Just curious on a couple cars I've owned the PO has pug a plug on vacuum advance. The MSD I have a feature to lock it out. What is the benefit of locking it out or not using it?
In performance driving (dragstrip, track) it is not necessary, or even desirable, but on the street it's a very good idea for driveability. People who fancy themselves driving hotrod cars eliminate vacuum advance. The only Ford that came with no vacuum advance, though, was the 289 High Performance. The 390GT, 428 CobraJet, BOSS 302, BOSS 429, BOSS 351, all had vacuum advance.
Have a c4 and a newly built 347 roller with MSD ready to run igntion, trick flow heads, RPM performer, and Holley 650 DP. Have not set full timing in it yet. Little new to the whole performance thing. 70 Mach with 351 Celveand I just sold - was basically all stock but PO plugged the vacuum advance.
At the dragstrip the vacuum advance would basically never be used, so it's only function would be to occasionally screw up your carefully set-up timing curve.
For street driving, you might consider having the distributor curved to the BOSS 302 specs, at least as a starting point. Use the vacuum and centrifugal specs, and ignore the retard numbers.
If your timing is on the edge of detonation and you use V/A you can damage you engine when going from a high vacuum situation to low, such as passing on the freeway. There is a slight delay before the vacuum canister/distributor can pull out the additional advance, which COULD cause detonation. Use a little common sense with timing and you will be okay running V/A on the street. It will help your mileage considerably. The only purpose of V/A is to increase mileage.
I have to check but what I got from builder was 10 to 12 degrees at idle and 32 total at 3000. Stupid question but is 32 total include 10 to 12 at idle + the centrifugal/advance . The springs and stop that come with MSD has 21 centrifigual at 4000.
How should I figure the v/a into the equation or do I need to worry about. Msd says it will advance up to 10 at 15 pounds.
Well, what sort of driving are you planning? If it's cruise night/weekend strip/club event car, you won't see much advantage in the vacuum advance. More than that, though, go for it.
Mostly cruise night and weekends. Gas Mileage not much of an issue.
If I keep the vaccum advance does the 10 degrees the vaccum advance adds above and beyond the Centrifugal + intial timing. Or is the max timing the centrifugal + initial?
As you can tell little experience in setting timing. Quick learner though.
It's not the max limit that's critical, so much as the curve getting there. Yeah, you don't want to exceed the total, but the rate of increase as the rpm or vacuum increases is what will ping, or lose power, if the advance opens too fast or too slow. This is where it really pays to have a pro with special equipment set up the distributor. I have adjusted quite a few Ford distributors, and I've never seen one that wasn't out of spec when I started. I have seen some well set-up engines run lousy, and all I did was bring the curves into Ford spec and they stated smoking tires. You can usually get a distributor done for under $100, and unless you're the one in a hundred that has a properly set-up distributor going in, you'll feel the difference.
Leave vacuum advance off until you have the mechanical advance dialed in. This is a must.
Yes, ~11* initial + 21* mechanical is what your builder is asking for when he says 32* total advance. You just need to speed up the rate so you have it at 3,000 rpms instead of 4000. Your MSD must have lighter springs you can swap out. I confess I've never messed with a MSD distributor. Some combination of springs will give you the 21* at 3,000 rpms.
Then you can mess with vacuum advance. Set it to the weakest setting and take it for a test drive, up hills, under load, etc. It should do nothing. Slowly adjust it up a little and repeat. When you start to feel something like the car lurching a little, back off and leave it alone. That's it. If the car starts to ping, get off the throttle immediately and back it off.
Just ordered a spring kit for MSD - - will swap out this weekend. The MSD instructions does not reference any vaccuum adjustments other than locking it out completely. SOme I'm guessing you can't. It show lighter springs to get 21* at 3000. Will set timing and put lighter springs in and take if from there.
Take out the vacuum module from the distributor. The MSD comes with an insert to eliminate it.
Use the medium spring settings on the MSD chart and tune from there.
32 seems low, but I do not know your compression, cam and head profile.
The vacuum is great for mileage, but if that is not a consideration then run intitial plus mechanical and leave the vacuum off.