# How does knock box ignition function to alter timing?



## laundryhamperman (May 15, 2010)

I have a distributor from a knock box set up, and the rotor doesn't budge relative to the shaft (like you would see with a vacuum advance or mechanical advance dizzy), is the ignition adjustment made at the distributor or is there some delay from the hall sensor to the knock box, back to the ignitor and then the coil? My question is how does this ignition system physically accomplish changing timing with engine speed/vacuum. It must all be electronic since the box gets a vacuum signal hose as well. I ask this in part because I am wondering does the model of distributor matter for a knock box car? Can you interchange a Digi dizzy for a CIS-E with knock box dizzy? My application is on a CIS lambda GTi '83.


----------



## ps2375 (Aug 13, 2003)

If both dizzy's are w/o vacuum adv/retard and have no mechanical adv, then yes they would be interchangeable. The "magic" happens in the knock box. The timing with a 4 window hall sender dizzy is purely a time-based function, the "delay" or "advance" is figured based upon rpm's and the load signal (vacuum level) off of a "map" that is loaded on a chip in the knock box. And when it "hears" knock, it retards the timing.


----------



## Glegor (Mar 31, 2008)

laundryhamperman said:


> I have a distributor from a knock box set up, and the rotor doesn't budge relative to the shaft (like you would see with a vacuum advance or mechanical advance dizzy), is the ignition adjustment made at the distributor or is there some delay from the hall sensor to the knock box, back to the ignitor and then the coil? My question is how does this ignition system physically accomplish changing timing with engine speed/vacuum. It must all be electronic since the box gets a vacuum signal hose as well. I ask this in part because I am wondering does the model of distributor matter for a knock box car? Can you interchange a Digi dizzy for a CIS-E with knock box dizzy? My application is on a CIS lambda GTi '83.


your car has a vac/mech advance distributor. if you tried to run the vac/mech advance dizzy with a knock box or digi, they would just be fighting each other the whole time trying to maintain steady timing..

and if you ran a non-advance dizzy on a car without a knock box, you would have no timing advance at all, and that would just be stupid.

the knock box receives a signal from the knock sensor on the engine block, and pulls timing (delays it) according to the amount of knock detected.. i believe that the knock box can pull 12* of timing out of the engine..

(CIS-E and Digifant run the same type of distributor, its the locked out, non-advance distributor)


----------



## ps2375 (Aug 13, 2003)

Glegor said:


> your car has a vac/mech advance distributor. if you tried to run the vac/mech advance dizzy with a knock box or digi, they would just be fighting each other the whole time trying to maintain steady timing..


There really wouldn't be any "fighting" going on to maintain "steady timing" as there is no feedback to anything to reference as to what would be "steady timing. You would however have the mechanical and vacuum timing ("mechanical map") overlaid upon the "electronic map" of the knock box making a pure mess of things. Twice as much advance and too much retard and the knock sensor in there to mess with it even more.

To keep the "old" style dizzy, you would need a purely knock control box to retard timing, like "Knock Sense" (I think that is one of them) to ONLY deal with the knock.

As it is with the VW components, you either have to go all fixed dizzy w/ knock box or all mechanical w/o it. You could find a good shop that can re-curve the dizzy, thise have been known to work well, and run a fuel that will satisfy the motor's need for octane to prevent detonation.


----------



## laundryhamperman (May 15, 2010)

So the timing is all based on delay? The hall effect sensor begins its square wave signal to the knock box, and the box, based on manifold vacuum and engine speed, then waits some milliseconds based on the map to accomplish the timing adjustment, then the box sends the signal to the ignitor to initiate the spark event? 

My wondering is how exact is this? The engine speed signal is the rate of 4 square waves per 2 revolutions of the engine? Compared to a crank angle sensor which has 100+ teeth on the flywheel and gets 100+ pulses per one revolution, is the knock box that good (precise) with such a coarse signal? 

Food for thought, If I had a mechanical and vacuum dizzy, I could lock the mechanical advance, and remove the vacuum to the advance, and set my static timing with the knock box and it would work fine? Or I could remove the vacuum hose from the knock box and run it to the advance on the dizzy, let the box do the speed advance and the dizzy do the vacuum advance....

Could somebody confirm the above is how the knock box works, it simply delays the spark...does it also advance it..i.e. count and fire the coil milliseconds prior to its next anticipated square wave from the hall sensor?


----------



## ps2375 (Aug 13, 2003)

It does the delay and advance based upon time. I don't know if the amount of time is great enough to "anticipate" the next trigger, but it could be enough to have less delay. And the 4 window dizzy is not as accurate and a crank sensor is, but it is not as bad as you would think, and yes the knock box is that good.

I would go either all one or the other and not mix. You'll also need the 2 switches on the TB for the box to work at 100%. They would be the throttle closed and the WOT switches.


----------



## laundryhamperman (May 15, 2010)

Thanks greatly for the info. This was a burning curiosity for me to compare the function of this setup to a more traditional EFI with electronic ignition.


----------



## ps2375 (Aug 13, 2003)

Not all EFI systems use a crank sensor. I myself and many other use the 4 window digi dizzy with MegaSquirt and control spark basically the same way as the knock box (but w/o the knock sensing). The advantage you can gain with the crank sensor and a single window dizzy, such as on an ABA or even with MS is sequential injection and some added accuracy with the timing. The dizzy window is there to signal #1 TDC to the computer. And the rest is taken off of the crank wheel teeth.


----------



## laundryhamperman (May 15, 2010)

So my last question, are all fixed distributors for say 85' GTI/GLI through the Digifont distributors for 1.8l engines the same? Are there distributors that have no vacuum advance but do have mechanical advance on the MKII cars (maybe the earlier base models without the knock box)? Reason is at a junk yard, you can easily identify a vacuum dizzy, but all the rest on the VW's look the same, three wires for the hall sensor.


----------



## WaterWheels (Aug 14, 2005)

laundryhamperman said:


> . . . are all fixed distributors for say 85' GTI/GLI through the Digifont distributors for 1.8l engines . . . Are there distributors that have no vacuum advance but do have mechanical advance . . .


Yes with some exceptions that have to do with height and design changes. No, but you can make one that way for running things like dual side draught carbs.


----------



## Glegor (Mar 31, 2008)

laundryhamperman said:


> So my last question, are all fixed distributors for say 85' GTI/GLI through the Digifont distributors for 1.8l engines the same? Are there distributors that have no vacuum advance but do have mechanical advance on the MKII cars (maybe the earlier base models without the knock box)? Reason is at a junk yard, you can easily identify a vacuum dizzy, but all the rest on the VW's look the same, three wires for the hall sensor.


 the distributors in my GTIs are SOLID.. no advance. mk2 w/ CIS-E/knock box.. both of them. swear to it. could be wrong tho. ive never paid THAT MUCH attention..


----------



## stark9dubber (Mar 27, 2007)

FYI: knock sensors generate AC voltage, timing is altered based on the voltage produced and read by the knock box.


----------

