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DEAR CAR TALK: Why is it that sometimes, when you see a car in a movie, the car is moving forward but its wheels are rotating backward?

-- Stan

DEAR STAN: Good question. It has to do with the "frame rate" of movie film.

Imagine you're in a room that's pitch black. And there's a single car wheel in the room at the end of an axle, so you can spin it. Now imagine you start spinning the wheel, and you turn on a strobe light. The first time the strobe light flashes, you catch a frozen glimpse of that wheel, wherever it happens to be the moment the light flashes on, right?

Now let's say the speed of the wheel is perfectly matched up to the speed of the strobe light. So every time the light flashes on, the wheel has made one complete rotation. Since you only see the wheel in the same position each time, the strobe light would fool you into thinking the wheel is not moving at all -- when it actually is!

Now let's say the wheel is turning a little slower than the speed of the strobe light. So the wheel makes a little less than one full rotation between flashes. Each time the light flashed on, the wheel would appear to be slightly "behind" where it was last time you saw it. That would effectively trick your eye into thinking the wheel was going backward, even though it's going forward, just more slowly.

Movies work kind of like that strobe light. Most film is shot at a rate of 24 frames per second. Normally, that's fast enough so your eye doesn't notice that it's seeing 24 individual pictures every second. What you see is smooth motion. But when you get something that's moving very quickly, like the spokes of a car wheel, you can get that "strobe" effect. And depending on how fast the wheel is turning, compared with the 24 frames per second the movie camera is taking pictures, the wheel can appear to be moving forward, backward or standing still.

We've tried to take advantage of this phenomenon at the garage. When we fail to fix a customer's car, and they call up to complain that it won't move, we'll sometimes ask them if it could be an optical illusion. Nobody's fallen for it yet, Stan.

DEAR CAR TALK: I have a project truck. It's a 1979 Ford F-100 with a 300-cubic-inch inline six-cylinder engine. Before I had it bored out to 30/1000s, it didn't ping. Now, using the recommended spark plugs, it starts pinging under very light load as soon as it warms up. I am assuming that's because of the increased compression ratio, which is causing a higher cylinder temperature. Everything else -- timing, carbon buildup, fuel mixture -- seems fine. I haven't checked the exhaust gas recirculation (EGR) yet, though.

So, am I right about higher compression and higher temps leading to pinging? I have been unable to find cooler spark plugs that will fit. Could I run a cooler thermostat instead? Thanks!

-- Don

DEAR DON: Good thing this is a project truck because I project you'll by tinkering with this for a long time. Here's the problem: We all know that the original engine was durable, but underpowered. Or, as we say in the business, with no disrespect meant to our canine friends, that six-cylinder engine was a dog. Most people who bought the truck with that engine wished they had opted for the V-8.

But instead of making your "project" replacing this engine with a V-8, you tried to increase its power. So you increased the size of the cylinders, and then I'm guessing you decided to replace the pistons with some differently shaped Mount Kilimanjaro ones that would generate more power. In doing so, you increased the compression ratio. And that's probably why it's pinging.

You should check the EGR valve and make sure it's working properly. The job of the EGR is to inject some exhaust gas into the fresh charge to cool it off and reduce pinging. So, hope that the EGR is not working, because that would be your easiest fix. Unfortunately, there are no cooler spark plugs or thermostats that will resolve this.

If the EGR is working correctly, then you have two options. Since you appear to have money to burn on this truck, Don, one option is to spend an extra 50 cents a gallon on 93 octane fuel, and see how much that helps. Alternatively, you can try to retard the timing. But that's going to have to be done through trial and error. You'll need to tinker with it and look for some middle-ground timing setting where the pinging goes away but you still have sufficient power.

If you're lucky, you'll be able to set it so the pinging stops, and you'll have almost as much power as you had before you re-bored the engine and installed those stupid pistons. Or you could just let it ping. And when the pinging melts your new pistons, put in a V-8 instead. Good luck, Don.

Ray Magliozzi dispenses advice about cars in Car Talk every Saturday. Email him by visiting:

HomeStyle on 11/10/2018

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