Timing Curve With Vacuum Advance Distributor

[Wreckless Warren]

I have a new MSD Ready To Run billet distributor with Vacuum Advance on the way. It will replace the MSD full centrif dist currently in my 445 with Ed heads and a Comp 294S cam. I'm currently running 16 deg of initial timing and I think it's set up with 38 deg total all in by 3K RPM. I will still run by MSD 6AL box and Blaster II coil.

MSD says the Vac Advance Dist will provide 10 deg of advance with 15" of vacuum. Do you think I will be able to retain the timing curve I have now with the new vac advance unit?

It currently runs 20" of vacuum at cruse and about 10-12 inches at idle  with a 50/50 mix of 93 octane pump gas and SUNOCO 110.

I know I'll have to tune to the new distributor but I was wondering what to expect. Thanks. ww

[Faron]

It depends on when the Vac adv starts and ends , my experience with the MSD units is that they do NOT come with an adjustable vac Canister ( its available but at an addition cost )

[Wreckless Warren]

The instructions say there will be 10 deg of advance with 15" of vacuum so it will come in fairly quick. But that will fall off as you get into the throttle. With a vacuum advance, what typical total advance is there at cruise RPM of say 2800 with a lite load?

And if there is too much advance and I can't adjust the Vacuum Advance, should the reduction in timing come from less initial or by reducing the total centrificial advance curve of the distributor? ww

[Faron]

The reduction ( if needed ) should come with the limit of the Vac unit , NOT the Centrifugal or initial , Vac adv is a MPG deal , not a performance OR drivability enhancer, I see so so much disinformation on that point , all the real performance motors were Centrifugal ONLY , Vac adv was added later to give MPG , and Smog Compliance  on the Performance motors, after 67-68 

[Wreckless Warren]

That's exactly what I'm trying to fix, not necessarily MPG but plugs wet with fuel after extended low speed driving. I know I need to look at the carburetor too but the added timing has got to help I figure. ww

[Faron]

Running more initial will  help with carb signal , allowing a drop in jet and throttle opening , it tends to tame high overlap cams / reversion, it will be easier to tune 

[Wreckless Warren]

If I give it more initial timing than I have now I get hard to crank when hot. My new distributor arrived this morning so I need to focus on getting it in and tuning with the vacuum advance. ww

[Faron]

If You are having trouble with Initial in the 16-22 range you have a weak Battery, a Weak Starter, Bad Connections or a combination  of , with the FE's 184 teeth Flywheel its got a Big Mechanical advantage , many people run the advance locked out without any trouble (34-42) and run 12-1 and Big inches

[Wreckless Warren]

The reason for this post was to ask, if my total initial + centrifugal advance is 38* while I cruise at 2800 RPM with 20" of Vacuum causing the vacuum advance to providing an additional 10* of timing (bringing the total advance to 48* at cruise) is this going to be a problem?

And if it is, should I start to back out total centrifugal advance? initial advance? or an equal amount of both?

Even if I I was running with the total timing (initial + centrifugal) to 32* at 2800 RPM, the vac advance is going to put it at 42* total. Now the vacuum advance is going to start backing out as I tip into the throttle or drop out completely at WOT, is that what keeps you from detonation.

Is this how the vacuum advance works and are those realistic total advance numbers you see with a vacuum advance distributor? ww

[jayb]

Vacuum advance can sometimes cause your total advance to run over 50 degrees while driving down the freeway.  That's pretty normal, I think...

[Wreckless Warren]

Thanks gentlemen, I'll follow up once I get it in and have some run time. ww

[Joe-jdc]

Warren, if you set your initial to 12-14*, and install the key in the slot to add 12*(12 x 2=24*) will give you 36* total mechanical, and then the vacuum will work for mileage.  I would set the total to 36-37*total with Edelbrock aluminum heads, and then check the ititial.  You can change slots to get the desired affect you are looking for with the little red key supplied by MSD.  Iron heads like 38* total with premium fuel, but to run more total mechanical timing you will need to add some octane.  If it still pings upon acceleration, you can adjust the vacuum to come in more slowly by increasing the spring tension on the dashpot with a small allen.  It is a small increment step by step to get it perfect without a distributor machine.  Joe-JDC

[fastback 427]

Thanks for the advice joe, we did exactly what you said yesterday! I picked up new truck and it ran like crap. Set timing at 14 initial and 38 total and could only get dwell to 30 cause dizzy is shot, but feels like a new truck!

[2VNTG68S]

Vacuum advance can sometimes cause your total advance to run over 50 degrees while driving down the freeway.  That's pretty normal, I think...

Just reading through this trying to learn more about timing curves and things to try on my build.

Something I'm still confused about.  If I'm targeting 38* total timing (18* initial + 20* mechanical), and the Vacuum advance is adding another 10* - 12* (Total of 50-52*) of advance in part throttle (high vacuum) conditions, isn't that hard on the engine?  What kind of total advance can an engine take under light throttle conditions, and how does that create MPG, throttle response benefits? Just trying to learn...

[jayb]

At light throttle loads the engine can use over 50 degrees of advance; this does improve mileage and isn't hard on the engine.  That was pretty much how all the stock advance curves were back in the day, except that the centrifugal advance didn't come in as quickly as 3000 RPM.