Vacuum Limitting Value opening pressure | FerrariChat

Vacuum Limitting Value opening pressure

Discussion in 'Boxers/TR/M' started by dradambbb, May 17, 2020.

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  1. dradambbb

    dradambbb Karting

    Apr 24, 2016
    124
    London
    Full Name:
    Adam
    As part of a tune of an 87 TR F113B no cats, no O2 sensor I've removed the the VLVs from both sides. I did a simple test with a hand held manual vacuum pump. I tried blowing down one hole while applying vacuum via the pump. This is somewhat inaccurate but both valves appear to be opening at around 50 kPa. I don't know how accurate the gauge is but the same gauge is showing about 60 kPa of vacuum in both manifolds at idle. This would suggest that the valves may be passing some air through at idle, which should not happen. Does anybody have metrics for VLVs for comparison?
     
  2. dradambbb

    dradambbb Karting

    Apr 24, 2016
    124
    London
    Full Name:
    Adam
  3. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,035
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    I don't think your test method is valid. The downstream vacuum applied to the large fitting helps to hold the valve closed so you need to apply the same vacuum to both the large downstream fitting and the small vacuum fitting at the same time -- then increase that vacuum until air is drawn into the large upstream fitting (i.e., there should be a limit to the amount of vacuum that can be applied). See the figure on page D17 of the TR WSM. In your method, you are only applying vacuum to the small fitting.
     
    turbo-joe likes this.
  4. dradambbb

    dradambbb Karting

    Apr 24, 2016
    124
    London
    Full Name:
    Adam
    Thank you for clarifying, Steve. Your explanation makes sense and I hadn't considered the impact of pressures between the larger openings.
     
  5. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,035
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    Please report the result of your (corrected) measurement -- I can't recall ever seeing a specification for what the maximum vacuum amount the VLV should allow.
     
  6. dradambbb

    dradambbb Karting

    Apr 24, 2016
    124
    London
    Full Name:
    Adam
    I bought a pair of vacuum gauges (for motorcycle carb tuning actually) to help me tune. These show vacuum in mm. It seems that at idle I'm seeing about 450 mmHg. While idling I used the hand help vacuum pump to draw air from the VLV. It appears to affect the idle speed (i.e. let air through) at 70 kpA or 525 mmHg. All in all not that much higher than idle vacuum.

    Actually it may explain a funny condition I experience sometimes. During deceleration to full stop, if I depress the clutch while the engine speed is above 2000 rpm, idle settles at around 1500 rpm. However, if clutch is depressed at under 2000 rpm then it settles to the expected 1000 rpm. I am beginning to suspect the VLVs for this as I can't image how else extra air gets in.. But again I might be quite wrong on this.
     
  7. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,035
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    You could always just physically block the air paths (by some other means) going thru the VLVs to see if this eliminates this sometimes 1500 RPM idle issue

    One other point is the idle switch portion of the throttle microswitch should be closed at idle, which retards the ignition timing, and slows the idle RPM. One simple test you can do is just unplug the throttle microswitch when it is wrongly idling at 1500 RPM:

    If the idle speed increases further to ~2000 RPM = the throttle microswitch isn't the trouble.

    If the idle speed does not change = throttle microswitch either bad or misadjusted.
     
  8. dradambbb

    dradambbb Karting

    Apr 24, 2016
    124
    London
    Full Name:
    Adam
    Wow, nearly a year has past since I posted this issue. Anyway, I can confirm that VLVs stay open when the engine is warm and is revved up. If I pinch the rubber hoses with a hose clamp the idle drops to a steady at 1000-1100 RPM (hard to be sure from the tachometer alone). The car drives totally fine with the VLVs disabled. I wonder if anybody else has tried the same.

    I've not tried the microswitch test but given that the engine has great pick up from idle it's probably safe to assume that opening the throttle correctly advances the timing giving that immediate power boost.

    Years back I used to own a D-jet Mercedes 280SE that gave an immediate 12 degrees of advance on throttle open. It gave the car a very peppy nature that differed somewhat from the elegant exterior. The electronic advance on the TR indicates to me that it was still a standard practice a generation later.
     

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