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Thread: Issues With Precat Decat on 1.9CDTI

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    Default Issues With Precat Decat on 1.9CDTI

    Hello Guys

    Im new on the forum, but have had my vec 1.9CDTi 150 SRi a while now.

    I was thinking of drilling out and removing the internals of my precat.

    I have one worry though. Will I get issues such as an ecu warning light due to the lambda sensors???

    Also is decatting the precat better than decatting the main cat?

    help would be very much appreciated.

  2. #2
    Regular Member nath-lav's Avatar
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    Decatting main cat will give you best results. .and if you only take out internals of precat but reconect lambda sensor etc there wiol be no probs..

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    Default

    ok thanks nath-lav.

    so im fine to rip out the internals of the precat and then fix the sensor back in place and all will be ok?

    I would rip the internals out of both precat and main cat but i think i would get issues with not enough back preasure.

    also i think one of the two sensors detects if thye cat is actually working or not.

    am i correct in saying this?

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    Regular Member Teapot's Avatar
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    Vehicle : Signum

    Trim : Design+

    Engine : 1.9 CDTI 150

    Year : 2007

    Mileage : 101000

    Default

    I've mixed opinions on de-catting depending on the type of turbo.

    For hybrid turbo's, removing or gutting all catalysts is essential. On my signum, I initally just gutted the DPF when fitting the hybrid, but it was exceedingly laggy with no power below 2200rpm despite proper mapping. As soon as I gutted the pre-cat, the boost threshold reduced by nearly 300rpm to 1900rpm with much more rapid spoolup.

    With standard turbo and standard mapping, gutting the cats can reduce lag and improve efficiency slightly but the turbo will tend to spool up more quickly and the boost pressure may spike a little just above the boost threshold.

    With standard turbo and remap that gives extra fuelling or even a tuning box, things can go horribly wrong. The boost pressure will tend to spike severely - in some cases I've seen it go above the limit of the TMAP sensor. In most cases this can be controlled by careful recalibration of the VNT maps, but in some cases the boost pressure will tend to oscillate, requiring further modification to the parameters of the PCR governor (closed loop boost pressure control algorithm). Its basically a 'PID controller' using the pressure reading from the TMAP sensor as the measured variable, and the pulse ratio of the boost solenoid as the manipulated variable. Look up PID control on wikipedia

    The reason that these spikes or instability occur is because in the absence of back-pressure in the exhasut system, the avalanche effect is much more rapid - i.e. the turbo rotating assembly accellerates more rapidly and so the measured variable (actual boost pressure) tends to overshoot the target valve (desired boost pressure) before the rather sluggish vacuum based VNT control mechanism can open the vanes enough to compensate. The PCR governor then tends to overcompensate due to the long delay in response from the VNT control system and so the boost pressure drops below the target value and the cycle repeats, causing an oscillation in the boost pressure which gradually reduces in amplitude until a stable pressure is reached.

    The reason that this issue is less likely to occur with a hybrid turbo is because the larger compressor wheel has a higher mass than the standard one, and so the rotating assembly accellerates more slowly than the standard one and the PCR governor and VNT mechanism response time is adequately fast to maintain a stable boost pressure,
    Last edited by Teapot; 3rd September 2013 at 22:03.

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    Regular Member Teapot's Avatar
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    Vehicle : Signum

    Trim : Design+

    Engine : 1.9 CDTI 150

    Year : 2007

    Mileage : 101000

    Default

    Also I should add that having now seen both, the pre-cat is much more restrictive than the main cat. The main cat is a simple square-honeycomb construction and so does not disturb gas flow excessively,

    However the precat is of very different construction. Imagine coke-can tpye metal corrugated on both axis like corrugated cardboard, wrapped in a spiral from the centre to the edges of the canister. This I believe offers much greater resistance to gas flow than the simple honeycomb of the main cat.

    And regarding the lambda sensor - as long as you leave them installed in the canister after gutting, or use a de-cat pipe that has a fitting for the sensor, there should be no issue. However if you want to completely remove the lambda sensor for any reason, it can be disabled in the ECU mapping so you can disconnect it without any fault codes appearing.

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    Default

    thank you littleteapot.

    great information. I knew that the precat was the most restrictive.

    I was thinking of gutting the pre cat, leaving all lambda sensors in place and leaving the main cat in place. then removng the centre silencer but leaving the standard backbox and tailpipe.

    After i was also going to get a remap.

    How do you think the car will perfrom after and also will I get any side effects?

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    Regular Member Teapot's Avatar
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    Vehicle : Signum

    Trim : Design+

    Engine : 1.9 CDTI 150

    Year : 2007

    Mileage : 101000

    Default

    It will perform well, but you need a competent remapper who will log boost pressure and other relevant variables, and make the necessary adjustments to ensure the boost pressure is stable and doesn't spike excessively, nor run-away at full throttle and high revs.

    If the boost pressure is allowed to spike, this will cause rapid wear of the thrust bearing. If boost pressure runs away (caused by the fuelling and hence exhaust gas flow being too high for even a fully-open vnt mechanism) this results in near instant destruction of the entire turbo.

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    VIP-Member Z80's Avatar
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    Vehicle : Opel Vectra GTS

    Trim : Elegance

    Engine : 2.2 DTi 16V

    Year : 2003

    Mileage : 75 000

    Default

    Just for information, I note none of this on the DTi. The only problem I had about bad VNT rod tuning was excess smoke, laggy spoolup followed by pressure spiking then strong pulling up all over the range, ultimately resulting in limp mode on sustained acceleration... The garage did a nice optimization job, as my fuel consumption has now fallen down by an additional 15%.

    But it never oscillated. Maybe it's because the turbo is larger, with maybe a heavier compressor wheel... The DTi also doesn't have a pre-cat, only a short, square-honeycomb, catalyser, and no DPF either, so the turbo is designed to work with minimal backpressure already. However, the catalyser still added a bit of obstruction, as after having guttied it out, the turbo spool delay has been dramatically reduced (after VNT rod fine tuning), now pulling from 1200 rpm and blowing full power from 1500 rpm.

    One the other hand, when the rod was shortened too much, I could achieve tremendous acceleration (but is was harder to have it reaching the necessary 1500 rpm) and high end power, so maybe something can be done in the aera between the current maximum boost given by the fully closed VNT and the value the ECU considers as being too much... The tolerance isn't excessively tight, maybe 0.1 bar or so, which leaves some margin in the 2 to 2.1 bar range, avoiding the error to trigger... Maybe a hybrid could address that.

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    Vehicle : Golf 7

    Trim : GTD

    Engine : 2.0 TDI 184PS

    Year : 2014

    Mileage : 40000

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    On a CDTI, Removing the pre cat yeilds better results since the pre cat is effectively butted up against the Turbo. Removing it allows the gases to get out quicker. The main cat is a bit far away and it too will relieve a pressure drop in the exhaust but I have to say you don't notice it at much especially if you leave the pre-cat in.

    Giving the DTI it's due, one cat and it sits a bit a away from the turbo so removing the cat from a DTI will give instant results. The CDTI's being of Euro 4 regulations means two cats with one on the 'doorstep' of the turbo.

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    Regular Member Teapot's Avatar
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    Vehicle : Signum

    Trim : Design+

    Engine : 1.9 CDTI 150

    Year : 2007

    Mileage : 101000

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    I've only seen the oscillation occur on Z19DTH engines. The PCR governor on some software versions has very short time constants and high gain (much faster than the vacuum mechanism that it operates) and so tends to becomes unstable during transient conditions. This will often be oberved as a surge of power, followed by a flat spot, then full power again, when full throttle is applied suddenly.

    Its a fine balance between responding quickly enough to prevent excessive overshoot, and slow enough to avoid becoming unstable (oscillating). My observation is that the other TD engines (2.0/2.2 DTI, 1.7 CDTI and 1.9 CDTI 120) have much slower PCR governor settings than the Z19DTH 150, which explains why this instability doesn't normally occurr on those engines.

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