Bench Size

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Ok, is there such a thing as an "optimum" size for the area between the orifice plate and the test piece? I know you want the area around the orifice plate to be as large and flat as possible, but how much is too much? And how big should that chamber be? I'll be flowing carbs that will be in the 25-50 cfm range.

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Difficult to give a meaningful answer, but the larger the settling volume before the measurement orifice, hopefully the lower will be the air turbulence entering the orifice.

On the other hand, orifice plates run at higher calibrated pressure drops will be more tolerant of some upstream turbulence.

A good setup will have large internal flow volumes and as high a pressure drop across the measurement orifice that is consistent with available blower power.

A poor setup might be very compact, and have a piss weak blower, forcing you to make all your flow measurements at very small pressure differentials.

Make it as big and powerful as you possibly can in the physical space you have available, but "big" is a relative term.

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Point well taken. What my "plans" are, is to go between 28"-40" test pressure.

The motor I have is the Ametek 115923. I think 1 will do well for what I have planned. Most carbs we deal with are .425" to 1" bore.

I'm thinking something along the lines of the SF-110 but with a larger bottom end. Something around 12" tall, 40" wide and I'm not sure on the depth yet, but something in the 36"-42" deep would work well with the floorplan I have.

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Forty inches test pressure is getting rather high, plus you need to add orifice pressure drop to that figure as well.

It may be a good idea to test your bare blowers before building the bench. You might find that blowers in series might outperform blowers in parallel at those sort of pressures.

There is no real way of knowing until you test the particular blowers you plan to run. They all seem to have different pressure/flow curves.

Another thing to be very careful of, forty inches is about 1.43psi, that is about 205 pounds per square foot trying to crush your bench. Make it nice and strong, or the panels are going to bend inwards, or worse.....

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Here is the link to the flow data for the motor I plan to use:


If I am reading that right (but I could very well be wrong) I should be able to get the 40" test pressure I'm looking for through a .500" test peice. And there will be only one motor so wiring series/parallel really does not come into play.

Now, I am confused about what you said about the pressure drop across the orifice. Let's say my test pressure is 28" and the flow scale I have is 6" tall, do I add 28"+6" to come up with the total test pressure?

Yes, you are correct about the test pressure and I plan to heavily brace this bench. That is also one good point to keeping it small as there will be no large surfaces. I can brace and support it rather easy since it will be a small bench.

Thanks for the info Tony, it has been a GREAT help. I hope to get this project off the ground this weekend and start taking shape.

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The blower sees the total pressure, that is test pressure (28") plus design orifice pressure drop (6") plus any other slight flow restrictions in the flow path.

That blower curve also assumes the full 120 volts is available right at the motor, and 68F entering air temperature.

The curve says 84CFM at 44 inches which is more pressure than you will probably end up having, so you should get at least that much flow I would think flat out.