Advanced Lessons
Every tune-up or cam change article I read in MM, or other car magazines say, "Set total timing at 32 or 35 or whatever degrees." I understand that is total timing at about 2,000 rpm with the vacuum advance disconnected. That way you have full mechanical advance. Here is my dilemma. I have a '69 Barracuda with a 340 sporting an Edelbrock Airgap RPM intake, 750 Edelbrock carburetor, Comp Cams Extreme Energy 268 cam, Mopar Performance electronic ignition, headers, 3.55 gears, and a four-speed. The ignition instructions say to set total advance at about 32 degrees, and then when you hook up the vacuum line, total advance at about 2,500 rpm will be about 55 degrees. With that setting, I get detonation.
I called Mopar Performance hotline, and they say, "Just unhook the vacuum line and go with 32-35 total degrees." I called up MSD, and they say the MP hotline guys are nuts and that I should hook up my vacuum line. It seems to run better with the vacuum line disconnected. And yes, I do have the vacuum line plugged into the correct port on the carburetor per the instructions. What's the deal? Michael Harrison via e-mail
If it is pinging with the vacuum advance, the simple answer is, it has too much advance. The vacuum advance is there to provide additional advance under low-load, part-throttle operation, obviously conditions under which the engine will develop vacuum. Under part-throttle, the induction charge and cylinder filling are greatly reduced, so generally a greater spark lead can be tolerated, improving mileage and efficiency. If you don't mind giving up the extra efficiency and mileage, then like the MP guys said, disconnecting the vacuum advance will be a quick fix. In this case all you have is the initial advance, plus the mechanical, generally about 34-degrees total. You'll get that much advance, and that's it, and it is totally dependent on rpm. So let's say you have 34-degress total at 2,500 rpm, and it doesn't ping, but with another 20 degrees coming from vacuum advance it rattles audibly. The problem is too much advance. What you need to do is dial-in the entire ignition advance system, and for a true street car, this is a lot tougher than with a track-only machine.
I'm going to have to skip the heavy theory lesson here due to space constraints, and go straight to practical solutions. First, get a timing light on the engine, and see at what rpm point the engine reaches full advance. A lot of guys go for fast advance curves, with full advance in by 2,000 rpm (or less), and that is not usually the best way to go. With your 3.55:1 gears and four-speed, at wide open throttle, you'll see under 2,000 rpm for a fraction of a second right off the line, and never again through the whole quarter-mile. Set the curve to come in a little later with a heavier spring combo. I like between 2,800 and 3,000 rpm for a street engine.
Only once the centrifugal advance curve is sorted out should the vacuum advance be dialed in. There are two considerations with vacuum advance: how much total, and how soon. How much total is built into the unit. The arm off the diaphragm has a stop built into it, which limits how much travel is available to advance the timing. There are a bunch of different units, and, fortunately, most are stamped with the amount of advance in distributor degrees, which is half of crank degrees. An 8.5 degree vacuum-can will give you 17 degrees at the crankshaft at full advance. There have been vacuum canisters manufactured with a wide variety of advance ratings over the years, unfortunately, I don't know of any source that has canisters of different advance ratings available and cataloged.
The rate at which the vacuum advance kicks in can have a significant effect. If vacuum advance starts at too low of a vacuum level, light part-throttle acceleration can cause detonation problems. The tonic here is to adjust the spring tension on the diaphragm by inserting a 31/432-inch Allen wrench through the vacuum nipple. Counterclockwise delays the action, clockwise and it kicks in at a lower vacuum level. Most, but not all, diaphragms have this adjustment provision, with one full turn typically changing the kick-in point by 1 inch of vacuum. You can adjust the diaphragm to slow the rate one turn at a time, until the pinging goes away. This should be all it takes to solve your problem.