Answered on Quora: Are there any risks to using engine braking to augment braking power on my 1992 Ford F150 with the 300 cu. in. I6 engine and M5OD manual transmission?

Are there any risks to using engine braking to augment braking power on my 1992 Ford F150 with the 300 cu. in. I6 engine and M5OD manual transmission?

Daniel Becker, Patent Attorney and Early-stage Engineering (2013-present)

Answered 18h ago

I am not sure why any particular transmission or engine would make a difference here, but in general, no. At least not as to the engine itself, so long as there is not some component that is particularly susceptible to breaking. Even if there was a weakened component, you have to consider that the loads which are placed on the engine during acceleration are the same but in the opposing directions as those which are used when fighting the acceleration of the same engine.

I SUPPOSE that there are components which might not operate correctly if there is a sudden decrease in manifold pressure. Vacuum lines suddenly lacking their one-way bias. Those seem unlikely, though. If an engine does not stall, all of its components still ought to be operating in the same direction. It is beyond my experience, though.

What I am more confident-of is that the engine braking itself ought to be just fine on any engine which is able to survive similar forces when being used for accleration. Engine braking just uses the decreased combustion force of a part-throttle operating condition to fight the inertia of the crankshaft.

It is POSSIBLE that there are some one-way engaging components in the drivetrain, such as syncronizers and the differential, where the play between the engagement surfaces of the teeth on those gears could take a hammering, being loaded backward, as that means that they are pushing on otherwise previously disengaged surfaces, and particularly where there is a style of frequently bucking between forward and rearward acceleration loads, if you are constantly switching between high forward loads and high rearward loads.

You question’s literal language does not limit “risks” to just mechanical driveline failures, though. You have an old truck with a big heavy engine and short gear ratios, and it has nonlinearly loading leaf springs and an independent rear axle. It is a vehicle which is highly susceptible to a forward-weight shift during braking which could unsettle the rear end of the truck and cause it to spin.

With conventional braking, the braking force is biased to match the weight distribution, and the tires are generally specified so that front wheel grip will break first. This means that the loss of traction will presumably keep the rear wheels from slipping out from behind the vehicle, and spinning the vehicle off the road.

With engine braking, though, particularly in the case of a rear wheel drive truck, you are imposing a braking force JUST at the rear-end. This means that it is using a significant amount of the rear wheel grip to resist the engine’s movement, bringing the rear wheels closer to their grip limit. In the event of severe braking, this means that the forward weight shift could decrease the vertical load on the rear wheels, the available grip that can be generated at the rear wheels would decrease, and the engine braking may consume the remaining grip, locking up the rear wheels.

If the rear wheels lock up before the front wheels, then the center of gravity of the truck would be behind the point at which grip is available, and any turning force involved might swing the rear of the truck around the front wheels, causing a spin.

Hope that helped.

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