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Old 11-23-2019, 06:56 AM
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DanH DanH is offline
Join Date: Oct 2005
Location: 08A
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Originally Posted by charosenz View Post
I am fortunate that this engine did not create any appreciable pressure at 5 lbs of boost with the 1/4" vent lines. If it did, it was less than what I could see on the MAP gauge I was using, probably 1 lb or less.
Case pressure is typically measured in units much smaller than PSI. For example, the Lycoming case pressure limit is something like 4~5 inches of water. One (1) psi is 27.67 inches H2O. Perhaps a re-test with a manometer? I'd expect a healthy Honda to exhibit less blowby than a Lycoming, so an accurate baseline captured now may be useful as a diagnostic tool later.
Dan Horton
Barrett IO-390
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Old 11-23-2019, 03:15 PM
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charosenz charosenz is offline
Join Date: Aug 2009
Location: Longview, Wash
Posts: 307
Default Crankcase pressure


Good point. I have read the high boost guys with high end dynos actually measure SCFM. Ultimately though when it comes to problem I think it is fair to say that PSI is what will cause things like seals and gaskets to leak and worse fail. I've seen quite a few charts that show the SCFM and PSI per hose diameter unfortunately most charts are geared towards high pressure. I have not found one with good specs on lower pressre (i.e. 0.5 - 3.0 psi) as it relates to say 0.125" - 0.50" hose size.) If you find one please post. Same with an easy way to measure, say, 0.2" to 2.0" scfm. Other than to extapolate the SCFM with known credible PSI and Hose I.D. numbers.

EDIT: I forgot to mention the manometer is a good idea. I think I detected some suction on low RPM. So the current home man manometer will have to be redesigned. And dang, allowing for suction and up to say 30In is one tall manometer! I just need more time to do all of this.

Aside from this, I have to admit pushing this set up in to boost is on the edge. Last time I crept up to 4500 rpm at 40" MAP I actually felt it starting to lift and get really light on the right gear. Needless to say I quickly reduced the power. Without wings on the fuse, it is just not stable enough in the roll axis to push it very hard. So higher MAP crank case pressure testing may have to wait until the wings are on it.



Last edited by charosenz : 11-23-2019 at 04:30 PM.
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Old 11-30-2019, 09:12 PM
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charosenz charosenz is offline
Join Date: Aug 2009
Location: Longview, Wash
Posts: 307
Default Digital low pressure gauge

I found this digital low pressure gauge. I bench tested it and it seems to read down to 0.30 psi and increases on tenths of PSI. It even quickly measure pressure from simply blowing in it. (even though it states it is 0.5 psi resolution. Cheap too readily available on Amazon or Ebay.

Dan's idea of a manometer is still a very good choice, this is a simple option as well.

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