PTS Flowbench Forum

Using the same CD for the orifice as I am a 3.1" orifice will flow 508.6 CFM at 16" H2O.

If you use a "square edged" orifice where you bore the hole straight through your plate where you have no bevel then your CD will be higher and hence the cfm will be higher than a "sharp edged" orifice at the same test pressure. I did a test back to back at the same test pressure and the orifice I was flowing (2.250"), the sharp edged flowed about 4 cfm less than the orifice with a .040" edge. It was not much of a difference but it did flow differently. Now if your edge on your square edged orifice is .060" to .080" and goes straight through then there could be even a bigger difference in flow compared to a sharp edged orifice with a beveled back. The orifices I was testing were bored in an .125" thick stainless plate. Both right on 2.250" both with a 45º back bevel to the orifice. The edge on one orifice was sharp and the other had a .040" thick edge. I would imagine the cd for a sharp edged would be around .595 -.600. Having a squared edged as thick as you are going to have the cd could be as high as .615 -.620. This is one of the reasons it gets to be an exercise in futility to compare one brand of bench to another as the internal cd's may be way different changing the calibration.

I was only flowing at 10" H2O so I only needed the 243 cfm orifice as the mathematical conversion would push my flow at 10" way higher. If you are flowing at the higher 28" H2O then you will need the bigger orifices to cover the higher range.

John

Thanks for that. I guess there would be some calculation or an online calculator that would give a close Cd based on the hole size, type of hole edge and the thickness of the hole.
I'm just wondering if I could bore a straight edged hole in a 125 thou thick (or even thicker) piece of material and what the CD would be.

I feel like the .125" would be best to give the stiffness for the operation in the bench you are building. In your case if I follow correctly you cannot use an orifice that is beveled on one side because it would not work in the reverse direction. What I might try if I were you is to just spot face around the orifice hole itself with a larger flat spot facer to make the area around the orifice hole thinner.

but this is one argument for building the PTS flow bench. Easy to flip the orifice over for flow in other direction.

John

A sharp edge orifice plate maintains it's Cd over it's working range of flow. The square edge orifice plate on the other hand is effected by the thickness of the plate so the Cd will change over it's working range.

Bruce, that is a very good point. I just don't know how you could use a sharp edged orifice in the MSD build, do you? That is why I suggested it would just be easier to build a PTS bench.

You would have to spot face the orifice disk in the MSD pretty thin around the orifice hole. The stainless orifices I was testing with were designed to be used in a pretty harsh environment. That is why they were only beveled so the edge of the orifice hole was .040" thick. I did not want to touch or mess with the orifice I got from you.

John

Ok, so how much change in CD would we be talking about over the working range of the orifice?
John I've got a few ideas about the rotating orifice, I'm just exploring what the limits of the holes would be first.

jfholm wrote:Bruce, that is a very good point. I just don't know how you could use a sharp edged orifice in the MSD build, do you? That is why I suggested it would just be easier to build a PTS bench.

John

This is why my design does not use a rotating disk

The PTS Design does away with all the problem areas of most if not all of the other flowbench designs that have been tried over the years. Using the info posted on the old Tractorsport forum and previous forums, I developed my plans and parts. Flowbench design has evolved since the MSD plans were posted in the Car Craft magazine years ago. Yes, you can still build a flowbench using those plans, make changes and have a good flowbench but looking at past posts on the forum it can be seen that a lot of people have taken it on as project and come here for the answers as to why is does not work like it should

Again, I'm not trying to sell my parts, only keep people from heading down a road we all know is fraught with problem areas and need to be addressed. But, if you are up for a challenge and wanting to learn and build something then by all means go the DIY route, do your research and have fun building and enjoying your end results.

As for orifice plates and plate thickness I have not done alot of research on this subject as I found my sharp edge plates work quite well so no further testing was needed on my part.

Ok. guess the best way is to just make up some test plates and flow them. I'll hunt around the net and see if I can find some info on cd's of straight edged orifices unless someone here can shed some light on the subject.

Bakerlite wrote:Thanks for that. I guess there would be some calculation or an online calculator that would give a close Cd based on the hole size, type of hole edge and the thickness of the hole.
I'm just wondering if I could bore a straight edged hole in a 125 thou thick (or even thicker) piece of material and what the CD would be.

Any calculation would be an estimate. You find the true CD when you calibrate the bench.

The main problem with that particular bench design is the air blasting directly at the orifices. Here's a sketch of away to cure that problem.

Add a raised section to the top chamber after installing the baffle.