msm z28

I have a potent 406 with a 730 ecm and and large solid roller cam 244/248 with .680 lift and 10.25 compression ratio that I rev to 7000 rpm. I'm using a GM 1103952 small cap distributor with a factory 369 module and correct latency values in tuner pro.

I'd like to hear some thoughts on where to set the initial base timing.. I always match the initial timing (Distributor setting) in Tuner Pro to the mechanical setting of the distributor in the car at 1000 rpm idle. My WOT timing is at 34 degrees total above 3800 rpm.

I've experimented with 10, 12 and 15 degrees initial and let the ecm adjust the rest of my timing map. Now I'm wanting to try 20 degrees initial and let the ecm adjust the rest.

My thinking is at higher rpms (above 5500) the higher the setting for initial timing the ecm doesn't need to adjust the curve as much to achieve the total of 34 degrees.

WOT timing curve example would be 20 initial + 14 ecm =34 degrees
vs 15 initial + 19 ecm = 34 degrees.

I'm thinking old school rotor phasing and getting the best spark timing for high rpm power.

As a side note... Are the latency values for the 730 ecm kind of like rotor phasing for crank trigger race cars?

I'd love to hear what everyone has experimented with on initial timing settings and the benefits.

Thanks for the help

Orr89RocZ

iTrader: (20)

I never felt it matters. I always gave the minimum required to keep motor idling when est was disconnected.

I thought the ecm started at initial advance for cranking and then referenced other startup advance tables to get starting timing. Most big cams seem to need 12-15 initial to have a chance at idling with est disconnected.

RBob

iTrader: (1)

> gave the minimum required to keep motor idling when est was disconnected.

Orr89RocZ, that is a decent idea. Between that and the following the base timing will be optimum.

I usually go by how the engine cranks and starts, as the timing is at the base until the engine RPM is over 400. On a cold engine it should start within a reasonable amount of time. If it just windmills then more base timing is needed.

Then on a hot heat soaked restart the engine should still spin easily. If it kicks back then less base timing is required.

Note that the ECM fires the injectors according to the base setting. Not important on a batch fire setup, but will affect an SFI setup.

The base setting also affects the minimum and maximum at-crank timing. The small and large cap V8 distributors can be programmed for -3.8* from base to +42* from base.

What this means is that with 20* base, the minimum at-crank SA is 16.2* (20 - 3.8*). It can not go any lower. On the other end of the scale the maximum at-crank timing is 62* BTDC.

So with the base timing at 20* BTDC the range of at-crank SA is from 16.2* to 62*. If that is OK and the engine doesn't kick back on cranking, then can use the 20* base setting.

As far as whether this is good for rotor to cap phasing, I don't know. I've never checked the phasing versus the programmed SA.

> Are the latency values for the 730 ecm kind of like rotor phasing
> for crank trigger race cars?

Not, the latency settings in the calibration make up for the envelope delay regarding the ICM filtering. See:

https://www.thirdgen.org/forums/diy-...ncy-table.html

RBob.

msm z28

RBob,
Thanks, That will help me in my quest for the best rotor to cap phasing and best power. I'll keep in mind the -3.8 to +42 adjustment range in my adjustments.
I looked at my timing table and saw that what the lowest spot was and set the base to be close to that.

So if I understand correctly, as long as my timing wont need to be lower than 16.2 at any given time and cranks over ok when hot, I might be ok.

I plan to put a hole in a distributor cap for viewing the rotor to cap phasing differences for 10, 15 and 20 degrees base initial timing while keeping 34 degrees timing for the whole timing map and see the phasing difference between the 3. I'll report my findings here. I'm curious if anyone's done it before.

Thanks,
Sal

RBob

iTrader: (1)

If you can get a clear distributor cap that can be used to check the phasing. Did this way back with points distributors when I removed the vacuum advance can. Needed to fix the breaker plate in place at the correct location for proper rotor/cap phasing.

RBob.

JP86SS

iTrader: (1)

I have good results at 12* base with a similar setup but with CDI.
You should get an MSD or other multi-spark box as the idle quality will improve dramatically and you won't foul the plugs with a ECM error in limp mode with larger injectors (I have 30# @ 50 psi)
Don't know if there is a clear cap available for the small cap setup but I don't know how useful that is. Last time I had one (into the wayback machine...) I saw nothing but a round arc inside there. Pretty to look at but didn't know what I was looking for at the time.

The_Punisher454

I would think that with a timing light you should be able to see the rotor position when it fires.
Personally I'd just use an old cap and cut a window in it with a dremel or something similar.

RBob

iTrader: (1)

I always used the clear distributor cap on the bench, not a running engine. Easy to use an ohm-meter to see when the points opened, and then look at the rotor to cap phasing. Could also use a battery and light bulb across the points.

With a centrifugal only distributor the rotor/cap phasing doesn't change as the timing changes as the point cam moves with the rotor.

As for a CC distributor it gets a little tricky. Need to spin the distributor fast enough to create a DRP, and also need to see the phasing. Could get close by observing the star wheel to reluctor timing then checking the rotor/cap phasing.

But if what The_Punisher mentioned about using a timing light on a running engine works, which it likely does, that would be the best way to go with a CC distributor.

Also, even with a standard non-clear cap, can examine the cap terminals and rotor tip for where the spark is jumping from. That shows the overall cap/rotor phasing.

RBob.

The_Punisher454

I have a buick 350 in my sons car that I will soon be upgrading to a '427 computer and TBI. I plan to modify the buick HEI by locking out the advance and adding a 7-pin module. I will be cutting a large window in the cap and checking rotor phasing with a timing light for sure, since I have no doubt it will probably be way off at first. My plan is to use the vac advance pull rod modified with a threaded end and jam nuts to lock it in place where I want it.
I will try to remember to shoot a video of the process. ETA Jan or feb 2017 (I got way too many projects right now).

JP86SS

iTrader: (1)

So you guys think that the ICM compensation/latency values could be that far off that it requires a phasing check?
I always thought of this as something you wanted to minimize the "jump" that the spark had to do between the rotor and the cap. With higher initial advance there is less angle at higher SA values. Finding a happy medium was all I thought was done.
Curious if I've missed something of value.

The_Punisher454

ok, but say just as an example, this buick 350 I'm converting to EFI. The distributor has mechanical advance and vacuum advance hardware that will have to locked out at some angle. Checking the rotor phasing and making adjustments for a good compromise setting will be needed.

Ok now what about the people that use varying different amounts of base timing. Those instances may sometimes end up a little less than ideal. For most stockish tpi/tbi setups its probably totally unnecessary. But i know that for some of my upcoming custom projects it could be critical.

just my take on it.

gbettner

iTrader: (2)

This is a good read for me, thanks guys. Most of it is over my head but one part helps a bunch. I'm unsure of my base timing. I do know that it starts good cold but struggles / jerky / won't hardly crank after heat soaked. After reading this thread I'll clock my dizzy until I get good cranking when hot.
Thanks fellers!