Dear Tom and Ray:
For as long as I can remember, my dad has known the answer to everything practical; he's not the guy to talk to about matters of the heart, but for anything else he remains my go-to guy -- with one exception. I can recall sitting between my parents in the relative safety of the front seat of our Vega, quizzing my dad about car stuff and asking a question that has remained unanswered for 35 years now: What happens if you press the brake and the accelerator at the same time? When I asked him, he said, "Well, it wouldn't be good, I can tell you that," and wouldn't elaborate.
Throughout the years, I have asked him again, and asked my male friends, boyfriends and even the jerk I was married to the same question, only to be told variations on the same theme: "Well, I don't know, but it wouldn't be good." Fast-forward to the present. A couple of weeks ago my 13-year-old daughter asked me what would happen if the brake and accelerator were pressed at the same time, and I had to confess my ignorance. However, I did promise her that this cycle of ignorance would end with her. Being a very bright child, she suggested that I ask you guys, and so I am doing just that: Would you please tell me (bearing in mind that it wouldn't be good) what would happen if I stepped on the gas and brake simultaneously? We thank you in advance.
-- Anthea and Rachel Miller
TOM: Well, it wouldn't be good, I can tell you that!
RAY: Actually, we do it all the time in the garage. If you plant your foot on the brake hard enough and then step on the gas, you are mimicking a situation in which there's a load on the engine (a load means the engine is working to move the car).
TOM: And there are situations in which it's helpful for us to see what happens when the engine is under load, without having to strap my brother to the underside of the hood and drive up a hill at 50 mph.
RAY: For instance, there are things called motor mounts that hold the engine in place. If we suspect that a car has a broken motor mount, we can check by putting a load on the engine and seeing how much the engine moves. If a motor mount is broken and we step on the brake and gas at the same time, we'll see the engine actually lift up out of the engine compartment.
TOM: Or, if the motor mount's not broken, we can break it that way!
RAY: Exactly. Now, when we step on the gas and brake simultaneously, we do it for only a few seconds. And if you do that and your foot is absolutely planted on the brake, nothing will happen. The brakes don't care. And the engine and transmission think they're working to move the car, so they don't care.
TOM: But if you do it for a much longer time, you might damage the automatic transmission's torque converter.
RAY: Right. When you're at idle, the torque converter spins slowly in transmission fluid. But if you make it spin quickly, without allowing the fluid to transmit power to the wheels, you could damage the torque converter. Or, at the very least, you'll overheat and degrade the transmission fluid, which will shorten the life of the transmission.
TOM: And that wouldn't be good.
RAY: So that's what happens if you step on the brake hard enough to keep the car from moving. What if you let up on the brake a bit, and allow the car to move while stepping on both pedals? Or engage the brake when you're already moving and stepping on the gas? Then you risk overheating your brakes.
TOM: Right. When brakes overheat, the brake fluid can boil. And boiling brake fluid doesn't work, so you'd have no way of stopping the car. And what can we say about that?
RAY: All together now: It wouldn't be good.