Golf R engine blown

[SMB]

Stickyfinger said:

Slamming on the brakes has an expected result, you are thus braced for that effect....try slamming on the brakes or down shifting 2 and dropping the clutch when your wife is doing her make-up and is not expecting it...... and tell me the result.

Yes, you WILL notice it.

The only reason you feel engine braking is the compression effort of the pistons trying to squeeze the air in the cylinders. We know in this case there is zero compression in ALL cylinders. We all agree it's an interference engine, so the valves and pistons have met and the top end damage to the point there is no compression. Now consider the engine speed, 8500 rpm, that means in 1/50th of a second the engine has done a complete cycle at least once if not twice. So in that 1/50th of a second all compression is lost.. so what is creating the engine braking now.? Answer nothing.

[Stickyfinger]

SMB said:

The only reason you feel engine braking is the compression effort of the pistons trying to squeeze the air in the cylinders. We know in this case there is zero compression in ALL cylinders. We all agree it's an interference engine, so the valves and pistons have met and the top end damage to the point there is no compression. Now consider the engine speed, 8500 rpm, that means in 1/50th of a second the engine has done a complete cycle at least once if not twice. So in that 1/50th of a second all compression is lost.. so what is creating the engine braking now.? Answer nothing.

So, like a quick Dab on the brakes then.....yeh, will not spoil the make-up ...ummmmmmm

[Gav147]

Blaster72 said:

It happens

https://youtu.be/uGVXYH5Mv1o?t=9m44s

9 min 44 secs in and 14.20 in. I suppose we'll find out from his future vlogs what the damage was on the GT86 for a similar mishap.

Haha I was watching that earlier and the first thing I thought of was this thread.

[SMB]

Stickyfinger said:

So, like a quick Dab on the brakes then...

Not really, damage done in an instant, compression lost no braking

[Stickyfinger]

Just the sudden loss of momentum during acceleration would be obvious.....I also think the human nervous system is a little bit better at detecting "the moment" than you seem to be.

[SMB]

Stickyfinger said:

Just the sudden loss of momentum during acceleration would be obvious.....I also think the human nervous system is a little bit better at detecting "the moment" than you seem to be.

He was decelerating not accelerating, so was expecting a loss of momentum.
other than the light forces required to bend valves there is nothing for the human to perceive.

Valves bent as they were still open, open valves equals no compression, once bent no compression .

[justinio]

Blaster72 said:

It happens

https://youtu.be/uGVXYH5Mv1o?t=9m44s

9 min 44 secs in and 14.20 in. I suppose we'll find out from his future vlogs what the damage was on the GT86 for a similar mishap.

Seems to be a recurring theme for GT86's

https://www.youtube.com/watch?v=X2elxbHxgC0&li...

[problemchild1976]

wasn't he downshifting though?

JJ

[TimmyMallett]

He was probably granny shifting and not double clutching yo.

[helix402]

That’s the problem when you live your life a quarter of a mile at a time.

[silentbrown]

SMB said:

Not really, damage done in an instant, compression lost no braking

Can't remember the vid I was watching but basically engine braking is caused by the throttle butterfly being shut so the intake stroke of the engine has to work really hard. Any braking caused by the compression stroke is largely negated by the same air expanding during the power stroke, so a loss of compression gives no significant change to engine braking.

This is why diesels (no butterfly) need something like a Jake Brake.

[SMB]

silentbrown said:

Can't remember the vid I was watching but basically engine braking is caused by the throttle butterfly being shut so the intake stroke of the engine has to work really hard. Any braking caused by the compression stroke is largely negated by the same air expanding during the power stroke, so a loss of compression gives no significant change to engine braking.

This is why diesels (no butterfly) need something like a Jake Brake.

Except both inlet and exhaust valves are shagged it can draw limitless air through the exhaust Your cant compare how a good engine would respond to one with open valves .

[lucido grigio]

Should've dominated the gear stick.......

[Super Slo Mo]

silentbrown said:

SMB said:

Not really, damage done in an instant, compression lost no braking

Can't remember the vid I was watching but basically engine braking is caused by the throttle butterfly being shut so the intake stroke of the engine has to work really hard. Any braking caused by the compression stroke is largely negated by the same air expanding during the power stroke, so a loss of compression gives no significant change to engine braking.

This is why diesels (no butterfly) need something like a Jake Brake.

Nice idea. Unfortunately it’s not really true.

If it were, the butterfly would create a vacuum on the intake which would pull the piston up on the compression stroke. Which is the opposite of the diesel theory.

In practice, diesels have lots of engine braking compared to an equivalent capacity petrol.

Trucks and buses can have systems that open the exhaust valve during the power stroke to release some of the spring effect, they can also have a valve or butterfly in the exhaust to massively restrict it during overrun conditions.
Both give increased engine braking.

[problemchild1976]

you see those signs in USA for lorries to only carry out engine breaking during acceptable times (i.e. not at night) as its soooo loud

jake braking


JJ

[silentbrown]

Super Slo Mo said:

Nice idea. Unfortunately it’s not really true.

If it were, the butterfly would create a vacuum on the intake which would pull the piston up on the compression stroke. Which is the opposite of the diesel theory.

I think you're right but you're wrong
The partial vacuum will indeed help "pull" the piston on the compression, so intake and compression stroke largely cancel out - But now for the power stroke you have an empty cylinder, so a vacuum is formed as the piston descends, and that takes power.

Wikipedia seems to agree with throttling as the explanation. https://en.wikipedia.org/wiki/Engine_braking

[helix402]

Has anyone considered that perhaps this thread is quite a lot of topic and should be closed?

[Blaster72]

What is it with random people asking for threads to be closed?

[Super Slo Mo]

silentbrown said:

Super Slo Mo said:

Nice idea. Unfortunately it’s not really true.

If it were, the butterfly would create a vacuum on the intake which would pull the piston up on the compression stroke. Which is the opposite of the diesel theory.

I think you're right but you're wrong
The partial vacuum will indeed help "pull" the piston on the compression, so intake and compression stroke largely cancel out - But now for the power stroke you have an empty cylinder, so a vacuum is formed as the piston descends, and that takes power.

Wikipedia seems to agree with throttling as the explanation. https://en.wikipedia.org/wiki/Engine_braking

True enough. Still, if that were the reason engine braking works, then a petrol engine would give more braking than a diesel of equivalent size, but it doesn’t. Even when taking into account the fact that the diesel will have a higher overall drive ratio to the wheels which also reduces engine braking’s effectiveness.

[liner33]

Wikipedia is dated , lots of cars have throttle butterfly valves that stay open whilst the engine is running ie Nissan 370z and are not used to control engine speed