Piston Slap: Swift Action on Engine Braking

Michael K writes:

I have 2008 Corolla with manual shifter. There is almost no engine braking power when I downshift. I know it’s not a malfunction (it’s how it’s designed), but I would like to understand why.

For example, when I’m approaching a red light and I downshift, the car barely slows down. Even if I downshift to even a lower gear to keep the RPMs high, there is still very little braking power provided by the engine. Or, going downhill and downshifting to keep the car’s speed stable without touching the brakes works only on small hills. On a steeper downhill the car continues accelerating. Of course, it’s not as bad as if the car was in neutral, but still…

Before this Corolla I had a 97 Suzuki Swift with manual. One of the cheapest cars on the road, but the braking power of the engine was good.

Sajeev replies:

The mechanics of engine braking are simple: instead of stopping the wheels at the source, you drag on them with the engine’s rotating mass. But there’s much, much more going on: factors like engine vacuum under negligible load and compression ratio determine the “plushness” of the piston’s pillow during the compression cycle. The softer the pillow, less engine braking. And I suspect the Corolla’s piston pillow is Hyatt-worthy.

But I reckon vehicle weight is the real problem. A low compression engine in a 2600lb sedan will not engine brake like an 1800lb hatchback with the rolling inertia of tumbleweed. Just an educated guess . . .

Bonus! A Piston Slap Nugget of Wisdom:

I may live in tabletop-flat Houston, but engine braking is only for killing speed over long periods of downhill travel. Approaching a red light on a city street (even the Streets of San Francisco) should involve coasting in gear until you can pop the shifter in neutral. This allows for maximum control and minimizes engine and clutch wear.

And if that sounds like a load of trash, consider the fuel economy benefit of low engine speeds. Much like a hybrid’s regenerative brakes, you can use the stop pedal to save Mother Earth. How great is that?

Or consider your wallet: brakes are one of the cheapest subsystems to fix. Powertrains are obviously not. The most brake wear is on the front axle: my last front brake job set me back $60 for (upgraded) Performance Friction pads and another $60 in rotors. Even if parts and labor is $250 for a front brake job every 30-40k, that’s but a chunk of the fuel and powertrain costs of unnecessary engine braking.

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