calb plates ontop of bore fixture?

[Brucepts]

Education of the "General Populace" they only care about CFM numbers as you so well know.

Those that actually use this tool knows what it is capable of. But, you have to deal with the others who foster the mind-set of "CFM is all that".

CFM sells parts, simple as that to most people!

[oldrusty]

Curtis, i couldnt agree with you more on cfm numbers being a fixation for the general populous but when you calibrate isnt this what we calibrate with, CFM? The reason i built my bench wasnt because i had no one around here with a flowbench, it was because of the 3 guys with benches close, not one of them had a velocity probe on their bench. I called them wanting a velocity map of my port and they acted confused and said well i can flow it for $$ so i said screw it and built my own bench with a dedicated velocity probe because the way they port without knowing where the velocity is i believe is shooting in the dark. Sure when they get done theyre heads flow xxx cfm but who cares if the ports are turbulent or way to big or the csa is way off the engine will be a slug and never carry any power past peak. ok sorry i kind of went off topic here but its something im opinionated on, even though im just a newbie starting out.

[blaktopr]

Here is where the calibration can screw you if it is wrong. Velocity. When probing and finding what the cross sections are.

If you do the math using airspeeds and cfm, the resulting cross section can be different than what is physically measured.

If one molds and measures a port, then begins probing it and finds cfm doesn't add up, then it may be time to recalibrate.

Or one may grind with one part of the equasion "off" and begin to get away from the "package" that is needed for a combo.

Math....350 cfm, 3 sq in pinch, 280 fps.
If say head flows 340 using different calibration method, does 280 fps, the calculated cross section is now 2.91.
Or 335 cfm at 280 fps, ....2.87 sq/in.
360CFM@280 fps......now 3.08 sq/in.

But when measured from port mold.......3 sq/in.

Hope this adds some more light.

[1960FL]

If the bore adapter 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, .. ..

Curtis i could not agree more... may I add in either direction!

Rusty,

The bench is the standard, we test the bench to accurately flow in a given range based on the capability of the orifice plate inside. We are testing a flow system be it intake or exhaust the bore adapter is part of the system we are testing so why would we use it as part of our standard?

In my case it is common for me to eyebrow the bore to reduce valve shrouding, i can measure the improvement of this change, thus my head flows different on a different bore.

All that said if you calibrate intake flow on the bore adapter, how do you calibrate for exhaust flow? Then how do you correlate your results?

As i have said before the better we our in our test methodology, the more strictly we stick to it, the more accurate and repeatable our results will be.

Rick

[comp262]

I was having a think about this the other night and because I always checked
my plates on top of my fixture I thought I would test it out.
After checking the calibration of all my plates directly on top of the bench,
I then bolted on my 4.155"bore fixture.
the smallest plate 75 odd cfm read the same, progressing to bigger sizes found the error to get bigger and bigger.
The biggest plate 419 cfm, went 440 cfm on top of the bore fixture.
All except the small hole flowed more. The plate that went 330 jumped to
343 cfm.
NOW i know why my flow numbers were down a bit compared to others.
I now know you should calibrate on the bench top first.

[jfholm]

That is exactly what happens to a orifice in a tube also. The closer the orifice gets to the tube size the bigger the error. I cannot remember what the exact percentage we found was, but in the back of my mine it seemed like if the orifice got to be larger than 30% of the tube id then you started getting those errors whch got larger the closer you became to the tube diameter. The tube will change the cd of the orifice.

John

[larrycavan]

For an orifice bench, the orifice should be mounted to the bench for calibration without the bore adapter in between. Generally speaking that is......

However, you can use an orifice that is significantly smaller than the bore adapter as a quick check from time to time. I would make sure such an orifice is no larger than .5 the diameter of the bore adapter though.

I've done that with my bench for years with both FP1 and PTS digital manometers being used on the bench. I've flow tested heads on my bench and then on a friend's SF bench. They're always within 1-1.5 cfm.

Ultimately it comes down to one thing.... Your heads make power for guys or they don't. There's no getting around that one simple fact.

In a shop that is setup to CNC port heads, there should be no need to get into a pissing contest with any customer over flow numbers. Providing that other guys with the same porting have had success with their heads and gave positive feedback. The head porter can't be held responsible for what a tuner can't do but with CNC, it's dam hard to point the finger at the port work if other people are making power with the same porting....

Larry C