The only downside is it can be really loud as you shift down the gear and engine RPMs bounce around. Hence, all the signs when you approach a town that say, "No engine braking".
When correctly muffled, engine breaking is not really any noisier than the engine operating normally.
Truckers will sometimes install a valve to bypass the muffler, so they can let someone know that they just cut them off, and that they should try moving out of the way.
Engine braking is definitely always much louder than regular braking. Don't piss off your neighbors.
The bypass valve on semi's you're referring to is not the exhaust. The huge BRAAAAAAAP that you hear coming from trucks when someone cuts them off aren't an exhaust bypass valve, its them slamming on their brakes to avoid a tragic 20-car pileup.
It is a release of the compressed air-fuel mixture momentarily before combustion called a Jake Brake or Compression Release Engine Brake[0], that Semis and large commercial vehicles are equipped with that provide for additional braking in an emergency.
What they're doing is releasing the compressed air-fuel into their exhaust, just prior to combustion. This causes the engine speed to drop instantly, slowing the truck down. It will surprise you to learn that semi trucks have pretty terrible braking performance, loaded or unloaded. They all use drum brakes.
It is strictly prohibited to bypass a muffler or emissions control device in the US. While cars get away with it all the time, a commercial truck carries additional registration, inspection, and license restrictions which makes this not only dangerous, annoying, needless and harmful, but extremely expensive.
Commercial Driver's face much steeper fines for equipment that is out of specification. Many commercial drivers are required to inspect their vehicles before they depart for even so much as a top indicator light which is burnt out, or they may face harsh penalties and fines that would make your skin crawl. $10,000 citations are levied daily.
That's actually not surprising at all for air brakes in good condition; braking pressure is supplied entirely by air, and drum brakes are used because they have a much larger area of friction material than disc brakes, and thus can provide a higher stopping force in the same volume. Their main disadvantage is brake fade, which is why heavy trucks use their engine brakes very often.
Another fact that might seem counterintuitive at first is that a heavily loaded truck can stop faster than an empty one. This is because it has more weight on the wheels, which means more force before they start to skid, and the brakes themselves are not the limiting factor.
Depends on the size of the engine and the design of the brake. Big engines are louder than small engines. Jake brakes are louder than regular exhaust brakes.
