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Thread: Cat / De-cat boost quickie

  1. #1
    Guest Rochester's Avatar
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    Cat / De-cat boost quickie

    I put the cat on for the yearly MOT and haven't got around to swapping back to the decat while at work the last week.

    What has surprised me is that with the cat on it has increased peak full boost from a nice steady 1bar / 15psi to 1.1 / 17psi.

    Nothing else changed, using an HKS adjustable boost controller in tandem with the good old Bren device.

    I realise that the pressures downstream of the turbo are altered by the presence of the cat but I rather assumed it would be the other way round, that the cat would slow down gas speed through the 'zorst and expected LESS boost as a result, not more....

    Am I missing something very obvious or just being stupid... ?

    It's not a problem, as I will be changing back to the decat just as soon as I have an hour or two spare to do the job, but it is puzzling me and if anyone does know then I'd be grateful to dispose of that niggle.

    Cheers all

    R

  2. #2
    Guest zeppelin101's Avatar
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    It comes down to the way the exhaust flow interacts with the turbine wheel and wastegate. Increasing the exhaust back pressure reduces the flow capability of the wastegate (imagine trying to push against a door which has nothing behind it, then pushing against the same door with someone applying pressure in the other direction) this has the same effect as reducing the wastegate size in terms of flow parameter.

    As a result, the turbine / compressor speed increase (there is a higher pressure ratio across the turbine wheel) which usually leads to an increase in boost pressure - although this is obviously a function of compressor efficiency and intercooler efficiency etc. The wastegate is likely opening a very similar amount to what it was before due to the way you are controlling boost pressure but the flow through it is slightly less.

  3. #3
    Guest Rochester's Avatar
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    I think I understand now.
    Once pointed out it's logical and makes sense.
    Given increased back pressure I wonder if it will make a difference to / increase / reduce fuel economy ? I may leave the cat on for a tankful of fuel to find out before decatting again, just to see.

    Cheers and thanks for the help.

  4. #4
    Guest zeppelin101's Avatar
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    Tricky one to answer on fuel consumption. Theoretically, at some speeds and loads fuel economy will improve as there will be more exhaust gas (door analogy again) in the cylinder after each combustion event, but we're talking fractions of a % here so the difference is almost certainly not going to be noticeable. Equally it may affect full load performance in some way, but tbh any change in knock behaviour would be impossible to separate out from the increased intake density versus the higher exhaust pressure.

  5. #5
    Guest Rochester's Avatar
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    Will report findings at the end of the tankful....

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    Is it spiking to 17 and falling back to 15 or rising to 17 and holding it ?

  7. #7
    Guest Rochester's Avatar
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    17 and hold if I keep the boot in - which I don't, as running H Dev stage 2 and 17 is not what it has been written for.

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    Hmmm. I can't see how the wastegate is going to result in you holding 17 unless its right on the limit of boost creep (which a healthy T28 wouldn't be).

    Where are your take-offs for the boost gauge and actuator and which order do the Bren device and the controller come in ?

  9. #9
    Guest zeppelin101's Avatar
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    It's not boost creep.

    Remember that the turbine wheel is the primary restriction in the exhaust circuit and the wastegate is effectively a pressure relief valve for the turbine wheel. If you increase the pressure on the other side of the restriction, the pressure ratio across the turbine will increase slightly and the wastegate will flow less efficiently.

    That's why high back pressure harms full load performance, because the pumping effort increases as the restriction in the exhaust increases and this impacts exhaust gas residual remaining in the chamber, increasing the charge temperature and increasing the likelihood of knock. For instance, this is the reason that small turbos which should be able to generate a lot of boost on a small engine cannot realise their potential in a lot of cases, because spark efficiency is reduced by the increased pumping work.

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    I know its not boost creep. That's not what I said.

    The controller is nowhere near the exhaust side and that should still be opening the wastegate when the same pressure is reached at the plenum/hotpipe/wherever he is taking the feed.

    The wastegate is sufficiently effective that it can allow the gases to bypass the turbine and stopping the turbo building pressure UNLESS ITS ON ITS LIMIT which it won't be.

    I cannot see any way that the back pressure in the exhaust can possibly be causing the pressure in the plenum TO BE SUSTAINED at 2 psi higher. Sorry, but I just don't see it.

  11. #11
    Guest zeppelin101's Avatar
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    Quote Originally Posted by Jonny Wilkinson View Post
    I know its not boost creep. That's not what I said.

    The controller is nowhere near the exhaust side and that should still be opening the wastegate when the same pressure is reached at the plenum/hotpipe/wherever he is taking the feed.
    The bleed valve isn't really a pressure controller in as much as it isn't actively controlling to a pressure. It just allows some air from the manifold to act on the wastegate canister.

    If, for the change in exhaust conditions, this results in a roughly similar wastegate opening angle, then the flow through the wastegate penny will reduce, increasing boost pressure.

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    Quote Originally Posted by zeppelin101 View Post
    The bleed valve isn't really a pressure controller in as much as it isn't actively controlling to a pressure. It just allows some air from the manifold to act on the wastegate canister.

    If, for the change in exhaust conditions, this results in a roughly similar wastegate opening angle, then the flow through the wastegate penny will reduce, increasing boost pressure.
    The "bleed valve" on Evan's car is a Dawes Device which is a ball and spring that allows pressure past it at a given PSI at the take-off. When the pressure gets past, it operates on the diaphragm in the actuator which moves the arm which opens the wastegate.

    Consequently, the pressure at the take off reaching a specific pressure will result in the spring being overcome and opening the wastegate. Its adjusted by preloading the spring.

    I think it extremely unlikely that a small increase in pressure behind the wastegate would overwhelm the diaphragm that is seeing 15 psi on the inlet side of it and hold the wastegate closed until it reached 17 psi especially as actuator pressure (with no controller) is normally 8 psi on a T28.

    If that is the case, its easy enough to test for. Remove the controller and see if it boosts to 17 psi
    Last edited by Jonny Wilkinson; 21-04-2015 at 10:40.

  13. #13
    Guest zeppelin101's Avatar
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    You misunderstand, the opening of the wastegate is likely following a similar opening profile to before in terms of %age wastegated flow. The cat brick will be adding some additional pressure behind the turbine wheel. Without measuring it, it's impossible to say how much but it could be up to 150 mbar pressure increase over the cat brick alone at maximum exhaust flow. That is noticeable when the entire exhaust back pressure might only be 3-400 mbar without the brick in at the same conditions.

    Without an active controller (which is actually measuring a pressure and adjusting the wastegate accordingly i.e. electronic wastegate control) then for the exact same flow conditions, the pre-turbine pressure will have increased, as will the exhaust back pressure in this instance. There is no way that with no change to the actuator arm effort on the boost controller side that the compressor will generate the same pressure, as it will be running at a different speed to before. Couple that with a change in flow parameter through the wastegate penny as well and it's entirely logical that the compressor out pressure is likely to be higher.

    It's a two stage effect, what happens on one side is directly proportional to the other (turbine to compressor), and what affects the turbine flow parameter has a similar effect to the wastegate flow.
    Last edited by zeppelin101; 21-04-2015 at 10:50.

  14. #14
    Guest Rochester's Avatar
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    [/very small and embarrassed font]Um....

    Turns out when I looked at it closer that there was a split in the rubber pipe from compressor housing nipple to Bren Device.... leaky leaky = overboostie.

    Replace pipe, all back to nice 15psi / 1bar stable max.

    Purely a coincidence it first showed symptoms after cat change.

    HOWEVER getting an extra 15 - 20 miles per tankful with the cat fitted than without..... should I keep it on or not. Hmmmm

  15. #15
    Guest zeppelin101's Avatar
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    I was wondering about this again the other day. You'd probably have to have been using a fairly basic wastegate system that maintained the same opening angle for it to be true so... glad you reported back.

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    Out of interest, are you running copper plugs ?

    If you are, you should change to platinum or irridium if you are going to run a CAT for any length of time.

  17. #17
    Guest Rochester's Avatar
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    Iridium one grade cooler than standard. Fit and forget..... well, for a longer interval than copper that is.

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    Do you have a 200SX RS13?
    Do you have a Silvia S13?
    Do you have a 180SX pre Aug '95?

    You shouldn't have fitted the cat at all.

    The only 13's that need a cat for MOT are late 180SX.

  19. #19
    Guest ANDY black s13's Avatar
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    13s are not listed on mot data unless it has recently changed so get tested as 14s

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    I'm pretty sure Evan knows that but they run OK with a CAT if you have one to fit.

    I wouldn't advocate buying one though as they're expensive and, as has been pointed out, not needed as long as you have a competent MOT man.

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