If I’m using timing control on my FiTech EFI, why do I need to buy the adjustable MSD rotor? Can’t I just lock out the distributor and then ensure the rotor aligns with the Number 1 spark plug terminal and make final adjustments to sync timing with the ECU using a timing light? Or, could I wire a stock GM small cap HEI to do the same?


Jeff Smith: I edited out the part of your question that began with “this is a dumb question,” because your question shows you really have a solid handle on what’s going on with EFI and electronic control over the ignition. The simple answer to your last question about the small-cap HEI is – yes. That’s where most tech writers would stop, but frankly with that answer we haven’t really learned anything. Let’s pry open this slice of investigative technical journalism and trace it back to the real issue – rotor phasing.

Your initial question regarded the adjustable MSD rotor. MSD originally created this product to allow moving the rotor to keep it near the spark plug terminal on the cap. This adjustable rotor was used mainly in conjunction with a crank trigger ignition. With fuel injection and electronic control over the ignition, the ignition timing can easily move from roughly 10 degrees before top dead center (BTDC) to as much as 50 degrees BTDC under light load cruising situations. Using these numbers, the rotor sweep is a full 40 degrees. The idea is at idle, to place the rotor effectively perhaps 15 degrees “retarded” relative to the spark plug terminal. Then as timing is advanced, the rotor will sweep across the spark plug terminal to roughly 25 degrees on the other side of the terminal.

This is an MSD photo of a rotor that is not phased properly.
Note how far the spark has to travel to reach the spark plug terminal on the distributor cap.

This keeps the rotor physically close to the terminal in the distributor cap, which shortens the distance the spark must travel between the rotor and the terminal and into the spark plug wire. If you think about this in terms of degrees of distance between each terminal (with 360 degrees in a circle and 8 terminals), that means there’s only 45 degrees between each terminal on the cap. That should help put this into perspective.

With electronic ignition timing control, there is no need for a mechanical advance mechanism in the distributor. That’s why all the aftermarket EFI installations require a locked distributor. You can easily do this with an MSD distributor by disassembling it and moving the advance pin in the mechanism 180 degrees from the slot into a drilled hole. This locks the rotor in place. Then the MSD adjustable rotor is required to phase the rotor to create the proper sweep.

Several companies like Accel, FAST, and Holley offer what is called a Dual Sync distributor. These are designed to be used with EFI because the rotor has been “pre-phased.” To explain how this works, we have to go into how the system triggers the ignition. In a traditional ignition system, when the trigger wheel on the distributor shaft aligns with the magnetic pickup, this triggers the ignition system to fire the coil. Mechanical and vacuum advance systems advance the position of the trigger wheel to create more ignition advance.

With a properly phased rotor, note how close the rotor is to the spark plug
terminal – requiring less voltage from the coil and far superior engine performance.

With an EFI system, the trigger wheel is locked in a fixed position. In order to make this system work, the usual approach is to pre-set the ignition trigger at an advance that is more than what the engine would use – like 50 degrees BTDC. The pickup and trigger are set at that point and then the ECU delays the ignition it to the actual timing the engine requires. So the trigger signals an event at 50 degrees BTDC, but the EFI controller delays this to the appropriate timing such as 30 degrees BTDC as determined by the spark map.

Here’s where the issue lies. On most traditional distributors, the rotor is phased to line up directly with the distributor cap terminal when the trigger wheel aligns with the magnetic pickup. Now if we align the pickup and trigger wheel at 50 degrees BTDC, the rotor is pointed well away from its intended spark plug terminal. So this requires us to “phase” the rotor. With most distributors, we will need an adjustable rotor that will allow us to position the rotor so that it sweeps across the terminal and minimizes the distance away from the terminal.

This 50-degree figure is most often used as the pre-position for the ignition because it allows us to work toward making sure the rotor is very close to the spark plug terminal at the rpm where peak torque occurs. This is important because peak torque occurs because the engine is making maximum cylinder pressure. Higher cylinder pressure also demands higher ignition voltage to push the spark current across the plug gap. Reducing the rotor gap reduces this overall voltage. It’s a somewhat complex relationship, but you can see why all this is important.

We’ve included a couple of photos pulled from an MSD video that shows the relationship of the rotor to the distributor cap terminal both when the rotor is accurately located and also when it is too far away. You can see how the spark has to travel a much greater distance when the rotor is not phased properly.

We were faced with a similar issue a few years back when we wanted to run an MSD electronically-controlled box that would allow us to digitally control the timing curve on a small or big block Chevy. As with any EFI system, the requirement was to run a locked-out distributor. We were looking for a way to do this when we realized – as you have – that GM did this first with a large cap HEI distributor and then later changed to a small-cap fixed rotor distributor with a separate coil as used on both the truck throttle body applications and the TPI Camaro and Corvette engines.

We did a quick test of this little unit and realized that the GM engineers designed this small-cap distributor with a pre-phased rotor. We lined up the rotor with the distributor cap and then reversed the engine until the trigger wheel lined up with pickup. This placed the rotor roughly 20 degrees “retarded” relative to the distributor cap terminal. This means that with roughly 45 degrees of total advance at part throttle, the rotor would swing from 20 degrees “behind” the terminal to 25 degrees “ahead”, minimizing the distance the rotor is from the spark plug terminal. This distributor is essentially “pre-phased” from the factory.

What is even better is that the GM pickup is virtually identical to the magnetic pickup used in the both the MSD and most aftermarket distributors. So we removed the module from a used small-cap distributor and clipped off the factory connector. MSD sells a replacement two-wire connector plug that we used. The wires coming off the GM pickup are green and yellow. The MSD colors are purple and green. Simply connect the green to green and yellow to purple, and you now have a locked out small or big block Chevy distributor with a phased rotor.

We made a simple little aluminum plate to take up the space created when we removed the TPI module and used the rubber grommet to protect the wires. The beauty of this simple conversion is that our distributor uses the stock pickup, distributor cap, and rotor from an ‘80s TPI engine that are available from any auto parts store so you don’t need to carry unique distributor cap or rotor spares.

Hope this helps with your understanding of rotor phasing.

This is a shot of our homemade locked out distributor using an MSD connector that works very well and is very inexpensive.
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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.