liquidh8

So I was going through some things, and notice the ebl SA latency table isn't the same as say the AUJP bin, small cap dizzy. Is there a reason why?? I thought those valus were decided by the manufaturer of he dizzy, and can't be changed? Do these valuse affect you overall timing scheme, at all times? Or just limp home mode? And if the tables can be changed, how/why would one want to?

Here is a link to a post with a stock small cap and big cap dizzy latenecy table.

https://www.thirdgen.org/forums/diy-...=spark+latency

here is a aujp table:
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

notice how the numbers are the same from 4400 and up, in the EBL.bin able, it goes to like 314 at 3600, then drops low again. where the stock tables, the latency continues to climb numerically as the RPM's increase.

Can someone please shed some light? I have found much in the search and on the web about this topic.

liquidh8

Ok, so I fired up my craptop and took a look at some stock tbi bins. the 5.0 and 5.7 bins share the EBL latency table, small cap dizzy. I see where RBob got it from, but, I open a 7.4 bin, 454, and the latency was way different. no the big block and small block chevys use the same dizzy, so why the different tables? And I had a 4.3 bin, and it to had a different table, but I can understand the v6 having a different table, it has a different module and only 6 cylinders. but the BBC uses the same dizzy and module, so why the change? I'm gonna keep searching the web in hopes of finding something more on this subject because it is bothering the crap out of me.

RBob

iTrader: (1)

The purpose of the latency is to make up for lost time. The way the distributor operates causes a delay between when the reluctor passes the pole piece and the signal reaches the ECM. This delay changes according to the RPM of the distributor.

The code works by subtracting the latency value from the spark advance value. However, at this point in the code the SA has already been converted from an angle to time. Instead of 30 deg BTDC, it will be in micro-seconds BTDC (In actuality it is usec's ATDC as it will be firing the next cylinder is sequence. And because of that a subtraction of the latency value has the SA event occuring sooner, making up for lost time).

Now, if my math skills are still OK, here are some calc's that show by how much they adjust the timing. Plug firing or 90 deg crank rotation intervals:

8 cylinders at 1600 RPM:

(1600 rpm) / (60 SecPerMin) = 26.67 Hz (RPSeconds)

26.67 * (4 CylPerRev) = 106.67 plug firings per second

1 / 106.67 = 0.009375, or 9.375 msec between plug firings

9.375 / 90 deg of crank throw = 0.104167 msec per degree, or 104.167 usec


8 cylinders at 6400 RPM:

2.34375 msec between plug firings

2.34375 / 90 deg of crank throw = .02604 msec per degree, or 26.04 usec




From AXKT the lastest '88 - '92 TBI Firebird/Camaro BIN the SA latency at 1600 RPM is 305.2 usec's (This is a small cap distributor).

At 104 usec per degree of crank rotation this is a little under 3 degrees of timing.


Oddly enough the latency value in AXKT at 4800 RPM is also 305.2 usec (there are different values at other RPMs). That value will be used from 4800 RPM through max RPM.

With that at 6400 RPM wiath a latency of 305.2 usec is 11.7 degrees of SA (substantial).

RBob.

liquidh8

Ok, so if i went through and used this formula, i would get totally different values. And they are different in the EBL bin. Like @ 400 rpm it would be 208 usec. But in the bin it is 0. So is there some spark timing tuning going on here from the general? If the tbi engines used crappy swirl port heads, and the corvettes/camaro's with the tuned port motors used different heads, or the vettes using the aluminum heads, there seem to be different Sa latency tables for these engines. I guess what I am trying to get at, if the latency is to catch up for the delay in the rotation of the distributer sending the signal to the ECM, then I would think it would be the same for all the chevy's running the small cap distributer, v8's that is. Induction and head/cam combos shouldn't matter. But it appears this is the case.
Like if you are using a vortec head, or aftermarket head that has a better combustion chamber design. You could fool with these numbers a bit to get the timing to react different.

6375 137.34
6000 137.34
5600 137.34
5200 137.34
4800 137.34
4400 152.60
4000 152.60
3600 137.34
3200 167.86
2800 183.12
2400 137.34
2000 137.34
1600 106.82
1200 45.78
800 0.00
400 0.00
0 0.00

This is the ebl sa latency table, same a few 5.7/5.0 tbi tables I have. But the aujp table is different, the numbers are higher, and there isn't a dip at 3600 rpm. I wonder if that is from having an engine with a better cylinder head design, or camshaft or induction?




So if I am understaning this right, the latency value

would have the total timing of say 30d, occuring 11.7 degree of crank rotation sooner?

Or is it the ecm making it's calcs to come up with the final timing number, the subtracting the 11.7 degrees from that number, i.e, right before the latancy adjustment, we have a 100kpa/64oo rpm timing of 32 degree, then subtract 11.7, so the total timing is really 20.3?
I think you mean this one.


Thanks for your reply in trying shed some light on the purpose of this table for me. I appreciate it!!!

RBob

iTrader: (1)

Subtracting out the latency is done right before the hardware is programmed. In this case it is not reflected in the SA value we see. Put another way, what we see is SA in an angular measurement.

The latency adjustment to the SA is in time measurement. This is because as the RPM changes the time measurement changes for the same SA in degrees (angular measurement). The SA hardware in the ECM works on a time basis. It knows nothing about angular crank position.

The purpose of the table is to make the SA values we enter into the calibration match what is at the crankshaft. For an example take the table I posted above (from AXKT). At 6400 RPM there is a correction of 305.2 usec (11.7 deg).

Lets say that the timing in the ECM at 6400 RPM 90 KPa is 30 degees BTDC. This is also shown by the WUD as the ECMs current SA timing value.

We put a light on the damper and sure enough we see the light flashing at 30 BTDC. Cool.

Now zero out the latency table, all values set to zero.

At that same 6400 RPM 90 KPa, ECM timing of 30 BTDC, and now the timing light is showing 18.3 BTDC (30 - 11.7 latency). Not so cool.

Without the latency table the SA values in the calibration wouldn't mean much. To get 30 deg at 6400 RPM we would need to put 41.7 deg in the table. Not a good way to tune. And, with the latency varying by RPM, the SA table isn't going to look too good either.

Another aspect is the vehicle manufacturer. Engines are set up on a dyno fully instrumented. The SA is then mapped according to maximum best timing (peak pressure at 15 ATDC, no detonation, etc.).

This timing map is given to the driveability guys that do the final tunes. They use it as the maximum amount of timing allowed. In this case the value that is placed into the calibration needs to match what is found at the crank.

Without the latency table that can't be done.


Back to the 64 thousand dollar question: why does the small cap SBC distributor have different latency values then the small cap BBC distributor? Simply put there is something different in the distributors. Which piece is different, that I don't know. Could be the pickup coil, pole piece, reluctor, module, or the combination of them all.

RBob.

liquidh8

So, what do you think would happen if I put the TPI SA latency table in my EBL-TBI bin? since the usec from 3600 rom on up are higher in a aujp bin, something like 216usec (not on my craptop)? And there also isn't that dip in usec @ 3600 rpm. I understand you are saying that something has to be different in the BBC distributer, but when I still worked in a shop, even now if I called the supply house, the P/N for the distributer, and even the replacement pick-up coil and shaft with relucter wheel, is the same part.

In an case, I appreciate all your replies to this subject thus far. Ithas helped me understand it a little better.

RBob

iTrader: (1)

I know exactly what you are saying. A couple of years ago I beat my head on the wall over this. Then came to the conclusion that all of the small cap distributors I've been using have come from fully size Chevy vans. And using those latency values let my head heal.

But, I wonder the same as you on replacement distributor parts. The pick up coil for a V8 car is the same for a V6 car is the same for a truck. Don't know about modules, although I really have my doubts that there is a different one for different applications (within the same distributor family).

By changing the latency table values you will be changing the end spark timing. A larger latency will advance the timing and vice-versa.

On a closing note, I was going through a bunch of TBI BINs last night and found some patterns to the latency tables. I need to look at it again to track where they matched up.

RBob.

RBob

iTrader: (1)

Looked through the TBI BINs and noted the patterns between vehicles.

RBob.

liquidh8

I figured I would have some advance timing. For the heck of it today on the way home from work, I tried a bing with higher latency values, advancing the timing. I didn't "feel" any noticable difference. This is from one drive. I couldn't pull over and get a dead stop WOT run, but I have a log from the last bin, everything being the same except the SA latency table to compare too. Most likely monday on the way to work, I'll try and get a run to compare the other one too. But if we are only talking 1/2 degree, or 1 degree here in different rpms, I don't think I'll notice too much while driving around you know?

I too an using a truck distributer. But in the future I am picking up an MSD for it, for the time being I am upgrading to a petronix module. So this in part is leading to this "want to know" deal about the SA latency, along with just wanting to understand it.

RBob

iTrader: (1)

If you have a GM ignition module I would stay with that. As for the MSD distributor, try an email to them about the latency values. It would be interesting to see what they have to say about it.

As for the 1/2 to 1 degree difference, no, probably won't feel that. What you can do is to use a timing light and compare what the WUD reports for SA timing and what the light shows. Try this at different RPMs and see if the two continue to match up.

The higher the RPM the more critical the latency value becomes. Can do the timing light check at a standstill, no need for a high load, just RPM.


In the mean time I checked the TBI small cap latency values against the TPI small cap latency tables. The TPI tables match the 3rd gen cars and 7.4l BBC tables. So it seems that the SBC trucks are the ones with their own set of latency values. The pic's show the two sets of values used for the small cap. Note that this isn't all inclusive. There very well may be other small cap distributors with a different set of latency values.

The TPI BINS checked: AUJP, AXXC, AXXD (5.7l & 5.0l)

RBob.

va454ss

iTrader: (1)

Wow. I could understand a small difference, but that looks substantial.

RBob

iTrader: (1)

Yes, at 6,000 RPM it is 6 degrees of timing difference (between the two tables).

Makes me wonder if I did all of the calculations corrrectly. I've also been thinking of a way to bench test this. Will need a strobe to 'see' when the reluctor & pole piece line up. Then use that to trigger the scope.

Measure from then until the DRP signal activates for coil firing. Then also check for latency between the incoming EST signal and the C- signal. This will also leave all ECM latencies out of the picture.

RBob.

Cflick

Let me try and add a bit of help.....

An inductive timing light, if you eliminate the pick up and go directly to the trigger inputs, needs VERY little voltage to fire. They *will* fire on both rising, and falling edges at slow rates, so watch that. Polarity seems irrelevant.
The coil pickup leads are adequate for reliable firing.
You could take the pick-up coil leads directly to the inductive light trigger, and simply watch the reluctor vs pole piece relationship directly.
That will give you the mechanical/magnetic latency of that dissy. ( assuming your young eyes are better than mine, and you can plot the shift in rotational degrees, or you add a 12 inch diameter degree wheel in place of the rotor )
As speed increases, ( freq. increase, or time decrease ) the reactance of the pick-up coil will dominate. The field should ( if memory serves ) advance up to about 22 degrees or so relative to the real reluctor position at very high speeds. Either way, R and L are fixed by the windings of the pick up, so +j MUST increase with speed. How much depends on the L/R ratio, and thus the relative phase shift.
Once you've plotted that, you have your strobe against which you can calibrate all other latencies through and including spark plug wire, and plug gap by simply using the reluctor itself as your scope trigger "strobe."
Add a compressor and bottle, and you *could* even include latencies for in-cylinder compression effects.
At THAT point, latency of the timing light becomes dominant, and I think we're getting into splitting hairs.

May I also add that I'm grateful for this thread. Being able to log spark firings relative to DRP, I have noticed a discrepancy, but not worried much about it.
Methinks I'm about to compare the latency table for my dissy ( complete system, actually, which I have always had ) against what's in my EBL. Might reveal a thing or two.
I'll concede that the possibility my spark tables contain unknown compensations for wrong latencies doesn't exactly make my day, even though it's my own d&^% fault.

One or three degrees "off" should be pretty irrelevant, but 11 point anything is well beyond the window of acceptability, even considering the weather changing.

RBob

iTrader: (1)

Being the adventurous type I decided to check out the latency table values & calculations on a live engine. The test unit: '85 Camaro LG4 converted to TBI with 350 truck parts (TBI, distributor, harness, etc).

BINs: used 2 BINs, both had the main, extended, SA set to 12 deg BTDC. Most other SA's set to 0.

BIN1 has above SA mod and stock latency tables (for SBC truck).
BIN2 has above SA mod and latency tables with +8 deg SA

The second bin (BIN2) latency table was increased in value that equaled 8 degrees of timing at that particular RPM. The calculations were done as posted ealier in this thread.

Results show that for the most part the latency values are correct. The low RPM area is where the timing lagged by about 1/2 degree. Instead of showing 12 BTDC it was around 11.5 degrees BTDC.

Switching to the second BIN the timing moved to 20 BTDC. Which matches up with the calculations.


A note about RPM. This being a tired LG4 with 200K miles or so on it, at 4500 RPM the timing went a little goofy. It retarded about 5 degrees and bounced between that and 20. Then by 4800 it smoothed out again, with 5200 the highest RPM checked (and briefly at that!).

In hindsight the events at 4500 RPM were probably due to the valves floating. This poor motor has a lot of miles on it. Makes a bench setup sound like a better idea.

RBob.

Cflick

RBob,
The latency table you posted for cars exactly matches my stock AFCB 5.7L 4WD 3/4 ton Suburban.
Just an FYI....

RFmaster

Rbob

Sounds like couple of 2x4, plywood and screws are in order. Since dizzy turns are 1/2 of the crank speed 0 to 4000 RPM variable speed motor (aka drill) is in order. An old tachometer, 12 volt supply, and ECM with harness pigtail should complement the setup. Oscilloscope tied to DRP line and tach
line.

//RF

liquidh8

This is an interesting thread i must admit, but I'm glad others are chiming in on the discussion. I wish I had the equipment to try out the test RF was talking about. I may make bin with the tbi car tables in it. Righ now I have a bin with values in between the two. I'll have to post ti and the xdf so it can be seen by all. So RBob, you are saying that there was an 8degree SA difference from bin 1 to bin 2, with nothing but the SA latency tables changed??

The latecy tables may just be there for the different pick-up coild used in the distributer. i know the is a green plug and a white plug, if I remember right. I'll have to look in my gargae, and at a few distributer I have to see the difference. I don't know if one was for the in cap hei, old style, or if it was a choile for the 7/8 pin moudule Fi distributor.

RBob

iTrader: (1)

With some masking tape on the damper and a timing light you can do the same as I did. That will verify if the latency values are correct or not.

Note that you shouldn't be posting an EBL BIN & XDF on any site. Copyrighted and proprietary information and all that.

So RBob, you are saying that there was an 8 degree SA difference from bin 1 to bin 2, with nothing but the SA latency tables changed??

Yes, the only change I made was to add enough latency at each RPM point of the table to equal 8 degrees of SA. This was done to verify that my calculations are correct (as posted above). And to verify how the latency table worked. Which it did both of.

Here is a copy & paste from Excel for the values used. Note that RPM of 0 is set to a 1 (can't divide by 0), and the BIN value was set to 255 (can't go higher).
The values in the 'usec for 8 degrees' column were added to the value already in the table. The result was a shift of 8 degrees more SA across the RPM range.

RBob.

RFmaster

Yes, it is very interesting thread. I'll put together a block diagram and list of parts together. I have several other projects in the cue (sick parts getter that's occupying garage stall and must be finished first). I also have couple 7747 with a long harness pig tails and three different distributors. One standard small hat, one of the 91 B-body and full size CCC. This will take a bit of time, but it can be done.

//RF

RBob

iTrader: (1)

For a bench setup I would leave the ECM out of the picture. It complicates things too much. The latency is the time delay between the pole piece & reluctor line up, and the REF+ signal to the ECM.

It would be interesting to see if there is an additional delay between the EST signal and C- going high. If so then that too should be included in the latency value.

For that matter, should be able to loop the REF+ back to the distributor EST signal. Then measure from the pole/reluctor alignment to the C- going high.

For the BYPASS signal just pull it to +5 volts for enabling the EST to control the module.

RBob.

liquidh8

So, I think I'll do as you said, and make 0degree BTDC, with tape of a paint stick, take my timing light (i have a snap-on dial back) and sit the lap top on the fender there, the just rev it to the rpms in the latency table, and recond what i see and what the wud is displaying, if they are off, then I can adjust the table to make it match.

This could be a good test for those who use an other that stock distributer, i.e., MSD, accel, ect.

I wonder if the different tables were used in the trucks, just to keep a tap on the timing, figuring the truck would be under heavy loads some of the time? hmmmm, who knows. But I am gonna check just to make sure I am using the correct values. Maybe make a bin with less timing in it all over, like you did, and check it in park, again, as you did. Now the thing is, should it be kept as the general did it, flattening out the latency value at the upper rpms? I wonder what the latency tables look like in a later model engine, right before they got rid of the distributers?

RBob

iTrader: (1)

Need to have that RPM measured, otherwise I'd leave it just flatten. Gets sorta' hairy at the higher RPM range. I used some protective gear as the timing light had to be shot down behind the water pump. Straight down from above (from the fan, pully's, belts, etc.).

The LS1 CnP smart coils run a flat 22 usec latency at RPMs up to 8,000 on an eight cylinder. Not sure what the pickup has for latency times.

There is also masks out there that have a single value for latency. And both DIS & distributor equip'd vehicles are involved. Not too sure what to think of that. It may be that some ignition setups don't have much difference in latency vs RPM. And don't require correction for the entire RPM range.

RBob.

RFmaster

OK, just been very busy here in the salt mines. I'll am trying to get Infinium scope on temporary loan once setup is debugged. My old Tectonics O-scope is still good but taking CRT pictures with a camera is so 20th century. I'll update a bit later this week.

//RF

liquidh8

Due to the weather, and time, I haven't went any further with this yet. Though I wil be renting the hobby shop on base Monday to fix my exhuast leaks (may be causing some erratic knock counts), change the oil, lube, clean the K&N, rotate the tires, and a few other small things. While I'm at it, I'll try and get this test done, and post my results.

The_Punisher454

So what I gather from this discussion is that many of us (especially ones doing retrofits) may have quite a discrepancy between indicated (aldl) and real world timing due to the latency tables.
Perhaps a database of latency curves for various common distributors would be handy.

RBob

iTrader: (1)

Have some interesting information to add this this thread.

First item to note is that in the opening post of this thread, the table shown as for AUJP is not. It is the latency table used for AXCN & ANHT. Which are Corvette large cap distributors. The poster in the thread that liquidH8 linked switched the two tables in his post.

AUJP uses table 2 in the following:


I have been able to identify two different ignition modules for the small cap distributors. They are the 048 and the 369. Check out the picture of the modules. It is the first 3 digits.

I ran 2 each of the 048 & 369 along with a Holley module through some tests. This was done on a live engine using a marked balancer, timing light, and the What's Up Display to show the commanded SA timing and the engine RPM. The EBL Flash had 5 calibrations banks set up for testing.

Bank 0 was the stock calibration to start up the engine and let it settle in.

The other four banks all had the main & extended timing tables set to 20 BTDC. Temperature comp, PE, launch mode, and whatever else was zero'd out.

Then each of the four banks had different latency tables. Banks 1 & 2 had the two tables shown above. Then banks 3 & 4 had the same tables with the low RPM latency values of 0 filled in with the first value greater then 0 from above it.


The reason for this was that it was found that the at-crank timing dropped off with the stock table values of 0. The was still latency at the low RPM's that needed to be accounted for.

With this change running the RPM from 800 through 3200 had each module with the appropriate table holding a steady SA. Switch the table and the advance moved around as the RPM changed.

The Holley module (part # 891-103) matched table 2. This was from 800 through 3200 RPM. Did not test past this RPM.

You may be thinking that 3200 is a low RPM. However, look at the engines/vehicles the ignition modes are used in. None of these engines are high RPM. Trucks, Vans, TBI 3rd gens, Caprices, Wagons, even the TPI engines aren't high RPM engines. Note what happens in the following tests. . .



Another test: while checking the timing as the RPM was increased, both the 048 & 369 modules held steady until a particular RPM. This was with 048 & table 1, then 369 with table 2.

The 048 module, it held steady timing until about 3800 RPM where it retarded 4 degrees. Then by 5000 RPM the retard had increased to 6 degrees.

The 369 module, it held steady timing until about 3200 where it gained 2 degrees of advance. This held steady to 5000 RPM, which is the highest RPM it was checked at.


If we could get a sampling of the ignition module in various 3rd gen engines it would be helpful. Under the cap check the 3 digit number and post back. Along with whether it is TPI or TBI. If you know the stock BCC that too would be helpful.

Conjecture:

I know that the latest f-body TBI calibrations use table 2. This matches module 369. Same as the AUJP TPI calibration. I wonder if the 048 modules are for performance limiting, and are set up to limit the engine RPM. With the timing retarded, the power drops off, and most people either up-shift or lift.

Where the 369 modules (the ones I have are from 5.7l full size vans) are more performance oriented. What is interesting is that the 369 modules I have are newer then the 048's. Some even look to be replacements (too new for the age of the vehicle). Maybe a GM upgrade?

RBob.

Cflick

Interesting.
For those of us interested, more study is certainly needed.
My original 1987 module failed some time ago, and was replaced ( cheap ) with an Autozone Duralast version.
Taking a timing light, and zeroing out all the advance stuff I can find, or know about, and setting a fixed timing of 20 advance, I buzzed the engine at each RPM in the latency table and recorded the difference. ( to 5200 )
Then, using Rbob's previously posted usec per degree at RPM, adjusted only the latency table, until the light showed substantial agreement throughout the range.
I'm not terribly fond of doing this too many times, as standing next to a screaming big block with a steel fan, timing light in hand isn't exactly comforting.
If it picked JUST the wrong moment to throw a fan blade, or a belt, well.....
Doing it this way includes *all* latency factors anywhere in the system, not just the dist and module, so there may be other factors at work, or I did something wrong.
The latency/RPM I found was...

305.20 5200
305.20 4800
305.20 4400
366.24 4000
412.02 3600
473.06 3200
442.54 2800
534.10 2400
518.84 2000
518.84 1600
549.36 1200
625.66 800
625.66 400
0.00 0

I'm surprised at two things.
One, how much latency there appears to be.
Two, that it's SO high at *low* RPM.
Methinks my next step will be to rig up a photo-transistor thing and a known fixed reference on the damper, so I can do this NOT standing where I'm not real comfortable, or just motoring down the road.
Maybe I'll try and find an "original" GM module, and run that, too.
Stories about the reliability of the Autozone replacement make this sound like a good idea anyway.

RBob

iTrader: (1)

That module is showing quite a bit of latency. And as you mentioned even low in the RPM range the latency is high. The must be a lot of filtering in that particular module.

Once I get a P/S together for the PM servo motor I'll be able to do this without leaning over a fender. At least with this car it has an electric fan (my original testing was on a different vehicle).

I also got a chance to test drive the car after some changes:

Used the 369 module
Used latency table 2 extended down to 0 RPM

Just in normal driving around the engine was smoother. It also pulled out from a stop better. With a stick the RPMs dip as the clutch is let out. By changing the 0 latency values to the ones shown helped in this regard.

For higher RPM, some spark table changes will need to be made. With the 048 module showing retard, the 369 module will be adding timing back in. So that 32-34 degrees of WOT SA will become 40 - 42 degrees of WOT SA.

Unfortunately I didn't test the Holley module at higher RPM (3200 max). Which are what the current SA tables are set up for. So I don't know what the SA did above that RPM. I think there's a can of worms here someplace

I was going to do as cflick did and massage the latency table to match through out the range (using the 369 module). But after the module swap-a-thon it wasn't in the plans for that day.

RBob.

liquidh8

You guys have a much larger jump at this than I did. I don't even know what number module I have under the small cap dizzy. But I agree on the high RPM test being uncomfortable, lol. Anyway. I tried checking my latency tables with a light, but I have an old snap on dial back light, not sure I should use something like this. I also failed to make the timing the same all the way across the board. I was just trying to check to see if it was really what I had set in the bin.

Way to go guys on really diving into this. As RBob had noted, if we are using a latency table that is a little off, there very well could be some performance improvement in having a correct table.

liquidh8

Well, I pulled my cap and I have a standard brand replacement module in there. I figured since a few replacement seem to use the second table, 369, then I plugged those values in my bin to see where it gets me. I did notice that I keep creepiong up on the WOT timing and it gets better. I have vortec heads I was conservative on the timing, afraid to go over 36 total. I may have been getting 28 for all I know with the 048 module latency tables. hmmm. I'll have to check my spare parts and see if I have a 369 module laying around.

I guess the worst that would happemn is I would gain 2 degree or timing at WOT, who knows. I have to put the light on it and see where it gets me I guess.

junkcltr

iTrader: (1)

Might be of some interested for users doing 165 ECM to 730 ECM small cap conversions. Using the tables from post one it would seem that the MAF (large cap) distr. module is slower than the MAP small cap distr.

Since the eqn is:

TOTAL_SPK_ADV + MODULE_DELAY - LATENCY_TABLE = CRANK_SPRK_ADV

Then, using a MAF dizzy (large numbers) with the stock AUJP / MAP dizzy (small dizzy) with the stock AUJP latency tables will add in MORE spark advance.

Example (tables from post #1):
stock AUJP at 4000 RPM: 198 usec
stock MAF at 4000 RPM: 290 usec (real module expected delay)
Want 30_degrees advance at the crank

3.75 msec/ign
41.7usec/degree of crank rotation

30 + (290/41.7usec) - (198/41.7usec) = 30 + 6.95 - 4.75 = 32.21 degrees at the crank

The user wanted 30* advance at the crank, but by using a stock AUJP with a stock large cap dizzy they got 32.2* at the crank without realizing it. Not a big deal for N/A, but things like that add up when using boost. It gets worse or better depending on the module used.

EagleMark

Found this information on GM ignition modules. The numbers match up to your findings and it seems the difference was intended for performance.

Ronny

iTrader: (1)

I just swapped in ACDelco #19179581 ign module. I believe it to be D196A as I found a post to that effect.

would the latency table to be used be that of #2 in RBobs post? I am using stock EBL.bin currently which is the TBI FBody. I presume it is later model.

Quote refers to table 2.

RBob

iTrader: (1)

For a large cap Corvette ICM use the table that is in the very first post of this thread.

RBob.

morgsie

iTrader: (1)

FWIW, I recently bought a D1943A ICM module. There was no markings on it aside from the [GM] logo, so hopefully it is indeed a "396" module.

EagleMark

I hope it's a 369!

I'm wondering if the newer 16197427 TBI systems also have a differant EST/ICM? Looking at the latency tables in those PCMs are again differant from everything here and also differant from SB to BB...

Fast355

iTrader: (2)

Not to my knowledge. I have 3 of the 369 modules and 2 of them came from 1995 model G20 vans with the TBI 5.7, same place I got my 427 PCMs from.

antman89iroc

Ok, I just checked my distributor/SA latency. It's within a couple of degrees until about ~4000 then picks up 4-6 degrees. My EBL tables are;

Low RPM to high:

213.64
213.64
213.64
274.68
305.20
244.16
274.68
244.16
305.20
320.46
289.94
305.20
305.20
305.20
305.20
305.20
305.20

If I understand this table properly I should DECREASE the latency value to REDUCE SA at the balancer- correct? Larger value = more advance

antman89iroc

Ok, here's what I did. I set every SA table to 20 degrees. Then I checked the advance at the balancer and manually adjusted the latency table until I got 20 at all rpms.

Here's the table I ended up with;
Low rpm to high
213.64
213.64
213.64
244.16
228.90
228.90
228.90
213.64
183.12
152.60
167.86
183.12
228.90
244.16
244.16
244.16
228.90

It ended up somewhere between the various tables in the EBL programs.

I'm using a GM small cap distributor that came on my TPI engine with a replacement module and a MSD blaster 2 coil (round style) Do you think the coil could be causing the unique table?

Cflick

Sounds like a good way to me !

Some of us suspect that GM may have used some values to compensate for something else. The tables may not necessarily reflect purely latency.

The coil, no. Not in and of itself. A coil will cause up to a 90 degree phase shift in the current in the coil, depending on the L/R ratio of the wire in the winding, but that's a little too deep for what we're about here.
In any case, remember the objective !
That the spark programmed into the calibration, modified by other values also programmed in, is accurately reflected at the crank as intended.
Latency is really just an error correction. That it's unique to your engine, or mine, simply tells us that, as we might suspect, one size may be "close enough" but doesn't necessarily fit all. Ultimately, if the engine gets the spark timing it wants at that moment, we've done what we set out to do.

Wow ! I just noticed how old this thread is/was !

antman89iroc

Yeah at least I searched! Thanks for your reply. I was starting to think my AFR heads were fast burn or something cause it only liked 30' or so SA @ WOT. There are quite a few tables like this which don't readily come to mind when tuning (like fuel and timing) but really do make a difference when trying to get that last 10-20% out of a tune. This motor/EBL combo has about a year and a half and it's still evolving.

Cflick

Only a year and a half ?

benheinzeroth

Great work, I used this table on the EBL 3006.bin for my DUI distributor in my 88 'Vette 5.7L TPI motor w/h L98 ported heads and now get a much smoother running motor through all RPM ranges - Thanks to your due diligence and effort!

InTech

I was pointed to this post, great read, didn't know anyone else cared.

Here was my reply, just trying to help.
"One of the guys in that thread mentioned he set the timing table to 20 degrees everywhere and adjusted the latency from there. Good idea but not on a 7747 style computer as it has a slope per 1k rpm and rpm limit for the slope...of course all that is programable but still needs to be noted and paid attn to. I'm not sure how many ecm's have that slope but 7747 (42) for sure. The mefi 1,2 computers have the same thing.

Wow, just noticed I've been on this forum for almost 19 years.

-Carl

SbFormula

I know this is a very old thread, however the information is still valuable for any tuner. So to add to this thread here's a test I did today. It was necessary before I get on a chassis dyno to tune AFR and timing at WOT. This is very interesting findings since I was getting audible knock at 4400 rpm, WOT, SA 26* with 91 pump gas (no 93 in my neck of the woods). I had to bring SA down to 22* at 4400 rpm which seemed low. I believed there was something wrong with my set-up and was very concerned. Well in fact, it was 28* and it was knocking at 32*. KINDA good to know isn't it?

TESTING:

Car is Firebird Formula 1991
Engine is a 2019 GM performance SP383 Deluxe with balancer GM#12498008. It was factory TDCed and is believed to be accurate.
TPI First intake with ECM 7730
Initial timing was set at 12* (tune was adjusted properly)
Tune is custom using S_AUJP v6. It is a modification from OEM ATNX. It is set for SD system with manual transmission (T56).
Knock retard and Low Octane Knock Retard disabled
Distributor is original from 1991 OEM GM#1103479
ICM is original from 1991 GM#10482827 (D1943A, no 3 digit number written on it)
Cap, rotor, wires and plugs in excellent condition (MSD and Accel)

For testing:
SA main and extended tables were set at 29.9* across the board
SA correction based on CT was zeroed-out
SA for PE was zeroed-out
SA Latency Correction was set at OEM ATNX (came factory with original distributor and ICM)
Car had hood up, brought to operating CT of 195DegF
Timing light was used while monitoring RPM, MAP, SA-TDC on laptop and observing timing on balancer
Throttle was operated by hand
Multiple readings were done across the rpm range except at Idle where SA is controlled by other tables (SA is bouncing for idle rpm correction)

Using tunerpro RT, testing showed:
******* SEE POST #50 for corrected testing results********
Commanded SA-TDC was 29.9 at all engine RPM and MAP. Consistent with user input.
From 1200 to 3200 rpm, observed SA on balancer was 32* (+2*)
From 3200 to around 4300 rpm SA on balancer was 34* (+4*)
From 4400 to 5000 rpm SA on balancer was 36* (+6*)
From 5000 to 5500 rpm, SA on balancer was 32* to 34* bouncing back and forth, getting worse as 5500 rpm was reached (closer to 32*)
At 5500 rpm and over, SA on balancer was 32* to 30* bouncing back and forth

Differences between SA user input and SA observed at balancer can be explained by, but not limited to:
User's bad eyes while setting initial timing
User's bad eyes while reading the balancer
ICM bias
SA Latency table not properly factory set
ICM ineffectiveness over 5000 rpm

No engine nor any vehicle were hurt during this testing

InTech

Is that the stock latency table you posted or where you ended up?

SbFormula

SA Latency Correction was set at OEM ATNX (came factory with original distributor and ICM)

Yes factory with ATNX in 1991. I had to set it up in S_AUJPv6 though.

InTech

Thanks, it still makes me chuckle sheepishly remembering the time I was back east tuning and had the poor guy standing next to the blower belt holding the timing light while on the dyno and had to set/correct the latency table..

SbFormula

Hahaha
My poor neighbour had to endure the engine revving in the garage. I had a gas mask and ear protection on and it was still loud. the garage was like a subwoofer. The sound carried across the lake like crazy!!!
Neighbourhood is getting used to it since the last drivetrain vibration test on the hoist from early spring. That was loud too lol

SbFormula

Follow-up Post #45

I tuned the SA Latency Table today using the same set-up and RBob's formula at Post #3. First off my initial timing was off by 1.0 to 1.5* because of my bad eyes lol. That was corrected from 13.5* down to 12*. So here's the results corrected from post #45

Using tunerpro RT, testing showed:
Commanded SA-TDC was 29.9 at all engine RPM and MAP. Consistent with user input.
From 1200 to 3200 rpm, observed SA on balancer was 30* (+0*)
From 3200 to around 3600 rpm SA on balancer was 32* (+2*)
From 3600 to around 4000 rpm SA on balancer was 33* to 34* (+3* to 4*)
From just over 4000 to 5000 rpm SA on balancer was 36* (+6*)
From 5000 to 5500 rpm, SA on balancer was 30* to 32* bouncing back and forth, getting worse as 5500 rpm was reached (closer to 30*)
At 5500 rpm and over, SA on balancer was at 30* to 31* bouncing back and forth

I corrected SA_Latency table using RBob's formula and testing showed SA reading on balancer was right at 30* across RPM range (1500 rpm to around 5700 rpm).

Mission accomplished.

Thanks to Thirgen.org!!!