Engine braking power

[dnegative]

Is there a rule of thumb on how much power is required to provide maximum engine braking?
Very vague, but say you have a modern 200hp L6 engine, will you require more power than the engine outputs to turn it over and provide vacuum at high rpm?

[fixeruperer]

i dont understand your question. power has nothing to do with engine braking. compression does. ? throttle closed at high rpm is going to have high vacuum on anything.

[oldcorollas]

will you require more power than the engine outputs to turn it over and provide vacuum at high rpm?

of course, otherwise there'd be no engine braking

do you mean power required to turn engine while making max power?
or power to turn engine at high rpm but throttle closed?

[dnegative]

I know this is as vague as shit...

To turn a 2000cc 6 cylinder engine over at 6500rpm pulling (idk... 40 inHg, throttle closed) must require decent power otherwise your car wouldn't have any engine braking yes?
Would the power required be more than the engine could produce?


Reason I'm asking, someone has said that under maximum engine braking you put significant stress on a gearbox. I'm trying to figure out exactly how much power would be driving a gearbox to do that. I think these days its probably less than what the engine would produce, so a box rated to cope with an engine of 200hp should have no issues with engine braking.

Which leads to another question, how much braking HP can a petrol engine of a given size give you?

[banana_socks]

The gearbox is definitely built to handle more than what you'd get from engine braking. You're limited by traction. If the engine braking was strong enough you'd lock up the wheels (which can happen, compression lockup) and that unloads the drivetrain. Doesn't matter how much stronger the engine braking is, you get no more load then the tyres can support.
Will it load up the gearbox? Yes. If you're constantly dumping the clutch in low gears and shock loading it, things might break. But generally its not a problem.

[dnegative]

That's what I assume, I've never heard of anyone lunching a box under braking. Peak torque and shockload yeah

[oldcorollas]

diesels can have more engine braking "power" than the power they make..

petrol engine, will depend on cam overlap etc..

I do remember seeing some power figures from people turning over V8's in order to study airflow etc... was not small.. can't remember where or how much tho

[dnegative]

diesels can have more engine braking "power" than the power they make..

Provided you crack the exhaust valve at TDC compression chugga chugga chugga
Think you get 10% on top of your rated horsepower


...........I think that answered my question though.
If a diesel with a compression brake can only get 110% of rated HP in braking what chance does a petrol have with the air helping push the piston back?

[boxh34d]

Diesels creating more power engine braking than what they can 'create' is only possible with the likes of a Jake brake. Exhaust brakes can get close to 80%.
Typical figures I've heard before for braking power of an 'unassisted engine' are around 20 to 30% of the available power at a given rpm. Eg, if your engine can create 200HP at 6000rpm, then you'll have 40 - 60HP engine braking at the same rpm.

[dnegative]

I've heard figures of 400+ of braking HP from 16,000cc
These days most make more (motive) than that though but a while ago I guess you could have more braking power than motive

[boxh34d]

As stated, heavy duty diesels with jakes do it easily. 15L cummins rated at 580HP can get nearly 650HP braking effort.

I've never heard of light duty stuff getting anything even close.

[oldcorollas]

...........I think that answered my question though.
If a diesel with a compression brake can only get 110% of rated HP in braking what chance does a petrol have with the air helping push the piston back?

indeed.. compression can be 10:1, ie 10 atmosphere pressure (talking static here)... but vacuum can only be "-1 atm"..

so there is a limit on the force that can be exerted on pistons in petrol engine.. ie 14.7psi

[Sam_Q]

what I want to know is why you would want to do any engine braking?

[motard_mike]

surely this can be measured like this...... time how long it takes to do 80-120 km/h in say third then lift off and see how long it takes 120-80. wont that tell you what you need to know? sure wind resistance and drive train drag and blah blah but im sure it will take longer to slow down than it did to accelerate? give it a go.

[banana_socks]

The maximum engine braking for a petrol engine will be near the redline. You could wring it out in first, then back off and see what happens.
Kind of a pointless test though, as I said before it can be more than the traction limit of your tyres. How much more is irrelevant for stress on the gearbox.