AMC V8 Timing, $42 ASDU, 7747?

[shimniok]

Trying to sanity check my TBI timing using the TSM as a baseline. However the max stock timing makes no sense to me.

I'm looking at my M.R.253, at the timing charts. They specify 16* static timing for high altitude (I'm in Denver).



Max timing @4400 on mechanical is 14-22*, and vacuum, 22-26*. With static timing of 16*, that means the max timing @4400 is between 52* (14+22+16) and 62* (20+26+16). Even at 4000 rpm, it's still between 50 (12+22+16) and 58 (18+24+16).

There's 45* between terminals on the distribtor. How the heck does this make any sense?!

[FSJ Guy]

Remember that if your BASE timing is 10* BTDC, your max total timing can be 52*, not 42* as with a Chevy motor which is at zero static timing.

I've always just timed mine by what the motor likes. I tried to figure out the OEM tables and couldn't make sense of it. I think they're of limited use since they're created with the limitations of mechanical distributor timing.

[Bill usn-1]

Base timing is all that is applied at idle unless you use manifold vacuum vice ported vacuum.

At WOT there is also no vacuum since the throttle is open there is no restriction so at WOT you have base + mech only.

Many....many....guys seem to thing their brand of engine is special, but the basic timing requirements remain the same for an old cast iron engine.

I have seen some really wild timing tables produced.

When applying timing tables to the ECM you have to keep in mind all the adder and subtractor tables like PE spark and spark slope.

A basic engine runs best about 32-34* at WOT. If you ever watch the car shows on the weekend power block you will note they adjust timing in 1* increments but are only watching WOT under the load of the dyno as the rpm rises.
So it has nothing to do with a DD running around town. But it does show you what your max WOT timing should be targeted to.


Your not reinventing the V8 engine. it still has to operate with the parameters of years ago with points, we just optimize the adv curve to match our engine.

So your not looking to have 50* timing in your table altho it has been used on some engines.

I think you will find the basic timing table I set up will be close to your AMC table once you figure out all the parameters your working with.

Best to look at the timing table and draw a line diagonally from the idle cell to the lower right corner.
On the right is accel/power and on the left is cruise/decal.

[shimniok]

I think you will find the basic timing table I set up will be close to your AMC table once you figure out all the parameters your working with.

I couldn't agree more. Your table was my starting point yesterday. I did not considering the PE Spark adder and Spark slope, among others.

However, my driving and tuning efforts resulted in... not a lot of changes from what you had.

Just a couple degrees here and there. And the truck's part throttle performance is much, much improved.

In fact, I suspect the only remaining issues pertain to the PE spark adder and spark slope so I will tackle that next. We shall see.

[FSJunkie]

I always warn people not to run too much timing, so I might as well do it again.

A common myth in the high performance world is to run as much timing as one can without ping, and call it optimized timing. Wrong. Early timing costs power, makes an engine run hot, and hammers on everything from the valves, to the rings, to the rod bearings.

I like the idea of 5-10* initial with centrifugal of 26-30 @ 4000 RPM. Do the math, and you're looking at 31-40 total. Add on 14-18* of vacuum advance at no load.

[mineral co]

I like the idea of 5-10* initial with centrifugal of 26-30 @ 4000 RPM. Do the math, and you're looking at 31-40 total. Add on 14-18* of vacuum advance at no load.

And things are right back up in the range that Shimniok posted from the TSM.

When I was considering the hot-rod MSD unit to do timing control, I came across a note that said that the unit had a maximum combined range of 30 degrees that it could control. Having also looked at the TSM's advance diagrams, I got concerned because it looks like the 360 wants a lot more variance than 30 degrees. So I called MSD.

To make the situation intolerable, they said that, in truth, their unit actually had a maximum combined range of 25 degrees! (And it took three different tech support people to reach this conclusion.) This unit is sold as a street/strip unit, and I know a lot of street cars that use a heck of a lot more than 25 degrees of range between centrifugal and vacuum advance. Get a clue MSD! It's just software, so change it!

Anyway, it is important to remember that, though the posts on the distributor cap are 45 degrees apart, that is 90 degrees at the crank. A dynamic advance range of 50 degrees is 25 degrees at the distributor, within the workable range to not have cross-firing between the adjacent terminals.

[Bill usn-1]

Correct!!
A lot of guys have a hard time understanding distr vs crank timing. Took me a while in the beginning to get used to it also.

There's lots of misconceptions that are repeated on the web.
While arc over is a concern, it's more important to have the proper phasing. just locking out the mech adv on your stock distr does not make it correct for EFI use.
And those big Ford caps on the distr are not really needed if the phasing and timing are correct. They were more for the .060" and higher plug gaps of the 70s and 80s and not for timing arc over.

And if you add the base + mech + vac adv you will find a lot of stock timing tables add up to 50* or more of total timing.
The biggest confusion for guys is WOT timing is much different than light cruise timing.

WOT will normally be 32-34* since it is base + mech adv only.
But add 14-16* of vac adv and your now near 50* under a light load cruise down the highway for those good MPGs.
Add EGR or highway mode and you can add a few more degrees of timing.

And your GM ECM is actually limited to 41* in the factory tune. So you can add you want in the timing table but if you don't modify the max timing scaler it will never go above that.

[FSJunkie]

While arc over is a concern, it's more important to have the proper phasing. just locking out the mech adv on your stock distr does not make it correct for EFI use.

YES! Thank you! I'm sure you are aware o how most people just stare at you like you are speaking a foreign language when you mention rotor phasing.

I'm not sure on the others, but on the Delco-Remy distributors the rotor phasing is not changed by centrifugal advance, only by vacuum advance. As the vacuum advance operates, it moves the trigger device (points, pick-off coil) independent of the rotor, shifting where the rotor is when the spark occurs. Most distributors are set up so the rotor tip is in-line with the cap terminal when the advance unit is halfway through it's travel, so it overshoots when fully out or in for the best compromise of rotor air gap.

Phasing being out not only can cause flash-over, but also weakens the spark and fries ignition coils and trigger coils, which is not good....

All those adjustable vacuum advance units people like to run? They have more advance built into them than the originals, so rotor gap is increased at full advance. Slotting the holes and shifting the unit solves this.

Some remanufactured distributors have point cams of the wrong rotation in them (among other problems) than REALLY screws phasing and rotor gap.

The best way to check phasing is to get an old distributor cap and chew a large (like 1 inch) hole by the #1 terminal and run the engine with a timing light pointed into the hole to see where the rotor is when the spark occurs. You can then shift the vacuum advance can in and out to adjust it, or better yet buy an adjustable rotor.

[Bill usn-1]

There are many things that some guys get away with during the conversion but just isn't right.

The rotor phasing is well documented in the FAQs on the binderplanet. And as noted, I keep a distr cap for each model of distr that I have drilled to ensure correct phasing during the conversion.


Another issue I dealt with a lot on the IFSJA board was guys wanting to try and use their jeep specs for base timing and apply it to the EFI when the GM system is setup for 0* base timing.

The misconception is that it helps how it runs when in reality it actually has no effect or can actually hurt the way it runs if other related parameters within the bin are not addressed at the same time.

I try to stress during a guys first intro to tuning, to only change what needs to be changed and not what you think needs to be changed especially if you really don't have a clue what other parts of the bin file may be affected by the change.