PTS Flowbench Forum

Hi Gang,

It's been fore ever since I've been over here, .. .. glad to see you're still here.

Question, .. or opening to a discussion, ..

Calibrating your bench with NO bore fixture, .. or calibrate WITH bore fixture, ..

what is everyone's feelings and what differences have you seen ?????

Curtis

I just did this- saw an 11% gain in flow by using the orifice plate on top of a 4" bore adapter as opposed to the 5" hole in the top of the bench. My own thought is to use the plate directly on the bench without the bore. hope this helps, J

I calibrate with the bore fixture. That is what the head chamber "sees".

now here is my thinking on this subject. If you used the same cylinder adapter every time you calibrated your bench then fine, you always calibrate back to the same setting. If all you are looking for is an improvement in what you are working on and always calibrate it the same way then you are OK. The Venturi bench I used to work on had a calibration hole in the bottom of a tall stainless can that was about 5 inches in diameter and about 8 inches tall. We always calibrated to that when using that bench.

I think Rick mentioned the effect of flowing on top a cylinder adapter and Jason just mentioned it also. I now calibrate by putting my calibration orifice right on top of my bench. The reason for this is two fold. I am not enhancing the flow of the orifice with a tube. We have a program that Ed put on the forum for us to download that tells us how much an orifice will flow at a certain test pressure. Those results are not through a tube but what an orifice would flow as sitting on top of most benches. That way you can test accurately what different bore sizes will do to the flow in a cylinder head.

Also if you calibrate this way and you have someone who you want to work with and you both calibrate together the same way then you can usually compare work. I love to work with other people as we always seem to teach other tricks we have learned. We are not high dollar racers so we need all the help we can get. Basically either way will work, but if you test with orifice right on top of the bench then you can check with Ed's sheet and know "what" that orifice should flow and not just that it is the same each time. This will help to keep you from having a "happy flow bench" one that reads way more than everybody else. I know a guy whose bench reads low by a few percent and leaves it that way on purpose.

John

Welcome to the forum again Mr. Boggs.

When using plate over "pipe" for calibration, the heads lost cfm. This especially happened when working with a "different" type of wet/dry bench that had a pipe as part of the internal system. The bench internal CD was lowered to match with the increased CD of the calibration plate from having the pipe below it.

My thinking now for calibrating over an open hole directly into the bench is due to the fact that the bore is part of the chamber. And as you know, moving valves around and increasing pressure recovery into the chamber/bore is part of the induction system. Its like instead of swapping only heads on and off the bench, you are swapping the head/bore as if it was one piece.

Ok here goes let see if I can make some points on what many of the issues are. First off I will not come out and say “You should calibrate on top of your bench” as it is not my business to and if I do I am sure someone will disagree as it does not work with their bench or some well known engine builder does it so it makes it right. Many of the members of the Society of Head Porters have beaten this subject to death and we all should know that the proper way to calibrate ones bench is with a NIST traceable calibration venturi http://en.wikipedia.org/wiki/Venturi_effect .

All that BS aside the next best way is with two known orifice plates manufactured as close as possible to each other, not size but process and specification (Thickness, sharp or square edge etc.). You may be asking why and it is because we want to regression test the CD of the two plates against each other. Once we know the dynamic CD of each plate we can use the spreadsheet to determine actual potential flow of the plate and calibrate the bench to them. The point is why would one try to calibrate a bench with two different plates that you have no valid knowledge of the plates CD, internal or external. This is the point validate ones bench. “This alone is one of the core beauties of the PTS bench design as it uses the same plate in either location and can be swapped back and forth to easily regress the CD” If you are unable to change your internal plate I suggest you buy one of Bruce’s sharp edge plates and have him flow it on a known calibrated bench using like plates to give you the actual CD of the that specific plate.

Now for calibration, we know that if you place the plate on top of an Pipe it will flow differently as you are effecting the post flow of the orifice thus effecting the Vena Contracta and eventually the plates CD http://en.wikipedia.org/wiki/Vena_contracta . In essence you are simulating ½ of an orifice in a pipe. http://en.wikipedia.org/wiki/Orifice_plate . There is a ratio of orifice diameter to Pipe diameter that once you reach it you will have little or no effect on the CD of the orifice. That is once the Pipe becomes so large that its ID does not disrupt post flow your, CD should not change the pipe is acting as a settling chamber at his point. This could be proven by flowing a .250 orifice on a 4.0 bore adapter, most likely it will flow that same as on top of the bench.

So if your bench is built with a large enough settling chamber and no obstruction to the inlet flow or potential flow disruption to the internal orifice; you can place the orifice plate on top of the bench. You are now testing a known plate against a known plate, thus your calibration would not be based on adjusting CD to reach calibration but maybe incline monometer angle etc or DM calibration settings. ( I am not saying you cannot adjust CD to calibrate but you should record what you did and why as this may affect future flow data).

Now if I place the same plate on a bore adapter I may and most likely will be affecting the CD of the plate but by how much?? What math tells me I am right? It is only ½ and orifice in a pipe?

Now lets say you did calibrate this way and you adjusted the CD of the internal plate to reach your calibration, you flow a head on a 4.0 bore adapter, you then decide your combination will use a 4.125 bore and you calibrate your bench again to the 4.125 bore again adjusting the CD to get calibration. You now flow the head; Could you actually compare the numbers from one flow to another? Are they even valid? How can you correlate one flow to the other? I do not know of any math that will tell me this but I am sure there are those smarter than me that will figure it out and tell me I am wrong. So be it.

If you are flowing the same head on the same bore adapter all the time and comparing to no other heads than sure this will work for your limited flow data. But for this you lose the ability to compare all your work data to each other and to me that is throwing away good data and potential knowledge.

You are calibrating the bench not your test setup. You are trying to make sure your bench is relative to itself and working properly and if done correctly will be relative to others if calibrated the same way. The concept here is to keep as many variables out of the calibration process so your end flow numbers can be compared against each other (head style “A” VS head style “B”) for education and correlation to your porting skills.

I told you I did not have an opinion

Remember the more error we take out of or bench, test setup, test methodology the more accurate and repeatable our data will be.

Rick

So Rick, .. .. how do you really feel about it ?

I brought this subject up to see what everyone thought, ..

I do have VERY strong feelings about it, .. and honestly it's dead on with what Rick posted, .
As far as I'm concerned you bolt a head on a bore, .. THAT is what the engine sees, .. simple as that.

Also, I brought it up as an engine builder got an expensive set of heads from
me then proceeded to rip me apart cause they didn't flow the same on his bench, ..
he was a little lost when it came to calibration.

Cheers,

Curtis

A little more on this subject on how bore adapters can affect flow. I have taken my SBC head and flowed it with different size bore adapters without touching the head and the flow changes. So bore does have an affect. Also one time when I was helping an SCCA racer with his Turner Ford we built a carb spacer to go under the carb. Instead of the carb opening into an open plenum it first had to through the adapter. I think it was 1.5" or maybe 2" thick. We machined it so it had two holes the same size as the throttle bores. Bottom edge of the bores radiused. It increased air flow through the carb as compared to being on an open plenum. Maybe this was because it affected the CD, I don't know but it did work and showed more HP on the dyno. This was under the carb not on top. You have to try everything and not just rule it out because you "think" it may not work. We had a whole shop full of parts that did not work.

Rick and I have both pointed out how important it is to have your bench calibrated so you can compare to each other accurately. Curtis has shown us how he was bit in the butt by someone who's bench was out in left field. I lost the calibration on a NON-orifice plate bench and was telling the guy how much air I had mysteriously found overnight He took it to another friend who my bench used to be close to and he called me and said "John, you had better check your calibration." I did and it was way off and I could never get it back. That is when I found this forum and built the orifice bench and have never regretted it.

John

Help me understand this better. Bruces cal plate is the reference calilbration. I will assume he flows it without a bore fixture on top of his PTS bench. Let's say he sends it to me and it flows XXX cfm @ 28" wc.. So I place it on my bore fixture. 3.680". which happens to be the bore of my best motor (now deceased) 330. I calibrate the bench incline test and incline manometers to each other in parallel. Ok, I then flow the inner plates to find their cfm and cd values. I know their orifice diameter and I know the delta pressure across them. Using the spreadsheet I find their respective Cd values in calibration. Might be .62, might be .595 or .612. Doesn't matter because now the inner plates are calibrated to the flow of Bruces calibration plate at that delta pressure. And the inner plates have no bore adapter effect. Just space above and below them. And his plate is referenced to his bench with no bore adapter fixture. So at this point I don't see the bore adapter swinging my inner plates reference values. All this is about to change for me because I am going Flowsoft. As an update though I sent all my plates to Larry M recently and he measured and flowed them. So technically the plates are referenced to his bench now.

I think this discussion of whether to test with a bore fixture or not is a little misguided. Let me explain, you're attempting to calibrate the orifice inside the flow bench, not the bore fixture or anything else and to do so, you're comparing that orifice to a calibration plate which is spec'd out to flow xxx cfm @ xx " W.C. in order for the comparison to be valid, we need to know the conditions under which our calibration plate is designed to meet its specifications and replicate those conditions as closely as we're able.

For example, let's says you asked Bruce to produce two calibration plates for you one designed to flow 200 cfm @ 28" when placed on a bore fixture 4" in diameter, 6" tall and a second plate which also flow 200 cfm @ 28" but designed to be placed directly on the bench top. I have a lot of faith in Bruce so I can say that both plates would be capable of accurately calibrating your flow bench but only when used under the condition for which they were designed. If you use the plate designed for the bench top with the bore fixture, you're going to get an erroneous calibration and visa versa.

IOW, part of the specification of a calibration plate should include a description of the conditions under which the plate will meet it's specification, if you're not sure, ask the plate vendor, they're not sure, get a different vendor.