what advance springs to use msd for stock sbc

For our budget-oriented purposes we notice that conceptually at least, GM'due south HEI distributor is a near ideal selection in terms of practicality. Even so in terms of high-RPM part, it falls well brusk of ideal. But before looking at the possible fixes lets consider why we might find it best to stick with an HEI-way distributor instead of just swapping out the entire ignition system.

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Start just ask yourself where else you tin find a benefactor and ignition organization packaged into i meaty, easy-to-use assembly. Installation involves little more than simply dropping the unit of measurement 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 utilize and has the potential to service a 9,000-rpm race engine, I'1000 non going to delve into endless other possibilities here. If you want to stray from my recommendations here, feel complimentary to practice so but consider this: if you are going to practice your own R&D, it'southward just that and you are on your ain!

There are a lot of fancy ignition systems available for the small block Chevy but the simplest and virtually cost constructive is yet the HEI—but it needs a little hopping upward to become the job done. A custom built Performance Distributors unit of measurement takes care of that.

Before going into a lot of details about what you can practice and what you should do let me set up the playing field hither. 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 and so you sympathise the fact that I am in a potent position to test all of the units involved. These companies are MSD, Accel, Pertronix, and lastly, Performance Distributors. All of these companies make proficient stuff, merely Performance Distributors has, by far, the best and almost personal client 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 demand. Yous volition pay a little more when dealing with Performance Distributors but when you come upwards against a trouble—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 whatsoever parts you are looking for, but from here on out, I am going to be referring to Functioning Distributors most exclusively. Just an bated here: I have been dealing with Performance Distributors for much of my HEI ignition needs and accept used its units in well over 60 magazine project engines. Never, in more than than 30 years, take I had 1 single problem with its products. Every one has functioned merely like it should!

This Scat cranked AFR headed 383 made 503 ft-lbs and missed 600 hp by just a couple of hp—all on a 10.5:1 CR and pump fuel. The sparks for all this power came from a Performance Distributors HEI.

HEI Shortcomings

Since we are dealing with upkeep builds, at that place are two factors that must be taken into business relationship. 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 brusk of what we demand for performance. Although outstanding in concept, the loftier-RPM spark capability information technology delivers about certainly is non. Depending on the whorl and module characteristics, a stock HEI's spark energy starts to fall off at about three,300 rpm. Although still capable of jumping a typical plug gap, the spark intensity begins to drop below what's needed for a higher-pinch performance motor. At about v,250 to five,750 rpm, a stock HEI volition drop sparks. Although a Band-Assistance fix is to close the plug gaps, this isn't what's needed to make practiced power.

Basic Rebuild

As a first step, permit's look at ways and ways of sourcing an HEI with which to piece of work. If you bought a consummate motor or already take an HEI because your project engine is already so equipped, you're in a good position. HEIs don't article of clothing their bearings as much equally points distributors considering the loading at the top is uniform, and at the lesser is well lubricated. This means at that place's a fair chance yous can hop up your existing unit to go the job done.

If yous don't already have a distributor, y'all have two options. Outset, pick upward a used unit at the salvage chiliad and rebuild it. This will have a couple of hours and volition 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 than just a hint of side-to-side movement it's a fair chance that the bearings need replacement. The best bet hither is to start with another used unit because at to the lowest degree 50 per- cent of any pile of used HEI even so have adequate bearing clearance. As well check the advance weights. These wear at the pin indicate and equally a result produce an inconsistent advance curve. Bank check the pivot points here. If the bodily 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 in that location is a point y'all might want to consider earlier tossing whatever unit you lot have. Functioning 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 y'all may desire to consider that route. If only the weights are worn then you tin can get a set up of replacement ones along with springs to build the accelerate curve needed from Performance Distributors.

Although it volition price a piffling more than, your second choice is to buy an Accel Bluish Print rebuilt unit of measurement. These go for nigh $150 through whatsoever of the big parts houses. At first, this may seem simply a shade more than plush than doing the chore yourself. The Accel unit of measurement, though, is more than distributor than just a stock rebuild. My own spin tests evidence it to exist capable of at about 750-rpm more than than a typical stock unit considering of a superior module. In improver, it comes with an adjustable vacuum accelerate and springs for the mechanical accelerate that will allow a dozen different mechanical advance curves. In all, this unit is a practiced deal.

More Spark, More than RPM

If the awarding requires more than 5,500 rpm, you lot'll need to replace performance-related parts such as the curl or module. That is what we volition deal with now.

At this point I'll cover ways to make a basic HEI unit of measurement run to college RPM. If yous're expecting dyno test results in this chapter, forget it. Information technology's already a proven fact that bigger, fatter, hotter sparks are unlikely to injure power. For this reason I'll rely on demote tests to show what tin be washed 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'due south spark output was dumped into a chamber pressurized with pure nitrogen at some 142 psi. Although this tin can be considered a valid A-to-B comparative exam process, there are some points to note and so you lot can more precisely relate results to actual use in an engine. Pure nitrogen is a far improve insulator than air, which basically consists of 20-percentage oxygen and almost 80-percent nitrogen. As well, cylinder heat (which was not simulated in our tests) makes information technology easier for the spark to jump the gap. This means it will be harder for an ignition organization to burn 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 almost 15- to 20-percent sooner than it does in a running engine. The results you see here have been corrected for this.

Scroll Test

Although the coil'southward turn ratio for high- output voltage is a prime business 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 plenty charging fourth dimension, a gyre won't fully charge between sparks at high RPM. A more rapid charging rate can be brought about by reducing curlicue inductance, which usually means fewer turns on the master side. Making such a change proves to be a double-edged sword. Although reducing the chief turns increases the primary-to-secondary turns ratio, thus producing a college secondary voltage, it as well increases the electric current draw. The coil design must be the consequence of a workable compromise. Subsequently testing, it was plant that the rate of current congenital up in the stock coil was 2.v amps per millisecond. The same test on a skillful aftermarket coil showed information technology to exist faster at iii amps per millisecond.

Swapping out the stock coil for a high output aftermarket coil is an easy 10-minute job. It's even easier if it is done at the same fourth dimension a new cap and rotor is fitted.

To see how this and any other ringlet blueprint 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 ringlet with the MSD coil raised this to six,100 rpm. The Accel and Performance Distributors coils, within i per centum, duplicated the MSD coils performance. This test so shows that all these coils are capable of delivering a spark-limited RPM increment of 25 percent or more over stock (Fig 10-ane).

Module Test

The part of the module in an HEI is much the aforementioned as contact breaker points in an older-way benefactor. Its outset job is to supply some 200 to 300 volts to the coil master and switch information technology on and off at the advisable time. The best module is one that reliably handles the high- est current possible, delivers the highest voltage to the scroll, switches every bit fast as possible, and has the longest dwell fourth dimension (switched-on time) possible.

I tested all these factors on a loftier- functioning module, but the well-nigh significant differences betwixt this and a stock module showed up in the electric current and voltage delivered to the roll. A stock GM module typically delivers a maximum of some 5.v amps. By comparison, the high-functioning module delivered 7.five amps, so its prospects for delivering a strong spark at significantly higher RPM obviously looked skilful.

Fig 10-1. This exam shows what a typical aftermarket gyre can practise to increase the spark limit RPM of an otherwise stock HEI (corrected from examination rig to engine RPM).

Fig 10-ii. By maintaining the voltage applied to the coil at higher RPM, an aftermarket module keeps the spark running to much college RPM. These curves bear witness that it'due south time to replace the stock module anytime RPM requirements exceed almost 5,000.

For really high RPM, it is necessary to upgrade the module. Shown here is the Performance Distributors Dyna- Mod unit.

The next tests nosotros're going to wait at relate to the module's power to deliver voltage to the whorl. The winner in this result volition be the module that produces the highest voltage upwardly to the highest RPM. For the results, cheque out Fig ten-2. Equally you tin see, the high-performance aftermarket module wins by a healthy margin once again. The exam results shown in Fig x-2 show what's required to produce more spark at higher RPM. Modules from Performance distributors, Accel and MSD all seek to do this.

Spark Exam

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, and so I will use this ratio for some existent-earth RPM capabilities. But there is a cistron I need to throw in here. The stock HEI numbers were with a stock-size plug gap of 0.040. A loftier-operation 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 Operation 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 lilliputian over viii,900, and the MSD components drove our exam HEI to the limit of our spin motorcar, which was ix,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 merely, say, seven,000 rpm, a system that goes to seven,500 will get the job done in exactly the same mode as one that goes to nine,500. The point I am trying to make here is you don't need to buy any more than RPM than about 500 higher up what yous perceive is needed. It's OK if the arrangement you cull goes more than, only there are no benefits to be had other than you lot know the system is more up to the job.

Plug Cables

Assuming yous take prepped the distributor every bit suggested with at least a high- functioning module, then your plug cable requirements for an affordable alternative kickoff to look good. The worst situation is to utilise old carbon string-type leads. These tend to decay in capability and inexpensive ones are often the source of a sizable ability loss when they've seen 40,000 miles or ii years. However, such cables, when used with the improved capability of a modified HEI, volition deliver results as proficient every bit the loftier-quality, wire- wound inductive suppressed cables from Accel, MSD, Moroso, etc. The simply departure is they won't terminal, but they will get the job done for a year at least. For a reliable name make at a depression price, check out Accel's Super Stock cables.

Mechanical Attributes

Once a high-energy spark tin be generated at sufficiently high RPM, the next issue to address is ignition timing and advance bend characteristics. Knowing what controls the advance bend and how to change it is almost common noesis.  The big questions are: what to modify information technology to, and why did it need changing?

The grassroots answers are: cylinder pressure prior to ignition, and, to a bottom extent, estrus. Reverse to popular belief, a charge doesn't explode, simply 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 earlier. The compression pressure significantly affects the speed of the burn down. The college the pressure, the faster the burn rate. This means that as the CR is increased, the ignition advance required is reduced. When longer menses cams are used, the low-speed compression pressure level 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 presume we have a short cammed 9:1 truck motor. The early closing of the intake valve volition hateful a lot of accuse is trapped at low RPM; this, in conjunction with the college-than-stock CR, will produce a cranking pressure of around 180 or more psi. At low RPM, the accuse volition burn faster than in a stock unit of measurement. Result: this engine now needs less initial mechanical accelerate, and the total required will probably be less, so the mechanical accelerate volition merely need to come in slowly. Initial timing may range from v-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 go fuel efficiency under such circumstances, information technology'due south necessary to advance the timing considerably. At threescore mph in high gear, a typical pocket-sized-block Chevy needs between 45 and 55 degrees of advance. Because of the brusque cam, the vacuum advance in our current case will not need to come in until about 7 to maybe every bit much equally x inches of mercury, and the amount required will be at well-nigh about 18-degrees distributor (36 creepo). This, added to the slow mechanical advance occurring at 2,000 to 2,500 RPM, should issue in almost 45 degrees under these conditions.

The mechanical or RPM-generated advance curve is controlled by the weights (white arrows) and springs (xanthous arrows).

Trimming the accelerate curve is usually just a question of changing springs. The plastic screws shown here supersede the metal ones that concur the rotor in place. These avoid any chance of a spark jumping from the rotor screws and firing the cylinder at the wrong time.

Here is how the vacuum advance operates. When the rod that comes out of the "can" and upwards through the pickup point indicated by the yellow arrow is pulled toward the tin under vacuum weather, it rotates the base (white arrow). This advances the timing.

This is the distributor motorcar that every Performance Distributors unit is tested on for curve accurateness before shipping.

If a relatively large cam is used, then the advance needs to come on faster at first, although little or no boosted total may be required. If the cam was used with the CR's recommended, the cranking force per unit area won't be significantly different. This means the quicker rate of advance needed by the longer cam can, to a large extent, be offset past the increased CR. Mechanical advance won't modify 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 first pulling in accelerate. A 285-degree cam may need the "can" to starting time applying vacuum at 3 to v inches with full vacuum advance in at around ten to 14 inches. At cruise, such a cam volition, even if the CR is well matched, require about fifty degrees.

At present if all these interrelated factors sound a fiddling confusing, just remember that Performance Distributors tin build what your engine needs at a very reason- able price.

Timing Controls

To accomplish the timing required, we have 3 system controls. These are: 1) the mechanical advance, 2) the vacuum advance, and, 3) the total accelerate terminate. Ideally, all the required advance characteristics should be determined by testing on a chassis dyno. Merely dyno time costs coin, and though I strongly recommend a dyno setting upward session fifty-fifty for the upkeep racer, information technology's worthwhile getting as many adjustments done as closely every bit possible before renting dyno fourth dimension. Failing that, let'southward wait at how the advance curve tin be gear up by test driving. The technique I'm virtually 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 tin can hear detonation should it occur. If the converter stalls at much above 3,500, the outcome of the ignition timing below this will affect throttle response only. The mechanical needs to exist done at the track, while the vacuum needs to be done on the freeway.

An adjustable vacuum tin is a boon to easier setting of the vacuum advance.

Mechanical Advance

At this point I'll assume yous have an HEI with a option of advance springs and an adjustable vacuum. The first step is to install the strongest springs at both locations on the distributor'due south advance machinery. It'due south necessary to deal with manuals and automatics differently. Automatics start. Stage the car a known distance into the staging light. Information technology matters little what this may be, but it must be close to the same for each run. You're interested simply in the first 60 feet, and the tires must grip well plenty to prevent bike spin. Brand a laissez passer and cheque the 60- human foot time. Next, supercede one of the springs with the i having slightly less tension. Make another pass. If the car is faster, repeat the procedure with a lighter jump yet. Go on with progressively lighter springs until either detonation is heard or the 60-foot times stop improving. At this bespeak, brand a pass the entire length of the strip. At present supersede the second spring with one having less tension. Make another pass and check the ET. If it's faster, echo the procedure until the best is established.

The advance bend at present could exist whatever- where from 90 percent as skilful equally it can be to the best possible with a mechanical benefactor. Information technology takes springs, weights, and a dyno to become really close. What we haven't dealt with at this stage is the total advance. This affects top speed more anything, and if the tests and then far were done at 28 to 30 degrees, the full will be short of optimum. If the whole distributor now is advanced, the low-speed initial advance may be too much. Go dorsum into the distributor and replace the light primary jump with one that's slightly stronger. Now brand passes and advance the distributor progressively until the best ET/trap speed is seen. Concluding, go back and evaluate the secondary jump that you starting time started with. You're now washed, and this is as good as it gets short of a dyno.

This 350 eventually made over 580 hp on pump gas and was nonetheless just virtually street drivable. Note the use of a Functioning Distributors HEI. This got the job washed on this 8,000-rpm-plus engine.

The distributor used in this engine was likewise a Pertronix unit but not of an HEI style. It required the use of a separate coil and ignition box.

Under all those wires is an MSD distributor for use with an MSD six box and an ignition retard for nitrous employ.

On the dyno or at the rails, this micro adjuster for setting the timing is a boon. It allows precise changes in timing every bit small-scale as 1/4 degree. This can be of import when trying to squeeze the final ounce of power from an engine. On some occasions I accept seen as much a 3 hp from a halfdegree modify in timing.

This is where the ignition has to bear witness itself out. If it doesn't work here, it's replaced!

If your vehicle is a manual transmission, all of your tests can be done on a lone stretch of road with a stopwatch. The technique involves choosing a suit- ably depression starting RPM and timing the vehicle between two speeds. Let'due south assume that the motor has the potential to pull from ane,500 rpm. With the strongest springs installed, take off and accept the clutch fully engaged in second or tertiary gear at an RPM lower than the test's starting point. For the example, we're working hither with 1,200 rpm. Floor the throttle and fourth dimension how long it takes to reach 2,500 rpm. Progressively lighten upwards the primary bound until the all-time 1,500- to 2,500-rpm fourth dimension results. At present repeat the tests using RPM from two,500 to 4,000. Take the automobile to the strip and set up the full. If this is more than a couple of degrees different than the setting used while developing the accelerate curve and any sign of detonation is apparent, go back on the primary spring to i slightly heavier.

Vacuum Advance and Mileage

Vacuum advance is critical if adept fuel economy is to be achieved. Although yous may not experience this is of great importance to a race car, having vacuum advance cleans upwardly 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 corporeality—non the rate—at which information technology comes in. It'southward a compromise, but it'due south a applied compromise. Trying a dozen cans in an endeavour to go the optimum wears sparse after a while.

To set upwards an adaptable unit, start with the accelerate cut way down and drive the vehicle. If no detonation is heard during normal, part-throttle driving, then add a footling more accelerate. Go along with this process until detonation is apparent and so dorsum out some of the timing until detonation has ceased. You're at present in business.

Written by David Vizard and Posted with Permission of CarTechBooks

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