I don't know if there were any posts on this or if i did'nt search right or what, but here goes anyway.
How exactly does engine braking or compression braking work? I figure with the throttle body closed there is'nt much air coming in and therefore not much fuel, so when your coasting what exaclty are the pistons firing on? the enigine is still rotating at a higher rpm than idle but its not getting anymore fuel, or is it? Since the engine is being sorta forced to these rpms is there any detrimental effects, or does the ecu compensate for the closed throttle body?Feel free to flame me if you find this stupid but i am just curious.

Engine braking (also known as compression braking) works because the engine has to compress air/fuel before it ignites it. When you engine brake, the ECU actually stops delivering fuel to the engine, it compresses only air, creates an extremely high level of vacuum, and slows the whole car down. The engine is still doing all the work to compress the intake charge but is generating no power from it.

There are no deterimental effects from engine braking.

Originally posted by admin2
creates an extremely high level of vacuum

well not really an 'extreme' level of vaccum....normally -25hg on deceleration (compression braking, engine braking, etc etc)

awesome explanation though :thumbsup

edit: and if you watch your a/f when compression braking it will normally dip to 20.0, or meaning pretty much just air and no fuel like dino said

Thanks, Thats exactly the type of explanation i've been looking for. Its just been something that i've been thinking about latley and could'nt come to any conclusions of my own.

Originally posted by turbo8u

Originally posted by admin2 creates an extremely high level of vacuum

well not really an 'extreme' level of vaccum....normally -25hg on deceleration (compression braking, engine braking, etc etc) awesome explanation though :thumbsup edit: and if you watch your a/f when compression braking it will normally dip to 20.0, or meaning pretty much just air and no fuel like dino said

yea it goes full lean on my autometer.........whoo hoooo