments done as closely as possible before renting dyno time. Failing that, let’s look at how the advance curve can be set up by test driving. The technique I’m about to describe only works if your vehicle is equipped with a converter that stalls at or close to typical stock levels and is muffled to the extent that you can hear detonation should it occur. If the converter stalls at much above 3,500, the effect of the ignition timing below this will affect throttle response only. The mechanical needs to be done at the track, while the vacuum needs to be done on the freeway.
At this point I’ll assume you have an HEI with a selection of advance springs and an adjustable vacuum. The first step is to install the strongest springs at both locations on the distributor’s advance mechanism. It’s necessary to deal with manuals and automatics differently. Automatics first. Stage the car a known distance into the staging light. It matters little what this may be, but it must be close to the same for each run. You’re interested only in the first 60 feet, and the tires must grip well enough to prevent wheel spin. Make a pass and check the 60- foot time. Next, replace one of the springs with the one having slightly less tension. Make another pass. If the car is faster, repeat the process with a lighter spring yet. Continue with progressively lighter springs until either detonation is heard or the 60-foot times stop improving. At this point, make a pass the entire length of the strip. Now replace the second spring with one having less tension. Make another pass and check the ET. If it’s faster, repeat the process until the best is established.
The advance curve now could be any- where from 90 percent as good as it can be to the best possible with a mechanical distributor. It takes springs, weights, and a dyno to get really close. What we haven’t dealt with at this stage is the total advance. This affects top speed more than anything, and if the tests so far were done at 28 to 30 degrees, the total will be short of optimum. If the whole distributor now is advanced, the low-speed initial advance may be too much. Go back into the distributor and replace the light primary spring with one that’s slightly stronger. Now make passes and advance the distributor progressively until the best ET/trap speed is seen. Last, go back and evaluate the secondary spring that you first started with. You’re now done, and this is as good as it gets short of a dyno.
If your vehicle is a manual transmission, all of your tests can be done on a lonely stretch of road with a stopwatch. The technique involves choosing a suit- ably low starting RPM and timing the vehicle between two speeds. Let’s assume that the motor has the potential to pull from 1,500 rpm. With the strongest springs installed, take off and have the clutch fully engaged in second or third gear at an RPM lower than the test’s starting point. For the example, we’re working here with 1,200 rpm. Floor the throttle and time how long it takes to reach 2,500 rpm. Progressively lighten up the primary spring until the best 1,500- to 2,500-rpm time results. Now repeat the tests using RPM from 2,500 to 4,000. Take the car to the strip and set the total. If this is more than a couple of degrees different than the setting used while developing the advance curve and any sign of detonation is apparent, go back on the primary spring to one slightly heavier.
Vacuum Advance and Mileage
Vacuum advance is critical if good fuel economy is to be achieved. Although you may not feel this is of great importance to a race car, having vacuum advance cleans up the way the motor drives in the pits or paddock and reduces the possibility of fuel-fouled plugs. Although an adjustable vacuum looks ideal, its adjustability is in terms of the amount—not the rate—at which it comes in. It’s a compromise, but it’s a practical compromise. Trying a dozen cans in an effort to get the optimum wears thin after a while.
To set up an adjustable unit, start with the advance cut way down and drive the vehicle. If no detonation is heard during normal, part-throttle driving, then add a little more advance. Continue with this process until detonation is apparent and then back out some of the timing until detonation has ceased. You’re now in business.
Written by David Vizard and Posted with Permission of CarTechBooks