Crankcase Pressure Levels

Discussion in 'Manton Push Rods Top Alcohol Tech Questions' started by TOL, May 6, 2019.

  1. TOL

    TOL Active Member

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    In a serious blown alcohol deal (screw or roots) has anyone ever monitored their net crankcase pressure values during a pass? Or even better, has anyone ever monitored their blowby gas volume rates?

    I would be really interested to hear more, either online of offline, if you don't mind.

    I'm curious about your values and flow rates, but I'm just as curious to know whether the typical twin 1.25" breathing tubes might be causing a net restriction or not ??

    I kind of gather that the current rules were written to suit typical chassis tube diameters. That's not a terribly scientific approach, but it has obviously worked.

    Got some ideas, but need some real world data to bounce them against.

    Thanks in advance !.........
     
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  2. TOL

    TOL Active Member

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    Case in point is this one ........

    I'm currently designing the ventilation circuits for my new/own block design.

    I have allowed for about 6.28 ^2 of external crankcase ventilation flow area inside the block, connecting the sump to the outside world. My interior airflow is not contra to any oil drain back flows. I'm just handling vapors and oil/fuel entanglement.

    Two 1.25" tubes on the other hand (typical of modern day breathers) is only about 2.45 ^2 of breathing area. Obviously there is a disparity here that I could take advantage of. I doubt IHRA/NHRA would balk at bigger diameter breathing tubes if so desired.

    I am wondering whether the normal twin 1.25's cause any appreciable back pressure (crankcase pressure) or not? Or are they just fine & dandy as long as there is not a catastrophic/explosive event (think nitro down the road)?

    It would be neat to draw a vacuum, or a near vacuum, but the baddest vac pump that I am aware of is only 100 cfm at max rated speed. I kind of doubt it would be up to the job. At 8 pounds each I would not want to run two or more.

    Thoughts ?........

    I'm thinking (and please chime in here to correct me) that it might be best to just simply freely vent the crankcase and get the tip of the puke tank into a low vacuum aero zone ? I'm mainly curious about the effect of the twin 1.25" current conventional breathers as a restriction.

    Open to ideas.

    Thanks.
     
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  3. jeffj

    jeffj Member

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    If your short block is correct... you do not need more than the 1 1/4 tubes. I would see from zero to 1 lb @ 6500 stage and zero right after the foot swap and then a bit negative at the top of low to minus 1 1/2 to minus 2 at the finish line with our T/AD. A big legal volume puke tank takes a bit of time to equalize pressure; I had the exhaust vent in the air stream so that it would help pull a vacuum @ 270.
    Jeff Johnsen
     
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