Timing light inductive delay?

Praxis · Post by **Praxis** » Wed May 25, 2016 1:50 am

Can someone explain how a timing light works? I always thought that it senses the spark via the inductive pick up,and this triggers a circuit to fire the light.

Why I am asking. I have 3 timing lights, and though all 3 show same timing at idle (locked distributor), 2 of them show timing retard slightly, progressively as I slowly bring the revs up.

So I searched this site, and saw an explanation of why dial back timing lights show "wrong" timing with wasted spark, suggesting they measure rpm to calculate delay of the flash.

But all 3 of my lights are non dial back, and I am watching 2 of them slightly retard (maybe 4 degrees from idle through 6000 rpm).

Got me thinking. If there is a slight delay between the spark event and the light flash, this would explain what I am seeing. For example, if a timing light has a delay of even 0.1ms, that would be 0.6 degrees at 1000 rpm, but 3.6 degrees at 6000 rpm.

Mentally, in my small mid, there must be SOME delay. Do some timing lights compensate for this in the circuitry?

What are your experiences?

user-23911 · Post by **user-23911** » Wed May 25, 2016 4:55 am

Everything has a delay in it....no such thing as instantaneous.
The more fancy and complicated it is, the more stages it has and the more delay.
EFI computers have lots of delay.
If your laptop tells you that spark advance is 30 deg at 7000RPM.......it won't be.

xenon timing lights can be really simple.......the flash tube has 3 connections........a high voltage DC across the ends provided by an oscillator / DC DC converter circuit (a bit like a CDI).......the trigger wire is close to one end....the trigger comes from the HT ...makes it conduct from end to end and flash.
It's normal for a timing light to read correctly at idle but delayed at high RPM.
The idea would be to test it on a (locked) points distributor....you'd expect the advance to stay constant.

As far as wasted spark goes, you might need to reverse the polarity of the trigger wire 'cos half the plugs fire on way, the other half the opposite direction.

Post by **In-Tech** » Wed May 25, 2016 5:08 am

joe 90 wrote:Everything has a delay in it....no such thing as instantaneous.
The more fancy and complicated it is, the more stages it has and the more delay.
EFI computers have lots of delay.
If your laptop tells you that spark advance is 30 deg at 7000RPM.......it won't be.

I think we talked about this before, lag time can be addressed but isn't in most controllers.

Lag_GM.JPG

He mentioned a fixed non adjustable timing light, sucks not to be able to trust one without a scope

pamotorman · Post by **pamotorman** » Wed May 25, 2016 7:40 am

some inductive timing light pickups have a arrow pointing towards the plug.

BigBlocksOnTop2 · Post by **BigBlocksOnTop2** » Wed May 25, 2016 7:51 am

I have heard it described as slew rate. Slew rate exists on all electronic/ electrical circuits. Basically it is the time it takes for a circuit to operate (the reluctor pole passes by the core of the mag pick-up and generates a signal (voltage). This signal goes to the amplifier. The amplifier ''deciphers'' this signal and sends out its signal to the coil. The coil charges and discharges sending power to the plug for spark.) All of this takes time and its called slew rate. It is constant. As engine speed increases slew rate causes timing retard.

engineguyBill · Post by **engineguyBill** » Wed May 25, 2016 7:59 am

First, check the inductive pickup clamp. If it has an arrow, make sure that it is pointed in the direction of the spark plug. Check the condition of your plug wires and test the resistance. Often times timing light fluctuation can be caused by bad wires. I am away from the shop now and I can't remember how many ohms of resistance the wires should have, but I am sure that someone here will chime in with that info.

Rick Finsta · Post by **Rick Finsta** » Wed May 25, 2016 8:44 am

joe 90 - If you have an ECU not delivering the commanded ignition timing, and no way to bring it in line with the commanded timing, then it is time to throw that in the trash and get a new ECU.

Zmechanic · Post by **Zmechanic** » Wed May 25, 2016 12:00 pm

BigBlocksOnTop2 wrote:I have heard it described as slew rate. Slew rate exists on all electronic/ electrical circuits. Basically it is the time it takes for a circuit to operate (the reluctor pole passes by the core of the mag pick-up and generates a signal (voltage). This signal goes to the amplifier. The amplifier ''deciphers'' this signal and sends out its signal to the coil. The coil charges and discharges sending power to the plug for spark.) All of this takes time and its called slew rate. It is constant. As engine speed increases slew rate causes timing retard.

Slew rate is quite literally the rate at which an electronic part can move it's output. Usually listed for analog parts as Volts per microsecond. Slew rate is only one type of delay. There is also propagation delay, which is the amount of time it takes a signal to propagate or move through a part. Parts like amplifiers have propagation delay, slew rate delay, and phase delay because they have limited frequency bandwidth.

To the original poster, if the timing light has a fixed amount of delay in time, that will look like an increasing timing retard in degrees. Since degrees, which is phase, is dependent on engine RPM. As engine RPM increases, the period between firings decreases, but the timing light's delay stays constant, so the apparent timing retards gets worse. IF that's what is happening. If it is going from right on, then somewhat jumps back 4 degrees and stays there, that is something different.

Post by **Baprace** » Wed May 25, 2016 1:31 pm

Why I am asking. I have 3 timing lights, and though all 3 show same timing at idle (locked distributor), 2 of them show timing retard slightly, progressively as I slowly bring the revs up.

If you use a distributor with an advance curve your retarded timming will not show up, it only happens with locked distributors.

pamotorman · Post by **pamotorman** » Wed May 25, 2016 2:04 pm

Baprace wrote:Why I am asking. I have 3 timing lights, and though all 3 show same timing at idle (locked distributor), 2 of them show timing retard slightly, progressively as I slowly bring the revs up.

If you use a distributor with an advance curve your retarded timming will not show up, it only happens with locked distributors.

could this be caused by slack in the timing chain and play in the cam/distributor gears ?

Post by **Schurkey** » Wed May 25, 2016 3:27 pm

pamotorman wrote:
Baprace wrote:Why I am asking. I have 3 timing lights, and though all 3 show same timing at idle (locked distributor), 2 of them show timing retard slightly, progressively as I slowly bring the revs up.

If you use a distributor with an advance curve your retarded timming will not show up, it only happens with locked distributors.
could this be caused by slack in the timing chain and play in the cam/distributor gears ?

If the problem was in the engine, I'd expect all three timing lights to behave identically--they're being fed the same signal.

In that vein, GM used to advertise that the old transistor ignition (K66) used to retard 1 degree per 1000 rpm. I see approximately that on HEI ignitions, even though GM doesn't talk about it that I'm aware of. And yet, it's there in all HEIs, but it isn't exactly 1 degree per 1000. Sometimes a bit more. Seldom less.

HOWEVER, since he sees retarded timing in two of three timing lights, I have to believe that the problem is in the timing light(s), and the more-retarded lights are the most-likely to be inaccurate.

Praxis · Post by **Praxis** » Wed May 25, 2016 6:04 pm

To be certain, i purchased a $200+ msd light (the battery powered one) and out of 4 cars here, it would only work with the one that has a cdi box. Lol. But damn, is it bright. Even on my chev truck, it only randomly blinks here and there.

user-23911 · Post by **user-23911** » Thu May 26, 2016 1:12 am

Rick Finsta wrote:joe 90 - If you have an ECU not delivering the commanded ignition timing, and no way to bring it in line with the commanded timing, then it is time to throw that in the trash and get a new ECU.

I agree but some of the delay will be caused by the computer, some by whatever ignition system you're using. Some by the timing light.

It's something that everyone should try out at some stage?
I don't think there will be one single aftermarket or factory computer out there which works as it should by giving the correct timing advance up at the red line.
"Correct" meaning that whatever you read off the logger for degrees advance is the same as what you read from the light.

Nick Campagna · Post by **Nick Campagna** » Thu May 26, 2016 7:00 am

Years ago, 35 ?, I had a timing light that went in series with the plug. No power connections, just a goesinta and a goesouta.

Circlotron · Post by **Circlotron** » Thu May 26, 2016 9:33 am

The only kind of timing light I trust is one that you remove the lead off the plug, put the timing light lead on the plug, then put the plug lead on the springy thing on the timing light lead. Mine is like this, and it sends the entire plug voltage up the light lead to a fine wire that is spiralled around the xenon flash tube. Inside it has a dc/dc converter that charges up a capacitor that is across the ends of the flash tube. The plug voltage triggers the flash tube directly, with no electronics in between it and the tube. If there is any worthwhile delay, I imagine this setup would probably have the minimum.

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