Use caps and rotors that have brass terminals. As has been already stated, most performance engines will work well with around 10-12 degrees of vacuum advance. It will also deter engine run-on, or "dieseling". Yes, you can remove a computer controlled HEI and drop an old-style mechanical advance distributor in its place; it will physically bolt right in. It will have no ill effects at WOT because there will be no vacuum at WOT (no vacuum = no vacuum advance added to the timing) so you will be running exclusively on mechanical advance. 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The statement of having about 50 degrees combined advance (initial, mechanical, and vacuum) at cruise rpm needs some clarification. A bad distributor will normally present a variety of symptoms, which can oftentimes be recognizable as distributor related. Other cams require a mellonized distributor gear. If the throttle blades are open too far, a poor idle and off-idle transition can be the result. If this is the case, the engine will idle high when out of gear and then the idle speed will drop down once it's put in gear, and the off-idle response will be poor. You will want to note that there's not enough vacuum at WOT to cause the vacuum advance to work. The "missing" 6 degrees can come from the vacuum advance. Bottoming of the distributor against the oil pump drive shaft (or on some engines the thrust surface of the block) has to be checked for and corrected if it exists. Steel cams require a bronze or plastic composite gear. This will be covering the 4-pin HEI module. In many cases that means there can be as much as 50 degrees of advance when the engine is cruising under a light load. Use these springs to give you an advance curve that starts at about 800 RPM and ends at 2800-3200 RPM. If there is, check the plugs. Non-computer controlled HEI setup for performance. Using a relatively mild camshaft and compression ratio matching the cam, if using an EGR valve, more vacuum advance may be needed to compensate for the diluted air/fuel mixture it causes- much the same condition that a cam having a lot of overlap can cause. To make sure it is not the hot wire, run a temporary jumper wire directly from the positive side of the battery to the distributor. New replacement HEI power and tach hook-up pigtails are available. Along with that, there's often a need to limit how much vacuum advance is supplied. The springs are located directly under the rotor and are easy to remove/replace by hand or with needle-nose pliers or hemostats. In a stock application this wire is often pink and a relatively large gauge. But it will cause the ECM to throw a code (the check engine light will be illuminated) because the ECM senses if the computer-controlled portion of the distributor is functioning every time you start the engine. To check the secondary resistance, remove the screws that hold the coil in the cap, remove the coil and touch the negative meter lead to the ring terminal on the black wire (between the red and yellow) and touch the positive lead to the bottom of the coil where the rotor bushing makes contact. Holley has you covered with the new Terminator Stealth Throttle Body Fuel injection System. Many times, a low-voltage issue can cause a myriad of problems. This will help keep the carb primary blades from being opened too far to get the idle speed where it needs to be. If you are in the 45-55 degree range (about 50 degrees is fine in most cases) of advance including 10-12 degrees from the vacuum advance, you’re in the ballpark. 1981-86 trucks/vans with 305s (RPO LB9) used a variation of the computer controlled HEI with electronic spark control with 5 pin modules. In the case of an OD vehicle where the cruise rpm is relatively low, you can supplement the advance curve by allowing the vacuum advance to give more advance than what's usually recommended. There will be a selection of shims; use whatever combination needed to get as close to the target as possible. The normal reading should be between 800 – 910 ohms. Each engine is different and what works for one engine might be a little different than what works for another engine. On a Chevy (and any other engine where the oil pump drive shaft is driven off the end of the distributor shaft), checking to see if the distributor is bottoming against the pump may be done by installing the distributor (cap and rotor removed for the test) in the engine with no gasket. Put the switch in series with the power wire and route the wires inside the vehicle so the switch is within easy reach of the driver. A typical advance curve for an engine built with a mild camshaft and having a compression ratio correctly matched to the cam will look something like this: The above gives 46-48 degrees of advance (including vacuum advance) under light throttle cruse/high vacuum conditions. insulating the hold down clamp and bolt from grounding the distributor to the engine. If the distributor fires using the jumper wire, run a new hot wire from your switched 12-volt source to the distributor. So fix it first or get another HEI to start improvements on; just make sure you are getting the right one for your engine- they were used on ALL makes of GM inline and V6/V8 engines and all look similar. P0741: TCC System Improper Mechanical Performance. There are 2 different designs of the HEI coil. The engine will run, although will have a miss and will detonate, with the #5 and #7 wires swapped. The vacuum advance will compensate for the engine load. The initial, centrifugal and vacuum advance work together overall but are independent of each other; each adds the appropriate amount of timing advance to supply the correct spark advance to the engine under all RPM/engine load conditions. The only other change you can make in the advance curve is to manually advance the base timing (usually worth a little HP by itself). The drawing below shows the transition slot as seen with the carb held upside down. On the SBC and BBC, a brace like was originally used by the factory is still available to prevent the starter from flexing in use. Ignition Coil Primary circuit fault. It should read between 6000Ω and 30,000Ω. Take the measurement of the gap and subtract 0.020" from it, the result is the shim thickness needed. In order to fix your car's problem, you must first know how to troubleshoot in order to determine what exactly the problem is. Pay close attention to the firing order at the distributor cap and at the plugs themselves. A gasket can be used under the shims. Note: Portions of this page appears to have been cut/pasted from a thread located at 72chevytrucks.com. The brace connects to the end of the starter opposite of the nose, using the studded through bolt to attach the slotted end of the brace to the starter. What vehicles came stock with a non-computer controlled HEI? The stock weights and advance plate are acceptable for many street/performance engines. If that occurs, the secondary throttle blades may need to be opened slightly to allow more idle air to be introduced into the engine. In these cases an aftermarket vacuum advance cam is required. This is done by measuring the amount of play between the distributor gear and the thrust washer. If the blades are opened too far, the idle quality and off idle response will not be good. For a stock or RV-type camshaft, 8 to 12 degrees initial is a good starting point. Currently, we're running a Mallory HEI ignition module in a distributor with a high-voltage coil with 8 ohms of resistance built into the coil. It won't hurt performance as long as the timing can still set correctly (vacuum advance can doesn't hit the firewall or intake) but plug wire routing might be more difficult. A relay that delays the ignition from being energized- giving the starter time to get the engine turning over- can be wired into the ignition feed wire. You may find as much as 16 degrees of vacuum advance is needed with a relatively mild cam if an EGR system is used. The HEI distributor is widely available from the aftermarket. TPI and TBI The Tuned Port Injection and Throttle Body… More on how ported may be preferable to manifold vacuum is here, by noted carb tuner, Cliff Ruggles. MSD and Crane have limiter plates (shown below) that do the same basic thing. It’s a combination HEI battery/tach pigtail that has a connector for both power to the HEI and the connection for the tach, molded together. Typically the centrifugal advance weights wear their pivot holes into an "oval" or eat a groove into their pivot pins (see green arrows in image below). In cases where the timing curve calls for a lot of initial timing with either a short mechanical advance or locked timing, using vacuum advance can be beneficial. The stock HEI uses a vacuum advance canister to further tailor the ignition timing. On almost any carb, there are vacuum ports that provide manifold and ported vacuum. Outside of that range toss the coil and get a new one. Some part numbers for a stock-type "990" series module: Use a heat sink paste (available from Radio Shack and computer shops) on the bottom of the module and be sure the surface of the distributor body where it mounts is clean. Using a manifold vacuum source will in many cases- depending on the cam and compression- allow you to close your throttle plates a little and still maintain the same idle speed. So always start adjusting initial timing without the mechanical advance adding any timing. For more distributor tech tips, check out Performance Distributors’ Professional Tips page on their website, it’s chock full of useful information. The primary resistance value should be between 0.6 – 1.5 ohms. That would be fried from a solar flare or EMP, but pop a new one in, and I’ll be back on the road in 10 minutes. The 2 wire plug from the coil to the distributor module is used to power up the distributor and provide a coil trigger for spark. To do this, you will need a 1/4-inch nut driver and a multi-meter. Many performance curves call for around 10°-12° of vacuum advance on top of the 32-40 degrees of total advance (initial plus mechanical), to give somewhere in the neighborhood of 50 degrees of advance under light throttle cruise conditions. Transmission problems. Set it so the vacuum advance starts to drop out at about the same point that the carb power enrichment circuit (Holley power valve, Edelbrock step up spring, or Q-jet power piston) starts to come in. They will soak with oil and compress more than when dry. This does a couple things: First, it will cure nozzle drip and a smelly, poor quality idle caused by the butterflies being opened too far at idle, which allows fuel to be pulled from the transfer slot. Unfortunately, there is no test that can be performed with an ohm meter on this part. The first area they suggest you check is to make sure all the wire connections on the cap or solid. … Depending on the cam specs, adding initial timing in addition to or instead of tipping the secondary throttle blades open more may give the best results. Right is the 12V switched power source (circled). The centrifugal advance mechanism on the HEI is a simple, robust design that is relatively easily modified. Another thing that is often overlooked, is if the cruise rpm is less than the rpm where the mechanical advance is all in by, the vacuum advance has to make up the difference to get the best mileage and drivability. If you experience surging under these conditions, that's an indication that there may be too much vacuum advance being used. This page has been accessed 536,110 times. No matter what we do (short of digital control), the timing curve is always somewhat of a compromise, being as how all engines and vehicles (and all the other considerations) are different from case to case. P0351: Ignition Coil A Primary circuit fault. Stock, the HEI distributor advance mechanism is pretty good but the stock springs are too strong, causing the advance curve to come in too slowly, if it ever gets fully advanced at all. Manifold vacuum is a good indicator of engine load. And your plugs can develop carbon deposits within just a few thousand miles. The all in by rpm is often said to ideally about 3000 rpm, or less if the engine will allow that w/o detonation. This article deals primarily with 1980-back cars and some '86-back trucks, using (or donating) a non-computer controlled HEI distributor. Customer service hours: 9.30AM – 10PM, Daily Technician service hours: Mon-Sat, 9.30AM-10PM (Last job 8PM) Telephone: 9067 4867 Price: From $250/unit for ChemJET service Aircon Astiquer website What is needed are the right springs, the right initial advance setting and the right amount of mechanical advance (vacuum advance will be discussed later). You want the mechanical advance in as soon as the combination will allow, without causing pinging. If you have disconnected the 4-wire ECM connector going in the side of the distributor or the wiring between the ECM and the distributor is damaged there will, again, be no advance (and the check engine light will come on). After determining how much mechanical advance your HEI is giving you, and it's determined it's too much for the amount of initial advance you want to run, the mods to the mechanical advance are shown in the image below (thanks to 69-CHVL of Team Chevelle). The more radical the camshaft, the higher the initial advance for a given compression ratio. Mechanical advance all in by =/< 3000 RPM. The increase in advance for a lightly loaded engine will increase fuel economy, lessen emissions, and can give smoother engine operation. Generally the vacuum advance can be tailored to suit the conditions after the initial and mechanical advance is worked out. Removal and replacement of the gear is covered in the link, Description of an HEI rebuild. A Delco module is a good choice for street/performance applications and is preferred over an auto parts store non-GM/Delco replacement, possibly unless it's a performance replacement. This isn't what's wanted for a performance timing advance curve. Stock, it's capable of about 35,000 volts and so-so total spark energy. The max mechanical at 3000 rpm is 18 degrees. That said, there are a few isolated cases where the vacuum advance plays a bigger part in the overall advance curve, like when the vacuum advance is relied on to provide advance at idle in order for the primary throttle blades to be closed down enough to keep the carb from idling on the transition circuit. This is located inside the distributor, and has the green and white wires from the pick up attached on one end, and a terminal block on the other. Typically, the distributor grounds itself to the engine when installed. P0736: Improper Ratio for Reverse Gear. Click toolbar for various cam duration specs. Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. We will safeguard your e-mail and only send content you request. If your centrifugal advance doesn't "snap" back when you twist the rotor with your hand and let it go then you have this problem. These recommendations can be considered to be a safe starting point. I drive a ’78 Oldsmobile Delta 88 Royale. Now you can have all of the benefits of EFI and maintain the traditional looks of a carburetor. Note: In almost every case, using the advance kit-supplied weights and cam will not work as well as using the stock weights and cam along with the different springs. Wider plug gaps (or faulty ignition wires) cause voltage spikes in order for a spark to occur. © 2017 Power Automedia. Great article! The centrifugal advance is used to advance engine ignition timing relative to an engine’s RPM. That also means that a fingernail file is no longer the repair tool for roadside fixes. Stock, it's capable of about 35,000 volts and so-so total spark energy. All rights reserved. This wire should be part of your original wiring or can be purchased separately. The HEI ground is through the distributor body to the engine, then to the battery negative cable. Vintage Chevrolet Commercial: What Makes The Monte Carlo So Great? But any way you slice it, it's still trial and retrial to get the curve dialed in. If ignition problems are experienced, the first thing to do is to install a timing light and watch for an irregular spark. Holley’s Terminator Stealth EFI systems have the same great … The heat sink compound (not dielectric grease) helps transfer the module heat into the distributor body which acts as the heat sink. ACDelco D-1906/GM p/n 10482820 (used in the GM ZZ4 crate engine). Initial advance recommendations, from: Demon carbs. The drawing on the left shows an overexposed transition slot. If you try to set your initial timing with the engine idling ABOVE the RPM that the mechanical advance has started to come in, getting a correct reading will be all but impossible. Also, the centrifugal advance plate (that rotates on the main distributor shaft as the centrifugal advance moves it), near the top of the distributor shaft sometimes gets gummed up and "sticky," slowing the advance curve and generally preventing the centrifugal advance assembly from working correctly. The LT1 coil-on-plug converters with adjustable retard and intelligent rev limiter still use the optical module in the OptiSpark distributor, but without high-voltage passing through it. If that's not your #1 plug wire, or your vacuum can is pointed in another direction, the distributor may be installed one or more teeth off. The correct gear material/treatment has to match the type of camshaft being used. The small spring on the button goes against the bottom of the coil. Installation is a simple one-wire hookup. Newer ignition systems tend to use an individual coil-on-plug or coil pack, which assigns one coil to each engine cylinder or pair of cylinders. Note: See link HEI vacuum advance specs, below. They also knew that by adding larger coils, there performance was enhanced accordingly. Testing the secondary circuit of the coil. The mechanical advance should be "all in" by about 2800-3200 RPM for a typical street performance motor (additional advance above this RPM point is neither needed or wanted; increased turbulence in the combustion chamber offsets the need for further ignition advance). If the voltage drops below 10.5, check your battery. It's fine for a naturally aspirated street engine that uses a 0.035"-0.040" plug gap and has a compression ratio compatible with pump gasoline (

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