NTBA

Distributors & Vacuum Connections

Most OEM distributors are a combination of vacuum and mechanical advance for ignition timing. The mechanical part is RPM dependant, while the vacuum advance is throttle "signal" dependant. I will cover both OEM and aftermarket distributors to give you a full scope of their intended use and proper setup.

It would be best to cover some functional logic first, in order for you to understand how distributor advances operate. I will start with the OEM Chevy distributor since they are popular in T Buckets. There are 2 basic functions which make up the TOTAL advance on the GM distributor. Vacuum and Mechanical. The vacuum part is controlled by a vacuum signal from the carb or intake manifold, while the mechanical is managed by engine RPM. The vacuum port on the carb is an active element, meaning it draws vacuum directly from the intake manifold. So, if you were to put a vacuum gauge on this port, you would see a vacuum signal. This vacuum signal actually "RETARDS" the timing on your distributor. Now when you open the throttle, the vacuum signal drops and the distributor "ADVANCES" until the vacuum signal rises again. Meanwhile, as the engine revs up the mechanical advance starts advancing proportional to the engine RPM. Under full throttle both the vacuum and mechanical advances are functioning. However while cruising, the vacuum advance will back off (RETARD) proportional to the vacuum signal and the mechanical advance will be advanced proportional to the RPM. This makes a good general use distributor especially for street driving.

One factor most overlooked when timing an engine is the TOTAL advance. While you could have your low side of the advance set correct, the total advance could be way off, due to wear or some other problem. It is imparative to check the total advance to be sure it is working within it's designed range. Too much total advance will cause serious detonation that will eventually damage your engine. For pump gas (the stuff you get at a gas station) won't handle lots of total advance too well. Try to keep your total advance under 38 degrees. On leaded racing fuel you can go over 40 degrees without worry.

Let's talk about vacuum line connections for a moment. Where your distributor vacuum line is connected to the intake manifold is important! First and foremost, it needs a continous vacuum signal. Second, it should have a stand alone connection to respond quickly and properly. Let's go into further disscussion here.First "continous signal" meaning there is a un-interrupted connection between the distributor and the vacuum within the intake manifold. Whatever your engine is doing is reflected in vacuum and your distributor needs to "see" this signal. Second, "stand alone" means the vacuum line has no other vacuum connections IE: Transmission modulator, power brakes, or PCV connections. The vacuum servo on your distributor requires a lot of vacuum signal to move the mechanical plate back and forth. If it has to share the vacuum signal with a transmission or other device, the distributor vacuum servo will not respond properly.

It's time to talk CONNECTIONS! All carbs which are designed specifically for street use have multiple vacuum line connections available on the baseplate. There will be one large nipple, and up to 3 small nipples. The large nipple is for your PCV connection. And, the others for Transmission modulator, distributor, and an optional timed port(Holley/Ford only). It is very important you connect these properly!

RULE 1: Do not "Tee" any vacuum connections except as a very last resort.

RULE 2: Never "Tee" anything to the PCV connection, I mean NEVER EVER. This line gulps huge amounts of vacuum to extract fumes from your block. Anything else connected here will virtually not function at various throttle positions.

RULE 3: check all vacuum connections with a vacuum gauge to be sure they have a continous vacuum signal, especially the distributor, transmission modulator, and PCV.

RULE 4: Wherever possible, direct connect the PCV and distributor to the intake manifold instead of the carb. The intake manifold is much faster at responding to vacuum signals than the carb.

RULE 5: (Holley Carbs) Never use the vacuum nipple located on the metering block for GM distributors! That port operates BACKWARDS to what your distributor needs. This is reserved for selected OEM Ford distributors ONLY.

RULE 6: Always check your total advance after setting the lowend timing. It is imparative that you do not exceed the limits of your engine/fuel type combination. Failure to check this could result in high speed detonation.

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Mechanical distributors are designed for 3000RPM and up launches off the line. While they work reasonably well on the street, that isn't their design intention. From a dead idle, mechanical advances can only do what the engine RPM tells it to do. Whereas a vacuum advance will go to full advance as soon as you open the throttle. This spontanous vacuum responce is very desirable for off-idle street hotrodding. A vacuum advance is much more forgiving while cruising as well. While cruising, the vacuum advance will retard to it's zero position leaving the advancing to the mechanical part. This makes for a slightly cooler running engine during cruise mode. You can think of a vacuum advance as a "power valve" for your ignition! Its function is IDENTICAL to that of a power valve in a carb. Both provide more "juice" when needed, and "conserve" when cruising. A full mechanical advance will tend to be more advanced than needed for cruising, and usually runs your engine a little hotter. This isn't a problem by any means, but it's not running as efficiently as the vacuum advance.

From a hotrodding perspective here's the difference; From a dead idle if you were to stomp on the accellerator pedal with a vacuum advance you would get instantanious power all the way up to where the engine vacuum rises and pulls the advance back. So, from an idle to near full power the vacuum advance wins hands down. The down side is, when you approach your power curve your manifold vacuum rises and pulls the vacuum timing back! From a dead idle if you were to stomp on the accellerator pedal using a mechanical advance, you would have a slower distributor advance thus taking longer to get in the power band. HOWEVER, once the mechanical advance reaches maximun, it stays there through out the entire RPM range. This is a major benefit, holding full advance. Since drag racers do not start from an off-idle, the slow RPM advance responce is of little interest. Drag racers much prefer to have 100% total advance when they launch off the line, which is something the vacuum advance cannot do under any circumstances.

Which is better, OEM or aftermarket? This is a subject of extreme controversy. The Big Three automakers spent millions of dollars to make their products reliable. And overall, they work very well for street use. The aftermarket ingnition systems are needed in some hotrod applications where intake manifolds take up lots of room. You may also note the aftermarket ignition systems are available in both vacuum & mechanical depending on your requirements. They too are high quality and very reliable. But the ultimate ignition is the distributorless systems! No cap, no rotor, no long spark plug wires, and no gears. The timing is monitored through an assortment of sensors centrally controlled by a computer. The firing order is managed by the crankshaft and directs the signal to one of eight coils directly above each spark plug. This is by far the most accurate and most powerful ignition avaliable today, and it's OEM!

But picture yourself cruising down highway 3 and your hotrod dies because of ignition problems. Here you are in Bubbatown, population of 6 1/2, and ol' Stevie Ray behind the parts counter thinks Mallory was his fifth or seventh wife... he can't recall. But you won't get any fixem' up parts for sure! But if you say, I gots to have points fer ma' Chebby... you'll be up & going in a flash. And Mallory? Well, she done run off with uncle Billy Bob...