Startup RPM

[nomadx38]

Yeah diesels don't do a O2 sensor warm up stage (from what I can tell with the diesels I have driven).

I'm taking a stab at it but I think it could be that there isn't any O2 sensors or no catalytic converter (as it has a DPF) on a diesel...? I'm sure it has to do with one of these.

Guess it could also be that diesel exahust gas is hotter and warms up the sensors quickly anyway.

[LunchboxVRS]

Diesels Still have a cat. Usually in with the DPF. The lambdas on both have a heater built in so the high revs are just for the engine to reach op temp quicker as thats when itsto emissions are at the right level.

[EDM12T]

It has something to do with the brake circuit controlling the turbo.

The revs don't actually change (well only a slight bit) but the exhaust / engine note changes quite dramatically. I think what happens is when you hit the brake it automatically opens the waste gate on the turbo and it vents the boost which changes the sound of the engine/exhaust.

Again pretty normal

Good to hear mate, relieved now. Ever since i had noticed it, i never layed a foot on the brake again until it had warmed up as i was thinking it was somehow stopping the cold start.. Thanks for that.

[amazeer]

There is no boost pressure at idle. Yes the turbo will be spinning just like a fan freewheeling in the wind but it is not making any boost. If you hold your foot flat to the floor with no load, you will still have no boost. Stomping on the accelerator in neutral you might momentarily hit 1 or 2 psi boost, thats it. The brake circuit has no effect on the turbo or waste gate. Put your foot hard on the brake with your foot hard on the accelerator. You are still making boost. Unless the new fangled funky polo has some kind of electronic influence.

The wastegate controls maximum boost pressure. When your inlet is seeing the desired maximum pressure the wastegate opens to bypass some of your exhaust gas around the turbine wheel. When you take your foot off the accelerator, there is no load on the engine, it is not making any boost, the wastegate is closed. When you snap the throttle closed there is a massive reverse shockwave that goes back towards the turbo compressor wheel and tries to stall it. Its that shockwave bouncing between the turbo and the throttle body that causes the flutter sound.

A small momentary change in idle when pushing the brake pedal could be caused by the brake booster stealing a little of the vacuum. If there is a leak somewhere somehow in the power brake diaphragm then it would be worse. If only 100 or so rpm then I wouldnt be concerned.

The difference in the size of a piston at a 20 degree darwin morning compared to a 0 degree canberra morning is less than 0.05mm. The cylinders have only shrunk 0.025mm. You have more clearance in cold start than hot. In any case the engineers know about thermal expansion and they wouldnt make the clearance that tight (would they?). These days they are making them looser than ever to reduce friction.

[nomadx38]

Actually yes, the turbo does have an electronic influence - that been because it is 100% electronically controlled by a electronic actuator.

And the brake circuit does effect it's operation as it opens and closes the waste-gate to reduce turbo lag. Also with DSG models you cannot brake and accelerate at the same time - the brake circuit disengages the throttle. It's this reason that there is no turbo flutter in these engines (or extremly little when the air-con is on - I have never hard it on mine though) - there is no blow off valve to reduce charged air after the turbo so instead the wastegate opens up automatically to reduce buildup.

And we're not talking about boost as such at idle but the difference in sound the engine / exhaust makes when the brake is pressed and it opens / or closes the wastegate on the turbo.

And when the engine is too cold the oil will be thicker which makes it hard to pump it to the places it needs to be which I assume is the reason it doesn't do an O2 sensor warm up (1200rpm idle) when the temp is under 10

[amazeer]

I checked my car on the way to work. Manual tsi. It too cuts the throttle with foot on the brake pedal! So I put my hand up as victim of talking through arse syndrome.

[ecks]

The real issue at hand here, is that we now know that when you heel-toe, you do so with the wastegate open.

[nomadx38]

Haha don't feel bad, this is how we learn. And in many ways what I thought was right wasn't exactly (can anyone say ohms!) haha.

Heel toe could be possible but it depends on how the turbo manages it. I'm not exactly sure how the turbo determines whether to open or close the waste-gate. It could close it thinking I needs boost from take off or open it until peek boost comes in so as to not starve the engine of air.

I do know you can't heel toe (or left for on the brake and right on the gas) with the DSG gearboxes. It's also the reason people complain of the DSG not engaging and slow to respond - that been their foot is probably just resting on the brake and cutting the throttle.

The diesel polo may be exempt from this system as there is no throttle body so excess boost and turbo flutter could never occur. It probably just a normal mechanical waste-gate instead of an electronic one

[VWplo]

wait so if i put my manual tsi in neutral , put one foot on the brake and the other on the gas, it wont rev? or will it just rev with the wastegate open?

[amazeer]

not sure the effect of the brake pedal but the engine will only rev to about 4000 in neutral... or was I in gear with the clutch in... not sure.