I need an Ignition timing for Dummies lesson

So, I'm trying to tune my 440 in my 72 Duster, I see the vacuum advance is not hooked up and do not know if it is locked out. HOW can i tell?
The motor has the MP 284 cam, headers, Airgap intake and 750DP.

Once I figure out if my dizzy is locked of not, where should my initial timing be?

From here I need to schooled... who has a Ignition timing for Dummies lesson?

Thanks in advance

PETE

Do you have a timing light and knowledge of how to use it?

It might still have the centrifugal advance and simply not use the vacuum.
Pull the cap and see if the breaker plate will move in either direction......this will tell you if it has been locked out , or fixed timing.
You would probably see a screw holding the breaker plate from moving if it is fixed.
If it is locked down , then your initial is the same as your total......not ideal in most cases.
I had one deal I ran that didn't use the vacuum advance and had a very quick centrifugal advance , where it was all in by 2000 r.p.m. , and I just shot for 32 degrees total at 2200 r.p.m.

Take the cap off and see you can make the rotor turn, should move counter clockwise and spring back if its not locked.

What kind of distributor is it? Do you have a dial back light or degreed balancer?

With that cam you want about 20-24 degrees initial timing and 34-40 degrees total. Most stock Chrysler distributors have about 22-24 degrees of advance in them.

Does the motor start and idle okay?
Ignition timing can make a motor perform well or not, it depends on several things like which compression ratio, what quality of fuel being used, which spark plug and a few other things.
Start with the basics first, figure out what you have now and let us know so we can "school" help you on how to make it the best it can be
Let us know what you find, BTW that 750 carb.ain't big enough for a mild 440, let alone future growth

What Cab said, find/note what you have now (1) initial (2) total (dialback or timing tape) (3) RPM it reaches max adv. I would assume that you have a regular functioning (not locked out) dist & that the can just ain't hooked up. Do twist the rotor CCW & let loose & see if it snaps back (I'm sure it is OK). I would highly suggest the FBO $25 plate to limit the total once you get things fine tuned & get a timing tape or a dialback light. Wouldn't hurt to get the 14MM TDC checker that screws into the spark plug hole ($10.00) but likely you are OK there. Look down into the dist & see what springs you have (OE is a light one & a heavy one with an elongated loop on one end). When you get there, setting the initial with the "vac gauge method" is fine tuning that part & 36 it a good ballpark total (checked with the can capped or non op like it is now). Bottom line you want as much timing as it will take (A) with out balky cranking especially when hot (for the initial) and mix & match the spring combo so you are staying just under the pinging point at WOT up thru the gears on your hottest/driest (most likely to ping) day. Peoples' hearing varies & I would assume octane varies somewhat so give it a cushion but you want to be close to that line (pinging) but NEVER over it. ANY pinging is extremely damaging.

Pictures would help and more info on Distributor. Assuming electronic and not points? Since you had or built the motor, it would be worth the effort to look in the service manual and take apart your distributor to check it out. The Cam stop will probably have a number stamped on for your mechanical advance. You may also have one stamped on the arm of the vacuum. As stated modern original MOPAR distributors had 24 to 30 degree mechanical advance, and Initial was set around 0 degree give or take a few degrees depending on motor and transmission. Roberts advice of keeping total without vacuum advance under 35 until you sort out the combination is smart advice.

I'll try to get pictures later today. It looks like a MP vacuum advance unit, it may be just a factory part.. not sure. I'll pull the cap to see if its locked out too.

Currently the timing looks to be set at 10 BTC and maybe 20-ish at 3000

It sounds like your distributor still has the big spring on it that isn't done advancing until above 4500 RPM or so
You can check that out by revving it up to over 5000 RPM to check total advance I do it on all my motors, revved up above 5000 RPM to make sure what the total timing is where I shift it, some ECU will retard also above a certain RPM

for a quick/immediate/significant gain right now: bump the initial up to 20

This is where you need to start.

Getting the ignition timing right for the engine takes WORK. You likely won't find a parts store item that will drop in and run properly with the engine specs you have.

Simple test. Get car warmed up. Twist distributor a little bit clockwise. If it picked up idle RPM, the engine WANTS more timing than where you currently have it set!

Ok, good information!

let say you have 15* of initial timing with the distributor vacuum advance hose removed and plugged. Then the weights and stop would allow for an additional 20*, that would give you a total of 35* total, and all in at a certain rpm based on spring tension. That said, how much would the vacuum add to the initial timing and cruise timing? did Chrysler set the vacuum advance to add an additional 10*?

So at idle with my above remarks, initial timing plus vacuum advance would be 25* at idle, and say 40-55* (high vacuum/low load) at cruise or there around. Would that be correct?

This what I have always thought, but not sure.

Thanks, Mark

Ma preferred ported so it'd still be at 15 at idle & the can would start adding in more when above idle the vac got to the amt needed to start pulling the diaphram (A Sears hand vac pump works good for working with this). (3) dist slot plates I have seen are stamped 11/13/17, 22/26/34 added crank degrees respectively & plus the can adds quite a bit at part throttle, good for mileage/driveability. Agreed, it takes work to dial it in spot on for max benefit, but worth it. EDIT dial in the initial/total/springs/vac can. IN ORDER.

Initial spark set by highest vacuum @ idle in neutral will always be slightly too much lead.
More accurate: in drive.
Why don't they recommend that? Too many idiots pinned against the back wall.

Yes set it in drive if you have an auto cuz that is what you will see stoplight to stoplight & on the vac gauge method I get max vacuum then rotate the dist back the other way till the vac drops 1 in hg.

Rapid Robert:

OK, so 15* initial in my above scenario at idle because of ported vacuum...so then how much after the springs allowed full movement, say a total of 35*, using my previous post. So how much more total would be added by the vacuum can at high ported cruise vacuum?


The reason I am asking, is for me to understand how to fine tune my my Holley Dominator ECU spark tables. See what I am getting at?


Thank you, Mark

Mark, the # of (dist) degrees added by the can would be stamped on the arm on it that pulls the top plate. remove the 2 screws & with a screwdriver lift up/tilt the top plate slightly to the right angle nub on the end of the arm is out of the hole in the plate so you can pull the can/arm out of the dist housing. The # is likely on the bottom side of the arm (that'd be too easy to be able to just eyeball it & not have to R&R it!). Double that # for what it will add for timing at the dampener. There are a bunch of cans available & that vac pump works good for checking that the can ain't got a leak & for the amt of vac to start it adding advance.

Hi Robert,

OK, I understand, however I want to know what the 440 A12's had as an example. since I am trying to simulate a vacuum advance in my ECU program. Total cruise timing, (Initial, spring/weight, and vacuum advance) say at 2800 RPM would be 50*???





Mark

"till the vac drops 1 in hg"

Thanks, good advice

Max advance total (full initial + mechanical + vacuum) was 60° in the smog days. If you're lean enough* 50° min, 55° probably safe if it comes off at the right time. Lexus: slight momentary tinkle on transition is "normal and acceptable", but this on engines with knock retard.

* If it misfires reduce Holley mains by one and try it.

You are leading & jetting for the worst (knock prone) cylinder: smallest chamber with the sharpest exposed edges, least water circ to the plug and exhaust port, narrowest exhaust port.
Anything you do is only a test to be verified in the field. 50 is safe, 2,800 sounds safe until you know better, but RPM will finally depend on converter lock-up, stall speed, axle ratio. A really big engine turning 2,000 at 70 MPH might like all spark in already.

I've posted A134 here.
https://www.tapatalk.com/groups/racingfu...s40.html#p16466

Polyyspheric, thank you for the information...60* light load is interesting!

That gives me something to think about went doing tuning the ECU.

Thank you, again!

Mattax,

Exactly what I was looking for! The information on Page 4 & 5 is perfect for me to set up the advance curves, and how to simulate a vacuum advance in the ECU Spark tables, including what the modifier tables do in relation to the MAP vs RPM tables.

Thank you very much!

Mark

If you have data recording you're looking for the highest vacuum at any constant RPM. Even if plus/minus small vacuum spark increments may not feel different, higher constant manifold vacuum reading always indicates lower load, coolant temperature, better mileage etc.

on my 330hp 383, I backed my light cruise timing down to around 52* I think. I had more to start, but watching the pyrometer and listening to it I dialed it back a bit.

Great! Thought it might be

Depending on what you've done to the engine, you can experiment with more initial or more along the whole curve, etc. I've found it useful to think about tuning from the combustion viewpoint. Basically trying to keep in mind the goal is combustion to develop such that maximum pressure is through the most favorable crank angles in terms of leverage. And if we know an engine has been built in a way that produces slower combustion in certain conditions, add more lead there.

For example an engine with more overlap at idle will exhaust dilution and less compression so will probably be a slower burn at idle. Same engine when fully warmed up may be more efficient in the mid to top range under load and develp pressure more quickly then.

To add to Polyspheric's observation. The higher vacuum should be for a given throttle open along with steady rpm. To some degree that will go hand in hand. That is, a small throttle opening at a given rpm should show as higher vacuum. I'm not sure how you evaluate the data when you are controlling using MAP as one of the inputs.