Ignition Timing

[superfragl]

I am getting ready to fire up the 440 for my Fury for the first time after rebuild. I will be using stock electronic distributor. I am not sure what is the best for distributor vacuum - manifold or ported.
If I connect to ported, I can set initial timing to 10 degrees and total mech. advance would be 36 (mechanical advance is 26 degrees on my distributor) which is good for total timing, but sounds low for idle.
If I use manifold vacuum I can have a lot more advance at idle (initial plus vacuum) and proper total mechanical advance, but I am not sure if I run into issues with starting the motor because initial mechanical advance will be close to zero (not much vacuum when starter is cranking).
What do you guys think?

 

[70bigblockdodge]

What cam and compression? 35-36° total advance is a good number, 10° is not bad initial timing for a stock engine with good vacuum. Usually you need more initial if you have a big lumpy cam to help it idle, but then the lumpy cam does not pull enough idle vacuum to activate the vacuum advance so the crutch does not work. Best to figure out how far you can advance it and not have hot starting issues then weld the slots up to limit the amount of advance to get you to total advance.

 

[superfragl]

The cam is Lunati Voodoo series with duration at 050 inch Lift:226 int./234 exh. and 0.494 int./0.513 exh. lift. Compression ratio is 9.6:1
The cam is not radical so the vacuum should be fine at idle.

 

[70bigblockdodge]

That compression is pretty high, I had lower than that on a 383 with 906 heads and it rattled at part throttle on ported spark advance, my mechanical was all in by 2500 though.

 

[superfragl]

The cam manufacturer suggested that total mech. advance should be set to 36 at 3000. As I understand - it should make no difference either it is ported or manifold at part throttle. My main dilemma is the idle and starting.

 

[twostick]

Vacuum advance has no affect on initial or total timing.

If your engine combo is happy with 10 deg initial, and you don't have a big lumpy cam, hook up to manifold vacuum and go.

The engine will start with 10 deg and as soon as it fires the vacuum advance will add about 40 degrees until you step on the gas and the vacuum advance drops off.

The only time vacuum advance comes into play is at part throttle/light load which generates high vacuum.

Ported vacuum was introduced mostly as an emissions control to reduce NoX.

Now if you have a big lumpy cam, manifold vacuum can cause a surge at idle caused by an erratic vacuum signal. In that instance you would choose ported vacuum.

Kevin

 

[superfragl]

Vacuum advance will never add 40 degrees at idle, in fact my vacuum can has 18 degrees max advance, so if I have 10 initial plus 18 vacuum - it will be 28 degrees advance ad idle, which I think is too much. Or it`s ok?

 

[twostick]

I probably exaggerated a little.

Your number is much closer and at idle will be no problem. As soon as you crack the throttle, the vacuum drops off and the advance reverts to the mechanical curve.

What heads are you running?

Kevin

 

[superfragl]

I got 902 heads, ports matched and bowls blended.

 

[twostick]

Is your CR based on actual measurements or published numbers of the components?

A true 9.6:1 with no quench open chamber iron heads could be right on the edge with pump gas depending on the camshaft.

Kevin

 

[superfragl]

Advertised CR was 9.6 considering the combustion chamber volume of 88. Actual measurement showed 9.5 because my heads measured 90.

 

[twostick]

How far below deck are the pistons? 9.5 is going to want the good stuff for sure. Can you get 93 or 94 in MA?

Kevin

 

[superfragl]

.010 below deck. I can get 93 and that is what cam manufacturer suggested to use.

 

[rags]

ported vacuum. no question.

 

[twostick]

ported vacuum. no question.

Because?

Kevin

 

[70bigblockdodge]

Play around with it see what it likes. I doubt you will pull the whole 18° at idle in gear and 28° will probably make it have a unstable idle, and a big rpm difference in gear and park, I would start with no vacuum at idle and then see what you can add and does it improve. Vacuum advance canister is adjustable through the nipple with 3/32nd Allen wrench if it comes on to easy/aggressively.

 

[65sporty]

The engine should start fine on 10 deg, you will have to try both ported and manifold to see which your engine likes best under light load

 

[superfragl]

What is the difference under light load? I am not getting this. As I understand, once you crack the throttle open the vacuum will be roughly the same for manifold and ported?

 

[twostick]

What is the difference under light load? I am not getting this. As I understand, once you crack the throttle open the vacuum will be roughly the same for manifold and ported?

That's right.

The difference is you get the extra advance at idle which depending on the combo is a good thing. Generally speaking it should idle leaner and cleaner and if you are stuck in traffic, cooler.

Ported = no vacuum at idle = initial advance only.

If you have a lumpy cam it can cause problems because if the vacuum is jumping around, so is the advance, which can cause surging. In a case like that, ported might be a better option.

Set your timing with the vacuum adv hose plugged. Hook it back up to manifold and see what it does. The idle should pick up 100 rpm or so maybe more, smooth out and sound "happier".

If after you adjust the idle speed and mixture, it still idles nice when you put it in D, leave it on manifold vac.

If it tries to quit or surges and carries on and adjusting the idle speed and mixture doesn't cure it, disconnect and plug the vac line and see if it behaves.

If it straightens up, chances are you have a cam that will play nicer on ported vac.

Kevin

 

[superfragl]

Kevin, thanks for a good explanation! The cam is mild and manufacturer assured I will have decent vacuum at idle - it was one of my requirements when I was picking the cam, because I got power brakes.