That I do not know Greg, but I bet it does. Things that make you go...Mmmm.
I just received a response from Pertronix to my question on: Is there a place I can contact for more info on the latency values since you do not have them? The response: Sorry no one to contact. I thought as much and wasn't too hopeful on a good response. The HUNT continues...

 

Ignition modules are not specific to whether the car is CA or not. There is no carb number on this part.

 

Could you not simply run this on a bench with a scope to figure out the values?

 

I’ve been thinking about how you could set this up and get reliable and consistent results.

 

8 spark testers, a good battery, drill press and a tach. Since you only want the module values this should work as it'll operate as if the est wire is out meaning it is only the module doing the work no?

 

My '84 FSM explains the operation of the ICM in the distributor (pg. 8A-18). The ICM has nothing to do with the ignition curve. It effectively only signals the ECM of the timing that is set by the user (which is recommended to be 6* BTDC). The ECM receives the inputs from the various sensors, including the ICM, and process the signal in accordance with the ignition curve that's programmed into the ECM. If the ICM is tested by itself, that would be the same as testing the advance at the harmonic damper at various engine speeds with the EST connector unplugged. As the diagram on pg. 8A-17 shows, with the EST connector (C474) unplugged, the pulse from the pick-up coil in the distributor bypasses the ECM and is routed directly to the ignition coil thru the ICM internal "electronic switch B".

 

There are some posts on the thirdgen i read through a year or two ago where rbob and some others did their best to bench test the modules.
at one time they also got some generic info about the dui module, stating it was very close to the vette module, but with their own super secret curve.

 

Hey Roy, did you happen to read the thread that I posted above? It does affect the timing on what is commanded and what it actually is in relation to the settings in the Latency table. Huge difference between small cap latency and large cap.

 

Yes I did read your post, Buccaneer, but it didn't seem to make sense to me. I'm trying to understand the '84 FSM pg 8A-18. Isn't the Latency Table in the ECM? It's my understanding that when the EST connector is open (and during "engine start"), the amplified pulse from the pick-up coil goes directly to the ignition coil primary, so any signal conditioning in the ECM has no effect on the ignition timing. I may be confused on the "small cap vs. large cap" latency. Is that explained in the FSM?

 

Does seem weird that they would put different latency in different modules.
maybe it gives mechanics / tsb's the ability to change the timing curve with a module swap vs a reprogram?

Could always check with rbob for his thoughts, probably some engineering reason, or even to keep certain things private?

 

Okay, I did some more reading in the referenced 3rd Gen info. It's obvious that I'm out of my element, here. This thread started out with a very simple question, and has evolved into custom ignition tuning! Waaay off topic!

The Latency tables are a major confusion by themselves. If I plot RBob's tables, the curves don't look anything like a beneficial ignition timing curve. I'm not into custom ignition timing, so I'm done with this.

 

I'm interested in what work has been done in measuring the actual latency in these modules. Where are you getting that information?

 

Hey Roy, I hear ya. This whole subject of latency gets real mumbo jumbo after a while and quite frankly, it hurts my brain after reading everything. There is a big difference between the two HEI caps as far as even the basic values go, but it gets better/worse once they start changing the values to accommodate various vehicles. The corvette values are much different than say a 305 Camaro and all has to do with timing (SA). I really didn't pay a whole lot of attention to this subject until after I had talked with Bob over at Dynamic EFI and then the subject peeked my interest and I found out that I was using the small cap values with a large cap HEI. WOW! what a real feel the difference it made when I got it closer to what it really needed to be. Now, on the flip side of that, DUI won't give out the info on their module and it also appears that Pertronix either can't or won't either.

If you look at some of the part numbers that Bob used to verify the values, they do vary in performance. Why is that? I'm not entirely sure, but I'm willing to bet it has a lot to do with the model of vehicle the module is in and the performance they want to give that vehicle. The SA is different depending on which vehicle like say for example a truck, which may tow vs. a corvette at WOT. That curve will be much different just like the VE tables. Bob is a very sharp guy if you ever talk with him and has some great ideas and has experimented with a lot of different things over the years, this subject was one of them.

I guess one of these days I will sit down and go through mine and be more granular and see how far off my DUI is from what I now have set in the latency table in the tune. I know it is fairly close, but not perfect and for now, I'm OK with that. If someone has a DUI module and decides to experiment and see what the values really are, post em up for everyone to see. I'm willing to bet that they are not too far from the corvette values, but who knows? The mystery continues...

 

Talk with Bob at Dynamic EFI, he has done a lot with this subject.

 

From what I recall, the actual differences observed with a timing light amount to degrees above a certain rpm. Iirc that was discussed in one of the third gen threads. I know swapping the large cap values on my tune made a massive difference in response down low. The thing went from drivable to borderline roasting the tires on pulls if you didn't roll into the throttle. I tried to understand and realized that I'm using stock parts so the stock values should be good enough on the curve. It is beyond what I know as far as electricity and the such... my knowledge there is a skim at best.

 

So, if you had a variety of ICM's, and swapped them out at the drag strip, and set your initial timing at the same point before each test, you'd see different performances on each run? There's a job for our good friends 383vette or 69427!

 

HRR, I think you've correctly summarized what appears to be the consensus of some. I'll confess I'm not on board with that conclusion.

Latency is an undesirable, but unavoidable thing, and excess/additional amounts are not designed into ignition modules. There will be slight variations between similar part number modules (due to component and manufacturing tolerances), but the differential effects should be small. I can think of no engineering reason why noticeable/significant performance differences would occur just by swapping modules.

I've asked a former colleague to dig through our old data sheets to give me some actual engineering information on a couple of the (ancient) ignition modules in question. Keep your fingers crossed.

 

I would love to see the data on all that as long as it pertained to GM HEI large cap. If it wasn't a big deal, it makes you wonder why module manufacturers are keeping the settings for the ECM tune a secret since it is apparent that the do vary in value.