Brakes on, brakes off – it’s easier than ever to brake automatically

Dear Car Talk:

I swear my old Jaguar had a parking brake that automatically set when I put the car in Park, and released when I took it out of Park.

This was very convenient, as I never had to remember to set or release it. And I never tried to drive away with the parking brake still on, which I have done a few times in my Audi A7 and my wife's Toyota Minivan. Why don't other cars have this feature? – J.C.

RAY: They do, J.C. We've driven a few cars that have that feature recently, including several Mercedes models. And it's easier to do than ever.

Traditionally, parking brakes were operated by cable. You’d either yank up on the lever between the seats, or you’d tighten the cable by pushing on a pedal with your left foot.

But cables weren’t perfect. They’d stretch over time and go out of adjustment. They’d rust and even seize up if they weren’t used regularly. Or they’d just snap.

Recently, most car makers have moved to electrically operated parking brakes. Instead of a cable, there’s a small motor on each of the rear calipers. And when you push a button in the passenger compartment, the motor activates and voila! The parking brake is on.

This not only eliminates all of the problems with the old cables, but – since all you need to operate your parking brake is a button – it also frees up space in the passenger compartment, which can then be devoted to more important things, like USB outlets and cup holders.

It also makes it incredibly easy to create an automatic parking brake.

You simply program the parameters into the car’s computer. For instance, you say “when the car is put into Park and the engine is shut off, engage the parking brake.” And “when the car is put into Drive or Reverse from Park, and the doors are closed and the seat belt is on, release the parking brake.”

That’s essentially what the Mercedes system does, with a few extra safety protocols.

And it’s hard to imagine a scenario where you’d need the parking brake to be off when the car was in Park. Or need it to be on when the car was in gear and you’re trying to accelerate.

So I suspect you’ll see more automatic parking brakes in the coming years. It makes a lot of sense.

Smoking valve seats need some time to seat properly

Dear Car Talk:

I have a 1952 MGTD that I restored about 10 years ago. I’ve put less than 200 miles on it since then. The engine has all new internal parts, and the head was milled out for hardened valve seats.

The problem is that it smokes a lot when running. Any idea what would be causing that? Thanks. – Joe

RAY: I'll bet your rings haven't seated correctly yet, Joe.

Unlike new cars, which basically come from the factory with zero space between the internal parts, cars of your MGTD’s vintage had parts that fit “close enough.”

Because of that, the so-called break-in period was very important. During that break-in period, you were told to vary the engine speed, and to not rev the engine too high. The reason for all that was to allow the rings to seat inside the cylinders.

By “seat,” we mean those parts wear against each other and eventually the rings conform to the exact shape of their cylinder walls.

How does that happen? Well, the rings, which go around the pistons, move up and down inside the cylinders and scrape oil off the cylinder walls just prior to each explosion.

And after going up and down hundreds of thousands of times, and scraping against each other hundreds of thousands of times, the rings and the cylinder walls eventually fit together better than they fit when they left the factory.

Until then, it’s likely that not all of the oil is getting scraped off the cylinder walls, so some of it will get burned up along with the gasoline.

Guess what burning oil creates, Joe? Smoke!

The seating process also requires that the cylinder walls be “honed” when you rebuild the engine. I trust you did that, Joe. “Honing” means to rough up their inner walls of the cylinders, which gives the rings a surface to work against.

I know this is not the kind of car you drive a lot, Joe, because, quite frankly, driving it in modern day traffic scares the hell out of you. And I don’t blame you. But the solution is probably to keep driving it and allow the rings to fully seat.

Back when this car was made, the break-in period was normally about 1,000 miles.

You’ve driven about 200 miles in 10 years. That’s 20 miles a year. So if my calculations are right, you should stop burning oil around Mother’s Day 2060. Write then, and let us know.

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