Dear Car Talk:
I have a 1982 El Camino. Two years ago, because it didn’t pass the California smog test, I had to take it to a designated repair shop. As part of the repair, the tech told me: “Some fool set the ignition timing to TDC (top dead center), or 0 degrees. I changed it to 15 degrees, where it should be, and it’s running better.” And it did run better. This year I went in for the smog test, and the technician told me, “Some fool set your ignition timing to 15 degrees, so you need to reset it to TDC.” I told him what I had been told two years earlier, and he told me, “The book says it has to be TDC, so you need to set it there or you won’t pass the smog test.” So I had him set the timing to TDC, and I passed the smog test. But the car runs like crap. So I’m thinking about changing the timing back to 15 degrees, where it ran better. I need some expert advice. Thank you. — Scott
Well, if you need expert advice, Scott, why did you write to me?
I’m sure the car’s specification is TDC, or top dead center. That means that each spark plug is set to fire when its piston reaches the very top of the compression stroke. Fifteen degrees before TDC means the spark plugs would fire when the crankshaft is still 15 degrees of rotation away from when each piston reaches the top. In other words, at 15 degrees the spark plug would fire early.
In reality, the timing of the spark is supposed to vary. At idle, it’s supposed to be TDC. But as the engine speed increases, the spark needs to fire earlier so that combustion is already in full swing by the time the piston reaches the top. Otherwise, most of the force of the explosion goes out the tailpipe instead of toward pushing the piston down and making the car move.
In older heaps like yours, the spark timing is automatically adjusted by something called an “advance mechanism.” Your El Camino has two: A vacuum advance, which uses the engine vacuum to advance the spark timing, and a centrifugal advance, which uses the rotation of the distributor shaft to advance the timing. And one — or both — of those is broken. I’d put money on the vacuum advance first, because they used to break all the time.
Manufacturers do a lot of experimenting to figure out how the spark timing should be set on any given engine. They’re trying to find a balance among power, economy and emissions. So at TDC, where it’s supposed to be, your emissions are good. Well, as good as they got in 1982, which is bad. But your power and performance stink because your advance mechanisms aren’t advancing the timing when you rev up the engine.
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