so i had a gift certificate for autometer and i used it on a brand new 2 5/8" sportcomp vacuum gauge. it's super cool looking and i know that there is alot to learn from them, which is why i installed it in the car. i just really don't know exactly what it is that i can learn from them. right now, at 750rpm's, the 280H cam in my 305 produces 10-11in/Hg with the car in gear. i originally setup the carb(holley 650 double pumper) on my car with a different cam in it that produced 13-14in/Hg with the same conditions and i opted to put in a 6.5" powervalve. i'm looking to squeeze some more power out of my combo by learning how to tune it better. with the drop in vacuum at idle, should i go down to a 5.5" powervalve (i'm using the rule of thumb of taking the in/Hg produced and dividing it by 2 to get my powervalve size) to make more efficient use of the engines potential?

i want to learn, someone with experience please educate me more!

 

Changing the power valve opening point will not change the WOT power. Once you floor the gas pettal manifold vacuum drops to 0 and the valve is open. But you can fine tune the power valve opening point to fine tune the part throttle to full throttle power enrichment point.

You can also use the vacuum guage to se just how much air your motor is actually using by reading the vacuum guage at Wide open throttle at high rpm. the guage will read something more than 0 at peak hp. Your carb is "rated" at so much airflow (cfm) at a standardized pressure drop (1.5")
At WOT and max rpm your vaccuum guage will read something other than 1.5" Indicating that the carb is actually flowing some percentage of the rated airflow at max rpm. Simple airflow pressure drop conversion will give you the answer. Actual engine airflow in cfm.
Now you have a real engine airflow number that you can enter in the "carb size" data on popular horsepower simulation programs (desk top dyno) and get much more accurate results.
You vacuum guage is also a great internal engine diagnostic device. Can detect sticking valves, weak or broken valve springs, High rpm valve float, ignition problems, broken rings etc etc etc.

Can help you dial in the vacuum advance too.

 

i'm no longer running a vacuum advance, the car picked up ET when i disconnected it. what would be the benefit of running the vacuum advance now honestly? initial timing is set at 16* BTDC.

 

here is a page with some diagnostic info.
http://www.centuryperformance.com/vacuum.asp

you should retain vacuum advance at all cost on a street driven car. Vaccuum advance drops to 0 at WOT and should not effect the ET. Unless the advance plate is unstable or some other reason the timing is being affected at WOT.
You could check to see if there is any appreciate amount of manifold vacuum (@ WOT) (thru the traps) at what ever vacuum source you used for vacuum advance. It should be near 0 and not cause any vacuum timing to be applyed at WOT) Unless the spring tension in the vacuum advance pot on the distributor is miss adjusted. Use ported vacuum for vacuum advance.

How much total mechanical advance are you running?
What is the difference in ET , 60ft and MPH with it disconnected or connected? Someting is not right. It should not affect the ET at all.

 

When the motor is operating at part throttle cruise 9 (like on the hiway) the carb is lean. and the cylinder are not being completely filled on each intake stroke (throttled). this weak, lean air fuel mix burns slower that the rich(er) high density mix at WOT. therefore the motor needs earlier ignition to best apply the power form the slower burning fuel to the piston. The vacuum advance on your distributor does a good job of modulating the amount and rate of needed additional ignition advance at part throttle. But you do need to dial it in to your motor's requirements.
A vacuum guage is good for this. Even two vacuum guages if ya got them. One on manifold vacuum the other T'ed in to the distributors vacuum source. (ported vacuum).
you can get fully adjustable replacement vacuum pots for HEIs from Crane. You need to dial in both the amount ( vacuum rod travel limit) and the rate of advance (diapharam spring tension).
Witout vacuum advance you'll shorten the life of your motor ( hot cylinder walls, valves and exhaust ports) reduce gas mileage. shorten the life of sparkplugs (fuel deposits) and pollute the air more.