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Mon Jul 08, 2024 2:42 pm

Anybody have any experience with blow by meters?

I noted that the orifice size is .040" which is the same as on the leakdown testers. Kinda measuring the same thing, but with the engine running instead of just one cylinder sitting at TDC.

A common number I found in the v8 world is 2 inches or less of water column or about 2 to 3 SCFM for a motor in good shape. Does that sound accurate? Any idea what a VW would blow? I think a magnehelic gauge would make it easy to hookup on a vw. Just curious about the numbers.

Mon Jul 08, 2024 3:16 pm

My initial thoughts are that someone would have to sit down and calculate a chart relating SCFM vs displacement vs RPM to get any degree of usable data out of that method of measurement.
Then you would need a chart to convert orifice size vs pressure to get SCFM values taking atmospheric pressure (altitude) into consideration. And there is the problem of temperature and gas density correction too. It's not simple or straight forward. Here is a taste:

https://www.engineersedge.com/calculators/gas_flow_rate_through_orifice_15779.htm

Gas flow measurement with an orifice plate is a long and complicated process to get any degree of accuracy. And as stated in the above article this is for a steady state flow. Pulsating flows are much more difficult. As said too, how much is too much? Has anyone actually tested and documented that? Needs to be done.

From my years of measurement experience you would be better off tying in a roots style gas meter or better yet, a positive displacement bellows style house gas meter. Then you would only have gas densities and temperature corrections to deal with.

Does that help you understand why it's not commonly done?

Mon Jul 08, 2024 4:05 pm

Found this on Speedtalk

https://www.speed-talk.com/forum/viewtopic.php?t=4607

Gauges on a 0 to 10 SCFM about $40 and up

https://dwyer-inst.com/en/series-rm-rate-masterr-polycarbonate-flowmeter.html

Alot of info here:

https://www.performancetrends.com/Blowby_CFM_Flow_Sensor_Meter.htm

Mon Jul 08, 2024 5:00 pm

Quote: Does that help you understand why it's not commonly done?

Yeah, its not as easy as I thought. A good digital gauge is around $500 or so.

I was thinking, if you had a newly rebuilt motor with good compression and leak down numbers, you could measure your SCFM output on the breather and have a reference number. Like a standard, then retest would be easier then doing a compression and leakdown test.

Maybe too many variables

Mon Jul 08, 2024 5:45 pm

How about just use one of those gas flow gauges where the ball rides up and down.

Mon Jul 08, 2024 6:15 pm

It really should not matter what KIND of gauge you use as long as its bore is larger than the outlet from the engine you are measuring FROM....so you do not create a restriction, and it needs to measure everything that comes out.

Velocity is not an issue....just volume in CC's or CFM. You can use virtually any accurate anemometer which is what that sensor is at that third link. I have a vane type that when placed in a cup adapter capturing all exiting air....would work just fine. Ray

Mon Jul 08, 2024 8:31 pm

chrisflstf wrote: I was thinking, if you had a newly rebuilt motor with good compression and leak down numbers, you could measure your SCFM output on the breather and have a reference number. Like a standard, then retest would be easier then doing a compression and leakdown test.

Maybe too many variables
Yes that makes the most sense. In reality who cares what the actual flow numbers are (getting accurate numbers is indeed too complicated for our purposes then who is to say what it should be), get a base line on a fresh engine, then look for change over time. Oil consumption has been my go to for deteriorating engine condition.