Porting School #2 - Super Cheap Flow Bench

[1bolt]

The home made flow bench is a little more involved than that. You need another U-tube to measure pressure drop across a known orifice size BEFORE the "stack" (the stack being the tube below your head). This is usually a tank with a plate through the middle one side connected to the vacuums the other side connected to the stack. The plate in the middle gets a hole through it of a known size. This will allow basic repeatability. Voltage changes and weather conditions (temp especially) will make a HUGE difference from test to test. With a second U-tube that measures the "pressure drop" across the hole in the plate you'll write down both test pressures (think of the tank pressure drop like a "baseline") your flow (or stack) U-tube's variances will be able to be compared to your pressure drop variances which means you'll get automatic (automagical heheh) self calibration from day to day. Then you need a speadsheet with conversion calculations that give you CFM based on the known "orifice" in the tank, and the unknown modified port...

The best part is its self calibrating just correct for temperature and barometric pressure. Or someone in the house running a washing machine on the same circuit as your vaccum(s) (you will end up needing more than one 6.5 HP vac, to pull enough inches with .5" valve lifts or higher, to compare changes at high lifts). How do I know this? I have two 6.5 HP shop vacs running my bench At high lifts the pressure drop at the tank side will only be a couple inches at best unless you have a tiny port. Which makes accurate measurement of small changes harder.

You also need zero leaks anywhere, including something sealing the head to the bore adaptor (the plywood). I use modeling clay. A rubber tube for your stack will vibrate and flex you need totally rigid PVC. Fluctuations are usually leaks, even insanely tiny leaks will cause movement... So will voltage changes as someone turns on or off other appliances.

[ThePinnacle]

1bolt,

Thank you for the reply.

I have been doing my research and I came to the same conclusion as your advice. Actually I had thought that the orifice plate setup was simply "the next level" if you will since you can calculate actual CFM from it as per this post DIY Super Accurate Flowbench

I didn't know it was an absolute must (though it seemed that way since my setup basically sucked)

Along the way I found some interesting information about orifice plate flow meters. I also found this great calculator to determine flow through an orifice plate of a given size amongst other parameters... you might find this interesting: Flowrate Calculation for an Orifice Flowmeter there are several generic tables for air density vs temperature. At 25 c (around room temperature) its 1.18... the default value in that calculator is 0 c

Anyway, do you suppose a single 6.5 hp vac will be enough to flow a head that will be under 220 cfm? The stock buick V6 head is pretty horrendous, I believe at 28" it only flows 150 -160 cfm and max ported heads only get somewhat near 220.

According to the calculations I made using the link above, I came up with a 1.25" orifice plate. What size orifice plate do you use or recommend?

I did figure out my manometer fluctuations. I put a restrictor in the manometer line.. All I did was get a needle valve with 1/8 npt threads where I installed 1/8 npt to 3/8" barbs. The needle valve allows me to precisely adjust the dampening. Of course this has no effect on the consistency.

One last question for you, what is the calculation to figure out cfm of the item you are flow benching? I see how the calculator I posted above arrives at the cfm value of the orifice.. but then how do you go from there?

I'd like to make a similar spreadsheet (or if you'd like to share yours, I'd appreciate being able to check it out)

Thanks again

[brucepts]

You can find all the answers to your questions including spreadsheets to help you design an inclined manometer and orifice plates on this site:

Flowbench Technology

All you need to do is follow the new member post in the General section to join. You will only see the standard forums till you become a member (there is no charge for membership). I have had a lot of forum members build some pretty serious home built benches!!

I also supply sharp edge orifice plates, velocity probes, plans and coming soon a 3 sensor digital manometer that will be priced for the DIY'er but offer Professional features! These parts are being used in quite a few Professional shops across the world now. No waiting list for the parts either, they are made in my shop.

[MorGo]

All I can say about Bruce is that the pitot probe is piece of art (and cheap!) and those orifice plates are just cool! To know how your bench relates to multimillion dollar Ford Motor Company bench creates somekind of self confidence:"My bench read very real numbers;how about the others!"
And service is quick and friendly!
(no,I am even on other continent so no ties to Bruce!)

[1bolt]

Bruce's site is chock full of home made flow bench goodness and I would recomend going there for all the answers to questions you haven't even thought up yet. I do