Turbo Backpressure?

[benough]

I have noticed something in my GT TDI.

If you are in 2nd or 3rd gear and you rev it to the redline, then back off and leave it in gear, as the revs come down, near 3500rpm, the engine shudders a bit.

It doesn't do it everytime but I can get it to do it often enough.

Is this back pressure causing the turbo to, um, shudder?

On the same topic, does anyone here experience if you shift into say 2nd or 3rd below boost, that when it does finally boost that the engine shudders as it kicks in?

Is my turbo a turkey? Is it a TDI trait? I haven't experienced this is in turbo petrol cars but that said I haven't spent much time in them

[Greg Roles]

I'll go to my private test track and get back to you. I notice my car is "rough" just around the 1800-2000rpm range, where it's starting to boost, and the injection and VNT vanes are no doubt all over the place. You'd kind of expect that though, given the wack that comes after that. If you wanted a smoother diesel, then there's the 103...which still goes pretty darn well for a weezy diesel...

[gldgti]

Is this back pressure causing the turbo to, um, shudder?

nope. apart from noise and boost, you would be unable to notice anything the turbo itself is doing under any conditions...... its far too small and light!

On the same topic, does anyone here experience if you shift into say 2nd or 3rd below boost, that when it does finally boost that the engine shudders as it kicks in?

ok.... a quick lesson in diesel.

diesels have a high compression ratio and big, heavy pistons. a diesel engine has a lot of work do do (in thermodynamic terms) to comrpess each stroke. because there is a large reciprocating mass, and a lot of compression to achieve.

in answer to your first question - the vibration you are feeling is most likely just the engine acting as a big pump. when you boost like crazy and then let off at high revs, 2 things happen.
1) the fuel injected drops to basically zero, resulting in a complete loss of torque production during the power stroke.
2) the turbo is still spinning really fast and making boost too, and theres a large amount of compressed air in the intake (in the feed pipes and intercooler) that feeds into the cylinders as usual. (remember theres no throttle body.)

imagine a graph of cranskshaft torque vs crankshaft angle (from TDC).

when the engine is decelerating, and the mass flow rate of air is changing dynamically and rapidly, the torque transmitted through the crank to the flywheel (this may be expressed as 'negative torque') is "biased" compared with the normal operation of steady engine speed. imagine large 'gaps' in the curve, as where there would normally be a power stroke, there is a much smaller positive peak due to normal expansion of the compressed air.

this means that there is a greater than usual "pulsating" input to the drivetrain, and as such you feel some vibration....

[jayjay]

haven't done thermo yet but i think i can now actually calculate all those torques... . lol

[Logzy]

Cant say ive really experienced what you have but then again im chipped.
I remember after chipping just how much smoother it felt. Now im used to it it just feels so smooth.

[Greg Roles]

Can't say I could notice it in my car today, on the private test track.

[benough]

ok.... a quick lesson in diesel.

diesels have a high compression ratio and big, heavy pistons. a diesel engine has a lot of work do do (in thermodynamic terms) to comrpess each stroke. because there is a large reciprocating mass, and a lot of compression to achieve.

in answer to your first question - the vibration you are feeling is most likely just the engine acting as a big pump. when you boost like crazy and then let off at high revs, 2 things happen.
1) the fuel injected drops to basically zero, resulting in a complete loss of torque production during the power stroke.
2) the turbo is still spinning really fast and making boost too, and theres a large amount of compressed air in the intake (in the feed pipes and intercooler) that feeds into the cylinders as usual. (remember theres no throttle body.)

imagine a graph of cranskshaft torque vs crankshaft angle (from TDC).

when the engine is decelerating, and the mass flow rate of air is changing dynamically and rapidly, the torque transmitted through the crank to the flywheel (this may be expressed as 'negative torque') is "biased" compared with the normal operation of steady engine speed. imagine large 'gaps' in the curve, as where there would normally be a power stroke, there is a much smaller positive peak due to normal expansion of the compressed air.

this means that there is a greater than usual "pulsating" input to the drivetrain, and as such you feel some vibration....

I have read this a few times now and I think I get it.

So it kinda turns into a compressor then? Like a truck brake?

What about when it's spooling up? The shudder is MUCH more noticable there than in the top end.

[gldgti]

I have read this a few times now and I think I get it.

So it kinda turns into a compressor then? Like a truck brake?

What about when it's spooling up? The shudder is MUCH more noticable there than in the top end.

yes- its fair to say that it acts a bit like a compressor - it uses some energy, and doesnt give it all back.

anyvibration you feel at low revs when spooling up is (likely) because you are producing large torque spikes at low engine speed. its more noticable and more easily transmitted through the engine mounts (because the frequency is low). traditionally speaking, this is why diesels have a reputation of being "rough".