Timing question

[85Silveradoin04]

Timing: What does using vacuum advance do? And how does it affect drivability? Thanks much.

[swervin ervin]

You have your initial timing set to whatever you set it to, 8, 10, 12 or more degrees. Then you have centrifugal (mechanical) advance which works off centrifugal force, hence the name centrifugal. This works by the force of the weights flying out under the spin of the distributor. I'm guessing you knew this though.

Vacuum advance is purely for ecomomy. It serves no purpose for the power side of the engine. Vacuum advance advances the timing with the vacuum from the engine, when needed. Timing advance on any engine is needed because the faster the engine rotates, the sooner the spark must reach the plugs in order to fire them soon enough to ignite the fuel mixture at the proper time, hence the name timing. So, vacuum advance advances the timing with the vauum need of the engine. High vacuum such as cruising around with very little gas pedal, increases the timing and increases power by igniting the mixture sooner. The more timing you can run without detonation (spark knock), the better. The quicker the fuel mixture is ignited, the more power a engine will make and the better the mileage and economy will be. Think of it as not taking as much pedal to make the vehicle go 45 mph with vacuum advance as it would with just mechanical. Sure you could not have vacuum advance and raise the mechanical up high, but at a cost of hard starting, spark knocking and more than likely, engine damage.

One thing is for sure I don't care what anyone says. Your gas mileage will go to he!! in a bread basket without the vacuum advance working properly. If you are only running at WOT all the time such as a drag racer does with no street driving and no worry about mileage, it's not needed at all. At WOT, you have no vacuum to make it work anyway.

[jimfulco]

It also tends to make an engine run smoother & cooler at low engine speeds, which is good on the street AND at the track (in the pits). It tends to increase idle speed (using manifold vacuum, not ported), so you can have an appropriate idle speed with less throttle opening, thereby lessening the chance of dieseling (engine still runs when ignition is turned off). More is not always necessarily better, though, because too much vacuum advance can cause it to miss at cruising speeds.

[swervin ervin]

Yes, Jim is most correct. I wasn't meaning the most vacuum advance you can have the better. Vacuum advance needs to be limited to around 12 degrees total if you have no EGR. With a EGR, the cooling effects of the recirculated gases will let you run more, 14-16 degrees total. Anything over these numbers is only hurting you because it makes you limit how much initial you can run. Remember, manifold vacuum is at idle and cruise so it must be taken into account when setting the initial. A stock canister will have 20-25 built in degrees of advance. Add 10 degrees or more of initial and you can see quickly what happens. Buy a Crane adjustable canister and limit the vacuum advance and be way ahead of the game. Can you say by to spark knock and pinging?

Folks will try everything they can think of to get rid of pinging and want high initial timing only to keep beating their heads against a wall with 25 degrees of vacuum advance. Then when all else fails, they move the advance hose to a ported source so as not to have the vacuum at idle to raise the advance. They will need to raise the curb idle to compensate and as Jim said, this only opens the throttle blades to where they are above the idle discharge ports wasting gas and causing a bad idle. This is a total waste of what the vacuum advance was designed for in the first place. Don't mask a problem, fix it right the first time and be done with it.

Mike

[71-longbed]

swervin ervin ... what should i do if i want to bypass my C.E.C valve (idle control solenoid) .. where should i plug my vacuum advance hose then ??

[swervin ervin]

Originally Posted by 71-longbed

swervin ervin ... what should i do if i want to bypass my C.E.C valve (idle control solenoid) .. where should i plug my vacuum advance hose then ??

Excuse me for acting dumb but but I'm not sure what you are calling a idle control solenoid. Is this on a Q-jet?

IMO, vacuum advance should always be run with manilfold (vacuum at idle) vacuum. Man, that sentence sure had the word vacuum in it a lot.

Running it on a ported source defeats the whole purpose of what it was invented for, to help economy in the cruise range. Sure some will say even ported will have some vacuum in the cruise range, but not much.

Hook it to manifold vacuum, limit it to 12 for non EGR, or around 15 with EGR, limit when it works to between 5 and 15 inches of manilfold vacuum.

I run 15 degrees of initial advance, 21 degrees of mechanical and 16 degrees of vacuum advance (since I have a EGR). I have around 9.5 compression ratio and run regular gas and even at 15 degrees initial I have no pinging what so ever.

Mike

[85Silveradoin04]

swervin ervin, Thanks for the info. I'm running on ported right now. I'll adjust to manifold per your rec. and see what it does.

[71-longbed]

im running a rochester 4MV ... what nipple on the carb would give me manifold vacuum ??? if you dont have a pic .... is it on the driver side/ passenger side .... more toward the intake or closer to the air cleaner

thanks a bunch

[swervin ervin]

Best way to make sure a fitting on the carb is manifold vacuum is to remove a hose from the fitting while the engine is idling. If there is vacuum on it, it's manifold vacuum. No vacuum, it's ported. If you feel no vacuum while idling, blip the throttle some and you should feel vacuum then.

[71-longbed]

thanks ...