On a recent episode of Mythbusters they were stopping a runaway car (a scenario where the driver was incapacitated but still had their foot on the accelerator) by driving in front of it and braking. They almost lost control but they managed to stop the car.
My question is how much does this hurt the powertrain of the runaway car? I couldn’t tell from what I saw of the episode if anything (besides the bumper) was damaged. If the accelerator was still pushing for the car to go 75 mph but the car couldn’t, I get the feeling something must have snapped, popped, cracked, or exploded from some pressure somewhere.
If the runaway car has a manual transmission with the throttle stuck on full, then the car will stall once the lead vehicle manages to get the speed below a certain point. That point would vary depending on what gear the vehicle was in before the driver lost control.
Excessive engine wear due to lugging is possible but not certain.
The vehicle’s powertrain being damaged is possible but not certain.
In the more likely (and more dangerous) scenario involving an automatic transmission, then once you force the vehicle to a stop, it would continue fighting you until something broke, you managed to open the doors and turn the key, or it ran out of gas.
The odds of something breaking when a given automatic transmission equipped car is run at full throttle depend on how long it takes the car to be stopped.
It could go either way. I can tell you that I’ve seen vehicles held at full throttle with the brakes applied in the past, and powertrain failure did not occur. At the same time, I never saw anyone do it for more than 5 seconds or so.
The tires would probably be the weakest link. The contact patch is incredibly small. And, assuming 2WD, there’s only two of those. The other two are just along for the ride and help with braking and cornering. I didn’t see the episode though, so this just IMHO.