Orifice plate range

[sully]

Hello All,

First, let me say that this is one of the best forums I have come across. I have been doing a lot of reading/learning on this forum, recently purchased a set of Bruce's plans for the PTS flowbench and hope to start construction of a bench soon.

I have a question regarding the range for each orifice plate. I remember reading in a thread from a topic in the old forum that each orifice plate has a "sweet spot" in it's range providing highest accuracy. So, this got me thinking about real world usage of the flowbench and performing flow tests at various valve heights. There is typically a wide range of flow between a valve height of 0.100 and say 0.700 (see below). Also, with a device such as Performance Trend's Electronic Automatic Valve Opener (sold as an accessory by SF), one would suspect that the whole range is being tested with a singe orifice plate. So, my question is: What level of accuracy can one expect using a single orifice plate when testing a port through a whole range of valve openings? Below is a chart (taken from the AFR web site) showing intake port flow for the AFR 315 cylinder head at different valve heights.

AFR 315
(Intake CC = 315)

0.100 76
0.200 162
0.300 232
0.400 282
0.500 321
0.600 348
0.700 368
0.800 370


Thanks,

Tim

[jfholm]

Tim,
it really depends on whether you are using an inclined fluid manometer or something like Bruce's Digital Manometer. If you use the electronic digital unit then you can use just 1 orifice and be accurate. If you happen to be using a fluid inclined manometer then you have to have an orifice for each lift range that will be flowing at least 40% of max flow of orifice. This being the case, if your orifice flows 100 cfm at your delta p then it should not be used for a lift that flows less than 40 cfm. Then once you have reached 100 cfm then you have to switch orifice plates to another that has a 40% capacity of 100 cfm and then go until it maxes out. Doing my SBC heads I usually had to use 3 -4 different orifice plates. I now use a Digital Manometer so I just use 1 orifice plate that will flow at least the max flow I will be doing.

John

[sully]

Thanks John!

So, given that a Digital Manometer (such as Bruce's DM) is sensitive enough to function accurately through a wide spectrum of flow, is it safe to assume that a single orifice plate of say 600 CFM could be used to test any device up to the plates maximum flow?

Thanks,

Tim

[hdwgfx]

Concerning orifice ranges - everything I have read on the old forum and other sites both bench sites and just general gas flow sites all agree about the orifice must be used within a higher DP vs a lower DP which is often referred to as the sweet spot, like John said above 40% on the inclined.

Thinking out loud here guys JMO

This to me suggest that the air flowing through the orifice at a low DP is affecting/changing the CD of the orifice and that the air is possibly not following the normal Vena Contracta shape and or path. The air at the low DP could possibly not even be using the entire area of the orifice or the circumference.

So how does just a more sensitive pressure sensor correct for the nature of a orifice or is there some more math involved in correcting this that I have not read about? The air is still moving in an unpredictable/unknown/unacceptable way at low DPs.

Has there been extensive testing in comparing multiple ranges with a manual inclined manometer to a Digital Manometer with one range. If these comparisons have been made was some math correction involved to correct for this in the design of a DM?

I just wanna know, please correct me if am off track here in my thinking

[Brucepts]

Just a quick reply for right now . . . the DM takes more samples and averages them so it pickups the smaller changes of flow with a large plate on the lower cfm range.

I have tested my DM with various size plates and ranges and have not seen a noticeable difference until you get down to around 25cfm with a 600cfm internal plate. Then sample rate and averaging needs to be addressed by taking more samples and less averaging. You can find a "sweet spot" with the two but each person is going to see it different.

Nice thing about digital, you can set up for your user style.

If I get some time on Saturday I'l see if I can do some testing and re-document it again as it's buried in the old forum.

[jfholm]

Just before Bruce did his test I had done a back to back test and posted it on the old forum. I flowed my head using my inclined manometer and my digital at the same time. I used different orifice plates to always keep the flow at 40% or higher for each orifice. The digital and inclined did read the same. I then left my largest orifice plate in for all lift ranges and flowed the head again. This time the water inclined manometer was off until I finally reached approximately 40%, but the digital read the same at every lift as it had done before. So yes, as far as I know there have been at least two of us do this test to prove the theory.

John

[JONATHAN]

Hi all
Has their been any more testing done on using a singel plate.?

Thanks
Jonathan

[Brucepts]

Quick answer is yes more testing was done, ALOT was learned about our DM in the process. Although it works we are not going to say it was a total success and still recommend a range of plates for utmost accuracy. On a 600cfm plate measuring 25cfm you are measuring around .00X" of water. You can not even see that movement on a water gauge!

When I get a chance I'll post some spreadsheets of some of the data. It was enlightening to say the least and it made us look at what other vendors are saying or NOT saying about their accuracy

I now get a chuckle out of posts on forums that say "my port flowed XXX.XX or XXX.X cfm"!!!

[hdwgfx]

Quick answer is yes more testing was done, ALOT was learned about our DM in the process. Although it works we are not going to say it was a total successand still recommend a range of plates for utmost accuracy.

Thanks for the info. Bruce, looking forward to the spread sheets and more details about your discoveries on the single plate...........

[Brucepts]

cfm.JPG

Ok, here is some data to discuss and this is for discussion purposes only. Bench was tested using different internal orifice plates with no change to Cd or digital manometer as well as the averaging or sample rate of the incoming data.

No changes were made in the interest of keeping everything the same so I could get some data to look at.

We were able to gain more insight than what these numbers show as the data coming in from the digital manometer was moving and the data does not show how long it took for the numbers to stabilize to the readings you see here.

I'm not sure I really know how to explain it to be truthful, seeing the data from the DM work on the screen would be the only way to get an idea of what's going on. Trust me Rick and I had some long conversations on the phone about our findings. We both decided it is best to setup your flowbench for the ranges you need to work in and not try and work across a whole range of 25-600cfm with one plate.

Testing was also done changing the Cd, small changes made large changes to the high cfm numbers and small changes to the low cfm. Tweaking the bench for "best fit" numbers would best be done in the middle ranges of your testing range.

Now this data is probably going to do a lot more to confuse people, I know it's confused me a few times!

Edit: Testing was done at 28", internal plates are listed on the left. Some of the internal plates are also used on top of the bench for testing. I have them calculated for flow at 16" and 28"