hi all
ive been pondering on this for a while,
Question, if you calibrate your bench with plate X directly on bench should u get the same readings with plate X on bore adapter ?
On the bench i have built if i calibrate to the 200cfm plate the 100 and 300 do not red correctly, but if i use the bore adapter they all read correctly (which is a bench design issue )
but my point is if we are all trying to compare notes shouldn't we be calibrating using bore adapter for this reason because now if you take the same head and put in on anyone's bench with the same diameter bore adapter you should get the same readings ?
cheers
You need to understand an orifice in a pipe and the major diameter on the vena contracta or CD, the concept of flow through an orifice is based on the pressure differential across the orifice and the affective CD. The issue with the bore adapter is you have half an orifice in a pipe and half not along with NO settling chamber. All that said you can calibrate your bench in this manor if you know the effective CD on the orifice for this condition and thus know what it will flow to calibrate to. But then again what happens to the flow next time you change bore adapters it will work if you have a special set of calibration plates to work with that bore. Not trying to be smart here just trying to get some people to see why it is important to have a sufficient settling chamber in there bench so they can calibrate at the bench top.
Rick brings out a very good point here. I was helping on trying to develop a orifice in a tube system and found some real issues. If the orifice got to be much about 25% of the diameter of the pipe (or in this case bore adapter) then the CD was drastically affected causing the flow readings to be off. What we were finding was we needed pvc tubing with an 8" inside diameter or larger to really work. We also found a real need to have 24" of tubing on both sides of the orifice. This is when I came to this forum and just decided to build Bruce's bench from his plans. Rick mentions the settling chamber as being important. I second that by the testing that I did. Having a larger settling chamber is the way to go.
After reading this thread i went out today and used my calibration plate rated at 450cfm at 28" and placed it on the bore adaptor and it read 450 right on the nose this was on a 4.200 bore, i took it off and placed it directly on the bench top and it read 448 cfm so i really dont see the point in taking my bore adaptor off if there was only a 2 cfm difference. (actually it was less than 2 but i know how people get when you post.xx cfm) Is this not the case on your guys benches??
the 450 orifice i was testing with was 2.504 i am sure that bore sizes closer to the orifice plate size would have more of an affect on the flow path through the plate itself but with this combo i seen like i said less than 2 cfm difference.
oldrusty wrote:the 450 orifice i was testing with was 2.504 i am sure that bore sizes closer to the orifice plate size would have more of an affect on the flow path through the plate itself but with this combo i seen like i said less than 2 cfm difference.
I feel the same - if the calibration is only 2 cfm difference keep doing it the way you are.
my internal orifice dia was 2.959 depression was around 22 in actual i just use the software to convert it to 28. i am working with a 4.200 bore adaptor for the heads i have now are going on a sbc with 4.200 bore and i calibrate at 450 because these heads are flowing around 425 as of now and thats the closest plate i had to that. i only do this for myself and not as a busniess so bore sizes are limited to when i make for my bench to match the motor im working on. this is a custom bench i built it runs a paxton centrifigual blower as a vacuum source powered by a 5 hp and a 1.5 hp motor together.the amazing thing is its alot quieter than the vacuum motor benches ive been around.
I think the really important point is being missed, .. to me, if my bench matches others or not
isn't the primary issue.
What is important is I want to know the ACTUAL cfm the engine sees, .. ..
If the bore adaptor has an effect on the flow of the plate, .. don't you think it's going to effect the "orifice" or valve
opening in the head too????????
I want to know the CFM the engine actually sees !!!
I think the idea is to calibrate the bench it's self, then whatever you put on top flows what it flows, .. ..
Unfortunately people are too fixated on damn flow numbers, .. and now with the internet EVERYone has a voice that can
be used to turn gatherings into mobs of rioters, .. not good for society in general, .. but that's another thread.
It's doesn't matter if the head flow numbers match on everyone's bench, flowbenches are yard sticks as far as I'm concerned, ..
the scale my be different but my data is all I care about.
On my bench my 400cfm plate will flow different if it's ontop of my small block bore adapter, .., 4.200" bore,
but take the bore adapter off or use a large bore adapter and it's right on.
Shows that different size bore adapters will effect the "calibration" if you calibrate on top.
