Newbie design help

[slracer]

OK, Now I'm confused!

Otto, you say "The Depresion input is the pressure differencial across your oriface plate. say 10 inches."

Are you saying that the pressure being measured for the inclined is the differential across the orifice plate? If so, then I understand. Otherwise, ??? Since the static pressure pickup (there is only 1 on Bruce's plans) is in the chamber above the orifice it cannot measure the differential across the orifice plate. IT measures depression (or test pressure) in the settling chamber under the head, carb, or whatever is being tested. The differential pressure taps are above and below the orifice plate and "measure" the flow as seen by the inclined manometer.

Since the flow through the orifice creates a pressure drop (in your example, 10") and air like water flows "downhill", the pressure required in the lower chamber has to be lower thus the 38" number you referenced. I have seen this before and it makes me believe my pitot bench is more efficient than an orifice bench. I have the 28" depression in my settling chamber and a pipe for a flow path which has the averaging pitot tube, flow straighteners and a single 90* turn into the vacuum chamber. I can't believe that there is a 10" delta P in that, so my motors are not working as hard to get the same test pressure. Is that right? Hmmm! -- Doug

[bruce]

Doug, You have it correct

[Otto]

[Otto]

the point i was trying to make was pick what dp across the plate you want and then figure your incline and oriface size then worry about test pressure

for something in his range I would pick 8" across the oriface for more sensitivity and less power requirments and then you figure your incline based on 8" with whatever fluid of his choosing and calculate his 125 cfm plate on that 8" dp

[bearwen]

Otto,
I get it thanks. Oh you said I could go ahead and make a square edge plate and use 1/8" works for me as for right now I am not really concerned with CFM as I want to flow my teams parts only and dont care to compare with others so I am just looking for a good and repeatable way to measure our parts make changes and see what happend and to flow all the stock parts that are not allowed to be machined to find the best ones to use. ok so the next questin is if you look at the sheet from SF there square edge diagram it shows that it is perhaps 3 times as thick as the sharp edge is this a factor or was it just drawn that way for illustration? If the thickness matters how do i figure out what thickness I need? Last question when i want to switch to blow mode to test exhausts all I do is reverse the flow through the bench and switch the hoses on the incline and flip the orifice plate if using a sharp edge correct? Do I need to move the vertical depression gauge to the other side of the box?

Thanks again everyone for your help.

[bruce]

The thickness of your plate will have an effect on the Cd of a square edged plate. For what you plan to do (comparison testing) I would not worry about it, machine your plate so it has a nice round clean hole and go from there. Machine another plate 80% of your test plate and use that for a "calibration standard" to set your incline.

Back to the thickness . . . you want a plate that will not flex under testing so don't go to thin.

Ed's spreadsheet will give you those numbers. Don't be worried about the Cd's as you are looking for comparison changes and not actual cfm numbers at this time.

[bearwen]

Bruce,
Dont I still need to have an incline meter that is deisgned for the plate i am using? I know how to do it in the spreadsheet but if I dont know the cd of the plate because i made it from a thickness that is incorrect then how do i set up an incline meter to be accurate and linear? Again the actual CFM is not important it would be nice but not required? Correct me if I am wrong make bench with orfice plate set up meters pull a run with no obstruction and make a incline scale from the spread sheet and readjust the incline based on a restrictor plate that is 80% of orifice plate that all makes sense but i am unsure as to how to change the incline meters scale once i get it ran with and withou the restrictor or flow plate? I know that without it should pull to 100% and with 80 plate it should pull 80% but is the scale linear it doesnt seem to be in the spreadsheet so how do i refigure the scale? Any idea how thick the plate should be for a given dia and a coeffiecent or maybe a formula to figure it out at least to get close.

[bruce]

Use the spreadsheet to get the scale length you want, this will also give you the vertical rise. Make your percent scale, then adjust your vertical rise to read 80% with the 80% plate. If you put the pivot point on the same axis as your zero mark it will be pretty close. You might have to tweak it a little to get it dead-on.

You will find that the calculations in Ed's sheet will be really close to what you end up with but, this way you get it set to your specific bench. So the plate Cd really plays no part. If you change plates though you might see a difference. Your incline manometer will be calibrated to that one specific set of plates.

Hope that all makes sense . . .

[49-1183904562]

[bearwen]

1960 Does this sound right I could flow 60CFM but at 28" and have a incline that is set up for 60 CFM and when I flow an exahust pipe of .75" ID it will show any changes to bends or what not better since I am flowing @ 28" correct? I am still unsure about the CFM as if the pipe is capable of flowing 100CFM but I am only able to flow 60CFM with the bench then I will not see any changes unless its a serious restriction since the pipe can flow high volumes but the bench cant flow as high. I am not sure I get it. Perhasp a laymens example as I am just trying to wrap my head around this flow stuff.

[49-1183904562]

[color=#000000]Bearwen;

First off you want to build your bench (single orifice) and make sure the orifice plate is capable of flowing more than you would need by at least 25% (Ricks personal rule or make it a changeable orifice design) SO if your dream is to get a head to flow 100CFM then make sure the bench can flow a max of 125cfm. My reason for this is that you will always be working closer to the midpoint of the benches capabilities and it should be more accurate (sweet spot).

The system as a hole is going to flow to the greatest restriction. If your carb is a .95 bore than everything down stream should be larger. There is no reason that you have to mount the carb horizontal, it can be flown vertically since this is dry testing. If you for some reason a need to flow horizontal then make sure the manifold piping is not going to interfere with the testing (bends etc). So it should be larger in bore than the intake and without the carb mounted able to flow a lot more than the carb itself. This will make sure the tubing is not interfering with your test process.

Last your max flow and CFM. When you design and build your orifice plate (the one I will still recommend you buy from Bruce) it is designed to flow X CFM at X Delta P. So if you build your bench, lets say for a 6