Re-Curving Rover Distributors

Re-Curving Rover Distributors

as published in British V8 Newsletter, Volume XIV Issue 3, December 2006

by: Jean Monfort

All engines must be equipped with the right distributor, giving the right advance curve. Especially on fast road engines or rally units, it is better to send 25,000 volts to the spark plugs at the right time than 50,000 volts too late or too early. So, in search of the best way to set fire to the gas, I collected some information.

First Issue: How Much Range of Advance?

In a previous article entitled "3. 2. 1. Ignition! Selecting a Rover Distributor" I explained how to enlarge the upper limit of mechanical advance on a Buick or Rover distributor. Without modification, these distributors only give a range of about fifteen degrees of advance. Such work is not necessary with Mallory distributors. Most Mallory distributors are factory set to limit the advance to 24 degrees, but can easily be adjusted from 0 to 28 degrees. A "fast-road" Rover/Buick (with 10 to 10.5:1 compression ratio) works well from 8 to 10 degrees (idle) advancing to 32 to 34 degrees at 3000 to 3500 RPM (and over.)

Second Issue: What about the advance "curve"?

Single-stage advance curves are common on 4 and 6 cylinder engines. Such curves are nearly straight and allow full or nearly-full advance from 2000 RPM, which is appropriate for race engines. With such straight curves, the ignitions on racing engines are relatively easy to tune. (Just use trial and error to find what advance gives the most power at high RPM.) Some single-stage distributor curves are so radical that they're only suited for racing V8's (e.g. Mallory 29.014 with YT or YH advance curve change kit) as shown in Figure 1:

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Advance Curves: Single Stage, Matched Springs

The following graphs demonstrate the importance of selecting springs - especially "paired" sets of springs. In paired-spring distributors, one spring is typically longer and consequently it remains loose until over approximately 1500 RPM. (Just to put ideas into the guys who are not running a Mallory...!)

Advance Curves: Single Stage, Paired Springs

By changing the spring combination on a two-stage advance curve distributor, one can achieve quicker advance to 1500 RPM, slower advance change at mid RPM, and full advance from about 3000 RPM. Studying this illustration suggest that a great distributor can be made without any sophisticated, digitalized or twist button systems.

Advance Curves: Two Stage

Third Issue: What about "dwell"?

The dwell angle adjustment determines the duration of the spark. (Dwell is normally set to 26-28 degrees on a stock Rover engine.) The only one way to have a long lived spark is to reduce the points gap or to modify the trigger's design (on an electronic ignition) - but I advise against doing that! It is better to go to a dual point system and subsequently to two sparks each cycle. For even more fireworks you must thrust your hand into your pocket and order a digitalized ignition!

Most stock Lucas get about 27 degrees of dwell angle, whereas the Mallory's vary by model as shown below:

  Point Gap         Individual Dwell         Total Dwell
Dual Point Vacuum 0.022 26 degrees 33 +/- 2 degrees    
Single Point Mechanical & Vacuum         0.018 29 degrees 29 degrees
Dual Point Mechanical 0.022 26 degrees 33 degrees
Old Mallory Dual-Point 0.018 17 degrees 34 degrees

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The following photo shows the Mallory spring kit (at top) and the Summit Racing kit (below). (Note: Summit kit part number SUM-G5212 fits GM HEI 74/79 non-computer distributors.)

Distributor spring kits

Fourth Issue: Mechanical or vacuum advance?

I personally prefer mechanical advance distributors for their simplicity, efficiency, and because the vacuum chamber presents a potential failure mode. The risk of rupturing the rubber diaphragm is real when running a four-barrel carb. One work-around is to install a Range Rover vacuum reducer (made in USA!) in the line. The part number is: ERC 6997 and it can be found on any 3.5 / 3.9 Discovery or Range Rover up to 1994.

Disclaimer: This page was researched and written by Jean Monfort. Views expressed are those of the author, and are provided without warrantee or guarantee. Apply at your own risk.

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