[EFI] Re: [EFI] Re: Building a Flow bench

Greg Hermann bearbvd at mindspring.com
Wed Feb 6 18:42:24 GMT 2002


At 4:51 PM 2/5/02, Brian Dessent wrote:
>Daniel Houlton wrote:
>
>> Yes, I would want some oxygen to get into the engine.  Which is
>> why I'm questioning why you'd use less than 1 psi to flow test
>> a head when there's in the order of 14 psi (on a NA motor)
>> pushing through the intake valve at WOT?
>
>There may be atmospheric pressure on the upstream side of the port but
>that doesn't mean the cylinder has a vacuum, in fact it's probably quite
>close to atmospheric as well, depending on about a million factors (cam
>timing, exhaust backpressure, etc...)  That nice 18"Hg idle vacuum is
>created by the throttle plate being closed.  The actual pressure
>difference across the head at WOT is probably quite close to 25" or 28"
>WC.

Gee--an echo??
>
>As far as a flow bench is concerned... I did a quick calculation, and it
>seems that if you took a 20 gallon tank, pressurized it to 120 psi
>(assuming you do it slowly or let it cool), then could somehow release
>it very quickly, you could achieve your 25" water column pressure at 200
>cfm for like 6-7 seconds.  The numbers are approximate, and the valving
>would probably be really tough, but it would let you take your time
>building up the air mass with any old compressor and then let it go in a
>hurry.  I've also completely avoided the temperature / condensation
>issue.

Not totally. The humidity (water vapor) in the air can vary, as we all know
(??), from zero to the saturation point at any given air temp. Because
water has a different molecular weight (18) from the average for air (29),
the presence (or absence) of its vapor affects the flow resistance of the
air as it flows through something like a port. Therefore--without a
reasonably accurate measurement of the relative humidity of your test air,
you CANNOT get repeatable flow test results.

Greg


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