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Thread: boost control

  1. #1
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    If you rally with a turbocharged engine like the one of the WRX you are enforced to use a air restrictor.
    It could well be Group N were you MUST USE the original boost control system, that is the 3 port solenoid (ideally) and a 32mm air restrictor, or it could be Group A were you could use alternative homologated boost control devices (like air injectors) and a 34mm air restrictor, or you can also have SCCA Open Class were you can use ANY boost control system and a 40mm air restrictor.

    I have some experience with group N rally cars, and in the very best case you will get 1bar of boost at 5200rpm. Boost climbs very very quickly and in the same way it starts to fall upto the 1bar point (which I use as a reference) which commonly is at 5000rpm. Probably with a better boost control system you could hold boost "a tad better" but at the risk of making the turbo overspin, remember since the inlet is reduced from 52mm to 32mm the turbo spins faster to produce the same boost level, so there is always a risk of overspinning the turbo if this is pushed to hard.

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    But lately on unrestricted cars (that is street cars) I have been using manual boost controllers, and for example on a front entry TD05-16g if I dial 25psi on midrange boost (that is 1.7 bar) it will drop down to 20 psi (that is 1.4bar) > 7000rpm.
    Now I don't know for sure if this is good for the turbo, but it does seem logical that boost drops as the turbo starts loosing efficiency. And this also seems to be the case when u use a Dawes type device with the stock TD04, boost also drops at top end

    So basically my question is will a manual boost controller like this one which can be set to behave a little different than your regular DAWES (which is my favourite one as it includes both a bleeder(fine adjustment) and a ball&spring mechanism(coarse adjustment)):

    give me a good boost control on a open class car that uses a 40mm air restrictor, that is will boost also gradually decrease as revs increase avoiding overspin ?

    Carlos H.


    [This message has been edited by CarlosH (edited 05 July 2003).]

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    For example I could set the ball&spring part of the boost controller to the maximum boost I want to hold ..... let's say a safe 1bar @ 7000rpm, and with the bleed valve create higher boost on the midrange which will eventually fall down at top end since there will be less resistance from the WG actuator to open ..... remember that air always follows the easiest way, and probably at higher rpm there is already enough EGBP to fully open the WG with just a little aid.

    Carlos H.

  4. #4
    22B Junkie Andy.F's Avatar
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    Hi Carlos

    I've found that the bleed function mostly changes the overshoot/damping in the midrange, the ball/spring setting the actual boost level.
    The best way to change the shape of the boost curve is to add/remove tension from the wastegate actuator, adding tension will hold more high rpm boost and vice versa.

    Andy

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    Andy, thanx for you reply ..... just to make myself more clear.

    When I use the stock 3 port solenoid on the group N cars (fitted with 32mm restrictor) I am sure that the turbo won't overspin and turbos last long enough too.

    What I don't want is to push the turbo, past the point were it will get damaged ..... in other words to avoid overspin. And if you are familiar with a boost profile on a rally car you will know that it is characterized by a big overshoot in the midrange are. Your typical boost targets on a group N car will be:

    3000rpm 210KPa
    3500rpm 250KPa
    4000rpm 250KPa
    4500rpm 225KPa
    5000rpm 210KPa
    5500rpm 190KPa

    That is why I was aksing if in the case of the 40mm air restrictor a good idea might be to set the ball&spring mechanism to the boost you want to hold at higher rpm which will be the lowest, because of the restrictor, (or u could also use an ajustable WG actuator, although it is easier to regulate a ball&spring and also makes the boost to come earlier) ...... and ..... to use the bleed part to overshoot boost on the midrange and give the "mountain" shape boost curve.

    Hope this makes sense, but I am just looking at options that are better VFM ..... probably a air injector setup may control boost better, and still let the ECU to control it, that is for example to lower the boost targets if the air inlet temps get past a certain temp ..... but still if I don't have the flexibility of controlling boost with the ECU, I can still lower the ignition advance if air inlet temps get too high .....

    Hope this makes sense

    Carlos H.


    [This message has been edited by CarlosH (edited 05 July 2003).]

  6. #6
    22B Junkie Andy.F's Avatar
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    Yes, I understand you (i think ) I believe that even on a 40mm inlet, using a simple ball/spring the turbo tends to regulate itself at the higher rpm due to high exhaust manifold pressure assising blowing the wastegate open.
    I don't like fitting heavy duty wastegate actuator springs on smallish turbos, this is what leads to overspin in my opinion.
    If my EMP gets high enough to overcome the std actuator, letting it escape out the wastegate is the best option.

    Andy

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    EMP as in exhaust manifold pressures ?


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    22B Junkie Andy.F's Avatar
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    Yes, to avoid confusion with exhaust system back pressure or the general term EGBP.

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    I also agree that fitting heavy duty wastegate actuator springs on small turbos may not be a solution, although a little stiffer unit that your common 8psi unit may work better.

    So I guess it should work, and like I said in order to creat a high peak of overboost a bleed valve may be nice too ..... I shall try this one out and post results later on.

    Carlos H.


  10. #10
    22B Junkie Andy.F's Avatar
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    The actuator on the TD series may only require 8psi to open but due to the area of the diaphragm this may result in a high force at the 'penny' ie if the actuator is 2.5" diameter then the area is 5x that of the penny at 1" diameter. So a manifold pressure of 40 psi would be required to blow open the wastegate (assuming the leverage is approx equal)

    Try leaving the actator signal line off and see what midrange boost you get before it blows open the std '8psi' actuator
    (don't do this!)

    Andy

  11. #11
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    Andy thanx for the reply, and I do get the point, but "unfortunately" the guy that asked me to help him with this little issue will be using IHI VF34's for his engine .....

    Carlos H.

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