VOES Switch Explained


BOT Machine
A good source of info about the VOES switch here...

VOES? What's That?

What is the Vacuum Operated Electrical Switch (VOES)? What does it do and why is it there?

All Harley Davidson motorcycle carbureted models since the 1984 model year have used the VOES to improve throttle response, increase MPG, and meet EPA requirements for emissions.

The VOES is a motorcycle part described as a vacuum ignition retard device. That is, under low vacuum conditions the switch is open and has no effect on ignition timing. Under high vacuum, the switch closes and advances ignition timing. Essentially, the VOES is like the vacuum advance in older type automotive distributors.

The VOES is a normally open vacuum operated switch that closes under 3-5" of vacuum, are adjustable and usually require different settings for different bikes. The switch is connected to a lead from the ignition module. Under high vacuum, 3-5 inches or higher, the switch closes. A lead from the switch to ground closes a circuit in the ignition module. This circuit advances the timing of the spark. The advance increases throttle response and decrease fuel consumption and emissions.

The vacuum hose is usually connected to a port on the carburetor or intake manifold depending on motorcycle year and carburetor. There are several different VOES switches used the mounting bracket style and operating vacuum being the main differences. The FLHT models have a different vacuum range than the other models. However, just about any VOES can be adapted for use by adjusting the point at which the switch closes.
As an example...
FLT: 5.0 - 6.0 inches of mercury
FXR: 3.5 - 4.5 inches of mercury

Your vacuum gauge measures in in/Hg (inches of mercury) and/or mm/Hg (millimeters of mercury).

The point at which the switch closes.

The switches can be adjusted by removing the potted plug and adjusting the setscrew. You will need an accurate vacuum gauge and vacuum hand pump. We have been able to set the operating point as low as 2 inches and as high as 7 inches.

Why would you want a VOES?

We have experimented with converting 1972 and later ignitions from points to electronic ignition. We have used Dyna 'S' conversions for H-D's. We have used Crane, Compufire, Spyke and other Harley Davidson conversion kits. With some of these such as the Dyna 'S', you still have to use the mechanical advance system. This requires service and routine maintenance.

Others work very well having digital advances and provisions for a VOES switch. We experimented with installing a VOES in a 1983 FX and found we have improved throttle response and better mileage. We also routinely install the VOES in custom built Harley Davidsons and even our own Kenny Boyce framed Harley FXR's use VOES with a Crane HI4 module. During Dyno runs, we found that part throttle roll on power was increased as was throttle response.

We believe that most street ridden Harley Davidson motorcycles will benefit from a VOES.

For racing applications or supercharged, turbo-charged or bikes using Nitrous Oxide, we do not recommend using a VOES. This is due to the possibility of a sudden timing change causing a backfire which can be a bad thing under these conditions!
I just made some modifications to my 97 DYNA....SE heads...EV 57 Cam...CV carb mods...inc. slide spring, jetting, and would like to know: What determines the setting on the VOES?? I know that there are switches available pre set to different settings, as well as the ability to adjust the factory one. But , again, what am I looking for? I set my total advance at 35 with a single fire Spyke ignition. :help
35 degrees @ what RPM. May be a little deep. Do you know what your actual dynamic compression is in PSI.
I'm waiting for an outer primary so that I can check the compression....And I'll have to check my Spykes lit. to see where it fully advances at. ...Hmmmm looks like I don't have enough info for you as of yet.
Sounds like your building a Performance bike/engine. Are you Really that concerned with getting the Best fuel mileage you can. At WOT and near WOT the VOES is disabled. And even when your running at part throttle when the VOES is being used, I bet it won't increase your fuel mileage by more than 2 miles a gallon. If your pushing your advance curve to the edge already, adding VOES spark advance can be tricky.

The way I would do it is, set up the basic timing & advance curve timing and get the bike performing at its peak with the VOES disabled. At that point, if you still want to, enable the VOES and see if the engine will tolerate any extra advance at part throttle. You may have to play with the vacuum switch threshold point.

I said adding VOES can be tricky because if you have loud pipes on your bike, it may be hard to determine slight engine knock before the VOES disables itself and kicks back the timing under hard acceleration. I do know for a fact, that even on my bike which has stock pipes, there were times I could not hear engine knock prior to the auto knock retard kicking in. Even when the SERT data logging said knock WAS in fact present, there were times I could not audibly hear any knock. And that's with stock pipes.

Sounds like your going to have a Sweet Machine when your done!

Which model SPYKES are you using.
After only getting 35,000 miles from a New in a crate S&S 96" Super Stock motor, I am interested in longevity. Sure, as you can see, performance is also up there in my list of wants. I may change the cam to something less agressive. The reason for the VOES, is to avoid detonation when accelerating ( rolling it on ). That is a function of the VOES (so I'm told )
I am using a Super Comp Spyke Ign. in single fire mode. full advance is reached at 2100 RPM as set at the factory, however, I can change that either direction. I also use the Spyke COIL, which puts out 80,000 volts.
I never actually used or installed a Spyke's ignition system but I went to the Spyke's web sight and looked at your unit and read the manual. The way I read it, the unit does not have a spark curve at all. It actually consists of 3 "Steps". Step 1 is cranking advance. Step 2 is a fixed advance # from idle RPM to 2100 RPM, then Step 3 is a fixed advance # from everything above 2100 RPM to red line. Their saying your advance at idle will be 20 or 25 degrees (depends on base setting) and the 3rd step is 35* at 2100 RPM but that is RPM adjustable. It did not state what the VOES timing retard is by itself. I can almost promise you that having that much advance (35*) at 2100 RPM under WOT will be destructive. Even if the VOES kicks it back 7 degrees, you could have 28 degrees at 2200 RPM under WOT, which is still a lot while under those conditions.
Maybe an activated VOES switch kicks the timing back more than 7 degrees, in which case you may be OK... I don't know what that number is. The manual does not mention what it is. I would call and find out.

Don't get too aggressive on the advance. Here are two examples of SE advance maps with a few cells of interest.

1) A SE Stage 1 map for a 2009 '96. At 3000 RPM/ high map load & under WOT, only 19 degrees advance. And at 6000 RPM under WOT the advance is 28 degrees.
2) A Stage 3 1856, 103+ Heads, Race exhaust, SE 264 cam has 19 degrees @3000 RPM @ WOT w/high Map load and 21 degrees @ 6000 RPM @ WOT.
It looks like you will be way over both those numbers.

Start out light if you can because you can always add more advance. Since you don't have electronic knock retard, all you have are your ears.

If you don't mind me asking, what caused the early death on the SS engine & what ignition were you using with that engine.
I used the same ignition. What I saw when I took the nose cone off, was cam failure. The hardfacing was coming off on the rear intake and the lifter roller axle had about .007 play (up and down) There was metal through out the motor. Pistons had too much play, and no sign of preignition. The wrist pin bushings were loose and the rods had a lot of wiggle as well as some noticeble up and down. I'm glad I checked it when I did.
I'll call the super tech at Power Arc Ignitions ( who are the manufacturers of Spyke ) and ask Gary about the "Curve" and the VOES retard in degrees. He seems real knowledgable, but I have to ask the right questions. So, your interpretation of the unit is that it "jumps" from 20 or 25( depending of the base setting) to 35 as opposed to a gradual increase. Is that correct? I have to ask the right questions ...as I mentioned above.
Yes, after reading the manual for your Super Comp single fire ignition system that is how I understand it. It actually states that once the system has seen 5 revolutions of the crank rotation, it jumps from cranking advance to the next "level" of advance. That new level is held until 2100 RPM at which point it jumps to it's final stage of advance. The switch point from level 2 to 3 can be shifted in 500 RPM increments by the user.

This is all on page 1 of the manual. Lets assume you use the "Quick Time Method". Your Base timing will be 5 degrees BTDC for starting. Then after 5 Revolutions the ignition will advance to 25 degrees BTDC. Then as set by the factory, the ignition will advance to 35 degrees at 2100 RPM. This is the "curve" that they recommend for Big Twins as stated on page 1 of their manual.

The question you need to ask is this. If Gary agrees with everything so far, ask what will happen to the 35 degrees advance when you ground the Blue VOES wire. I am thinking the ignition timing will back off 5-10 degrees (?). Lets say the answer he tells you is 5 degrees so instead of 35, you will have 30 of total advance. My next question would be this. Lets say I permanently attach the blue wire to ground, does that 5 degrees of retard also effect my "stage 2" of advance meaning is my advance 20 degrees instead of 25 for stage 2. And just for the heck of it ask what your cranking advance will be.
Does grounding the blue wire knock off a set amount across the board and if so by How Much.

You gotta tell me what oil you were using in your SS engine.
I have been using Valvoline 4 stroke motorcycle oil for years, ever since it came out. Before that I used Valvoline racing oil....both in the 20-50 WT. I run an oil cooler as well. It's a LOCKHART..though it says Harley Davidson on it. Whenever the temp drops below 60, I block it off.
I was under the impression that grounding the blue wire actually lets it run up to 35 advanced. It's only when it's open( it opens under low vacuum, and closes at the preset value) that the timing would retard. How much, I don't know.Maybe I can catch Gary tomorrow.