> i have a nissan altima 2008 - it's an automatic that can be thrown into a
> 'tiptronic' mode where you can shift up and down. while test driving it, the
> salesman said he like to use the tiptronic for engine braking.
>
> so, i've played with it some, and it just sounds harsh on the engine. i mean,
> there's obviously no gas being used, but tossing it into a low gear while going
> 30-50mph (i go down gear by gear... 5th, 4th, 3rd... etc) the rpm's still ramp
> up and the engine sound binded up.
>
> is there any risk of harm to the gear by engine braking? or is it something
> that can be done very slightly, or is it ok to just tap down into 3rd gear doing
> 45mph?

The gears and transmission, kinda. The engine, definitely. Compression braking isn't all that great for the engine. It puts more wear and tear on the engine, particularly because you're making it run at high revs. Also, I don't know about the more modern semiautomatic transmission designs, but older slushbox automatics were kinda harsh when dropping gears like that, like downshifting a stick without blipping the throttle and matching revs. Any kind of jerks like that are bad for the drivetrain.

Also, you are using gas. The ECM keeps flowing fuel to the engine even at closed throttle in order to keep it lit. Granted, it's small compared to wide open throttle, but it's there, and two to three times more than if you left it in the higher gear.

That said, it's a valid thing to do to get more braking power, and they do it all the time in racing, but normally it's engine braking in conjunction with the middle pedal, and they're not really concerned with longevity past, say, a few hours.