In old cars, spark timing probably not cause of poor performance

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

RAY: 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.

So rather than have the timing set back to 15 degrees to simulate a working vacuum advance, go to a nearby nursing home and see if you can find a mechanic who knows what a vacuum advance and a centrifugal advance are.

Fix them, and that’ll solve all of your performance and timing problems, Scott – until the next problem arises. Good luck.

Which is better for oil change – cold or warm?

Dear Car Talk:

I was taught that when changing the oil, it is best to let your truck sit and let the engine cool, so that all the oil drains down into the pan before you drain it out. But I recently read online that you should let the engine run for about five minutes so that the oil heats up and thins out right before you start an oil change. Which way is best? – John

RAY: Wow. Somewhat accurate information online! I’m stunned.

The five-minute rule is a good one. You warm up the oil so it’s less viscous and it flows better. That way, less of the old oil remains inside the engine – stuck to the walls of the oil pan and other engine parts.

Recently circulated oil also picks up more contaminants and holds them in suspension. So you’ll remove a little more unwanted gunk when you drain out warm oil.

If you want to be really fanatical about it, John, then you’d want the oil to be fully heated up when you drain it out. In that case, you’d want to drive the car for 15 or 20 minutes and get the engine up to full operating temperature, then pull over and immediately remove the drain plug. Then you’d proceed right to the emergency room after the 300-degree oil ran down your arm, seared a pathway down your right flank and pooled in your underwear.

That’s why we strongly recommend against getting so fanatical about changing your engine oil, and why we endorse the five-minute rule for DIY’ers. The difference between changing warm and hot oil is not worth the trip to the emergency room and the permanent disfigurement.

Alternatively, if you’ve been driving the car and it’s hot, let it sit for at least a good half-hour. Or more. This is the equivalent of sitting in the Jiffy Lube waiting room, catching up on Brad and Jen’s recent breakup in the 10-year-old People magazines while you wait your turn.

Then put on a pair of gloves, carefully remove the drain plug and get your hand out of the way. And be especially careful when removing the filter. Even “cooled off” oil still can be pretty uncomfortable when it puddles in your Fruit of the Looms, John.

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