Vacuum Advance and why to use it on the street

[VintageMusclecar]

My 496 uses a OE distributor with a 22° mechanical advance curve which is all in by <4000 rpm. I run 12°-14° initial and 34°-36° total advance, and with ~11.2-1 compression it runs fine on 93 octane gas. (I also run a 160° thermostat to help ward off detonation)

When I had the distributor curve set, we installed a vacuum advance can which provided very little advance--if memory serves, only around 8°-10° degrees since I knew I would have to keep the total advance limited to be able to run pump fuel.

I have tried hooking up the vacuum advance several times since I got the car running, and every time, simply hooking the hose up literally renders the car un-driveable. Running mechanical advance only, even with a 256°-264° @ .050&quot; cam and 108° LSA, the engine will pull down just fine to &lt;1500 rpm in 4th gear with only an occasional minor hiccup--by 1600 rpm it is totally smooth. Hook up the vacuum advance and it instantly becomes a bucking, snorting Bronco Bull ride from hell. No doubts here, this combination does <span style="text-decoration: underline">not</span> want, or need vacuum advance.

As far as super quick advance curves and/or locked out advance curves;

Years ago I had a 75 Monza with a nice running 400 small block in it....very mild build, never saw the high side of 5500 rpm. It had an HEI distributor with an aftermarket curve kit installed, and I originally had it set up with the medium weight springs which provided a similar curve to what I now run in my Chevelle.

I tried experimenting with the advance springs one day, and simply swapping the medium weight springs for the light springs resulted in a full .4 loss in e.t. At first I thought it was a fluke--there was no tire spin and my butt-dyno was telling me the engine was <span style="font-style: italic">FAR</span> more responsive at lower rpm. I made another run--again, it was .4 off. I stuck the medium weights back in and the .4 immediately came back.

Again, that combination simply did <span style="text-decoration: underline">not</span> like a fast advance curve--despite what my butt-dyno was telling me.

In regards to locking out the curve entirely, one of the more well-known and respected members here tried that recently with less than stellar results. I'll leave it up to them should they wish to divulge the details. I personally have used a locked out distributor one time with successful results, and that was on my old ~800 HP 572 in my last Chevelle. (Of course, the 4K+ converter probably hid whatever low speed driveability issues that may have been present)

I'm <span style="font-weight: bold"><span style="text-decoration: underline">not</span></span> saying there is <span style="text-decoration: underline">never</span> a time or place for either a locked out distributor or an aggressive curve. What I <span style="text-decoration: underline">am</span> saying is that it is entirely dependent on the combination.

My $.02 worth of real world experience. [img]<<GRAEMLIN_URL>>/smile.gif[/img]