in my bin aujp there is something called
spark latency correction. Can anyone tell
me what this is for ... thanks

 

It corrects the timing based on the A/C alternator effect of the pickup/ reluctor.
Something you'll never have to worry about, unless you change to a different type of triggering devise for the ignition.

 

thanks grumpy

 

WHAT IF YOU CHANGE THE VALUES IN THE "SPARK LATENCY" CHART? i have changed my values below 3000 rpms, seems to have helped

 

Has anyone seen an effect where the timing seems to wobble back and forth with a period matching the distributor? I was doing some derivative processing of some WOT pull WAVs I recorded off of the coil wire and noticed a fairly regular wave in the timing between cylinder firings. In fact, it seems to swing as much as 20 degrees of crank rotation. To prove to myself that I wasn't crazy, I reprocessed the data to plot each cylinder individually, and it plotted 6 relatively smooth lines with no waves. At first I was thinking it was just play in the cam/dist drive, but it seems way too regular to just be scatter from play in the gears. Could it be something as simple as a bent or badly made trigger wheel in the distributor? Is there some sort of filter value I'm not seeing that could reign this in some?

For the record, I'm running a 4.3L TBI V6 with a 7747, MSD 6a, MSD coil, and MSD pro-billet distributor.

The 3k plot is compressed in the x-domain. The markings for the X axis are for the 5K plot, and I was too lazy to try to re-scale it.

Teeleton

 

Is this what your refering too?

It came from this https://www.thirdgen.org/forums/diy-...k-advance.html

 

The cam isn't always under the same load, there are 12/16 lifters all opening, closing, are in some state between those. With a chain driving the whole operation, and the slack it allows, I can well imagine seeing a *pattern*. Chains are really poor at transfering motion, compared to say gears. Even if on was to change to a gear drive you still have the tooth lash, and lifter movement dithering the motion.

 

When I first read this thread, I was hoping I could use it, but the problem is cylinder to cylinder, and the table is per RPM. The problem seem to be consitant across all RPMs. If I take a snapshot of a single revolution (or two, since it takes two crank revs for all cylinders to fire), each cylinder seems to have a different timing advance. 4 firings take progressively longer than the previous (retard), and then the last two reverse that trend and take a shorter interval (advance) to end up back where it started and start the cycle all over again. Ideally, the spark pulses should be evenly timed, with a slight decline in time as the RPMs increase.

Teeleton

 

20 degrees seems like an awful lot to attribute to gear lash and chain stretch, though. I'm just going to have to tear into the engine and start testing and measuring things, I guess. Maybe run the distributor on an electric drill and see what the pulses look like, and failing that look for chain stretch. It's a double roller with maybe 30k on it, but anything is possible, I guess.

Teeleton

 

It's a problem with the 90deg V6. The firing interval is not even. Look at the distributor, and the posts on it. Not even.

 

You're thinking of a 229, which was a "semi-even fire" motor. The 4.3L is a true even fire. On the pre-96 motors, if you twist the distributor 60 degrees, and move all the spark plugs around one post, the motor will fire right up, and run like nothing's changed (96 and later distributors don't twist, and have a cam sensor).

Teeleton

 

My bad.

Try sticking a timing light on each cylinder (and put some timing tape on the balancer). See if it's really as what your other measurement says.

 

You asked about this before. FWIW, here's my impression of the phenomenon:

1. A GM HEI pickup generates an AC sine-type wave, one complete wave for each cylinder firing
event. And in fact, if you look at them on a 'scope, as the engine speeds up the signal's amplitude
increases.

2. Mathematically however, the length or period of a wave is inversely proportional to its frequency.
That is to say (frequency) X (wavelength) = (a constant). IOW, the more signals you generate
per second, the closer together the signals have to be; wave length is related to frequency, not
wave amplitude in this case.

3. HEI uses a 'zero crossing' trigger point in order to make sure the spark index location does not
vary with the frequency or size of the ignition signal from the pick-up. You can read about this
trigger scheme in the application notes for the Motorola MC3334 4-pin HEI module here:
https://www.thirdgen.org/forums/atta...g?d=1086192457
http://www1.jaycar.com.au/images_uploaded/MC3334R0.PDF
Or in the Nat Semi LM 1815 chip (7-pin & Northstar modules) notes here:
https://www.thirdgen.org/forums/atta...g?d=1086106771
http://www.national.com/ds/LM/LM1815.pdf
Contrast that design with the way MSD runs their ignition box triggers.

Speculation:

4. With respect to latency, as I looked at the AUJP hack (table @ 8129) the high-to-low difference
in time was < 100 micro sec IIRC. 100 Usec on a V8 @ 6000 RPM = 3.6 deg of crank rotation, if I
managed to get all the units correct. The table is pretty flat WRT RPM - indicating it's a fixed delay,
possibly due to the charge-time required by the pick-up coil(?). Or maybe GM doesn't use the exact
center of the 'zero cross' - my scope doesn't have enough resolution to see the difference.

5. If you notice in the application notes above, the HEI pickups feature an LRC network on the input.
Without actually trying to model it, my first thought is that the LRC introduces some phase-shift
as frequency varies and that this is a possible source of the minor 'spark latency'change with engine
speed (frequency).

HTH

 

Why would the spark latency be different between ANHT/AXCN and AUJP?

That is, it's the same for ANHT an AXCN, but different on AUJP?


AXCN/ANHT
-------------
RPM usec
800 214
1200 275
1600 305
2000 244
2400 275
2800 244
3200 305
3600 320
4000 290
4400 305
4800 305
5200 305
5600 305
6000 305


AUJP
-------------
RPM usec
800 122
1200 168
1600 183
2000 198
2400 259
2800 229
3200 198
3600 198
4000 198
4400 214
4800 214
5200 214
5600 214
6000 214

 

They use different distributors.

RBob.

 

So it's large cap vs small cap?

ANHT and AXCN are Corvette, IIRC

AUJP is obviously Camaro.

Oh... Hmmm... I've been basing my cal off of AUJP, but I'm running a Vette distributor. I guess I should change over to ANHT (since I have an automatic)?

Interesting. Are there any other ignition hardware differences I should be accounting for in my calibration?

 

Yes, large vs small cap.

Yes.

Yes.

Yes, I would use the large cap values in the latency table.

None that I know of.

RBob.

 

I was looking at the AXXF bin and the spark latency table looks more like the ANHT table than the AUJP table. The AXXF was a small cap as far as I know. That appears to make it more of a module change than a small cap vs. big cap change.
According to my calcs the change in degrees from the various tables is about 1* to 3* of advance.

 

The tables may be different because of federal vs. CA emission modules.