See Tips On How To Adjust Vacuum Advance On HEI Distributor

See Tips On How To Adjust Vacuum Advance On HEI Distributor

Do you know how your car moves? Internal combustion engines produce motion by burning the gasoline inside the car engine. It does this by using a spark in the spark plugs to ignite the fuel-air mixture. 

A distributor in your car’s engine delivers the right amount of current to the spark plugs at the right time for ignition to take place. In addition, modern engines come with computerized ignition systems. 

One notable feature of modern ignition systems is an ignition timing control mechanism known as vacuum advance.  Adjusting the vacuum advance unit ensures optimal performance of automobile engines.

In this article, we will discuss how you can adjust vacuum advance on HEI distributor, including other things you need to know. Keep reading!

How To Adjust Vacuum Advance On An HEI Distributor

One of the first and vital adjustments for optimized engine performance is the vacuum advance on high engine ignition (HEI) distributors.

So, how can you adjust the vacuum advance on the HEI distributor? The steps below describe how.

  1. Set the engine’s mechanical timing by disconnecting and plugging the vacuum advance hose at the distributor and plug the hose. Then, connect the power lead to the vehicle battery.
  2. Loosen the distributor hold-down bolt Kick start the engine and ensure it heats up to standard operating temperature.
  3. Switch on the distributor and use the timing. Ensure you have set the vehicle’s timing to the manufacturer’s specifications. Most cars come with a decal in the engine compartment, which shows the timing specifications.
  4. After making all necessary adjustments, tighten the distributor bolt. Rev up your engine to 2000rpm with your assistant observing the time reading on the harmonic balancer. Record the time settings. Then, remove and reconnect the vacuum advance hose plug to the distributor.
  5. Pull out the vacuum advance hose from the carburetor. Proceed further by connecting the long hose vacuum gauge to the carburetor’s vacuum advance port. 

Ensure you have placed the vacuum gauge in the passenger’s side of the car in a location where your assistant can easily read it when the vehicle is in motion. In addition, ensure the vacuum gauge does not Interfere with any operating parts of the engine.

  1. Drive the vehicle while having your assistant observe the maximum amount, not vacuum, displayed on the gauge when the engine functions at average highway speed.
  2. Safely bring the vehicle to a halt, apply the brakes and switch off the engine. Next, connect the timing light inductive pick-up to the number one spark plug wire and connect the power lead to the vehicle’s battery.
  3. Ensure the vacuum pump with a gauge is attached to the distributor vacuum advance. Kick start the engine while having your assistant Rev the motor up to 2000rpm. Pump the vacuum to match the driving test reading in step 5.
  4. Record the timing on the harmonic balancer. Record the total advance displayed on the scale. From the recorded reading, subtract the reading value recorded in step 3. For example, if the task in step 3 was 35 degrees and from step 8 is 42 degrees, then 42 minus 35 equals 7.
  5. Detach the vacuum pump and connect a 3/32 Allen wrench into the port on the vacuum advance where the hose clamps. A small adjusting screw is present in the vacuum advance. Adjust the screw in a clockwise direction to decrease the vacuum advance and anticlockwise movement to increase the vacuum advance.
  6. Carry out steps 9 and 10 repeatedly until the absolute difference between the mechanical advance and vacuum advance has been set to 10 degrees of advance at standard highway operating conditions.
  7. Remove vacuum pump and timing light and connect the vacuum advance hose back to the carburetor.

What Is Vacuum Advance?

Vacuum advance is an ignition control mechanism that automatically adjusts the sparks from the intake manifold vacuum. Vacuum advance provides the extra needed refinement when an engine is not running at full throttle.

Vacuum advance comprises of three components:

  • an advance plate,
  • vacuum canister, and 
  • a linkage.

Vacuum advance is an essential component of the ignition platform that offers both performance and economy. Leaving vacuum advance unplugged is synonymous with trashing free engine efficiency.

About Ignition Timing

Ignition timing is a method of delivering a spark at the right time. It aligns the pointer with the position of the piston and the velocity of the crankshaft.

As a result, ignition of the air-fuel mixture occurs at the best possible timing. Excessive vibrations arise when the ignition occurs too early or too late. These vibrations might be harmful to the engine.

How Does Vacuum Advance Affect Performance?

Vacuum advance improves throttle response, increases the engine’s efficiency, and regulates the engine temperature.

Vacuum advance helps High Energy Ignition (HEI) distributors to increase engine responses at low RPMs. Vacuum advance works best with street vehicles as they do not operate at full throttle than their racing counterparts.

A vacuum advance unit can increase advance up to 15 degrees, although this depends on the vacuum applied. Thus, you can use a vacuum advance together with a mechanical advance.

Note that vacuum advance does not affect full-throttle power and provides no benefit for engines that spend most of their life at RPM (race engines).

How To Test A Vacuum Advance On An HEI Distributor

It is a commonplace for a vacuum advance to wear out, but you can quickly test it using a vacuum gauge. You can do this by attaching the vacuum gauge and checking the reading against it.

In cases where vacuum reading doesn’t correspond, then you may have found your problem. You can test an HEI distributor for vacuum leaks following the steps below.

  1. Detach the vacuum advance from the distributor.
  2. To the vacuum port on the distributor, attach a vacuum gauge.
  3. Continuously rotate the engine until operating temperature is attained.
  4. Pay attention to the time taken for the manifold pressure to drop below 10 inches of Mercury and then return to normal.
  5. Take this as the baseline reading, which typically should be around 15minutes but may vary from one engine type and age to another.
  6. Disconnect or remove the vacuum engine from the distributor and replace it with a new or rebuilt vacuum advance, a reversed order with the gasket facing down towards the manifold.

Note: Before you test the vacuum advance on an HEI distributor, touch it to ensure it is warm. You do this to check if power is flowing through the system.

What Is High Energy Ignition (HEI)?

High Energy Ignition, known mainly as HEI, is an electronic ignition system made by Delco Remy, a division of General Motors. This ignition system was a feature of all North American GM models between the seventies and the mid-eighties. 

The designs changed with each passing year, and at some point in the late 1970s, GM added computer controls for some applications. Then, in 1972, they released an optional predecessor system known as utilized ignition for Pontiacs.

Most HEI systems come with an ignition coil mounted on the distributor cap. They also have control modules and magnetic pick-ups on the distributor in the breaker points and condenser of the ignition systems.

HEI control modules come in four types – the 4-pin, 5-pin, 7-pin, and 8-pin modules.

The four-pin modules:

This was put to use in carbureted engines. It uses vacuum and advanced centrifugal mechanisms, which are conventional mechanical timing controls.

The five-pin modules:

GM introduced this module in 1978 as an early attempt at electronic timing controls. It had a provision for connecting a knock sensor.

The seven and eight-pin modules:

Early computer-controlled engines used these modules and a fixed timing distributor as the computer controls the ignition timing.

How Does The HEI Distributor Work?

The high Energy Ignition (HEI) system comprises a star-shaped wheel in the center shaft, passing in front of a magnetic pick-up.

Each time the star point passes close to the pick-up, it produces a pulse that triggers the coil. The coil is an inherent part of the distributor and is present in the distributor cap.

The control unit for the magnetic pick-up is a module located in the distributor. The distributor requires only one wire to supply power to the module.

The module sends a voltage to the magnetic pick-up, which the stay wheel interrupts each time it causes a pulse. This pulse reverses to the module, which grounds the coil, causing a spark plug to fire.

The HEI has a vacuum advance that operates the base of the distributor. Therefore, on starting the engine, the distributor will be in a semi-retarded position.

The produced vacuum turns the base plate band and increases the advance for the spark. When the engine starts to run, the RPM increases to over 2000rpm, then the mechanical advance takes over and further increases the advance to 32 degrees.


High energy ignition (HEI) is an electronic ignition system designed by Delco Remy, a division of General Motors. The majority of HEI systems come with a mounted ignition coil present in the distributor cap.

Vacuum advance is an ignition timing regulation mechanism that varies the instant a spark occurs as an intake manifold vacuum.

Vacuum advance is present only in vehicles with partial throttles. As a result, they are absent in racing cars as they possess a full throttle.

Vacuum advance improves vehicle performance. For example, it contributes immensely to how fast vehicles accelerate and climb hills.

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