Timing issue

[LS1 TA]

On an L98 when you rev the motor should your timing advance or retard. Because on my car as I rev it up it retards the timing about 10 degrees and sometimes more. If this is suppose to happen any ideas of what the problem may be. It has a new knock sensor on it already.

[95TransAm]

I'm not familiar with an L98 motor, but I am familiar with how a knock sensor works. It will retard the timing if you have a knock coming from somewhere.

Hear any noises coming from your engine bay lately?

[LS1 TA]

No noises under the hood dude. Its a new motor barely has 4000 miles on it.

[Jeff_in_Atl]

I didn't think the L98 motor had a knock sensor.

The ignition should advance when revved. My 95 runs about 25 degrees advance at idle. Crack the throttle and it jumps to 40 degrees.

My 85 Mustang GT came from the factory with the vacuum to the distributor advance wrong. It's even on the under hood sticker wrong. I'm wondering if yours could be wrong too

[LS1 TA]

The knock sensor on the L98 is down by the starter. No vacuum advance line it's all electronic. 1989 was a crappy year, GM couldn't decide what they wanted to do, so the cars are half and half.

[Injuneer]

Remember.... the timing "map" has two axes.... engine load (MAP) and RPM. When you depress the accelerator suddenly, the MAP rises, and the ECM reads that as increased load. It then moves to that MAP value on the timing map that will typically reduce the timing advance compared to the "low load" value. So, your scan will show reduced advance (= "retard", but not really, just reduced advance). As the engine comes up to speed, and the RPM increases, you will now be on the same MAP ordinate, but you will see the timing starting to advance as the RPM comes up.

You need to look at both MAP and RPM to understand how the advance value is picked off the timing map.

[LS1 TA]

So is it doing what it's suppose to do by retarding the static timing?

[Injuneer]

Yes... its "reducing the advance".

I have the stock timing map (at least up to 4000rpm) for an LT1 in my online writeup on scanners:

http://members.aol.com/InjuneerZZ/ScanMast.htm

If you look at the timing map, you see that if you are just cruising at 2,000rpm, and the MAP is 40kPa, the spark advance it 43deg. If you press the throttle and open the blades, and MAP increases to 90kPa the timing advance is reduced to 24deg. A loss of 19deg advance from stepping on the throttle.

If you were at 4,000rpm the spark timing at 40kPa is 44deg advance. At 90kPa its 35deg advanced..... a reduction of 9deg advance when you step on the throttle.

[LS1 TA]

I thank you for your help.

[Jeff_in_Atl]

I think the statement "rev it up" needs to be more specific as to RPMs. I stick by my statement that the timing should advance when you "rev it up" vs the advance setting at idle. I don't understand why it would think there is a load on it just revving it up. I realize my LT1 is different than the L98 but thinking back to my 85, the Mallory dual points and Chrysler electronic ignition they all advance why revved. My 85 did when cold but not when warm due to the vacuum being wrong. I fixed it and it revved a lot better after both under load and not under load. If the ignition retards when you crack the throttle it would usually stumble. The L98 may not have been good enough to sense whether it was under load or not but if it retards just sitting revving it to 2,000- I don't know 5,000 I would say that is not right. It may be as designed by GM though.

[LS1 TA]

It doesn't stumble or sound different, the timing mark just moves backwards.

[Jeff_in_Atl]

It doesn't stumble or sound different, the timing mark just moves backwards.

I still say that it shouldn't do that. I am not saying that that wasn't the way GM designed it though.

[LS1 TA]

I still say that it shouldn't do that. I am not saying that that wasn't the way GM designed it though.

I'm thinking the same thing, I was just curious as to what was wrong or maybe, like you said, that's the way gm wanted it.

[Injuneer]

Jeff.....

Let's go back to the "good old days".... totally mechanical distributor. I'm old enough to remember those .

First you set the initial (static) advance by rotating the distributor on the mounts..... You put in 10 or 12deg advance.

There were two devices that added or subtracted timing. One was the centrifugal weights inside distributor. As RPM went up, the advance went up, because the weights moved the distributor baseplate relative to the drive shaft. The second device was the vacuum modulator. That either ADDED or SUBTRACTED advance, based on the vacuum in the intake manifold. This is the "load adjustment" to timing.

All that the modern distributors do is turn over part of the work to the computer. In the L98's it appears that you still set initial advance the "old way" - mechanically, and you rely on the ECM to interpret the timing map to adjust for RPM and engine load.

The LT1 goes one step further, and eliminates the mechanical "static" advance, and controls the whole mess through the timing tables.

You are correct..... for a fixed MAP level, as RPM goes up, advance goes up. But, if you just "blip" the throttle, as the throttle opens, MAP goes up (vacuum drops), and the timing table pulls timing out. Then as the RPM goes up, the MAP will start to drop (vacuum increases) and the timing will be put back in, and even more added as RPM increases. But the initial reaction to the disappearance of vacuum is for the advance to be reduced.

The ECM assumes that intake manifold vacuum is a measure of "engine load". It uses MAP for things like the fuel map, timing map, and power enrichment, etc. When you open the throttle, MAP increases (vacuum drops). Doesn't matter whether you are standing still, or on the highway. Throttle opens, MAP increases. It will not stay increased for long if there really is no load on the engine, but the initial reaction, until RPM starts to build will be less advance.

As I said, to trace the path of the advance you will see in a computer controlled system, you need to have the timing map in front of you and look up each combination of RPM and MAP to deteremine what the ECM is calling for.

[Jeff_in_Atl]

..... The second device was the vacuum modulator. That either ADDED or SUBTRACTED advance, based on the vacuum in the intake manifold. This is the "load adjustment" to timing.

I don't agree.

I remember the good old days too. I also remember plugging the vacuum modulator up to the timing port on the Holley carburetors (I've never used anything else). I've used an 1850, 3310, 700 cfm double pump and the Holley 6 pack on a Dodge. All of them had a vacuum port on the side of the metering plate which had 0 vacuum at idle and the vacuum increased as the throttle plates opened. When the vacuum modulator on the distributor is plugged in to the vacuum port on the carb, when you open the throttle the vacuum increases and pulls the modulator adding advance to the timing. It will not pull the modulator fully at WOT though.

What you are describing is what would happen if you plugged the vacuum modulator to manifold vacuum which decreases as the throttle plates are open. That causes the engine to stumble when the throttle is abruptly open.

For more info;
Holley used to have a page on their site talking about how people plug the distributor in the wrong port but I can't find it.
http://www.tpub.com/content/construc...s/14273_67.htm
http://ncfto.org/Articles/manvport.htm
http://www.tpub.com/basae/46.htm
http://www.junkyardgenius.com/holley/tune01.html
"Ported Spark Vacuum. (The 4th biggest mistake made)
There is a vacuum port on the upper left side (as you stand in front of the vehicle looking in the engine bay) of the primary metering block.
This is ported spark vacuum.
It is drawn from the Venturi bore, not the intake manifold or carb base plate.
This is where your vacuum advance for the distributor should be attached."

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. A I said, it may work as GM designed it but I still think it is wrong whether by design or by a part failure.