TDi Torsion value.
Discussion
After reading a few posts mentioning ‘torsion value’ on VAG engines with regards to rough running and fuel consumption after cam belt replacement, I thought I would give a simple explanation, especially as VAG doesn’t include this in the syllabus for technicians at Milton Keynes.
‘Torsion value’ as displayed in MVB (4)on the 5052 VAG machine or VAG COM is basically an accurate timing value for the cam that has a sensor pointing at it. On the 2.0 Tdi this is normally the inlet cam.
When the engine is still on its original timing belt this value is normally 0° or very close to it. When the timing belt gets replaced with the original VAG tools, this value quite often moves to values up to -3° or +3°. This happens because the locking tools and timings belts are not quite as accurate as one is led to believe.
Popular belief that this changes injection timing on PD engines is wrong. The cam lobes that compress the injector do so for about 45° and the actual injection timing is done by the ECU closing off the main return flow in the PD injector.
However, cam timing itself does have a fairly large effect on smooth running, fuel consumption and torque curve. Older people that have played with cam timing on Ford Pinto, Alfa Nord etc. engines will know what I am talking about.
On the VAG 2.0 Tdi PD engine it is quite clear that when the ‘torsion value’ is around -3° (inlet cam timing retarded by 3°), peak torque is moved around 200 revs further up the scale and the engine is a bit happier to rev to higher RPM, albeit with slightly less power. This together with the odd misfire at idle, depending on temperature and idle load.
The same engine running a torsion value of +3° (inlet cam advanced by 3°) will display decent low down torque, but is not happy with higher revs. This also seems to provide for more pronounced misfires at idle and seems to increase fuel consumption (as viewed in MVB 15).
When the cam timing is set back to 0° (+- 0.5°) the engine regains its original torque curve and power, as well as the engine running smoother.
To keep things simple, I suggest that when adjusting the inlet cam timing on the twin cam TDi engines, the exhaust cam is adjusted a similar amount. For the really anal ones amongst us, but without the use of old fashioned cam timing tools, it is possible to fine tune the exhaust cam for optimal fuel consumption as viewed in MVB 15 with very, very small adjustments.
‘Torsion value’ as displayed in MVB (4)on the 5052 VAG machine or VAG COM is basically an accurate timing value for the cam that has a sensor pointing at it. On the 2.0 Tdi this is normally the inlet cam.
When the engine is still on its original timing belt this value is normally 0° or very close to it. When the timing belt gets replaced with the original VAG tools, this value quite often moves to values up to -3° or +3°. This happens because the locking tools and timings belts are not quite as accurate as one is led to believe.
Popular belief that this changes injection timing on PD engines is wrong. The cam lobes that compress the injector do so for about 45° and the actual injection timing is done by the ECU closing off the main return flow in the PD injector.
However, cam timing itself does have a fairly large effect on smooth running, fuel consumption and torque curve. Older people that have played with cam timing on Ford Pinto, Alfa Nord etc. engines will know what I am talking about.
On the VAG 2.0 Tdi PD engine it is quite clear that when the ‘torsion value’ is around -3° (inlet cam timing retarded by 3°), peak torque is moved around 200 revs further up the scale and the engine is a bit happier to rev to higher RPM, albeit with slightly less power. This together with the odd misfire at idle, depending on temperature and idle load.
The same engine running a torsion value of +3° (inlet cam advanced by 3°) will display decent low down torque, but is not happy with higher revs. This also seems to provide for more pronounced misfires at idle and seems to increase fuel consumption (as viewed in MVB 15).
When the cam timing is set back to 0° (+- 0.5°) the engine regains its original torque curve and power, as well as the engine running smoother.
To keep things simple, I suggest that when adjusting the inlet cam timing on the twin cam TDi engines, the exhaust cam is adjusted a similar amount. For the really anal ones amongst us, but without the use of old fashioned cam timing tools, it is possible to fine tune the exhaust cam for optimal fuel consumption as viewed in MVB 15 with very, very small adjustments.
Same on the previous generation 1.9 8v PD's.
However folklore exists that there is a sweet spot away from a sync angle of 0.0 but what it is nobody knows. Also on the old 8v engines on the cam cover it is quite often written (in biro) what many believe to be the factory sync angle setting.
That said, I don't believe the folklore and always set my cars (we have two old 1.9 8v PD's in our houshold) to 0.0 normally only with the locking tools but sometimes it does require a tweak of the (single) cam pulley to get it spot on.
I'd be interested to hear more about if the folklore is tosh, the factory setting being written on the cam cover and anything else you may know.
However folklore exists that there is a sweet spot away from a sync angle of 0.0 but what it is nobody knows. Also on the old 8v engines on the cam cover it is quite often written (in biro) what many believe to be the factory sync angle setting.
That said, I don't believe the folklore and always set my cars (we have two old 1.9 8v PD's in our houshold) to 0.0 normally only with the locking tools but sometimes it does require a tweak of the (single) cam pulley to get it spot on.
I'd be interested to hear more about if the folklore is tosh, the factory setting being written on the cam cover and anything else you may know.
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Useful information but I'm a little confused by something I have experienced.
You have stated that a negative figure is retarded and a positive figure advanced.
I have recently done my timing and set the torsion value to zero, I've found when I rotate the cam, using the center bolt, in a clockwise direction (advancing it?) the value goes negative and vice versa.
Am I misunderstanding something?
I've also found that my car is happier and more responsive with the exhaust cam slightly advanced (turned clockwise) and the intake cam retarded (turned anticlockwise) by about 1 degree. Thus reducing valve overlap and also spool time. My current torsion value is 1.
Am I getting advance and retard the wrong way round? I've always understood advance to mean it happens earlier in the cycle. That would therefore suggest that negative is advanced (0 - n, where n is the number of degrees of rotation) and positive is retard (0 + n). So a positive number would mean that the cam is opening and closing at "n" degrees after the factory setting.
You have stated that a negative figure is retarded and a positive figure advanced.
I have recently done my timing and set the torsion value to zero, I've found when I rotate the cam, using the center bolt, in a clockwise direction (advancing it?) the value goes negative and vice versa.
Am I misunderstanding something?
I've also found that my car is happier and more responsive with the exhaust cam slightly advanced (turned clockwise) and the intake cam retarded (turned anticlockwise) by about 1 degree. Thus reducing valve overlap and also spool time. My current torsion value is 1.
Am I getting advance and retard the wrong way round? I've always understood advance to mean it happens earlier in the cycle. That would therefore suggest that negative is advanced (0 - n, where n is the number of degrees of rotation) and positive is retard (0 + n). So a positive number would mean that the cam is opening and closing at "n" degrees after the factory setting.
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