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Hair-brained venturi idea, will it work?

  1. Oct 14, 2011 #1

    I know a little about a little and nothing about a lot, but I've got an idea and I thought this community may be able to lend a hand. Perhaps even find it interesting, who knows.

    I'm working on a race car, and I need to deal with pressure that can build in the crankcase because piston rings don't seal perfectly.

    Typically the crankcase pressure is vented through a tube back in to the intake behind the throttle plate and thereby recirculated. That is not an option on this car as the vented pressure contains an oily mist which reduces the effective octane rating in the combustion chamber, and this is very unhealthy for race car engines.

    A common alternative is to simply vent the crankcase to atmosphere. In time however this smothers that oily mist all over everything under the hood, and generally makes a mess.

    A more enterprising idea is to use a vacuum pump to draw pressure from the crankcase. Unfortunately it's also a rather expensive idea to purchase a pump for this purpose. Some have tried to re-purpose smog pumps (exhaust air injection pumps) but they are designed to run dry. So when their innards are exposed to that oily mist, they shortly fail.

    Now here's my hair-brained idea. Rather than using a smog pump directly as a vacuum pump, use it to blow through tubing. Step down the diameter of the tubing after the smog pump to create a venturi effect, and provide a tap at the choke point where from a line can be run to the crankcase. Finally terminate the run of tubing in to the exhaust system.

    Here's a picture that I whipped up.


    Would it work?

  2. jcsd
  3. Oct 15, 2011 #2


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    Welcome to PF.
    It seems to me that the introduction of a "smog pump" (whatever the hell that is) would be a wasted effort in an otherwise good system. Back in the 70's, we just vented from the valve covers to the headers. We called them "scavenger pipes" or "pan evacuation systems".
  4. Oct 15, 2011 #3
    You're absolutely right. But I'm told the class that this car will compete in requires all the factory accessories to be present, which means the smog pump stays.

    I'm assuming the rules don't allow for one of those exhaust evac systems or we'd just use that. I'll double check.

    Meanwhile are you saying that the venturi design would not work?

  5. Oct 15, 2011 #4


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    What are the pressures from the crankcase, at the exhaust headers, and from the smog pump (when connected to the headers as shown in your diagram)? As mentioned, is the smog pump really needed?
  6. Oct 15, 2011 #5


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    Does it say that they have to be hooked up? :devil:
    By the bye, I looked up "smog pump". It's just what I knew of as "air injection". While I have some very vague knowledge of it, I've never owned a vehicle new enough to have one and thus am totally lacking in experience with it.

    Not at all. Sorry if I gave that impression. It's just that the headers themselves provide the venturi effect without modification. I don't know whether or not that would apply to a supercharged engine, though. As Rcgldr mentioned, it's a matter of relative pressure. He has demonstrated, in other threads, that he is more knowledgeable than I in automotive matters, so I'm going to turn the answering over to him at this point. Pay attention also if RangerMike or Stingray chirps up; they're both serious gearheads.
    Happy Hunting on the track. Keep us informed of your activities.
  7. Oct 16, 2011 #6
    I'm afraid I have no idea precisely what the pressures are. The smog pump by design blows in to the factory exhaust, so I can know it will produce more than what's in the exhaust. Factory exhaust is not particularly free-flowing either. I expect the exhaust is slightly above atmospheric, the smog pump is producing a little more than that, and the crank case is slightly above atmospheric as well.
    None of these I can readily measure directly except perhaps crank case pressure. I could run a line to a pressure gauge in the car and see where it's at under load. At that point though I may as well build everything and see how it changes.

    So about the smog pump, it does need to be installed. It does not need to be hooked up to anything. So, just connecting the valve covers to the header collectors would be an option, BUT, the shop owner doesn't want to do that because the clearance to install them is so tight, he doesn't believe the headers could be installed and removed with the extra tube installed, without lifting up the whole motor.
  8. Oct 16, 2011 #7
    No worries! Thanks for taking the time :)
  9. Oct 16, 2011 #8


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    My pleasure.
  10. Oct 16, 2011 #9
    I bet your not old enough to know how the old steam locomotive used steam released from each cylinder to suck a vacuum on the fire box and blow the smoke out of the chemney.

    Use your car engine exhaust to suck the vacuum you need. Lets assume your car exhaust pipe is 2" diameter. Slide a 3" diameter pipe over the tip end of your exhause pipe at high speed with the engine running several 1000 RPMs all the exhaust moving through the 3" pipe will suck a vacuum on one end. The exhaust comes out one end of the 3" pipe and you have a vacuum on the other end of the pipe. If you run a piece of tubing from your engine vent to the vacuum end of the 3" pipe it will do what you want at high speed but not at idle speed.
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