Vacuum Advance

[Wietse]

The vacuum can I have is the one that came with the MSD distributor.
In the picture I send in previous post you can see the chart of vacuum vs. advance.
As per that it should come in starting from 6" vacuum.
I have no idea what vacuum to expect during a constant speed cruising @ 50mph for example.
Simple test to hook up the vacuum gauge and zip tie it on the hood scoop and see.

 

[Paul_G]

Something to keep in mind. Vacuum advance has two functions that can help an old street car, increasing fuel economy, and help the engine run cooler. It can also help reduce emissions, but not really a concern on a 40, 50, 60 year old car anymore. None of those functions help a drag car go faster, so drag racers dont use it, that is why so many people think it is not needed.

Another thing, radical cams, and cams that produce low vacuum can cause a choppy vacuum signal. In that case when vacuum advance is conected to manifold vacuum, the flucuating signal can cause the engine to surge and sound like it is missing at idle. In that case ported vacuum is a better choice.

 

[Wietse]

My car is like a drag racer and i want to convert it back.
I think i will try manifold vacuum first and see if i can get it to run decent.
Currently the timing is set @ 20deg idle with 18deg mech advance @ 2500 rpm all in.
I have tried to go beyond 20 deg at idle and found it was still running nicely, maybe even better then at 20 deg but going beyond 20 puts the all-in to 40 or higher wich is maybe a bit to much for high load and WOT.
If the vacuum advance adds a few deg in idle should be no problem.
At idle with tranny in Park i get around 8" vacuum and in Drive 5-6".