Timing questions

[srobertsfsj]

I am thinking it would be fun and challenging to build my own timing table and see if it would work with my TBI install.

Looking at this stock 360 chart, I had two questions.



One, if my Jeep idles at about 11-12Hg (5200ft), is this chart telling me that around 13Hg vacuum advance is all in?

Second, I understand that with the TBI, base timing has no effect on the main spark advance table but in the context of translating from mech/vacuum timing to TBI, do these charts take into account base timing? So if I have my base timing set to 10*, lets say at 2000 rpm, my total timing should be around 42*???? base timing + mech + vacuum? 10 + 8 + 24 = 42?

Am I going down a rabbit hole here and wasting my time?

[babywag]

I have some information regarding this...

I had the same exact thought, and even started a thread on another forum.
Another member on here contacted me, and sent me the work he did on this.
Shoot me an email and I can forward what he sent me.
It's in spreadsheet form.

EDIT: Those charts won't reflect altitude changes.
Jeep issued a TSB regarding advancing the timing for high altitude operation.
I would not consider it wasting time, knowledge is never a waste of time IMHO.

[Bill USN-1]

You need to understand the difference between manifold vacuum and ported vacuum and how it applies to the timing table.

The other forum referred to was binderplanet.
IIRC you have already been there and viewed the AMC timing table thread.

Bmalta applied the same tables to come up with his version of the basic timing table. We spent many hours on the phone helping him understand the differences.


As you pointed out, if you use manifold vacuum as a reference for your table then you will have base timing + full vacuum adv at idle.

so if you run 10* BT + 15* VA = 25* @ idle.
Most engines will now fail emissions testing due to high HC with that much timing.

Ported vac ref has 0 in/hg at idle so you will only have base timing at idle.

Altitude is compensated for by the map sensor.
If additional timing is needed at high altitude, the distr can simply be rotated about 1* for each 1,000ft altitude increase.

[FSJ Guy]

Altitude is compensated for by the map sensor.

Does the ECM adjust TIMING according to altitude, too?? Or just fuel?

[srobertsfsj]

Ported vac ref has 0 in/hg at idle so you will only have base timing at idle.

That makes sense now, I see what I have been missing. So if I was running manifold vac to my dizzy it would be much further advanced at idle.

[Bill USN-1]

If you think about the timing table it is map/rpm.
If set up correctly, at WOT/100map the timing is less then at 80 or 90 map.
At altitude you will never hit 100map due to the atmospheric pressure.
So your WOT is now going to be 80-90 map.
So it is actually adjusting the timing.
Something that can be done is to adjust the map setting lower for PE timing to kick in to better match the engine and the WOT map.

But this still leaves the lower RPM timing to compensate for.

I have wheeled from sea level to the 14rs in Co.
What i normally do is as suggested and just rotate the distr when I get to base camp at altitude. That way when i return to sea level i just turn it back down.
If you live at altitude and never drive down to sea level then the timing just stays up.

[Bill USN-1]

That makes sense now, I see what I have been missing. So if I was running manifold vac to my dizzy it would be much further advanced at idle.

Yes, the vacuum adv will be fully applied at idle and then decrease as you hit the gas. At WOT it will still drop to 0*.
ported is 0* at idle and at WOT and increases at part throttle.
That's what the vacuum adv is for is to help driveability around town at part throttle.

A race engine or boat engine never uses a vac adv since the engine is always under a load and there is no vacuum.

[mud89]

For some years according to the jeep TSM, to improve warmup driveability, when the engine is cold it's the manifold vacuum which is applied to the distributor advance unit, and then the ported vacuum when the engine is warm, thanks to a CTO switch (Coolant Temperature Override)

[Bill USN-1]

Not needed with EFI.
it's all in the programming already.