[Cliff R]

Despite all the good info out there folks still do not understand how VA really works.

The factory was much better at the source location for ported vacuum advance than the aftermarket. If you look very closely at the source location in a Q-jet for example it will provide FULL MANIFOLD VACUUM to the distributors Vacuum Advance with the slightest movement of the throttle off idle.

One has to realize that there are numerous other ported sources on many Q-jets that were NOT designed to operate the distributor VA. They were for other things like the EGR valve and the source location is higher and/or they have a "bleed off" slot cut vertically to lower the signal so the EGR didn't fully apply and lean out the engine in the "normal" driving range.

If you are using a correct source location for ported vacuum you will have FULL MANIFOLD VACUUM to the Vacuum Advance for all "normal" driving scenarios and it will have the exact same vacuum reading as all other manifold sources below the throttle plates.

So the only real difference between ported and manifold vacuum will be no VA idle and coasting.

Some engine don't care at all where you source the signal to the VA, others are finicky if you add too much timing idle/coasting and others thrive on it.

Since I use higher compression with my engines and camshafts with improved vacuum at idle (wider LSA/less overlap) they do not typically like, want or need any additional timing at idle speed. So in most cases I hook the VA up to a well located ported source. I also don't need to add much timing with the VA since I'm using tight squish and efficient dynamic compression at lower RPM's, etc.

I take in a LOT of troubled combinations here for custom tuning and find that despite all the attempts to make them happy before they got here the biggest problem with most are the distributor not solely with the carburetor. The root evil is often a poor cam choice for the CID and compression ratio as well.

A lot of engine "builders" and "guru's" still stick to "old school" methods to set up these engines and eliminate the VA and install super-quick spring/weight kits with a LOT of initial timing to offset the negatives from camshafts on tighter LSA's and more overlap that the engine likes/wants/needs at idle speed to be happy.

The cure for nearly all of them is to add idle fuel to the mixture screws so the engine can get plenty for complete combustion at the lower vacuum reading, and a little more timing at idle speed. NONE of them really like, want or need these super-quick mechanical advance curves well meaning tuners have put in them so I slow things down there as well.

I ALWAYS use the vacuum advance, and in most cases will test it both on a ported and manifold source to see how it responds. I also find myself reducing the amount of advance it adds "on the pull side" as mentioned above and use and prefer cans that start early and all in around 10-12" vacuum or so.

At this point I've cured so many piss poor running engine combo's I've lost count as I opened up my Saturday mornings to custom tuning troubled set-ups a couple years ago. It sort of got out of control and I found myself working almost every weekend and getting vehicles in here from great distances. To date the furthest cam from Atlanta Ga and most recently one from Canada.

I've only ran into a couple that I couldn't completely cure but was able to get most of them spot-on without a lot of effort. Few that I couldn't nail down had piss-poor cam choices, either WAY too small and/or LSA was too tight.

Something that's hard for me to get my head wrapped around is that with all the excellent info out there folks still think a 454/455 is going to "make more low end power" with a tiny cam in it on a tight LSA. What's really going to happen is that you cut the balls off of it and spike cylinder pressures up pretty high, and pinch off any potential for good upper mid-range and top end power using a cam in it that would be better suited for a 350 build with 8-9 to 1 compression in it.

Anyhow, what I recommend when it comes to this topic is to make absolutely sure that your distributor adds NOTHING below about 900-1000 RPM's with the mechanical advance, nice smooth curve all in around 2800-3200rpm's or so, and about 10-14 degrees from the VA. Set the initial timing so the total timing isn't going to pound the rod bearings out of it, then start testing manifold and ported VA sources to see how the engine responds. Some engines will enjoy 20 degrees or so timing at idle, others will not. Another thing that deal can get you on is that adding a bunch of timing at idle requires that you lower the idle speed screw and nearly close the throttle plates. With some combo's this leads to difficult restarts as the timing is retarded a bit and throttle plates closed down and not supplying enough air until it fires up and gets the additional timing to the VA via manifold vacuum. This is something you may encounter but is seldom discussed with this topic.......Cliff