Vacuum Advance or Not?
 

I just ordered my new MSD tach drive distributor with the vacuum advance module and it includes a plate to provide the option to remove the module.
MSD also offers the distributor without vacuum advance.
Talking to my local engine builder, he told me it's better to run the vacuum advance on the street as it will increase fuel economy.
I'm building the 496 for my 73 as a hot rod and fuel economy is not something I'm worrying about.
I'm also trying to get rid of as many vacuum lines as possible and wondering if there is any other advantage or disadvantage to removing or keeping the vacuum advance.
What are your thoughts or experiences?

Run vacuum advance, connected to manifold vacuum. Your car will run cooler.

I just read alot about this. The others may or may not happen according to Ruggles book but you will have a better idle. He recommends using it for street use

there is no reason to NOT keep the vacuum advance on a street driven car. they can only benefit you on the street cruising.

I ran an MSD without and switched over to a new distributor with vacuum advance and I can without a doubt say the truck drives better with.

It only needs a vacuum advance if it has a license plate.

The instructions for my ZZ502 specifically say to not use vacuum advance.

And I run my ZZ502 with the advance and it works well.
I would like to know what kind of customer complaints GM want´s to avoid with this statement. Maybe it´s harder to dial it in propperly?
And of course you need the right vacuum advance curve and travel, matching to your other setup.

In a boat where you are at 3/4 throttle all the time, vac advance is not necessary. In a car, any car you are at 10 to 25% throttle almost all the time. Gas mileage will suffer terribly without vacuum advance.

Instructions say you will get 32 deg with centrifugal only. That is with the GM dizzy. I have a MSD dizzy so I'm not sure what to do other than block the tube to set the advance at idle.

No reason other than they say you get enough without vac.

I can only recomend: ask Lars.

I have the TI distributor with MSD AL2.
Have full advance of 36°, starting from ~18° at idle.
Vacuum adds 15° (don´t remember it ecaxtly)

I have made a lot trial and error to get to those numbers.

Of course GM gives a shit on your gas milage and wants to avoid complaints.

32 is not enough for light load gas mileage. ALL distributors will give you 32 full throttle above 3000 degrees of advance. At least they can all be timed to that point. You need around 50 degrees for part throttle gas mileage. And you MUST be set up to limit total advance to 32 to 36 degrees at hard throttle. Set your mech advance too high cuz you have no vac advance, you are roasting pistons.

I would guess you would be way more prone to ping when cruising with a vacuum advance especially if you stomp on it and transition to full power. That being said, I don't think I would eliminate on a street driven car.

A lot of crate engines are assumed to be for other-than-street-street applications. A strip car has no need for vacuum advance, since it spends a lot of time at WOT or close to it.

Having said that, there is still no penalty for vacuum advance even on the strip - it won't hurt performance, it just isn't necessary if your goal is to go a quarter mile as fast as possible.

Different story on the street - the majority of time is at part throttle, so the vacuum, which changes based on RPM and engine load, allows the timing to be modulated based on load. This can help with fuel efficiency and allow your car to run cooler.

Learn how to set one up they are an asset
Dont have one on mine but not worried about mpg and overlap is nuts anyways. Same with a carb, well tuned they seem just as smooth as EFI save for extreme hot/cold conditions big deal.

Get the best of both worlds. Use a programmable Ignition module.

I have a MSD 6530 Programmable ignition with a MSD 85551 Distributor. Added a 2 bar map sensor to program the vacuum advance as required.

No need to replace parts to re-curve anything. Complete control of RPM curve & vacuum curve. Great for tuning at your local track or dyno booth as you can make changes between run using your PC.

Timing information here.
 

Same as my ZZ454. I have a Davis HEI with vac advance on manifold vac port. 12 degrees initial and it idles around 26 degrees or so. No ping at cruise, even lugging in 5th gear.