Get In Phase: How To Set Rotor Phasing On Several Different Engine Configurations

Words: Jeff Smith, Photos: Jeff Smith and MSD

“Security – set your phasers on stun!”

Okay, so we’re not on the Star Trek set and you’re not Captain Kirk. But in the real world of high horsepower engines, we can show you how to set your rotor phasing so that your engine isn’t stunned into running poorly. What? You don’t know about rotor phasing? Then this is a great time to learn.

Rotor phasing has always been an important part of setting up a high performance ignition system, but a subject rarely talked about. But now with a galaxy full of affordable, self-learning EFI systems that control both fuel and ignition, it’s important that the distributor rotor be phased properly with the distributor cap. Let’s begin by looking at what happens underneath the distributor cap of a simple mechanical and vacuum advance ignition system.

How it Works

With most typical mechanical advance distributors, the weights and springs are located on a plate that also mounts the rotor. As rpm increases, centrifugal force pulls the weights against spring tension and moves the plate, which advances the rotor in relation to the spark plug wire terminals in the distributor cap. The plate that mounts the rotor is connected to the distributor shaft that also spins what is sometimes called a paddle wheel, or reluctor, with 8 lugs on it (for a V8 engine). When each lug passes by the magnetic pickup, it generates a signal that triggers the module to initiate a spark from the coil through the rotor, across the gap to the distributor cap post and on its way to the spark plug. . In a standard non-EFI application, the reluctor tabs pass the pickup at exactly the same time that the tip of the rotor lines up with the intended terminal inside the distributor cap. These two are in a fixed relationship with each other. Mechanical advance moves not only the position of the reluctor in relation to the pickup but also the rotor in relation to the distributor cap spark plug wire terminal.

Vacuum advance works in a similar fashion. When engine vacuum is applied to the canister, it pulls on the plate that mounts the pickup. By moving the plate in the opposite direction of the rotating shaft, this also advances the timing.

All of this motion affects the position of the rotor in relation to the spark plug wire terminal on the inside of the distributor cap at the moment that the signal is sent to the CD ignition box to “fire” the coil. Stock distributors align the rotor with the spark plug terminal on the distributor cap with the engine at the initial timing position, and since the reluctor and rotor are fixed together, mechanical advance does not cause phasing issues because as the trigger is advanced, so is the rotor. But performance engines with EFI or power adders that require retarded ignition timing combined with high output spark demands place undue load on the ignition system and place a premium on rotor phasing. Let’s take a look at a typical situation.

Rotor Phasing with EFI

On any EFI system, the distributor no longer uses mechanical movement inside the distributor to advance timing because the ECU determines when the spark signal is triggered. These EFI systems require a locked out distributor which eliminates mechanical or vacuum advance movement within the distributor. This fixes the position of the rotor relative to the reluctor. If the position of the rotor in relation to the distributor body at the point of ignition did not change with advanced or retarded timing, then we would not be having this discussion. But even with digital control of the ignition timing, the rotor position is fixed in relation to the crankshaft position, while the spark events vary in relation to the crankshaft position. This relationship between the rotor and the distributor cap when the spark is transferred from the coil to the intended spark plug wire is what rotor phasing is all about.

On a V8 distributor there is only 45 degrees of rotor movement between the centerlines of each spark plug wire terminal. The MSD illustrations we’ve included reveal why we must keep the rotor as close to the spark plug wire terminal as possible. Even with only 36 degrees of total advance on a street engine, this will move the rotor a significant distance away from the spark plug wire terminal. Larger diameter caps put more distance between the posts but this also means the rotor moves farther for every degree of timing change. The ideal situation is to align the rotor with the spark plug wire terminal roughly where the engine makes maximum cylinder pressure – which generally occurs at or near peak torque.

The reason we want to keep the rotor near the spark plug terminal on the distributor cap may or may not be obvious. Torque and horsepower result from cylinder pressure and as cylinder pressure increases, this requires greater ignition voltage to jump a spark across the gap on the business end of the spark plug. Before this voltage can get to the plug, it must also jump the gap between the rotor and the distributor cap.

Symptoms of poor rotor phasing do not always fall under the category of noticeable misfire. Other issues can be lazy performance despite the fact that engine seems to otherwise run well. Cross-firing inside the cap can also occur which can cause serious damage and contribute to engine kick-back during starting. Excessive rotor gaps will also generate electronic noise that can cause havoc with other electronic devices. Curing these problems is relatively easy.

A commonly accepted procedure for rotor phasing in EFI applications uses 30 degrees as the reference number. For our first example, let’s say we have a normally aspirated small-block with 36 degrees of total timing programmed in the ECU using an MSD two-wire distributor and an MSD-6A ignition box. MSD distributors are designed like OE units so that when the distributor is set at its initial timing (let’s put it at 12 degrees Before Top Dead Center (BTDC), the reluctor will line up with the magnetic pickup and the rotor will be pointed directly at the distributor cap spark plug terminal.

With an EFI system, the reluctor no longer triggers the ignition box to fire. Instead, the ECU interrupts this signal and delays it so that the ECU can vary the timing as programmed by the user. Obviously, to accomplish this, the ECU must see this signal before the intended time of the spark event. This is where the confusion often originates. A common “reference angle” used in aftermarket EFI systems is 50 degrees BTDC. This gives the ECU time to receive the signal at 50 degrees BTDC and command spark events in a normal range (10 – 40 degrees BTDC). When the reluctor passes the pickup in the distributor, it now is signaling the ECU that “The crank is at the reference angle (usually 50 degrees) so get ready to fire the coil when programmed to do so.” Without modifying the relationship between the reluctor and the rotor, the rotor alignment will always be too far advanced in relationship with its intended lug or terminal when the spark event occurs. This can lead to the previously mentioned weak spark or cylinder mis-fires.

Because the timing varies and the rotor phasing is fixed, you must select a compromise crankshaft position where the rotor will be in perfect alignment while creating the proper signal with the reluctor at the required Crank Reference Angle. The best way to select the optimum rotor phase position is to consider the entire range of your programmed ignition timing, and where the timing will be at the estimated peak torque rpm. Peak torque is used (with normally aspirated engines) because that is where maximum cylinder pressure occurs, which places the greatest demand on ignition system voltage. So this is where we should place the rotor the closest to the spark plug terminal inside the cap. For rotor phase position in a boosted application, replace the peak torque programmed timing with the timing programmed at peak boost.

In most naturally aspirated applications, 30 degrees BTDC is a good average rotor phase number. In a high boost or other application where lower timing is employed, 20 – 25 degrees BTDC may be more appropriate.

For this EFI example, we will use 30 degrees BTDC for the rotor phasing position and 50 degrees for the ECU Crank Reference Angle. Because we have to use an adjustable rotor, this is the procedure for an MSD style distributor and phasable rotor. To begin our EFI phasing procedure, drop the distributor in the engine and turn the crank until Number One cylinder is on the firing stroke at 50 degrees BTDC indicated on the harmonic balancer. Now remove the rotor from the distributor so that you will have a clear view of the reluctor and the pickup in the base of the distributor. Turn the base of the distributor until one of the reluctor tabs aligns perfectly with the pickup. At this point, tighten the distributor clamp because you have now set the Crank Reference Angle.

Mark the position of the Number One spark plug terminal position on the distributor body with a Sharpie. Now turn the crankshaft to 30 degrees BTDC (or the phasing figure of your choice). Using an adjustable MSD rotor, bolt it on and move the rotor until it lines up with the spark plug terminal position mark on the distributor body. Tighten all the screws and your rotor phasing is complete. It’s probably a good idea to use a little thread locking compound on that single screw to make sure the rotor does not move.

Phasing the rotor is crucial for all engines, but is especially for power-adder applications. As an example, let’s assume we have a big-block with 36 degrees of total ignition advance but once the centrifugal supercharger makes boost, we pull 1 degree of timing out for every 1 psi of boost. If the centrifugal will make 20 psi and total timing before retard was 36 degrees, then the total timing at max boost would be 16 degrees BTDC. A good place to start would be to phase the rotor at 20 degrees BTDC since this is where the engine will make peak power and place the greatest demands on the ignition.

Rotor Phasing with a FAST Dual Sync Distributor

The FAST Dual Sync distributor is designed to be used with EFI systems that will control ignition timing with a computer. While this distributor is also locked out, it is designed to make rotor phasing very easy and it does not need an adjustable rotor. We will run through the procedure as if you were installing this distributor in a small-block Chevy using a FAST EZ-EFI 2.0 throttle body fuel injection system.

To begin, turn the engine over until it is at 30 degrees BTDC on the Number One cylinder compression stroke. Mark the position of the Number One spark plug terminal position on the distributor body with a Sharpie. Install the distributor in the engine with the rotor pointing to the mark you made on the base of the distributor. Tighten the distributor hold down clamp and that’s it! Since the relationship between the trigger wheel and rotor are pre-phased for a FAST EFI application at 30 degrees rotor phasing, locating the rotor at 30 degrees forces the trigger point to occur at the proper time for the ECU to control timing.

Crank Trigger Rotor Phasing

Setting rotor phasing with a crank trigger is the simplest of all the systems to adjust. With the MSD system, it uses a wheel fitted with four flying magnets bolted to the harmonic balancer that acts as the ignition trigger. The pickup is mounted on the engine and either sends a signal to the ignition box to fire at a fixed timing point, or sends a signal to an ECU indicating the crankshaft is at the required Crank Reference Angle. A crank trigger also narrows down the distributor’s job to merely directing the spark to the individual cylinders. But this still requires rotor phasing.

After mounting the crank trigger and checking the clearance to the pickup, set the wheel so that the flying magnet is directly across from the pickup. In a non-EFI application, setting this trigger point at maximum ignition timing will lock the ignition at that point. Let’s use 36 degrees BTDC. The distributor’s only job now is to transfer spark from the coil to the plugs. With the crank set at 36 BTDC and the distributor in the engine and set on Number One cylinder, move the distributor body until the rotor lines up with Number One spark plug wire on the distributor cap and lock it down. Rotor phasing is now set and you can fire the engine. Remember that unless you are using digital timing control, this timing is locked in at the set point.

In an EFI application using a crank trigger, the setup procedure is slightly different. Instead of aligning the flying magnet directly across from the pickup when the crankshaft is at the point of your desired ignition timing, you would do this procedure at the Crank Reference Angle required by the ECU (50 degrees BTDC in our previous example).

Then, using a rotor phase angle using the previous guidelines, rotate the crankshaft until the timing pointer indicates your desired rotor phase angle, and rotate the distributor base until the rotor tip aligns with the terminal inside the cap for the intended plug wire.

Obviously, we have covered several ways to phase a rotor. If all this seems a little confusing, the best thing is to drop a distributor in your engine, place the crank at TDC and then 30 degrees BTDC and look where the rotor is in relation to the spark plug terminal on the distributor cap. That will help you visualize what is happening in the engine. With all this accumulated knowledge, you can now unleash all the power your engine is capable of cranking out and both you and your engine will now live long and prosper.

*Special thanks to David Page at FAST for his technical input with this story.

Parts List

Description	PN	Source	Price
MSD two-wire distributor, SBC, BBC	85551	Summit Racing	239.97
MSD locked out distributor, SBC, BBC	85501	Summit Racing	265.97
MSD phasable rotor for std. MSD cap	84211	Summit Racing	34.95
MSD phasable rotor for Cap–A-Dapt system	8421	Summit Racing	33.95
MSD wire connector	8861	Summit Racing	7.95
FAST Dual-Sync Distributor	30505	Summit Racing	345.97
ACCEL Dual Sync distributor, SBC, BBC	77100	Summit Racing	319.97
MSD small-cap HEI, Street Fire	5591	Summit Racing	183.99