To make a long story short, I had to make up a new timing curve since Im really lazy about backing up old, archived data on my PC and we all know how murphy is. Hard disk crashes, data go bye-bye

Anyway, I wanted to get some opinions on this timing table that I made up. I know some of you guys have set up zillions of old school distributers so you have a good idea of what kind of timing works well for starters. Im usually too conservative so Id like to know what everyone thinks.

The cars specs are:

-350
-Crappy smog heads with 8.5:1 compression
-stock-type cam listed in my sig
-Performer dual plane intake
-headers + restrictive exaust

Its metric so:

30 kpa = 21 inHg of vacuum
50 kpa = 15 inHg of vacuum
70 kpa = 10 inHg of vacuum
90 kpa = 3 inHg of vacuum
100 kpa = atmospheric pressure

Mainly what Im looking for opinions on is the advance at idle, cruise, and WOT. Do I have too much, not enough? Normally id test it in the car but since the car is in pieces, Im stuck being a bench tuner.

:lala:

Heres the tables. The small table is the one my motor idles off of, the larger table is the one used while the car is in motion. All timing showed is the actual engines spark advance, in degrees, of course.

 

sorry about the size. Didnt realize it would be friggin huge when I posted it.

 

If I had more time I'd be able to give a more thorough reply, I have to get to wk tho.

Mine's 12* - 14* @ idle, with the tube connected to full manifold vac.
Some say you shouldn't do that, some do anyhow... I do it that way cause I've tried it both ways and it seems to work best like that.
At about 6"- 8" of vac or less the vac can will be fully retarded.
At 15" of vac or more the vac can will be fully advanced.
The bottom line for spark curve is how it works at WOT ... IMO. From there you add in more timing advance to suit your needs at part throttle. I've fiddled with mine a bit and what I've concluded at this point is that a decent compromise is what to hope for.
About 36* @ 3000rpm @ 0" vac


I'd have to look a few things up to tell you the rest, I'm sorta late for work now.

 

I think you're on the right track as far as trying to emulate an old springs-n-weights distributor curve as a starting point.

Do this an ADDITIVE process, putting these two curves on top of eachother to create your final advance table....

Centrifugal advance: Start at about 12* initial and hold that constant up to about 1000 RPMs so that it isn't fluctuating at low RPMs and maybe causing a "rolling" idle. Ramp it up to 32* by 3000 RPMs. This is not linear, but rather advancing quickly at lower RPMs and then kinda flattening out the higher you go. Kind of a curve, rather than a stright line. Something like this.....

1000 - 12*
1500 - 20*
2000 - 26*
2500 - 30*
3000 - 32*
3000+ stay at a constant 32* all the way up

You'll have to fill in the rest of the points along the curve (a piece of graph paper with the curve drawn on it and mapped out precisely can really help you visualize what you're doing). That's a little conservative for a low compression motor and I'm sure you could bump it up to 34-36* maximum when you get to the actual power-tuning of the combo but this should be a reasonable starting point. If you're the nervous type, for get about the curve and just do a straight-line up from 1000 to 3000.

The "vacuum advance" part of the curve should probably look something like this......

Start the advance curve at about 3" of vacuum. End it at about 13" of vacuum. You'll want it to give about 14* of advance up at 13" vacuum and 0* down at 3" or below. This one CAN be a linear curve so it's just a straight-line plot on your graph paper to find all the in-between points.

You'll notice that with the 2 curves added to eachother you can get as much as 46* of maximum timing at higher RPMs and high manifold vacuum (light load). This is as it should be. Most SBCs will give best WOT power at about 32-36* total timing and best fuel economy with about 45-50* of total timing at light loads. That's about what you'll get if you add the 2 curves together, above.

 

Back...

Damon said it pretty good.

I might add. If you want to get fanatical about your centrifugal/vac timing curve, a good device to have is http://www.msdignition.com/tc_images/pn8680.jpg It's used with the 6-A box and it lets you vary the timing from the drivers seat +or- 7* That way you can see what's really going on, and make compensations for changes in weather, altitude, grade if nessecary.

 

As far as compensation goes, this is on my cars ecm so as of now I do have compensation for temp changes. Actually, I wrote/write a good deal of the code that runs on the computer so I can have as much control as needed, which is great because im flat broke and I could never afford an ignition control box.

Anyway, heres the curve with just the centrifugal advance:

 

Heres teh curve with with both the vac and centrifugal advance added together:

 

The last curve is teh actual timing as stored in the computer. The plotter is the built in graphing feature in tuner cats. Is this about what you had in mind?

On a side note, this is the most timing I will ever have run with this car. I think before it was like 35 degrees of advance at cruise with around 24 degrees of total advance at WOT at 3200 rpm.

 

One of the cool things about that MSD adj timing control is that you can just floor it and get your total at WOT dialed in first and then work from there.
I don't think that 24* @ 3200 @ WOT is right.
But mebbe there's something about your combo that's different than mine.
I totally go along with the curve that Damon said. Abt 45*- 50* @ full vac/fully advanced.

 

Before I was running just enough timing to get the motor to run well and not bog. The throttle response was excelent, it had reasonable power, and ran on anything I wanted to toss in the tank without getting any knock counts (detonation). Took alot of work to get it to do this. Alot of the other efi guys run conservative as well, which isnt really a bad thing IMO.

As for the present table. Does that look more like what would be present with a non-cc dist.? I guess I can always tone it down if the motor doesnt like it.

 

Now I know why Im so bothered by the above curve. Forgot there is no vac advance at idle with a standard dist. hooked to a ported vac source. Gonna say, with 26 some odd degrees at idle I dont think the motor would ever want to idle down. Probably would be hard to keep running when all that jumped in after the motor started. Put 16 some odd degrees in at idle instead and smoothed the curve back out.

Ive always told people with non-stock engines to start out with a mechanical dist. type timing curve with their ecms when nothing else is available. Guess Ill be putting that to the test myself. Sure know that there is no way in hell Im going to use the stock LO3 timing curve.

 

I run 14 to 16 degrees static advance with 15 degrees of Vacuum advance ( Crane adjustable vacuum can ) hooked up to manifold vacuum...giving a total of 29 to 31 degrees total at idle. Car idles down just fine. No problem at all.

Fairly high CR...10.34 to 1.

Power-Master gear reduction starter ( Mini starter) spins the engine over just fine...even when hot.

The reason the engine can handle so much advance at idle and cruise is that the engine is " choked" off by the nearly closed throttle blades. Cylinder charge density is greatly reduced. This takes longer to burn, thus has to be lit off much sooner.

 

I was running something like 20-22 deg at idle and it ran ok, but it would want to die when the computer immediatly advanced the timing when the rpms came up above 400 rpm after startup.

 

With your mild 8.5:1 compression ratio you should have no problem running 26* of advance at idle. Lots of carb guys like me run the vacuum advance to a ported source on the carb for different reasons. Emissions is one, dieseling or run-on when you turn the key off is another (carbs don't stop feeding fuel when you turn the ignition off- your FI application will never diesel becuse the injectors get shut off as well when you turn the key off) and sometimes large cams with low vacuum can cause problems at idle with full manifold vacuum advance.

On many of my applications I run ported advance, but not on all. Low compression applications like my roots-blown 383 I run full manifold vacuum to keep the idle reasonable and have plenty of "snap" when I lay into the pedal gently from a stop.

Also wondering..... where do you set the initial timing at the distributor? 0*? 6*? I assume you're taking that initial advance into account in your programming.

BTW- those curves look real nice in full color 3-D. Lots better than mine done on graph paper with a pencil! I blew at least an hour at work yesterday mapping that out on my desk blotter. People kept coming in asking what I was doing. I told them all to go away and leave me alone. They'd never understand.

 

Yeah, the initial advance is important since the ecm will only let 42 degrees of advance at the dist. Having the base set to 6 deg BTDC will allow for around a max of 48 degrees to be run.

The graphing feature in tunercats is really cool. You can even use your mouse to drag points up and down on the curve to do editing. Only problem is that it saves all the previous changes so after awhile, my PC will crash because it takes up all the memory with the stored changes.