while doing some research on distributors i found this info.

"A vacuum advance distributor senses changes in the engine's LOAD -- and does so almost instantaneously. Mechanical advance distributors, which sense only changes in rpm, are meant for special purpose applications, such as as drag racing.
If you try to use a purely mechanical advance distributor in your daily driver you will have to jack-up the output of the accelerator pump, since providing a jolt of raw gas is the only way you can bump up the rpms, which is the only way you can get the advance to kick in. But before the revs can build up you waste a lot of gas. This plays hell with your fuel economy and shortens your engine's useful life at the same time. "

Right now i am at the middle of swaping a 5.0 ho from a 95 mustang and i am planning to go carb. so i am looking a the right dizzy as well as carb and intake to buy. However still there are few things that i must know. While searching for carb some of them saying secondy vacuum, Do i have to consider thissince i am using a C4 trany.
thanx

 

IMO a vac. advance is best for street driving, as well as a vac. secondaries carb

 

where is that quote from?

I have mech advance on my msd distributor and mech seconds on my carter afb (with counter weighted baffel)

they both run great. Im no timing expert but Doesn't make much sense to me.......if rpms are not changing why the hell would you need to advance the timing? I thought we advanced timing because it takes time for the fuel charge to burn and reach peak pressure... so at higher rpm we set the spark off a bit sooner so peak pressure is reached at the right time and not after the fact.

RPM and Time are depended on each other more rpm less time.....less rpm more time.

What am i missing.

Thanks

Gun

 

IN GENERAL...vacum distributors are better for the street. A properly set-up(depending o engine requirements)mechanical unit will get all the advance in by3000 rpm, with the mechanical unit you will have less advance at cruising speeds and more at max rpm...just what you don't need. What you do need is lots of advance up to the max rpm with a way to retard it in high gear to stop detonation.
If you use ported vacum you will get a slight retard just off idle as you drop vacum and it will build with vacum up to the point where you open the throttle enough to loose vacum. This retards timing under load to prevent detonation at low rpm. As rpm build you have acentrifical device to advance timing at higher rpm.
Now..what does all that mean? Use a good vacum advance unit that you can tune both vacum and centrifical advances. BTW..I NEVER heard the part about an earlier pump shot to aid your centrifical advance.

 

I run a straight mechanical in my daily driver, never had a problem.

 

Mike......thats an odd quote no matter if you use mech or vac. Last i check more fuel and air IS the only way to bump up the rpms. If you guys find a way to get more rpms out of you car and still use the same amount of fuel you do at idle let me know okay....

I had stock vac dizzy setup and then I switched my whole ignition system to MSD with mech timing. Really couldnt tell a difference never had a problem with the Vac or the Mech systems.

Gun

 

That original quote doesn't make a lot of sense.

As far as ignition timing is concerned, engines need to be able to vary that based on both RPM and load. More RPM needs more advance (generally), while greater load at the same RPM demands less. Centrifugal (or mechanical) advance takes care of the RPM side, and vacuum covers the load requrement. But it isn't a simple relationship, so different states of engine tune will require different tuning of both advance mechanisms. Sometimes for a narrowly defined type of driving (most typically racing) you can eliminate either or both, but the wider range of street driving really needs both for best economy. Don't forget - gaining economy via vacuum advance does not cost anything at WOT, assuming that the tuning of both advance mechanisms is right.

The use of vacuum-operated carb secondaries is also a load-based mechanism. But it too has tuning adjustments, so it isn't an all-or-nothing feature. It does make carb CFM choices simpler though, as you can choose a slightly bigger carb than normally necessary and get away with it, again for street duty.

IMHO, an automatic tranny is even more requirement for both a vacuum advance distributor and a carb with vacuum-operated secondaries. Partly, it's based on the extra load placed on an engine at idle by an auto tranny in gear at idle.

One other thing - ported vacuum is purely and simply an emissions control feature. Less spark timing generally results in lower emissions.


Norm

 

That's not quite accurate. Under load, lessadvance can result in less NOx emissions, but under low-load cruising conditions you want more advance to start the burn process earlier, resulting in improved economy and emissions.