Distributor advance

    It doesn't matter what kind of ignition you have, if the advance curve is not set properly it won't make any power. The ignition is advanced so you reach peak cylinder pressure right after TDC (top dead center). If the spark comes too soon the burning fuel will try to push the piston back down the cylinder before it reaches TDC, resulting in a loss of power and possible engine damage. If the spark is too late you will not reach full power and get poor gas mileage. There are two major factors that effect how much advance is required, engine speed and load. You increase advance with rpm and decrease advance with engine load. You don't need a distributor test stand to curve a distributor. All you need are some basic hand tools, a timing light, and a tach.

    The first thing you should do is find a shop manual for your particular car. Read about the advance mechanism and make yourself familiar with the different components. Before doing too much you should check the condition of your advance mechanism. If your vacuum diaphragm is bad replace it. Lube all the components and make sure they are moving freely.

    You probably won't have enough timing marks to test total advance. You can buy stickers for the harmonic balancer or you can make one. I made some on my computer. If you are really cheap you could just draw lines on a piece of tape. Trace them from the timing marks you already have to get the right spacing. The easiest way is to buy a timing light with an advance dial. They are kind of spendy, about $60, but a big time saver.

    The mechanical, or centrifugal, advance adjusts the timing based on engine speed. The faster the motor spins the more it will advance the timing. The factory advance curve is very conservative. By using a more aggressive advance curve you can greatly improve your engines performance. To setup your mechanical advance you first need to disconnect your vacuum advance line. Next you should hook up a tach and timing light and see what your timing is set at. Simply watch the timing marks with the light. The reading at idle is your initial advance. Rev the motor up until the distributor stops advancing and note what speed it maxes out. This is your total advance.

    Total advance is the most critical setting. Short of a dyno the best way to find what total advance you need is some track tuning. The MPH reading at the end of the track is your best indicator of engine output. Make a run or two to get a baseline then increase your total advance and make another run. If the MPH increases advance it some more and run again. Continue advancing the ignition until the MPH starts falling off then pull it back to the point where you had the highest trap speed. It may take quite a few passes until you find the optimal setting. Total advance is initial advance plus mechanical advance. There are two ways to change the total, adjust the initial advance by turning the distributor or adjust the mechanical advance mechanism. The easiest, and cheapest way is to just turn the distributor until you get the total you want then just leave the initial wherever it ends up. If you want more control over the initial then you need to adjust the advance mechanism.  If you have a total of 36° and want to run 12° initial then you need 24° mechanical advance.  Aftermarket distributors will have replaceable bushings to adjust mechanical advance.  Stock Ford distributors have two slots. There is a pin that sits in one of the slots to limit travel. To change slots you simply remove the armature assembly turn it 180 degrees and reinstall it so the pin is in the other slot. The slots are numbered as to how many distributor degrees it will pull in, double it to find the amount of crank degrees. So if you want 24° mechanical advance you need to find an armature assembly with a slot marked 12L. My distributor is setup with the stop in the slot marked 13L, this means 26° mechanical advance. The other slot is marked 18L which is 36°. I wanted less than 20° so I made the big advance spring a stop. I wrapped the coils with copper wire then soldered it so it was solid. Now I can adjust the total advance by bending the adjustment tab through the hole in the breaker plate. This makes it easier to adjust than the factory setup, and most aftermarket distributors for that matter.

    Initial advance isn't very critical. Usually you just set the total where it needs to be and leave the initial wherever it ends up. Once you found the best total advance setting you can play with the initial. Basically you want to run as much as you can before the motor cranks over hard. With the motor warm pull the timing way up and try to start the motor. If it cranks real slow then pull it back until it spins normally. Lets say that is 20° and you ran best with a total of 36°, that means you need 16° mechanical advance. Any time you adjust the initial you need to adjust the mechanical advance so the total stays the same, this is easy to do with my distributor mod above. Now if you're running tall gears and/or a heavy car you will probably encounter some sort of pinging if you stand on it with that much advance. If it happens at real low engine speeds then you need to pull the initial timing back. If it happens at moderate RPM levels then you can probably fix it with the advance springs.

    The springs adjust the advance rate. Where the distributor is bolted down determines the initial advance, the amount of travel in the mechanical advance determines the total advance, and the springs determine what RPM total advanced is reached. Lighter springs allow the mechanical advance to move more easily so you will reach total advance at a lower RPM. Stiffer spring will delay the total advance. Most distributors use two springs, a small one and big one. The big one will usually have a bit of lash so the small one does all the work at low speeds. This allows the advance to come up quickly off idle. Once the lash on the big spring is used up the weights will be trying to pull both springs so the advance rate will slow down. The chart below shows a typical stock advance "curve". It is the dual springs that give it the curve. If there were only one spring the chart would just be a straight line.

curve.gif (2675 bytes)

    In this example it idles around 500 rpm and has 6°initial advance. At this point only the small spring is holding back the timing. It is a light weight spring so the advance rises fairly quickly until it hits 18° at 1800 rpm. Once you hit 18° the lash on the big spring is taken up so the advance rate levels off a bit. If you increased the lash on the big spring the advance will go further than 18° degrees before hitting the big spring, less lash will hit the big spring sooner. To adjust what rpm you hit the big spring you would change the small spring. A lighter small spring will cause you to hit the big spring before 1800 rpm and a heavier small spring will delay it until after 1800 rpm. In this example you hit total advance (28°) at 4000 rpm. As mentioned above the total is limited by the mechanical advance mechanism. The rpm total advance is reached is determined by the big spring. A heavier big spring will delay total until after 4000 rpm and a lighter big spring will allow total to come in before 4000 rpm.

    If you experience pinging just off idle you should lower your initial advance. If your motor pings around the point you reach total advance you have two options; lower the total advance or put in a heavier big spring to delay the total. If its pinging well above this point you will need to pull back the total. If your motor is fine at low and high speed but pings in the mid range then you need to either reduce the lash in the big spring to lower the mid advance point or install a heavier small spring to delay the mid advance point. Making these fine adjustments can be a pain in the neck because adjusting one element often changes the others. If you want to bypass this whole mess you could use my programmable digital ignition. It allows you to adjust any of these points independent of the others and from the comfort of the drivers seat.

    The mechanical advance is adjusted for high load WOT conditions. Under light load, part throttle conditions the manifold pressure is lower so volumetric efficiency is lower so the cylinder pressure is lower so the fuel mixture burns more slowly. This means you need to light the mixture sooner so you reach peak cylinder pressure at the ideal time. This is the purpose of the vacuum advance. The lower the load is the more it will advance the timing. Vacuum advance will improve gas mileage and drivability of a street driven car. A lot of guys think a vacuum advance hurts performance, this is not true. The vacuum advance is entirely independent of the mechanical advance. They are two separate systems that perform two separate functions. The mechanical adjust timing based on RPM where the vacuum adjusts timing based on load. Under high load, WOT, performance conditions there is almost no manifold vacuum so the vacuum advance does nothing. The only time the vacuum advance adds timing is at part throttle, low load conditions when there is manifold vacuum. So unless you race at half throttle a vacuum advance will have no effect on performance. It will however improve part throttle drivability so unless your car is a 100% race car I would recommend running a vacuum advance.

    You're probably thinking, "Sure there is no manifold vacuum at WOT but aren't I supposed to use ported vacuum for the vacuum advance." Hold onto your hat, THEY ARE THE SAME THING! Except ported is shut off at idle. There are a lot of misconceptions when it comes to the ported vacuum source. After hearing 20 different theories I decided to hook up two vacuum gauges, one to manifold and one to ported, then drive my car and watch it. I found out they are the same, except the ported is shut off when the throttle is closed. Even then I had a hard time convincing guys so I hooked up a couple MAP sensors and a throttle position sensor to a data logger and recorded them while driving then dumped it into a spreadsheet and made a graph. As you can see, there is a direct relationship between throttle position and vacuum. When the throttle is closed vacuum is high, when the throttle is open vacuum is low, and ported vacuum is the same as manifold except when the throttle is closed. So which one do you want to hook it to? I prefer manifold vacuum. This pulls in more timing at idle which is good since there is virtually no load. Your motor will idle smoother and cooler with the extra timing. One night I was at the drags and my car was running hot in the staging lanes, I swapped the vacuum advance from ported to manifold then it would idle all night at 175°. Believe it or not the purpose of ported vacuum is to raise the temperature at idle, to lower hydrocarbon emissions. If you're like most hotrodders that is of no concern to you. If you have a big cam with a choppy idle then a vacuum advance hooked to manifold vacuum can really help. It will idle smoother and requires less throttle to maintain speed. Often a big cam requires you to open the throttle so far that the curb idle adjustment needles won't work. Hooking the vacuum advance to manifold vacuum will allow you to close the throttle some which may be enough for the idle mixture screws to work. Someone told me he noticed less dynamic braking with the vacuum advance hooked to manifold. I didn't notice it on my car but it makes sense. If the motor is running more efficiently with the added advance it will make a less effective brake. So which should you use? Try both and see which you like best.

    Once you have the mechanical advance setup to give you the most power, and no pinging, at WOT then you should setup the vacuum advance.  A stock vacuum advance will pull in 20° or more. If your car is pinging or running rough after hooking up your vacuum advance then you need to turn it down. Most vacuum canisters are adjusted by sticking an allen wrench in the vacuum tube. Turning the wrench counterclockwise will reduce the timing. Just turn it down a bit at a time until the problem goes away. I had to turn my vacuum advance down until it only pulled in 5°. 

    If you have any questions or comments e-mail me at mrriggs@gofastforless.com.

Update: 12-6-2016 - I recently bought a vacuum advance canister [Standard VC221] for my daily driver. The adjustment screw in it does not work the same as the vacuum advance I was using on my drag car way back when I wrote this page. Turning the scew does not limit the vacuum advance, it is a fixed 18°. All the screw does is change what vacuum level the 18° is reached. Out of the box, screw turned fully clockwise, it reached 18° at 2.5"Hg. At 12 turns out, it reaches 18° at 15"Hg. With that limited adjustability it can't be set high enough to match a stock vacuum advance [typically 20° @ 20"Hg] and likely couldn't be set low enough when hooked to manifold vacuum on a built motor.

   I've had several people comment that ported vacuum is not the same as manifold vacuum because at low throttle positions the throttle blades act as venturis which makes ported vacuum stronger than manifold vacuum. Referring again to the graph, this can be seen in the middle when the throttle was held around 10%. Ported vacuum does pull an inch or two more vacuum but the vacuum advance is already maxed out at this point so it has no effect on the timing. Ported and manifold vacuum will deliver the same amount of vacuum advance any time the throttle is open. The advance only differs between the two when the throttle is closed.

   I mentioned before why hooking the vacuum advance to manifold vacuum can be advantageous when running a bigger cam. Some have interpreted this as me saying EVERYONE should ALWAYS hook their vacuum advance to manifold vacuum. Sometimes, especially with stock cams, ported vacuum will work better.
   Manifold vacuum allows you to close up the throttle blades on a built motor, but if your stock motor is already running the throttle blades in the right spot then switching to manifold vacuum may require closing the throttle too far, completely blocking off the transfer slots. Depending on how much air is coming through the PCV, you may not even be able to close the throttle enough to keep the idle from racing.
   When I said before that I didn't notice any difference in engine braking when switching to manifold vacuum, I had a built motor with limited vacuum advance. Now that I am running a stock engine with full vacuum advance, I can definitely feel a loss of engine braking when the vacuum advance is hooked to manifold vacuum.
   Running vacuum advance from manifold vacuum on a built motor can also lead to troubles. If your engine pulls 15"Hg but your vacuum advance doesn't max out until 20"Hg then you may have an unstable idle. A small dip in RPM will lower the vacuum, which lowers the advance, which further lowers the RPM, which lowers vacuum which lowers advance and on and on until the motors dies. This positive feedback loop can work the other way as well; rpm goes up, vacuum goes up, advance goes up, rpm goes up more, etc and you are left with a racing idle. The solution is to limit the vacuum advance so it maxes out at a lower vacuum than what the motor pulls at idle. If your vacuum advance canister works like the one I just bought, limiting only the advance point and not the actual advance, then you will likely have issues at part throttle acceleration from too much advance. You really need to be able to adjust both the timing and amount of advance to properly tune a vacuum advance hooked to manifold vacuum. Without this capability you may be better off hooking the vacuum advance to ported vacuum.