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]

[Hemicolt]

&quot;I'm not saying there is never a time or place for either a locked out distributor or an aggressive curve. What I am saying is that it is entirely dependent on the combination.

My $.02 worth of real world experience.&quot;


Very well put. A little knowledge can be a dangerous thing. These blanket statements saying that &quot;one rule applies&quot; is the wrong thing to be telling people.

It's like the carb. trick he told me about using on the tunnel ram setup. I can't argue if adding the wire to the metering plate will or will not work when it comes to cutting down the fuel at idle.
We will however try this when we run the engine on the dyno. Before and after results will be recorded and I will report the findings for everyone to see. The reason I will try it both ways is because of what was said above. Every combination will require different things.
I'm still thinking the amount of of vac. at idle is going to determine how much fuel is pulled through the idle circut. I may be wrong in the end, but I'm not going to tell everyone you can or can't before trying it first.

To go in and blindly make the modification without testing it both ways is foolish.

[WILMASBOYL78]

I will certainly bow [img]<<GRAEMLIN_URL>>/worship.gif[/img] to the experts and can only offer my &quot;real life&quot; experience. We have a number of L78 Novas which are street driven and one which is occasionally run in the 1/4. They all have factory points distributors...we run vacuum advance on all of them. The engines all have the factory L78 cam, 11 to 1 pistons, big heads, etc..basically stock L78 motors...nothing tricky. For fuel we run a mix of AV Gas or race fuel mixed with 93 pump gas...no issues. Only one of the L78 cars has a &quot;curved&quot; distibutor...our black 1970 TH400. I have never tried running the cars without the vacuum advance...never saw the need to. Most of the factory BB solid lifter motors (68-70) had the #355 unit which had 15* advance..this is the vacuum unit that is in all of our stock distributors.

Again...my info is directly related to driving these cars for many years and knowing how they run. I can't comment on how other people's cars run or perform...only my own. I maintain that &quot;fuel alone&quot; does not determine the results related to vacuum advance or the lack of it...there are many factors that impact performance and tuning.. and each situation is a little different.

wilma [img]<<GRAEMLIN_URL>>/[/img]

[Salvatore]

Tom, did you write that on your own or was someone there coaching you?