you mesure vacuum in inches of water.
i am not saying you are doing it wrong, but on a performance motor manifold work's better IMHO.do a search well ok i will post what a person named I-man posted. and he is very right and on the ball on this. i have goten the same info from other's who build street rod's and distridutor people.
"I only have a business to keep going, and not all the time in the world to read magazines and articles by dyno kings, so if it comes to a little time in my response vs posting, I'll keep the time and the business going, thanks.
Now, for the rest of us: Vacuum advance.
Ported vacuum advance was, in fact, used by many makers of performance and stock engines in the past. Reason, the fuels were so good that the burn temperatures of the combustion chambers actually lowered as upper rpms were reached, and the added timing from ported was needed to heat the burn properly. In those days, the amount of degrees added in the cannister were fairly large, with some engines having 20+ degrees, but this number was usually not attained, as the total amount of vacuum added by the ported port was lower than that needed to get the vacuum advance to pull all the way in.
Today's pump vended fuels, and their additive packages, or lack thereof, have just the exact opposite of the earlier fuels, they always burn much too hot, even stuff like the Sunoco 280 have this problem. Add vacuum advance into the lace most succeptable to detonation, upper from mid to upper rpm ranges, and you can take detonation to the bank, you're gonna get it.
The only time today that ported timing is beneficial, to a point, is on Exhaust Gas Recirculation designed engines. These engines recirculate partially burned gasses from the exhaust system into the intake tract again to be re-burned and reduce emissions output, and the partially burned fuel needs the added combustion chamber heat to more completely burn the exhaust fuels recirculated.
In today's non-EGR performance engines, stock to performance, we do not require any upper rpm added timing for burn temperature control, in fact, it is detrimental to proper running.
So why use a vacuum cannister at all? Well, there is a simple silver lining to vacuum advance, engines as we are working with like timing at idle in the 18 to 24 degree range, but this isn't all that easy to do with some parts packages. How do we do this? Well, first, we need to think of the vacuum advance as not that, but an idle timing supplement to the initial timing.
As we now know, the idle timing can now be raised to a decent level at idle, to help with a stable idle, pull against a bigger than normal cam, pull against a converter and help cool the engine better from more complete idle fuel control. There is a catch, though, too many degrees of supplemental idle timing can be very counter-productive.
If we use the 24 degree figure, we can do things like set the initial timing at, say, 14 degrees, starts easily, then add the 10 extra with the cannister. OK, how do we get the 10 crankshaft degrees into the 20 degree cannister? We use either a Crane scroll plate to stop the advance pin that conects the diaphragm to the point/pickup plate to restrict plate movement down to the number of degrees we wish different from the static initial timing and the total idle timing.
OK, now that we have the number of degrees restricted, do we want to be able to change the vacuum level the canister comes in goes out with? Yes. We do this in two different ways, we look at the information of the stock cannister pull rate, and select the proper one for our vacuum levels, or we add the adjustable cannister from Crane, with the scroll plate that comes with it. Adjust away, you will find the correct amount of added timing for idle to make your engine run correctly.
Well, doesn't the idle added vacuum timing stay in the system when the gas pedal is hit? No, actually. It is immediately dropped as the off-idle vacuum goes away, and the centrifugal advance acts as usual . Only 180 on this is an engine that has no idle vacuum from a realy radical cam duration, and then, the vacuum actually rises after the engine is brought off idle. We just wouldn't use any kind of vacuum advance on an engine this radical anyway. Static idle timing would also be jumped up to 18 or so with the redical engine anyway, to try to stabilize the idle quality. Once again, adding initial timing helps the idle, just depends what you have available to help.
All HEI distributors manufactured for GM vehicles as production ignitions have way too much advance, mechanical degrees and vacuum degrees, and usually cause mucho problems in our performance and stock engines until they are set up correctly, and have full intake manifold vacuum sourced.
99 percent of all vacuum advances have way too many degrees of timing available and need to have the total number reduced to work with full manifold vacuum, as the manifold vacuum will pull all the degrees available into the cannister, not like the partial vacuum pull available with ported vacuum sources does at rpms.
Now, to dispell some myths about dyno tests and ported vacuum advance. Dynos always use more jetting to keep the chamber, engine and water temps cooled off, and ported timing just counteracts the rich mixtures of the rich jetting meeded for the dyno. In all the experiences I have had with crate and built performance engines, ported vacuum advance has been simply not productive, no matter what the engine did on the dyno.
If you are forced to use ported vacuum for your supplemental timing surce, then the curve and initial are not correct, and/or the total amount of vacuum supplemental timing is too large foruse as an idle supplemet from full intake manifold vacuum."
this is from a post he made last year. i hope it help's you to understand why you use manifold vacuum. as you noticed this apply's to non EGR and emision car's. in anthor post he sugjested you limit your vacuum advacne to 10* allso. a proplerly curved and adjusted distributor useing manifold vacuum will give better performance and milage then one useing ported. i know this from my own wxperiance and people that i know that have some prety fast car's. they all use manifold vacuum. i gained 4MPG and smother idle. also the motor has alot more power that i can feel and is faster off the line.
[This message has been edited by ilbl8 (edited 04-09-2001).]