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2.5TDi Sluggish

Discussion in 'Diesel forum' started by PAULF, Jun 8, 2012.

  1. PAULF
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    PAULF Active Member

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    My wife has a 25TDi Avant, and she is sure it is not running right.
    It was not the best starting in the cold, but it would(we're talking down to -15). The MPGs are around 39-40. It sounds to me to have a bit more of a 'bark' than it should, as if the pump timing is a little advanced. It doesn't really smoke when you plant it, but it feels gutless until about 1500-2000 RPM. After that, it is a scalded cat - faster than mine, I think.
    I can compare it against my saloon, and they are totally different, mine has 120,000 miles on the clock, it pulls steadily throughout the rev range, I get 40-45 MPG, starts on the button and is less noisy!

    To date, the engine is a remanufactured 'zero hours' (as new) unit, done by Vege Engines, as is the turbo - now on 30,000 miles. The glow plugs are less than a year old, from Audi. The MAF is less than a year old, from Audi. To double check the MAF, I swapped it with mine, fault didn't transfer. N75 valve tested on VCDS, and turbo actuator seen to move full travel. After a chat with Addamss4, I checked the actuator rod length against mine, and shortened it. The pump and injectors were overhauled by Diesel Bob about a year ago. The pump timing is bang on the line, against my saloon being at the retarded end of the scale. There does not appear to be any leaking hoses - although I have not had a pressure check - but I would not expect a boost leak to cause low down torque problems and nothing at the top of the rev range. The fault does appear to get worse over time - but with swapping between the two cars, it may be psychosomatic!

    It hasn't helped diagnosis that during this time, at least one of her brakes has been dragging at some point - even after fitting new parts!!
    I'm pretty sure that isn't the case now, though......

    There are no fault codes on VCDS.

    I'm starting to wonder if one of the banks has a cam out by a tooth?

    I really would like to get to the bottom of this, as there has been a LOT of money spent on this one, and my wife is starting to lose heart over it all. (We're going through a 'jinxed cars' stage at the moment, if you see some of my other posts!)
    Last edited: Jun 8, 2012
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  2. adamss24
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    adamss24 Well-Known Member

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    It could be the cams are out but the engine will have a much noisier metallic sound than usual- the pistons kiss the exhaust valves ever so slightly. I suggest you advance the timing towards the top blue line and see if it's improving. The stop screw on the turbo also can lead to poor performance at low revs and spiking. I would swap the turbo's around and take it there. If no change then i suspect your engine rebuilder has used cheaper valves/cams and guides and they are dragging a little. When i have rebuilt Martin's/FJTwelve's a6, i found that many valves were stiff to move in their guides and i replaced them as i fought they're bent. It proved they were not machined to the same tolerances like the factory valves. The engine had rebuilt heads at some point and new guides/seats fitted at the same time- you can tell as the head cracks next to injector and glow plug due to stresses.
    Start by swapping the turbo's then adjust the timing then move to the cam timing. If the cam timing is out the engine will sound like a bag of nails. I have swapped the injectors in my allroad with Bosio pp683 which were fitted in older AFB injectors. They were fitted/balanced by dieselbob. Upon removal i found a rocker arm not seated on the lifter- the engine has been rebuilt by me before X-mas- so it must have fallen whilst engine built pressure. I had to remove the exhaust cam on passenger side and re-seat the offending rocker. When i started the engine the nozzles were so noisy that i thought the cam is out at least a tooth so i whipped off the rocker and cam just to make sure. Now it's done a few hundred miles, the injectors are getting less noisy but it still has that "barking" sound which wasn't there before. I also found the engine does not have as much power/torque low down as before which require a different driving technique especially when towing cars- need to slip the clutch a little to get the tow rig moving !
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  3. PAULF
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    PAULF Active Member

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    Thanks Chris,
    I think the turbo swap will be the first thing then, so deep joy ahead trying to access the ****ty 13mm nut! When you suggested adjusting the actuator, I swear there was an improvement for a bit - but it is so subjective, it's hard to tell.
    It'll have to wait a while, as I'm up to my eyes in stuff for couple of months.
    Obviously I don't know what Vege used but I would hope they would use good gear, as they claim their engines are at least to OEM standard.
    Are there any VCDS measurements that I can take to get any more clues? The cylinder differences show one bank of three about 1.80 above the reference, whilst the other two on the same bank are 0.50 to 1.0 above ref. I'm not too sure about how to log and post them, but I was wondering about the relationship between MAF, demanded boost, actual boost and injected quantity?? (Basically air/fuel requirements)

    I'm lucky in that I have another car for comparison, as if not I would have just thought the 2.5 engine was a bit **** low down.
    Mrs PaulF kept on saying the car was slower than before the engine replacement, but I put that down to her being used to the power. I still seemed to shift compared to my L200!

    Oh, I forgot to say. Regarding the poor starting when cold, I have fitted the worlds biggest tractor battery which has something like 980 CCA, as I know slow turning over can effect diesels and that didn't really help either. It's not as if there is an actual fault with anything, it's just a bit **** at starting cold and low torque - really hard to put your finger on.
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  4. adamss24
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    adamss24 Well-Known Member

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    When you take off the slow boosting turbo, crack it open on the hot side and check the VNT mechanism is free of soot and rust. I had that on my allroad after running petrol/oil and diesel mixture as the un-burnt ash will clog it rather soon.
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  5. PAULF
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    PAULF Active Member

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    I'll do that when I eventually get it off - although it has been like it from the first installation and the mechanism moves freely by hand in-situ.

    Kerosene ( and indeed, your petrol/oil mix) runs quite a bit hotter, and a few of tankfuls should clean out that sort of ****. Don't worry about the lubricity too much, as you shouldn't be running it long enough to cause grief.
    Bear in mind HMRC frown upon this though, even for maintenance purposes..........:angrymod:
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  6. adamss24
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    adamss24 Well-Known Member

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    There is a special cranked spanner used to remove that difficult to access nut, have a look on ebay !
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  7. adamss24
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    adamss24 Well-Known Member

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    I want to see on what grounds they will take me to court, disposing of the waste oil in a green maner so it does not get into a drain pipe or thrown in one corner in a dark alley or because they cannot prove what my engine was running on ! They cannot do f*ck all to prove in a court of law that I broke any laws by dumping oil and petrol mixture into the tank for maintenance reasons or not ! I still buy tons of fuel a year so it would not be difficult to prove them wrong whatever the case !
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  8. PAULF
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    PAULF Active Member

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    Thanks for the spanner gen, I'll probably get one.

    I'm not having a go, just saying they can be a bit miserable! It does help clean out the engine of soot and rubbish (at least I've heard kero does)
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  9. Tom_B
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    Tom_B Member

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    Are you really really getting 40-45. Mpg from your 2.5 tdi?
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  10. PAULF
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    PAULF Active Member

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    Yes, rolling average over 15 months is 41.5 MPG , although for the last couple of months I've had a sticky brake or two. Before that it was about 43 MPG. I work silly times, so almost no traffic, which helps. I can't keep my foot of the throttle long enough to get high forties!

    Mrs PaulF's 2.5TDi Quattro Avant is getting 38 MPG rolling average over 2 years, but she has to go into Aberdeen city during rush hour and I'm sure hers is not running right. (Hence this thread)
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  11. Tom_B
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    Tom_B Member

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    Well I will have to start tracking the consumption carefully because i would be surprised if I am getting much more than 35mpg as an average. I do have a most delightful commute through twisty a roads though.
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  12. PAULF
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    PAULF Active Member

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    We use her i-phone app 'road trip' quite religiously. You must try and fill to the same level each time. I fill to spill, she fills to click. Any differences will average out over time. (If you under-fill one time, you get higher MPG, next time you take more fuel, so MPG is lower, average is correct)

    I have tweaked the DIS to read more accurately too - mine wasn't bad, but hers was a good 5-10% out to start with.

    I don't hold back on the Aberdeenshire roads either, although with no traffic, it's bit like slidey motorway driving! Her lack of oomf at lower revs is the other reason for this thread!
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  13. KGB
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    KGB New Member

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    I had similar issues with my 2002 2.5 Tdi, though the lack of performance started at the lower revs you mention it soon increased higher through the revs too, also sounded rough running, almost as if there was an induction leak, cause of the problem was the brake servo had filled up with water.

    KGB.
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  14. PAULF
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    PAULF Active Member

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    Thanks KGB, that's an interesting one.

    It shouldn't be to hard to check either.
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  15. KGB
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    KGB New Member

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    It's easiest with a vacuum guage Paul, we just tested sections of the vacuum hose back from the vacuum pump on the engine, all was good up until we re-connected the servo then showed no vacuum at idle and only a slight increase as revs were lifted.
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  16. PAULF
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    PAULF Active Member

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    Hmmm, this is getting interesting....

    I decided to check some engine values against my saloon, to see what is going on. The Injected Quantity on the Avant is about 6.8-7mg/s, against the saloons 5mg/s. Whilst it is in spec, I decided to try and adjust it down (increase fuelling, improve torque)and test the car. I logged in using 26242, and adjusted the value in 01 to max. The IQ did not appear to change much (about 6.3 now), but I swear there was a little improvement. I then decided to remove the MAF plug, and got a good running car!

    I had already tried this a few months ago, and bought a new MAF from Audi. As there was no improvement, I then decided to swap the new MAF with my known good saloon one - no difference, my car ran fine with the new MAF, the Avant sluggish with my old one.
    There is DEFINITELY some issue with the MAF/Intake Temp signal as the car really perks up once it is unplugged.The problem is, when I look with VCDS, all the values seem reasonable, and change with throttle or disconnection. I don't believe the sensors are at fault, as the MAF was fine in my car, and her unit went into mine with no problems.
    I also don't believe there is a short or open circuit in the loom as the values vary.
    Any ideas??

    I would also like to adjust the IQ down a little further - 5 mg/s would seem about correct going by mine. There are a few guides on how to knock the arm across from our Transatlantic cousins, but they are all for 1.9s, and it appears the 1.9 also responds better to VCDS value changes!

    Again, anybody have any clues?

    There is a chap with an AKE with a similar strange fault over on the VW Audi forum AKE woes, VAGCOM IQ querey.
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  17. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    why would REDUCING the injected fuel quantity give you more torque? Surely the exact opposite is true.
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  18. PAULF
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    PAULF Active Member

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    If you reduce the IQ value, you increase the fuelling.
    I've no idea why!
    It seems similar to the old mechanical part-load governor, where you could screw it up half a turn or so to increase off-metered fuel. The metered fuel then decreases at idle. (cue Chris for a proper explanation)
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  19. diffas
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    diffas Member

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    Well. In simply words, reducing IQ you receive more sprayed fuel at the same fuel amount shown in ecu. Let say you have IQ 7mg/str and max. amount of fueling on wot is 50mg/str. And now we have IQ set to 4mg/str and still its 50mg/str at wot. 3mg more fuel will be sprayed. That's what I've heard. :) Not sure if correct info but close enuff :)
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  20. PAULF
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    PAULF Active Member

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    Thanks Diffas, that's pretty much as I think I understand it.

    On the older mechanical systems it was the same - but you screwed up the part load governor. That meant when you planted the foot, you had your 'Injected Quantity' plus the unmetered fuel. The more unmetered fuel at idle, the less 'Injected Quantity', or metered fuel.
    Older-timers than me will probably remember 'Diesel Runaway' where if you screwed up the part load governor too much, the unmetered fuel was sufficient to accelerate the engine - to destruction!. The faster it revved, the more fuel it pumped. A similar effect can be had if the turbo oil seals go and engine oil leaks into the turbo inlet. My mate had a Montego revving out at about 10,000 RPM on engine oil before he stalled it!!! (the injected quantity was zero, as the ignition was switched off after it ran away)

    I still don't know what my fault is, though :uhm:
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  21. doldis
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    doldis New Member

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    Hi, I am new to this forum, actually joined up just because of this tread.
    I have just the same car as you, with exactly the same issues.
    Have been replacing just aboaut everything, the only thing that helpt so far, was a set of new injectors( costing a leg
    and two arms )
    Another thing, adaptations ch4, change the value in direction to raise the injection timing( 100 is similar to 1 deegre)

    best regards
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  22. PAULF
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    PAULF Active Member

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    Right.

    Firstly, Chris, you were spot on in one of your earlier diagnosis.

    You suggested I should adjust the VNT actuator, and I shortened it by about half-a-dozen turns to get it to about the same length as the one on my other car. There was a little improvement, but not enough for me to be sure.

    I have carried out various diagnostic checks, including checking the cam timing, to no real avail. The only thing that appeared to help was unplugging the MAF. I bought a new one - nil fix, so swapped it with mine - no change.

    I was getting to the hair-pulling stage, when I came across this VNT TDI turbo: how to adjust actuator & cure boost problems

    I did some log runs, but to be honest, was struggling. Redline in 3rd is 60 MPH, 4th is 80-ish, which was interesting on Aberdeenshire back roads and I didn't really get good results. The Poster talked about aiming for a N75 Duty Cycle of 70%-90%, but that was a 1.9 engine. However, figures aside, the theory is the same
    I logged about 40%, so to test things I ran my other car the same route. It was higher - 45-50%.
    I shortened the rod by 2 turns, and now have a decent running car. It is still not 100%, but it's close. Another run showed a duty cycle of about 45%
    Once we are both off work, I'll play around and see what I can get.
    There are a couple of guys on the VWAudi forum with similar problems - and we all had fitted new/recon turbos.
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  23. doldis
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    doldis New Member

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    I've been testing out the vnt-actuator adjusment ( just by try and fail though) Adjust, out for a drive and adjust some more, both ways.
    Doesn't seem to cure my engine problem, so I've put it back to where it was.
    Strange thing is that everything( I can possibly think of) is checked out/ replaced, TURBO ( actuator ok, vanes cleaned) N75, Map,
    Maf, injection pump, injectors, Water temp sensor, oil temp sensor. ecu, egr system ok, also regarding to wag-com all make sence
    and seems to be working as it should. And I still have this annoying issue ( engine is lazy and barking at low rews when warm)
    That said, beeing a heavy german tank ( quattro, tip) it is actually very fast. ( this is my fifth 2,5 tdi, third AKE) None of my
    previous a6's was even close to this.Its, like you said, a scaled cat, so there is definitly something odd here. Fuel economi is none
    existent.
    The only odd thing I've found is in measuring blocks-14- IQ at idle. cyl 4,5,6 ( also the left bank) is all around +2.
    Should bee within +- 1,5.
    You may suspect the camtiming is out, No it's definitly not.
    Injection pump timing is also ofcourse spike on.
    Could this issue may be anything with the camshafts themselves?
    Would you happen to know what make your camshafts are? ( mine is Kolbenschmidt, INA pushers and rockers)
    #23
  24. PAULF
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    PAULF Active Member

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    I'm afraid I don't know what parts they fitted, but they were a reputable re-manufacturer - so good ones, I hope!

    Re the actuator, when Addamss24 suggested adjusting it, I set it to the same length as my other car. It meant shortening by about 5-6 turns. There seemed to be a tiny improvement - but nowhere near enough to justify more - I thought.

    I have since shortened it another 3 turns, and am not finished yet! Have a read of the link above, and do some log runs. I found that I was looking more at the relationship between demanded air, actual air and RPM holistically.
    I will post logs at some point, but it may be a while before I get around to it.
    I believe if you shorten the rod too much, your duty cycle will be very high, and you should go into limp mode from overboosting. This will also show on your log. (I spotted this on my 'good' car without any symptoms yet, so will be Mr Muscling the turbo vane ring, to ensure it is giving full range)

    I can't believe it has taken me 30,000 miles to finally put this to bed!
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  25. PAULF
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    PAULF Active Member

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    As promised here are some graphs of the logging runs.

    [​IMG]

    Here the rod was too long. The car felt gutless at low revs, picking up as the rpm increased. The N75 duty cycle can be seen to be around the mid-forties with RPM increase speeding up past 2000 RPM, in a 'hockey stick' curve.

    I then shortened the rod by about 10 turns, making it far too short.

    [​IMG]

    Here, the car was much more responsive at low revs, but very quickly the boost rose above 2500, the N75 tried to limit boost to its 10% minimum, but could not slow the turbine enough, so the ECU chopped fuel - limp mode

    [​IMG]

    This time the response was still good at lower revs, but again there is a slight over-boost that the N75 couldn't quite keep on top of.

    [​IMG]

    After lengthening the rod a little, the car now drives well. The testing was done on a steep hill, on Scottish 'B' roads, so consistency was a bit difficult. Redlining it in 3rd gives about 60MPH, in 4th nearer 80MPH. This can make life interesting on these type roads. It did mean, though that the boost requirement was high, so as there was no over-boost and the car picked up fast, I decided to leave the rod as is.
    It can be seen that the N75 just controlled the over-boost after the gear change, by limiting the boost to about 13%. I suspect the rod should a tiny bit longer, but I will leave it as is unless I have over-boost issues.
    Many, many thanks to Fenwick458 on uk-mkivs, his info was instrumental in my solving of my problem. Chris diagnosed it here first, but I didn't shorten it enough first time until I was able to understand the figures.
    I was just about to bin the thing, after spending buckets on it over the past 2-3 years. The car now drives great. Incidentally, I also found out that my saloon is just getting to the point of needing another Mr Muscle, as the N75 has just topped out a couple of times without actually giving me the over-boost code.
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  26. diffas
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    diffas Member

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    Jesus...got any emp readings?
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  27. PAULF
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    PAULF Active Member

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    I'm still building the prototype EMP transmitter :ninja:

    (Sorry Diffas, as a child of the 70's, I only know Electro-Magnetic Pulse - not the EMP reading I think you mean - tell me what it is and if I can measure it, I will)

    It took a while to get real info, and then apply the knowledge. I still think I really should lengthen the rod until the duty cycle runs at over 70% as the original thread said, but as the car runs well as is, I'll leave it.

    Interestingly for you, Diffas, it also works with hybrid set-ups. As what you are looking at is the optimum boost without overboosting, not actual pressures.
    As I see it, too much boost (low % N75 duty cycle) means you are wasting energy spinning turbines for no reason - hence the optimum being around 70-90% However, reducing this too much (long rod) also means problems with spool-up at low revs - I think I now have Mrs PaulFs' about where she wants it .
    Time will tell............
    #27
  28. PAULF
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    PAULF Active Member

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    Sorry, I don't have equipment to measure Exhaust Manifold Pressure. :readit: :applaus:

    A before and after EMP would be useful - although remember, my both cars are stock so the 'cat' will be very restrictive - especially on the 170k mile Avant.
    #28

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