NastyZ28.com - GM Ignition System Tuning

There were two types of ignition systems used by GM during the 1970s: 1) the old style single point Delco-Remy distributor, and 2) the High Engergy Ignition (HEI) distributor. You should consider using the HEI if you have one handy, however we're supposed to be saving money by using what we have on hand... anyway, the concepts or tuning objectives will be similar for either model distributor. If you decide to change to an HEI distributor, make sure your tach is compatible with the transistorized HEI.

The ignition system on smogged motors is very retarded. Advance is usually still zero (0) at 1000 rpm, and full (10 degrees) at 4000 rpm.

In general, you want full advance around 2000 - 2400 rpm for good throttle response and good performance throughout the engines rpm range. On the other hand, dialing in too much advance too quickly will burn holes in your pistons if you're not careful.

Other items which affect how quickly you want the advance to come on are compression, rear axle ratio & gasoline quality. High octane gas can help prevent detonation even on engines with radical timing dialed into the distributor.

GM Points Type Distributors

Stock points bounce around 5000 rpm. Throw the stock points out, and replace them with "Super Duty" points. If you don't care about RFI, throw the RFI shield out too, this will help prevent heat build up around the points. Crimp the circuit braker plate hold down screws to prevent unwanted movement and keep the dwell accurate. Remove the bushing stops under the weights so the weights can advance through their total travel.

Remove the stock centrifugal weights and springs. Replace them with high performance (heavier weights, lighter springs) items. Lubricate the centrifugal weight base and distributor cam lobes.

With the new weights and springs, advance should begin just off idle and be fully advanced (13 degrees) at around 2000 rpm. Set dwell at 28 degrees, replace the cap and rotor with a quality cap with brass contacts.

GM HEI Distributors

The HEI distributor is a breakerless system using an all electronic module, internal pickup coil and timer. Because of this, there are no points to wear out. The distributor advance curve is still severely retarded, and the system runs out of gas around 4500 rpm. The advance curve is controlled by weights and springs, however, with the HEI, you want to keep the stock weights and simply change the springs. By doing this, you keep the total advance (11 degrees), but the lighter spring allows the advance to come on quicker (just off idle). The curve should look something like, 2 deg @ 800 rpm, 6 deg @ 1100, 10 deg @ 1900 and full @ 2200 rpm.

Spark boxes.... what are they good for? MSD 6A, Mallory Hy-Fire.


(Adjust the spark advance curve prior to getting to the track).
Inspect your ignition system to make sure it's working correctly.
Adjust timing upwards until the engine rattles (be carefull).
Spark advance is critical to running well.

Look at electrode for excessive burning.
Look inside header primaries from the flange side, you'll get an indication of flow distribution and fire.

First, get a baseline in timing, if you want more off the line and less top end, advance the timing a degree at a time, just remember running out of timing on the bottom end, limits your amount of timing on the top end.

Once you establish a baseline, check the ignition and decide on a starting point. In most cases 34 - 36 degrees total timing will be in the ballpark.


Last updated: 10/20/2011
Author: MadMike Maciolek


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