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 Floating Depression? 
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Joined: Fri Jan 08, 2010 10:36 pm
Posts: 1118
Location: Maryland
As usual many miss the point and only look at what transpires to there benefit, My example video was not about selling cold air intakes or show yet another way to test air flow.

The point here is that this is a live example of how the ever changing CD of an apparatus (Pipe, Port, Venturi, Orifice) has an effect on flow and or it’s retraction to. In this example the CD changed and for the same flow amount we saw almost .4” change in depression. The whole point is in using the variable object (your port) as the reference you could be forever changing and chasing your tail, but in the event where you are continually comparing the flow against a known standard the fallacy of change (In this case) would shine through. Yes I know that in this example the author of the video used restriction to make his point as the air flow change might not have had the same impact to his customer. The pipe acting as a 2.9” +/- orifice the .4 change in depression is only worth 8 CFM and buy gaining 8 CFM of airflow when putting the filter on most likely does not make the same point as showing the increase in restriction when taking it off. All in the light of selling the product.

I have never said the VD test apparatus could not depict changes in effective restrictions, but I do believe it is a far stretch to compare that to valid CFM numbers.

What concerns me is when testing at a static depression that may be higher or lower than what the engine is seeing at the same valve lift, The flow patterns that develop at that valve lift and that depression around the valve and in the port could be quite different and perhaps misrepresentitive of what the engine will see.


This is an issue with test methodology and not bench design, a quality flow bench can provide more data than one knows what to do with. A valid test theory implemented through a grounded methodology and followed up with flow test results, dyno results and winning race vehicles are a core part of a successful R&D program


Wed Jul 27, 2011 12:08 pm

Joined: Fri Feb 19, 2010 7:36 pm
Posts: 1628
Location: Grantsville, Utah 45 min west of Salt Lake City
My biggest issue with floating depression testing is you do not actually replicate what the engine is doing anymore than using a regular orifice bench. Yes, an engine's depression (vacuum) is different at everylift, but also rpm, cam, carb, port size and on and on affect this. You do not really know what it is. So why test floating depression? Don't get me wrong, if you want to then go right a head. I just find the standard orifice bench much less complicated and very repeatable. You are basically trying to track positive changes you have made so you want to do that in the most accurate and easy method.


Wed Jul 27, 2011 7:14 pm

Joined: Sun May 29, 2011 7:52 pm
Posts: 41
Location: central Illinois
Bruce, would it be possible to move this thread to the "other stylesOf flowbenchs" section? ,might be better there?

Thanks Jason.


Fri Dec 16, 2011 2:27 pm

Joined: Sat Feb 25, 2012 8:34 am
Posts: 63
BigBro74 wrote:
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.

Ok, I had some spare time on my hands this weekend so I decided to do some testing with the FD style bench. So I made one.
Now before we get into more discussion about voltages and repeatability etc there is one thing that I noticed as soon as I ran this style of bench. If your using 2 X 1600W ( will take 4) vac motors as I am then you will start to have problems with the motors pulsating at around 115" WC. I guess they started to stall and then back flowed. I could run them harder for higher depression but the pulsating noise would just get worse and I think this would make repeatability harder at high test pressures.
Now to me this was a limiting factor and I guess it would come down to motor power.
So what I was limited to on this design was :
5 CFM @115"WC and 320 CFM @9.5 "WC These were using sharp edge orifices.
That leaves me to ask , how did the poster above get a stalled value of 50.5"WC and 10.5"WC at 320CFM???
That does not sound correct to me.

Mon May 07, 2012 12:19 am

Joined: Sat Feb 25, 2012 8:34 am
Posts: 63
I will be testing over this week at various times to see what the "WC difference is between these two extremes. One motor runs straight from the mains and the other is using a phase controller.
My next step may be to run them both from the mains and introduce an air bypass to set the depression.
I think having 10.5@320 is not really all that good and I would like to see it at least over 20"WC.
So what I may do is split up the CFM readings. What I will do is run enough motors to get say 28"WC on the 320CFM plate and then see where it goes when I use a plate of half that value, around 150CFM. Then I will reset the motor speed to get 28"WC on the 150CFM plate and see where it goes against the 10CFM plate.

Mon May 07, 2012 9:19 pm

Joined: Sat Feb 25, 2012 8:34 am
Posts: 63
Well I've been playing around with this thing for the last two days at various times of the day with both motors running straight off the mains. At the moment my Volts are very steady and pretty much the same for each test and I'm just using my 320CFM and 160CFM test plates. I can't run the 10CFM plate until I put in a valve to bleed off some depression.
To be honest I'm a bit mystified because each time I run this thing it gives me the same reading for each plate 39"WC on the 160 plate and 15.25""WC on the 320 plate. There's very little variation.
It's beginning to look like I'm going to have to make a CFM reading plate in 5CFM increments if I want to test this thing further.
If I get time to make a bore adapter this week then I might stick a head on there and take some readings and see if I can repeat them over a few days.

Wed May 09, 2012 9:49 am

Joined: Sat Feb 25, 2012 8:34 am
Posts: 63
Wow, looks like no one is interested in this subject anymore :(

Fri May 11, 2012 8:29 pm

Joined: Sat Feb 25, 2012 8:34 am
Posts: 63
Here are some results of some testing. These tests are run with 2 Vac motors directly plugged into the mains:
These are run on different days, one run a day.
1) 160CFM - 39 "H20.
320CFM - 15 3/4 "H20
2) 160CFM - 39 "H20
320CFM - 15 3/4 "H20
3) 160CFM - 39 "H20
320CFM - 13 1/2 "H20
4) 160CFM - 38 1/2 "H20
320CFM - 13 1/4 "H20
5) 160CFM - 37 1/2 "H20
320CFM - 12 1/2 "H20
6) 160CFM - 37 1/2 "H20
320CFM - 13 "H20
7) 160CFM - 38 1/2 "H20
320CFM - 13 'H20
So just testing using the plates from day to day without changing anything we have a variation of 12 1/2 to 15 3/4 (20.63 % variation) on the 320 plate and 39 to 37 1/2 ( 3.84% variation) on the 160 plate.
These tests just show how much variation you can get from an uncontrolled power source when using this bench design, but we all knew that.
Ok to pull more depression @320 CFM I have added a 3rd motor. Two motors are power controlled and one is on the mains.
Initial testing shows 24" max at 320 CFM and 58 1/2 ' on the 160CFM plate. So if you want to pull at least 28' on the 320 you will need a 4th motor or you could limit your max test CFM to around 280CFM which would be at the limit for most production iron heads that a first time ported would want to do anyway.
I'm now doing some controlled motor tests with my 3 orifice plates.

Sun May 13, 2012 4:08 am

Joined: Sat Feb 25, 2012 8:34 am
Posts: 63
Ok, I've done multiple tests using variable speed/ volts settings on this bench and these are the results.
After setting a H20 level on the 320 plate and then swapping to the 160 plate I'm getting a variation of 0.47% . Then when setting a H20 level on the 160 plate and then swapping to the 10CFM plate I'm getting a variation of 0.2%.
These results are way better than I expected and I have no reason to believe that this design could not be used as low budget bench for heads up to 300CFM in it's current state.

Tue May 15, 2012 1:21 am

Joined: Sun May 29, 2011 7:52 pm
Posts: 41
Location: central Illinois
BL- haven't looked on here for awhile so have missed all your posting. cool testing! 115" stalled and 9.5" @ 320. interesting. motors have different performance curves so you could have that where it pulls a high stalled depression and yet not as much flow all open. on my setup i was using two shop vacs for those results so.... it just ended up there.

you posted that two of the motors were controlled, what did you find to control them?

nice efforts

BTW-still interested just busy!


Sun May 27, 2012 12:29 am
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