How Does a Dual Sync Distributor Work and When Do You Need One?

10 min read

How Does a Dual Sync Distributor Work and When Do You Need One?

10 min read

Conventional distributors use centrifugal weights and springs to affect the engine timing. Essentially the faster the distributor spins, the more the weights move away from the center of the distributor, and that alters the engine timing in turn.

Although that design worked well enough for decades of automotive applications, it’s not exactly cutting edge tech by today’s standards, and it isn’t particularly user-friendly when it comes to tuning. “Back in the day, if you wanted to change the advance curve, you had to put in different weights and different springs to change the advance curve,” explains Doug Flynn of Holley Performance.

“So you’re fiddling around with it a lot, and most people don’t really know what they’re doing with this stuff. You really need a distributor machine if you’re going to do it right, and you also have to know how to use it. Most of the time you’re basically stuck with the combination of springs and weights that you have.”

Dual Sync vs standard distributor

In a standard distributor, like the MSD 8352 Ready-To-Run (right), you have wear items that will have to be addressed down the road, like the cap, rotor, and vacuum pot. A Dual Sync distributor (left) eliminates these problems.

And that means that you’re also stuck with a pretty linear timing curve, whether or not that’s ideal for the engine in question.

“The other thing that conventional distributors add on is a vacuum advance canister,” he says. “So you have an additional mechanical device on the distributor which can add more timing with higher manifold vacuum – that’s typically what you want to do with a timing curve. It’s supplemental to the springs and weights so that, when you go to wide open throttle and you have no vacuum, it doesn’t add any timing. But when you’re at idle and cruising speeds, it allows you to feed more timing in that the engine can’t use at wide open throttle because of potential detonation.”

The upshot here is that, with a conventional distributor, you have several mechanical systems that are trying to establish an ideal timing value at any point in the engine’s operation. And that’s far from precise by modern standards.

Fundamental Differences

“On a V8 engine, you need eight crank pulses for every two engine revolutions. And with a cam signal, you need one pulse for every two engine revolutions,” says Flynn. That’s what a dual sync distributor provides.”

Here, the timing tables in the ECU essentially replace the archaic springs, weights, and vacuum canister used in a conventional distributor to establish the curve. And because of that, you aren’t limited to the linear and imprecise timing curves provided by a mechanical system. “You can really make whatever base timing curve you want based on engine RPM and load,” he says. “That can look however you want it to, and you just can’t do that with a mechanical design.”

Dual Sync timing table

Software-generated tables provide a level of precision and control over engine timing that’s simply not achievable with a conventional distributor.

Although you could, in theory, block off the mechanical elements on a conventional distributor to make it work with an EFI system, it still wouldn’t function the way a dual sync distributor does. “There’s two output signals in a dual sync distributor,” Flynn notes. “It has a crankshaft position signal, and it has a camshaft position signal.”

That cam signal is what makes it a dual sync design, and it provides several functions. “First, it’s used to give you timed sequential operation in a multi-port application. You can run a mutli-port injection EFI system without dual sync, but you don’t know where the engine rotation is, so you’re just kind of firing the port injectors randomly, and there are gains to be found by timing the fuel injector to fire in relation to the intake and exhaust valves opening. Without the cam signal, the ECU can’t know where the valves are, so it’s just going to spray the fuel somewhat arbitrarily.”

Dual sync capability is also a requirement with coil-on-plug ignitions. “The ECU has to know the position of the engine at any given time, exactly when cylinder #1 is firing and that sort of thing. Otherwise the engine simply won’t run right – you’re going to be firing it on the exhaust stroke, and so on. You just can’t do it. So the cam sync element of a dual sync distributor is what allows you to run coil-on-plug.”

dual sync LED lights

During installation, LEDs on the circuit board of a Holley or MSD dual sync distributor will light up when the crank and cam sensors are correctly positioned.

And pairing a dual sync distributor with a modern coil-on-plug system offers some significant advantages. “You get the potential for a lot more ignition power because you’ve got eight coils instead of one,” Flynn says. It also allows you to ditch the things that have the potential to go bad and cause issues, like the rotor and cap.”

Other Benefits

dual sync timing control 2

Utilizing computer-controlled timing can allow for useful and exciting racing functions. For example, Holley’s new V6 software has built-in provisions for wheelie control and other functions that go well beyond simply adjusting the timing curve for maximum efficiency.

The level of control that dual sync distributors provide goes beyond simply fine-tuning the timing curve.

“Now you can use the ECU to do a number of things,” Flynn says. “You can use it to retard timing for nitrous, or use it to change the timing for launch control, or for wheelie control – the list goes on and on.”

You can even use it for traction control functionality as well – not only to make sure you’re putting the power down out of the hole, but also further down the track. “You can take timing out during shifts to make sure you don’t lose grip. Computer-controlled timing is what makes that all possible – you’re doing all of this with a keyboard, not messing around with a bunch of greasy junk.”

But it’s important to note that not all dual sync distributors are created equal. “There’s two common sensor types,” he tells us. “One is variable reluctance, and the other is called a Hall effect sensor. The main advantage to Hall effect-style sensors is that they’re much better at preventing electrical noise from screwing up the signal, and that’s why Holley and MSD use them. The Hall effect sensors just don’t have issues with noise from the plug wires and all that stuff.”

Making The Switch

Dual Sync MSD 6A product

Dual sync distributors require a capacitive discharge box, such as this MSD 6A ignition controller (p/n 6425), to fire the coil. Holley has other ignition box options as well.

Stepping up to a dual sync distributor is a fairly straightforward proposition, but there are a few things that would-be installers should keep in mind. “You need to have an ignition box with it,” Flynn points out. “Typically that’s going to be an MSD 6A, 6AL, or a Sniper Hyperspark box. Today’s dual sync distributors do not have something to fire the coil, so you need one of these boxes to take care of that.” On the programming side, Holley has made the process as hassle-free as possible with a simple drop-down menu option in their EFI software that dials the configuration in for a dual sync distributor.

The other important element to keep in mind when swapping over to a dual sync is that it needs to be installed in the engine in the right orientation.

“The cam and the crank sensors need to be in the right spot, that’s really important,” Flynn says. “The instructions for Holley’s products cover all of it – there are LEDs on the circuit board, and you’re walked through the process – ‘Rotate this until this LED turns on, drop the distributor in here and rotate that until this other LED turns on, and tighten it down. After that you’re just about done.”


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