Timing issue

[Injuneer]

I don't understand how the computer controlled systems work with the MAP sensor getting it's vacuum reading from manifold vacuum not venturi vacuum.

Look at the timing map. The computer reads the MAP sensor, and adjusts the timing according to the reading. MAP is the opposite of vacuum in simple terms. If you are looking at the MAP value on the timing map, as MAP increases, advance decreases. As vacuum disappears, it pulls timing. That makes sense. At cruise, you have full engine vacuum.... 20-21"Hg. The timing for the LT1 goes up to 46deg advance under those conditions.

You want a lot of timing advance at low loads for fuel efficiency. But when you open the throttle, increasing the load on the engine, you want less timing. If you kept it at 46deg advance under full load, you'd have a lot of detonation. Not sure why you would increase advance under load.

I gave two examples of the actual LT1 timing map....

2000rpm:

35kPa (20"Hg vacuum) = low load cruise
Advance = 43deg

Open the throttle, and MAP increases. Let's assume it goes to:
45kPa - Advance goes to 41deg
55kPa - 39deg
65kPa - 37deg
75kPa - 31deg
85kPa - 25deg
95kPa (2"Hg vacuum) - 23deg

In effect, as you open the throttle more, the PCM pulls out advance.

Now, as the RPM increases, it puts the timing back in....

For a constant 85kPa:

2000rpm - 25deg
2200rpm - 28deg
2400rpm - 32deg
2800rpm - 35deg
3200rpm - 35deg
3600rpm - 37deg
4000rpm - 37deg
....and so on.

Based on the table, as you open the throttle, and before RPM starts to build, it will reduce the advance by as much as 20deg. Then, as RPM comes up, it will put the advance back in, in proportion to RPM, but if you are watching the timing mark on the damper, as soon as you open the throttle, timing advance is reduced.

My analogy with the older full mechanical distributors was "simplified" to avoid the complication of the ported systems, but the end result is the same.

[Jeff_in_Atl]

QUOTE=Injuneer]My analogy with the older full mechanical distributors was "simplified" to avoid the complication of the ported systems, but the end result is the same.[/QUOTE]
I disagree. It's not the same. The results do not act the same. I have never seen that on a full mechanical distributor unless the vacuum modulator is plugged into manifold vacuum instead of the ported vacuum. This was a common mistake that people made on a fully mechanical distributor. I wish I could find that page on the Holley web site that talks about this.

I've never tried to see it on the crank damper on the LT1 but I don't see it on the scanner. The scanner may not be reacting fast enough to see it though.

I hope I'm not irritating you here and I truly respect your knowledge with cars. But I don't agree. I've got many hours working the mechanical distributors and watching the advance curves with a timing light.

When I say I don't understand the fuel injection, I'm not looking for an explanation. I have seen and read yours many times and I believe it. You don't have to explain it any more. I got it.

By your explanation of how the fuel injection works, the L98 is doing what it is programmed to do. I see that. I don't agree that it is the correct way to do it. If I were the designer/engineer I would have put a MAP sensor where it could get ported vacuum and use that for the spark advance table along with the manifold MAP sensor to detect load to back off the advance to stop detonation. That would prevent the advance from the blip drop which does not help the way it runs and could cause a hesitation and it would give a truer conditions to the ECM as to load.

But when you open the throttle, increasing the load on the engine, you want less timing. If you kept it at 46deg advance under full load, you'd have a lot of detonation. Not sure why you would increase advance under load.

It's a little different with the computer controlled ignition vs. a mechanical ignition. It appears that the car manufacturers figured out that one thing they could do is increase timing to increase gas mileage. The computer engine runs a lot of timing anyway.

On the mechanical distributor, you'd dial in an initial timing of whatever 6 degrees on a slant 6 (LOL no comments allowed there) to 10-12 degrees on a SBC (small block Chrysler LOL) like you said. If you did not plug the vacuum advance on the distributor to the ported vacuum on the carb, when you cracked the throttle the engine would stumble. There was no timing advance except the spring weights. When it was plugged into the ported vacuum when the throttle was cracked vacuum was applied to the modulator and the advance would go up to like 30-35. This would stop the engine from stumbling. The crazy thing was the instructions said to unplug the advance from the vacuum, set the initial timing and plug it back in. On my Mustang the vacuum was plugged into manifold vacuum. You'd unplug the vacuum, set the initial timing at like 10 degrees, then plug the vacuum in and the timing would go up to like 30 (I don't remember the exact number). When you cracked the throttle, the vacuum would go away causing the ignition to retard down to the initial setting which is exactly what you describe the computer does. This kept causing a stumble whenever the throttle was opened. I re-routed the vacuum line to get ported vacuum (The way Holley describes) and it stopped the stumble. I also dialed in another 10 degrees of initial timing which up to this very second never dawned on me why my car seemed faster than other Mustang GTs. Hmmmm. Maybe that's why it seamed the factory fluke running 14s. That also explains why it would jump my Camaro by 2 car lengths in 1st gear. It never spark knocked either. I'm getting side tracked here.

Under load you don't want the timing because of detonation but you want the advance when the throttle is cracked to keep the engine from stumbling. Back in the old days you needed that kick of advance to keep the engine from stumbling. Maybe the computer controlled ignition having so much initial timing that it is enough to keep that from happening. The computer does a good job in taking out timing at WOT to prevent detonation which is a full load condition but it's the partial load condition where I believe that it is taking out too much timing at the throttle opening condition. Anyway this is why I think the design of the fuel injection is flawed.

[Jeff_in_Atl]

Through this conversation I have learned something that I didn't understand before. I never could understand why Ford plugged the distributor advance up to the manifold vacuum when Holley provides a port on the side of the carb labeled "timing port" and the instructions say to plug that into the distributor just like all those links I sent earlier said. Ford was using the modulator as a load management tool to back the timing off to prevent detonation. I think Holley saw the stumble that this produced and thought it would be better to plug the advance diaphragm into ported vacuum. (The difference in theory world and real life world.) That caused a loss of initial timing because with the Ford design it would run like 30 degrees of timing at idle. I think I carried it the last step by accident dialing in more initial timing. Your explanation of how the MAP sensor works perfectly describes what Ford did. I still think that is not the right way to do it but at least it explains it.

[LS1 TA]

Come to find out this weekend, in 1989 the L98 didn't have a MAP sensor. They didn't start using a MAP sensor until 1990. It does however have an electronic spark control module that reads the two knock sensors and adjusts the timing. I think that's where my problem may exist.