Norm Peterson
corner barstool sitter
I think a better description than "My driveway isn't very steep" is in order before this goes much further. I've had a car creep about one car length down a slope over the time it took to eat lunch (different car, MT, P-brake not set).
Engine compression and ring friction both contribute to an engine's capability for resisting the car rolling in gear (engine off). Consider the two together to be an "engine braking torque", where the friction is more or less constant but the compression of air in the cylinders is not. If the static friction component isn't enough to hold the car still you're left counting on compression - which can and will leak down whether the car is moving or not. If/when it leaks down low enough, the car will roll slightly until the next cylinder in the compression stroke sequence starts adding its compression resistance. The cycle can repeat, with the car doing a jerky little start-stop creep downhill.
Transmission gearing matters - the lower the gear the more effective the engine is as a P-brake. Reverse may or may not be a better choice (the ratio still matters, but the reverse gear profiles may not offer as much friction as the forward gear helical gears do). Same goes for axle gears, so if you've got 3.15's you're not in as good of a situation as an otherwise identical car with 3.73's would be.
Car weight matters - are you needlessly carting around any heavy stuff?
Tires have a little influence (tires that are taller than stock or of much lower rolling resistance putting you at a disadvantage here).
Norm
Engine compression and ring friction both contribute to an engine's capability for resisting the car rolling in gear (engine off). Consider the two together to be an "engine braking torque", where the friction is more or less constant but the compression of air in the cylinders is not. If the static friction component isn't enough to hold the car still you're left counting on compression - which can and will leak down whether the car is moving or not. If/when it leaks down low enough, the car will roll slightly until the next cylinder in the compression stroke sequence starts adding its compression resistance. The cycle can repeat, with the car doing a jerky little start-stop creep downhill.
Transmission gearing matters - the lower the gear the more effective the engine is as a P-brake. Reverse may or may not be a better choice (the ratio still matters, but the reverse gear profiles may not offer as much friction as the forward gear helical gears do). Same goes for axle gears, so if you've got 3.15's you're not in as good of a situation as an otherwise identical car with 3.73's would be.
Car weight matters - are you needlessly carting around any heavy stuff?
Tires have a little influence (tires that are taller than stock or of much lower rolling resistance putting you at a disadvantage here).
Norm
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