Floating Depression?

[jfholm]

How many people are going to be shipping a head back and forth to someone else and therefore need to be able to compare numbers off of two different benches? I think that for most of us repeatability within our own bench's numbers is going to be of far more importance than traceability to NIST.

Maybe if we're a Pro Stock team with big money at stake it would become more important to have a 100" bench. On the other hand, having any bench that lets us quantify flow is a big step up over nothing at all.

I don't need a better bench, I need to understand what the bench is telling me and what I need to do to make the port happier.

cheers,
Michael

Very well said Michael!

John

[BigBro74]

Micheal and John-
That is exactly the case!! I know you can convert. That is what i am doing in essence (only simpler in practice.). The plate used flows X amount of cfm when pulled to 28". I put the plate on the bench by itself and open orifice holes in 5 cfm@28 incriments. I then record the reading seen on my manometer at every 5 cfm interval with vac motors no restriction other than the plate on top of the bench. While my bench is pulling a different depression (Substantially higher over half the capacity), it is already converted!! no need for math. Write it down, make a graph. I then know the flow thru the WHOLE SYSTEM converted already. if the depression with a head on the bore adapter is the same as with a plate flowing on top with the corresponding area of orifice, THE HEAD IS FLOWING THE SAME AMOUNT (provided voltage same and no leakage). since the orifice plate is already measured at 28 it can be used in place of doing the math.
my chart is like this-
CFM@28"-MANO reading
0- 50.5"
5- 48.5"
10- 47"
15- 46"
20- 44.5"
25- 43.5"
30- 42.5"
35- 41.5"

Etc. etc all thru all the orifices on the plate being open = 315 - 10.25"
(there is a pic of the plate on page one if it sounds odd)
It is very simple. open the valve to a certain lift and take a manometer reading. go to the chart and read the cfm.

I could use delta p across an orifice and figure the % and the flow and then convert to 28. but this is faster and gives repeatable results. and i am still testing at high depression at low lift.

I like this style of testing.

I don't believe that static testing is bad or wrong or anything. It is great! I just think this is closer to what a running engine sees. testing at 28"h20 static depression is conventional. it is CONVENIENT. I suspect this is part of the reason it is conventional. That dosn't make it necessarily better or worse. People tested at lower depressions before Smokey said 28. I am not sure but i have a guess that this is simply the first depression that started working the port/ valve seat hard enough to give good results for him. I do feel however that low/mid flow is just as or more important than high lift flow. I know for certain it is on my engines as they are rules limited to flat tappet cams, and really good valve lift is non existant on the budgets the racers work on. different classes of racing require a different line of thought.

[jfholm]

I personally feel, and this is just my not so humble opinion , the reason most people now flow at 28" H2O is so the "can" compare to each other. It is always "mine is bigger than Yours" mentality. Velocity is so important with what you are doing. If you don't have enough velocity then the car will not come off the corners well or not recover on shifts when drag racing. It is all combination. If you only need 260 cfm to feed you engine why put 300 cfm heads on it.

I would flow at the highest rate I could possibly do. That takes money and my money will probably let me flow my heads at 40" H2O. I used to use the FlowQuik all the time but my unit started having calibration problems and then I think one of the pressure sensors inside went south. That is when I found this forum and built a bench from Bruce's plans. Love it as I can walk away from it for months at a time and come back and it is still calibrated.

John

[SSR]

I agree with the 28" comparison with others (it's only natural), but surely it is chosen because it is closer to what the actual port sees when on the working engine?

Is it not fair to say that the air going around a bend is the same as a car going around a bend - it might make it round at 40 mph, but not as tidily (if at all) at 110mph?

I publish figures at 10" as a comparison (in the UK), but test at other much higher test depressions for my own personal comparison tests.

[Michael Moore]

Here's what I've got for conversions
100 CFM @ 28" converts to:
56.76 @ 10
70.71 @ 14
73.19 @ 15
84.51 @ 20
94.49 @ 25
103.51 @ 30
119.52 @ 40
122.47 @ 42
141.42 @ 56

I stuck in .5, 1.5 and 2X the 28" even though I've not seen anyone use those, just because it was easy to do and I was curious how things scale.

cheers,
Michael

[BigBro74]

SSR- It is my understanding that the tuned exhaust on a race v8 can pull in excess of 100" water on the cylinder toward the end of the exhaust cycle(during overlap). This i have seen with my own eyes. Rig an evac system to a headder collector and measure the vacuum for yourself. We did this to a freinds 5.0 ford and saw over 10"hg at just abou 2000rpm. Supposedly the peak piston demand by itself (only intake open) is only around 15- 20"h20 at about 70-75 degrees after tdc and the valve is now near wide open! This i can only take on faith from a well known scource who would be in the know(Vizard). Knowing this it is my personal opinion that testing at static dep. is less realistic as it pertains to a running engine.

[1960FL]

I love the discussion this is creating the forum needs a little transfusion to keep the blood flowing.


Ok let look at what you have said and help me out here maybe I don’t get it?

“SSR- It is my understanding that the tuned exhaust on a race v8 can pull in excess of 100" water on the cylinder toward the end of the exhaust cycle(during overlap). This i have seen with my own eyes. Rig an evac system to a headder collector and measure the vacuum for yourself. We did this to a freinds 5.0 ford and saw over 10"hg at just abou 2000rpm. Supposedly the peak piston demand by itself (only intake open) is only around 15- 20"h20 at about 70-75 degrees after tdc and the valve is now near wide open! This i can only take on faith from a well known scource who would be in the know(Vizard). Knowing this it is my personal opinion that testing at static dep. is less realistic as it pertains to a running engine.
“

You are using an example of air flow (Exhaust gasses under heat expanding) across a venturi and correlating that to be the same as the affect on a cylinder that may be in split overlap? I do not think this is realistic. Even engines with highly functioning crank case evacuation systems suffer from reversion and effects of poor exhaust ports and design. I think you need to rethink just what is happening at this juncture in time. Oh, and yes I agree that a properly sonic tuned exhaust add huge benefits if not used properly it too can over scavenge and add the adverse effect of poor mileage. Remember this aspect only plays a small role in the sprint to get VE over 100% and ram effect only makes its play when piston speed/demand is on its way toward to 0.

“Engines NEVER pull a fixed depression on a port. for this reason alone I believe that testing at a floating depression with a high starting depression is more appropriate. It MUCH more closely simulates what happens in a running engine.”

Jason, Though I agree that the engine never pulls a fixed depression it does not in any way correlate to the that of a floating depression tester in fact I believe it is more the inverse. Engine Demand at split overlap is at near 0 as piston speed is at its 1st slowest point and remember that any benefit from exhaust must first overcome the negative offset by any vacuum accumulated it the intake tract ahead of the valve. (not an issue on single or individual runner intakes) In all my testing I have never seen an engine have 0” of vacuum at full load WFO (that is while they are still running).

So if I understand this correctly your testing has it highest depression when the intake valve is closed and air speed at its lowest; And has its lowest depression at full valve lift and your slowest air speed. I just do not see how this correlates as more realistic a test method.

Jason if you told me you were only using this system to test exhaust ports on a blow thru bases I would have more faith in it as I can see the correlation to blow down and the exhaust valve cycle.

All that said if you can get this thing calibrated and against some standard I think you are on your way to have a viable testing device. If your money is tight try to find a better way to create a precision orifice, it does not have to be sharp edge you just need two identically made to regress the CD values and calculate the air flow across them.

Rick

[1960FL]

A friend suggested that I keep a "flow check" head on hand. It really only needs to have one useable port with guide so you can saw it out of a bigger head or find a cheap junk head somewhere. It doesn't matter if it is stock, ported or what, you just want it to flow the same every time you put it on the bench.

Michael,

I could not agree more.


Rick

[Michael Moore]

It occurs to me that the "checking" head/port is for after you've run your sharp-edged calibration orifice on the bench and it would serve as a cross check against that orifice. I don't know that you really need a head with a valve that could be lifted. Perhaps doing a bare port test (no valve, blocked guide bore) is all you need to get another "it should be this" number.

But if that's all you need then you could probably put a straight tube with radiused ends onto a plate (the lower end protruding through the plate) and let that be your test "port". It would be like the piece I made in this thread:

http://www.flowbenchtech.com/forum/view ... ?f=7&t=370

As long as you calibrate to your sharp edge orifice and then get the expected reading from the test port (probably best to have the flow in the test port significantly different from what the calibration orifice does) you could presume you were ready to move on to a real head.

This sounds like a good job for some scrap PVC pipe.

cheers,
Michael

[1960FL]

Michael,

I agree a scrap head with no vlave and a blocked guide would be a good test of your benches consistancy and ones ability to place the head in the same spot every time.

But i am sure others from the sociaty of head porters would disagree.

I am often heard yapping about standards and it is just that I despise the fact that our industry so waffles about a standardized testing procedure and data recordation. I am all for R&D and this is where the gains are made, but if my 100cfm plate only flows 95cfm on your bench then I know the math to correct my data to yours. Problem is this makes both of us honest and some just need to say they have more. Don’t get me wrong I can think of 10 other places for error or discontinuity in the average testing procedure including the shop floor 2 X 4 turned into a rough looking ¾” radius….

I am with you the only thing I know is if my numbers go up on my bench it means I found more air either in my work or as a leak. Outside of this all the other numbers are fun to look at and talk about but with the radiuses, head adapters, valve opening fixtures and flow benches I have seen over the years it is hard to put faith in most posted numbers.

Rick