Do you guys run vacuum advance?

[FLYNAVY53]

I've got a 1964 Corvette with the stock 11:1 327 solid lifter motor with a 4 speed M21. Carb is freshly rebuilt, stainless works long tube headers, and I'm working on installing an MSD distributor with the 6A box. These motors like 36-40 degrees advance if you can get there without detonation so I plan on starting with the two lightest springs in the MSD to have 25 degrees all in by 2200 RPM and set the idle somewhere between 13 and 15 degrees.

My question is, is it worth retaining the vacuum advance in this particular set up? I've done all the research, and fully understand the theory behind the vacuum advance system. Obviously in the world of Corvettes, everything is better the exact way it rolled off the showroom floor, so any conversation to the contrary on the Corvette forum is an exercise in futility. That being said, I know two local individuals, both running the same ignition set up in their C2 Corvettes, with none of the over heating at idle or horrendous gas milage problems people wax poetically about on the Corvette forum.

Just curious what everyone's experience over here is as there is obviously a lot more diversity as far as the cars go on this forum.

Thanks for the help!

[andrewb70]

I've got a 1964 Corvette with the stock 11:1 327 solid lifter motor with a 4 speed M21. Carb is freshly rebuilt, stainless works long tube headers, and I'm working on installing an MSD distributor with the 6A box. These motors like 36-40 degrees advance if you can get there without detonation so I plan on starting with the two lightest springs in the MSD to have 25 degrees all in by 2200 RPM and set the idle somewhere between 13 and 15 degrees.

My question is, is it worth retaining the vacuum advance in this particular set up? I've done all the research, and fully understand the theory behind the vacuum advance system. Obviously in the world of Corvettes, everything is better the exact way it rolled off the showroom floor, so any conversation to the contrary on the Corvette forum is an exercise in futility. That being said, I know two local individuals, both running the same ignition set up in their C2 Corvettes, with none of the over heating at idle or horrendous gas milage problems people wax poetically about on the Corvette forum.

Just curious what everyone's experience over here is as there is obviously a lot more diversity as far as the cars go on this forum.

Thanks for the help!

I see no reason not to run vacuum advance. By that I am assuming you mean manifold vacuum, which is higher at idle and goes down to zero at WOT.

Andrew

[anguilla1980]

I always ran mechanical advance on my performance cars and vacuum on my daily beaters. I like slightly heavy springs and being all in by 3K to avoid pinging on acceleration.

[andrewb70]

I always ran mechanical advance on my performance cars and vacuum on my daily beaters.

There is no reason not to use both on everything.

Andrew

[FLYNAVY53]

Yes, I was referring to direct manifold vacuum. In that case, I'll probably swap the original vacuum canister off my factory distributor and put it on the MSD as they have different advance characteristics.

[68Formula]

For big cams I prefer running running stiff springs with higher initial. As long as your initial timing isn't so much that it causes the starter motor to lug during cranking on a hot engine (fully warmed up), that's what the engine prefers.

I'm not a fan of trying to use really light springs to bring in timing super-quick. Bring it all in too quickly like 2200rpm with light springs, it could have more than it would tolerate so then you're backing off your total to compensate and giving up power. Plus really light springs can cause the centrifugal to begin engaging at an idle speed, so effectively you've engaged some advance right where you're setting it, thus in effect losing total rate increase. Also the light springs can fatigue over time causing them to take a set in a slightly stretched position changing the curve later on.

Vacuum advance compensates for the engine's need of additional timing light loads, and should only be set after the initial and centrifugal curves are dialled in. It helps improve fuel economy and part throttle tip-in response.

In most cases vacuum advance should be hooked up to ported advance of the carb. Ported should only allow vacuum once the throttle has been opened past a certain point, therefore not adding to the initial timing at idle (always confirm this to ensure everything is okay). This add on top of the mechanical curve, so once you change those settings, you need to get an adjustable vacuum canister to optimize the new combination for part throttle.

There are rare cases where connecting to non-ported source can help (full manifold vaccum). If you had a low compression engine with an aggressive camshaft coupled with an automatic, then sometimes you can't reach enough initial static timing set mechanically to get a stable idle (especially when you transition to in drive). So you can set the initial where the engine prefers it, then connect to a direct manifold source to add on top.

[68Formula]

If it were me, here's what I'd do given your information. The MSD shows 4 bushing choices. I'd pick the 25degree bushing and use the two blue springs. Then I'd set the initial at 12 degrees without vacuum. Increase engine speed up to 4500rpm to verify how much centrifugal is added and by what rpm. Should be around 35 total. Take it for some WOT runs and bring the timing up a couple degrees until max power is achieved. Then do a timing verification at idle and rpm sweep again to see where you're at for max timing.

Next, while the engine is fully warm, increase initial timing in 2 degree increments. Each time, do a restart listening for starter load. (DO NOT DRIVE!) Once it seems like the starter is struggling a little, back it off 2 degrees. Now you know your highest initial and best max timing for power. Change to the closest bushing size that gives the same curve or slightly more. Now set and record the desired idle speed. Then start changing spring combination to bring it in a little sooner based on power and listening for detonation. I record all values as I go as well, so I can track all the changes through the process.

To illustrate. Let's say I find the best total to be 38degrees and the most start timing it will tolerate is 16 degrees. This gives me 38-16=22 degrees centrifugal possible. The closest to this without having too little sweep is the 25 degree bushing. Therefore I can install the 25 degree bushing, set initial to 13 degrees (38-25) and set to desired idle speed. If it liked 34 total and 22 initial, I would use the 18degree bushing with 16 initial.

Now that I have the initial and centrifugal set, I can focus on the vacuum timing. First I'll connect it to a ported vacuum source, and then verify with it connected I see no change in initial timing. Then open the throttle slowly and I should see a large jump in timing compared to before when it was disconnected. Now drive it down the road. At low speeds in 4th gear (35-45mph), do some throttle tip-ins of about 25-30% and listen for knock. If there's knock, reduce the timing added in small increments until it goes away. If no knock, continue to increase in small increments until knock is heard and then back off once knock is detected.

Final check, I can see how it likes vacuum advance connected direct to a manifold source. Disconnect and plug the ported source and connect to manifold. Restart the car. The idle speed will probably jump up from before. You can reduce it back to desired. If it doesn't have a miss or runs rough, take it for a drive switching from no throttle (decel) to part throttle to feel if you prefer the response. If you like the idle smoothness and driveability this way you can leave it. Make sure to write all the final settings down for reference later.

[FLYNAVY53]

68Formula, I appreciate all the info! Because my motor is essentially stock, I'm doing my best to replicate the curve of the stock distributor with the MSD unit. The car actually runs great, but breaks up above 4000RPM because the bearings in the old distributor are shot and the shaft starts to move around at high RPM, changing the advance all over the place....see dyno sheet:

179767


Below is the graph of the factory distributor, as well as the factory vacuum advance canister. Full mechanical advance of 24 degrees is in by 2300 PRM.

179768

The MSD distributor with the two lightest springs most closely matches the factory distributor with the 25 degree bushing.

179769

My plan is to swap the original vac canister off the original distributor and put it on the MSD, with the light springs and 25 degree bushing, which should get me pretty close to the factory set up with somewhere between 12-15 degrees at idle. The hard part is that the L76 won't idle below 900 RPM, so even at idle, you have a few degrees of mechanical advance...even with the stock distributor.

[ctcz28]

Take a look at posting #38 in this thread for some helpful information:

https://www.camaros.net/threads/question-about-distributor-advance.462693/page-2

Also, the information in the attached article on timing is very good. I swapped my vacuum source to full manifold and made a vacuum advance stop as described in the aforementioned posting and it made a huge difference in how well my car runs.

[ilikeike]

Although mine is using a MAP sensor for computer controlled timing advance, I basically have it set up like full manifold vac on a traditional distributer with vac advance.
Hi load pulls timing based on manifold air pressure, light load hi vac adds timing during cruise and idle. good advance at idle helps a bit with the watery eye stinky exhaust on a larger cam.
So my engine cranks over with about 8deg. as soon as it fires it goes to 20deg.(and stays there between 500-1200rpm)and a nice clean AFR and reasonable smooth idle, this allows me to keep my carb throttle opening just right exposing just a bit of the transition slot.
Full throttle full load is limited to 36.