The simple version is that he's wrong, in general. The more accurate version is that when doing it
everything his way it can work out OK.
So here's why it can work. Lets say you have a distributor with a long advance curve, one that the factory intended for 5*BTDC at 750 rpm.
You dont care about emissions anymore and want pre-smog initial timing without reworking the slots and springs. One way to do that is connect the vacuum advance to manifold vacuum. That can be a way to get the intial up to 10 or 15* BTDC and will be reasonably stable if the manifold vacuum at idle is strong and steady.
RRR answered this.
Chrysler illustrated his answer this way.
View attachment 1715759140
You need to plot out the timing curve. I'd prefer to see you do that without the FBO plate.
I mentioned plotting out the curve in your previously here
Too much timing?
Let me borrow from a post I made yesterday to illustrate why you need to measuring timing every 200 or 250 rpm or so.
Circles represent the timing we measured for a 1967 440 with the correct distributor set at 12.5* BTC @ 650 rpm.
View attachment 1715759141
That distributor is going to work fine on that engine when the vacuum advance is connected.
But if we accidentally had been given the distributor intended for the '67 440 with Clean Air Package, and set it to 12.5* at 650 rpm, we'ld get a plot like this.
View attachment 1715759142
If we limited the advance to 38* BTDC, it would be OK at the drag strip.
It may or may not be OK on a long trip with this much timing.
When vacuum advance is connected this engine will be very unhappy on the road.
Knowing how the timing between 1400 and 3000 rpm advances is critical in deciding how to modify the distributor.
The FBO plate is not helpful for street performance when dealing with a situation as shown above. (It would be OK for drag strip as long as the vacuum advance was disconnected). In other situation the FBO plate can work.
Take the timing readings from as slow as you can get the engine to run, and then in regular increments of increasing rpm.
