|Setting Total Mechanical Advanced Timing|
|Home : Practical MoPar Tech : Setting Timing Advance|
Initial timing, total mechanical advanced timing, and vacuum advanced timing
Tweaking your timing will not cause your engine to gain more power per se, but rather to run cooler, improve gas mileage, make the motor burn cleaner and give you a better throttle response.
In order to check your timing, you'll need a dialback timing gun capable of setting timing advance and a distributor wrench of the right size to loosen the lock down on your distributor. Disconnect the vacuum advance hose from the distributor and plug the hose. Hook up the timing gun (I'm going to assume you know how to do this) and shoot the harmonic balancer to get a reading. This is your initial timing. By loosening the lockdown at the base of the distributor, you can adjust the initial timing by either retarding it (clockwise) or advancing it (counterclockwise). On an engine that is stock, the service manual should be referenced for the factory initial timing setting. Note however that the addition of performance parts (such as a new cam) will render that factory timing setting pretty much useless. At that point I'd set the timing by ear, turning the distributor until the engine idle smooths out without stumbling. Shoot the balancer again with the timing gun take note of where your initial timing value is set.
Move on to setting your mechanical advance timing. Insure that the emergency brake is set so no one gets killed. Set the dial of the timing gun to the 35 degree mark, shoot the balancer and slowly bring the engine up to 2500rpm. This will clearly be easier with the extra help of someone sitting in the car to bring the revs up. Turn the distributor until the light falls back down to the 0 degree mark on the balancer. Secure the lockdown on the distributor, tell your buddy to get off the gas, and consider your mechanical advance set.
Total timing equals initial plus mechanical, not including vacuum advance (that's why the vacuum is unplugged). So in this example the total advance is "all in" by 2500rpm and for most small block MoPar engines, this is a good place to have it.
If initial timing is 17 degrees and mechanical advance (total) timing was set at 35 degrees @ 2500rpm, then: Total timing (35º) = initial (17º) + mechanical (18º) "all in" at 2500rpm.
Plug the vacuum hose back into the distributor. You will notice the timing mark shift to well over 50 degrees. It's cool, vacuum advance has just been introduced into the system and its function is to provide extra advance when a vacuum signal is present to improve fuel economy (or so I've been told). Because this is in addition to your initial and mechanical advance (aka your total advance) you notice the degree spike when it's plugged in. The amount of advance can be adjusted by swapping out different vacuum canisters or by putting on an aftermarket adjustable canister. Unless you are a hard core racer, most of the time this is not worth the trouble.
While we're still on the subject of vacuum advance, another point of controversy amongst hot rodders is where to actually draw your vacuum signal from. Ported vacuum, where the vacuum signal is taken above the throttle blades and usually plugged into the side of the carburetor, gives you no advance at idle and increasing advance (to a point) as the throttle increases. As the throttle approaches wide open, the vacuum goes away and so does the vacuum advance. Others choose to get a vacuum signal for their vacuum advance by connecting it to the manifold. Full manifold vacuum takes its signal from below the throttle blades and is highest at idle and gradually drops to zero at WOT. According to discussions I've followed on this topic, full manifold vacuum generally isn't used except for some factory stock applications, and even then it's often not a good idea. Personally I run it off the carburator, the way the factory did, but you can feel free to experiment.
Recommendations for how much timing you should set obviously vary from engine to engine and the only way to find out what's best for the combination of parts you have is trial and error:
On a car intended for race only, whose sole purpose is to blast down the quarter mile at full throttle all the time, you can plug the vacuum advance up and set your timing to have the mechanical advance "all in" by your torque convertor stall speed and the initial set to whatever yields the lowest ETs *without detonation*. The vacuum advance is plugged on a drag/race only car because there is no vacuum at full throttle (WOT) and therefore it doesn't come into play.
On a street driven car. The 2500-3000rpm range is a good starting point to have your mechanical advance "all in". For MoPar small blocks, a widely accepted starting point is 35 degrees mechanical advance "all in" by 2500rpm. In general, set your advance to be "all in" somewhere between 2500-3000rpm and set the timing advance on the gun between 34-38 degrees; the combination that's right for you is in there somewhere. Take test drives to find out what feels best. You may choose to hook up the vacuum advance for a little bit of improved gas mileage, but from what I understand the benefits are negligable and many really don't bother hooking it up.
In both cases, strive for enough timing to make you go fast without experiencing detonation and pinging or overheating. Detonation and pinging is NOT a good thing. Back off on the timing.
Back to Practical MoPar Home
"Horn broken...watch for finger"