Sharp edge orifice CFM @28" per square inch?

[rwdford]

Hello guys
I am new to the forum but have been reading with interest, I know there are some very knowledgeable helpful people on here

I am in the process of building a new 6 motor vari speed orifice style flowbench but am using a basic floating depression 4 motor for the time being

Would like to get some fairly decent flow figures from some race carbs I am modifying, Weber 45 DCOE and 48 DCO/SP side draft carbs with the full range of choke sizes available and some very high flow + high velocity chokes I am working on

Anyway I am just wondering with a true sharp edge orifice cut into a 2mm flat steel plate with a 45* chamfer and a razor sharp edge but with no raised burr what CFM does a well made orifice like this flow @28" per square inch?

I have read 84CFM @28 on the Speed Talk forum which appears to be pretty close but just wanted to hear what you guys think is the most accurate figure for this, if 100% flow coefficient equals 147 CFM @28" then this would have a coefficient of 0.571 which sounds realistic, maybe a little on the low side as some people use up to 0.62

Are the 84CFM and 147CFM values correct or at least pretty close?

Machining the orifice plates is not a problem as I have an engineering workshop with a decent lathe and bridgeport mill with good tooling and DRO, I have already made 2 different size plates cut from flat 2mm steel plate, bored in the lathe to exactly 51.80mm and 55.20mm both with a 45* chamfer, the edge is a perfect 45* angle with no burr, razor sharp, I lightly honed the top surface dead flat to remove the tiny burr

I flow tested a 45DCOE with 38mm choke, aux vent and trumpet in place with all holes sealed off in the carb and was getting a little more flow than the 51.80mm ID plate which flows 239CFM @21.3", David Vizard states in one of his books that this combination flows 242CFM @21.3" (1.5" HG) which is telling me the 84CFM figure may be correct but I would like to confirm this if possible

I know floating depression benches are nowhere near ideal but until my new proper orifice bench is finished I just want to get some basic readings so that I know which combination flows more air when A, B, A, B, A, B testing and also to get a pretty decent idea of the CFM @21.3" within reason

Thank you Bruce for the flowbench plans that I bought recently, it is a great design with minimal chance of any air leak and user friendly + simple rather than super complicated and a high risk of air leaks, tbh your design makes a superflow 110 look like a well over priced toy lol!

Best regards
Jason
Ireland

[Brucepts]

I machine my plates to a .62 Cd number and it seems to have become the industry standard with the amount of plates that are now in the field.

[rwdford]

Thank you Bruce

Is the 147CFM @28" for 100% coefficient correct?

0.62 x 147 = 91.14CFM @28" per square inch

In that case I am getting a lot more flow than I thought

[86rocco]

Those numbers sound just about right. If you want to play around the numbers a bit, download the spreadsheet from THIS THREAD

[Old Grey]

Is the 147CFM @28" for 100% coefficient correct?

!47cfm is for an orifice with a radius entry and a square edge exit - like a trumpet on a Weber - and in testing I have found it to be true.
Now, if you use a orifice with a radius entry and a radius exit, it would be higher because the gradual declaration of the air on exit reduces the turbulence and makes it flow more. In this case something like 153cfm/sq inch - from memory I did a rough test that produced 151cfm, but the orifice wasn't optimised -.