Limit total advance???

[azfordman]

Originally Posted by Excaliber

I think the vacuum advance and your HG readings at various speeds done when the engine is NOT UNDER LOAD, are a red herring. A snipe hunt. A,,,, well you get the point.


The reason is simple: As the HG (vacuum) drops, due to increased throttle opening, climbing a hill, accellerating, heavier load on the engine, the vacuum also drops. The radically advanced timing goes away, drops back to mechanical advance only!

It's unlikely your motor will be pulling anywhere near the HG at 2,500 your reading in the garage as it will at 2,500 rpm under load! Try this, get a really long vacuum hose and mount the vacuum gauge on the window or in the cockpit so you can see what the REAL HG is under various conditions, rpm, load, cruise, WOT, etc.


The other thing you need to know is: When does the vacuum advance begin to advance? At what HG? Second is: How MUCH advance does it offer at what HG?

I appreciate the informative post! I had always thought that the intake vacuum decreases considerably when under acceleration. Unfortunately with this car that is just not the case!!! I took the car out after reading your post and here are some results from a drive with vacuum gauge in cockpit:

In 1st gear up to 3000 RPMs the vacuum reading is 20" Hg
In 2nd gear up to 3000 RPMs the vacuum reading is 19" Hg
In 3rd gear up to 3000 RPMs the vacuum reading is 17" Hg
At this point I am already at 55 mph and speding.....

As far as putting a load on the motor, from a rolling start of 25-30 mph

In 3rd gear vacuum drops to 15" Hg
In 4th gear vacuum drops to 10" Hg
In 5th gear vacuum drops to 7" Hg
this is under very light throttle.

Under heavy (WOT) acceleration the vacuum will drop to around 10" Hg initially, but as I am ready to shift, around 5500-6000 Rpms the vacuum is back up around 20" Hg. The vacuum reading rises steadily as the RPMs increase. When shifting, between gears (clutch in and throttle off) the vacuum will jump up to 35" Hg (35" is incorrect...it is actually 25" Hg) then drops down to the 10" Hg as the next gear is engaged and throttle is applied again.

Normal cruising in 4th gear, 2500 RPMs, 60 mph the vacuum is a steady 20" Hg.
My question is, do I have an issue since this motor seems to pull so much vacuum???

As far as the advance goes, with the vacuum line disconnected and plugged, the mechanical advance only goes up to about 28 degrees BTDC, it begins to advance almost initially and is at full advance around 2000 rpm (a guestimate since I do not have a hand held tachometer.) With the vacuum advance hooked up, the timing advances the same way, but goes up to 45 degrees BTDC. So it seems with the vacuum advance disconnected I don't get enough advance, but with it connected I get too much.