Cylinder Head flow rates

[S2central.net]

@RWD - can you help those of us without qualifications in gaseous dynamic equations and flowbench tecnobabble understand what you are saying here... This is a very specialized area of course - and not many folk will comprehend the statements you are making unless you have a better way to explain it to uneducated rabble (myself included).

[LHD_Kid]

Putting that aside it isn't about peak flow but total area under the curve gains/curve shape and port velocity. Heads with less measured cfm but correctly adjusted port velocities can produce more power and deliver it smoother especially on a FI engine when correctly matched to the ramp rate and lift of a cam profile.

Precisely.......

[S2central.net]

For example - is there a standard recommended way to get flowbench results or does each operator / bench have their own settings ?

What parameters can be tweaked for overinflating these kind of tests ?

Replies in plain english please

[JP]

OK my best attempt at basic

First of a flowbench comes in two types the main type is an orifice style bench where the market leader is considered to be Superflow. A company that stems back to the founder using a hoover for porting his VW heads subsequently going on to win all the races he competed in etc...

So I will try and simplify what goes on in the most common flowbench.

First of all a flowbench does not actually measure cfm (flow) it measures pressure. By interpreting the pressure results using mathematical formulae you can convert these into a recognised flow figure that you see widely published around the world.

In order to calculate this cfm you need to have various pressure readings to compare. These come to a total of 4.

If you can imagine your cylinder head sat on a tube that simulates your bore size, bolted to a box that consists of 3 levels.

Top level is where the air ends up as it is sucked through the port and subsequently the tube.

Then there is a restriction of known diameter (your orifice) before the air can pass into the middle level.

Finally the lower level houses the vacuum motors which is accesed through a large adjustable valve.

First you need to measure the pressure difference between the upper chamber and atmosphere using a water manometer, one end is left open to atmosphere and the other end just under the opening that goes to the tube your head is bolted to. The more vacuum you pull the more the water in the manometer is displaced. The difference between the two levels in inches is your test pressure. The industry standard being 28" and for smaller less powerful benches 10". Some people have started to use 25" aswell but 28 and 10 are the most common.

The 28" industry standard came from a poor sod that did loads of R&D work around the 70's iirc that worked out that calculated cfm gains tested at 28" made the most significant hp changes on an engine dyno.

The next measuring point of pressure is that of the top and middle chambers measuring the difference of pressure across the orifice metering plate. The metering plate will have a preciselly machined 45 degree chamfer coming to a sharp edge creating a known internal diameter. The accuracy in this plate is immenselly important as this is where the biggest assumptions are made.
For the purpose of this explanation we will use a diameter of 1.181" which gives 100.7cfm of flow @28" pressure drop through a sharp edge (but for ease we'll use 100cfm as the baseline).

The top and middle chambers are then connected to what is called an incline manometer. Which means instead of being perfectly vertical it is set at an incline as the relationship of pressure vs cfm is not a linear curve. This manometer becomes more and more accurate the higher up the scale you work.

So now measuring flow of your cylinder head is as follows If you can imagine that there is no head bolted to the bench and you adjust the pressure to 28" on the first manometer, your incline manometer will now show 100% on it's scale. If you where to place a restriction over the bench opening namely your cylinderhead you end up with less pressure in the top chamber relative to the atmosphere so you will see a pressure drop on the manometer, lets say it drops to 20" in height difference. You then have to open the valve to the vacuums more in order to re establish the 28" pressure difference.

Of course as you do this the pressure difference between the top and middle chamber changes aswell resulting in a change on the incline manometer which for example drops to 70%.

This is then interpreted that the restriction placed on the bench flow 70% of the orifice plate. As we know this to be a 100cfm your head at this point must therefore mathematically flow 70cfm.

This is where the problems start arising as the claimed 100cfm of the orifice plate is based on maths not on reality. Where it is assuming that a thin sharp edge has a .62 efficiency. However depending in which direction the air is moving this efficiency may change making wildly inacurate results the more you move up the range due to the non linear relationship. Likewise a 1.18" orifice mathematically makes 99.9cfm vs the 100.7cfm of a 1.181" orifice so you are already out by .8cfm on the whole scale by being out by just 1thou of an inch. Get the quality of the sharp edge wrong and say make an efficiency of .52 and your 1.18" hole now only flows 83.79cfm @28" Almost out by 17cfm when flowing at 100% of the scale!

To put it totally into perspective take an orifice that is capable of flowing 300cfm which is what is required to flow these heads comfortably within range say 2.05" @ 28" which is 301.52cfm at .62 efficiency. Now change the co efficient to .52 and you only make 252.89cfm @28" almost out by 50cfm over the whole scale!

Which means the head that was making 270cfm @ 28" on a properly callbrated bench is actually only flowing 227.6cfm @ 28".

This is where the problem lies comparing flow figures from one person to the next as the variables are as wild as those on rolling roads. Someone that has seen lots of flow figures will spot the anomalies and know why they occur. Unfortunatelly most flow bench operators dont actually understand how their equipment works and therefore do not compensate the results.
The only way this can be done is by using a callibration plate that has been measured to a known cfm and comparing to the results of the bench being used as this will let the operator know whether to increase or decrease the end results.
Much like callibrating a set of digital scales one will go out and buy a 2kg callibration weight which has been made to a specific standard by a fancy labratory it is up to the flow bench operator to invest in a callibration plate to ensure true results.

hth

[jogi730]

Well you did ask!
That's a great explanation.
Many thanks
Chris

[Jamo]

So they don't only have different size horses but different sized cfm as well?

[zlobo]

tonight i am going to flow test the exhaust side.

[JP]

Here we go some figures from a calibrated bench measured at 70 fahrenheit ambient and 10" pressure drop

Intake
50 thou 18.6cfm
100 thou 38.9cfm
150 thou 56.9cfm
200 thou 76.9cfm
250 thou 92.2cfm
300 thou 104.2cfm
350 thou 111.1cfm
400 thou 114.4cfm
450 thou 114.7cfm
500 thou 114.3cfm
550 thou 113.9cfm

Exhaust
50 thou 17.3cfm
100 thou 43.5cfm
150 thou 50.2cfm
200 thou 65.3cfm
250 thou 83.5cfm
300 thou 78.8cfm
350 thou 79.6cfm
400 thou 87.6cfm
450 thou 86.9cfm
500 thou 81.4cfm
550 thou 79.9cfm

Note on the exhaust the up and down figures near the top end of the lift range this comes from the turbulance caused by poor exh port design. The last half of the lifts from 300thou will always jump up and down due to turbulance so each pull will be slightly diffeent however they always average in that neck of the woods. Any standard I5 10v or 20v exhaust flow curve which is smooth has been doctored. Or the measuring equipment was not sensitive enough to pick up the variations.

[S2central.net]

Eyyy up - need to put my brain in a jar while reading this thread... Will take a few more reads - good stuff though !

[zlobo]

finished the flow test tonight.

[newt]

For RWD19T:

Just to clarify, those numbers we have and use are from a Superflow 600 bench, that has been calibrated against a number of other 'top' benches in the states here. The measurements we use are at 28". Here's the rundown of the range plates and percentages for the info hungry:
Intake
Valve Lift Intake Range Percent CFM
0.050 69.500 62.500 43.438
0.100 129.500 62.500 80.938
0.150 129.500 92.000 119.140
0.200 259.500 59.000 153.105
0.250 259.500 69.500 180.353
0.300 259.500 77.000 199.815
0.350 259.500 80.000 207.600
0.400 259.500 82.000 212.790
0.450 259.500 83.000 215.385
0.500 259.500 83.500 216.683
0.550 259.500 83.500 216.683
0.600 259.500 83.500 216.683

Exhaust
Valve Lift Exhaust Range Percent CFM
0.050 71.000 45.000 31.950
0.100 126.700 46.000 58.282
0.150 126.700 68.000 86.156
0.200 126.700 84.000 106.428
0.250 256.700 45.000 115.515
0.300 256.700 46.250 118.724
0.350 256.700 47.500 121.933
0.400 256.700 48.250 123.858
0.450 256.700 49.000 125.783
0.500 256.700 49.500 127.067
0.550 256.700 50.000 128.350
0.600 256.700 50.000 128.350

Given your 10" numbers when multiplied by the somewhat standard factor of 1.67 to convert to 28" numbers, the bench your on is reading a good deal higher on the exhaust side, FWIW. Obviously numbers will inevitably vary from bench to bench, we use the numbers as a reference for our own internal information. We don't typically publish them for the very reason seen here, lots of people have lots of opinions on what the 'correct' number are. No problem there, but our flow bench operator's test methods are the same as the top guys in the industry, and the bench reads withing a couple CFM of some very well known shops. We're comfortable with our numbers, and use them accordingly; as a measurement of relative improvement or loss based on our own data.

[S2central.net]

I hate that I don't understand this all yet

[JP]

^^^^ @ Newt

Nail on the head,
same reason I say to people flow figures are for inhouse R&D not for publishing as it is an incomparable process within reason.

The flow figures published above by myself where measured using digital manometers as opposed to fluid ones where the co efficient of the orifice plate is programmed in using a callibration plate and it adjusts the results accordingly. Obviously flipping the callibration plate over when reversing flow, which a lot of bench operators forget to do.

This is where it was discovered that the metering plate flows at different efficiency's depending on the direction of flow. So it's pretty much like what the 600 does but on a smaller scale.

Exhaust figures have always been a curious one as when blowing through the exhaust ports 2 sets of results can be achieved depending on the port design @ 10". If the Exhaust seat sits at an obtuse angle to the port exit the figures are always very stable at 10". When they sit at a right angle to the port exit they jump about at 10" but not at 28" as much, mainly due to the port turbulance being overcome by the higher pressure. Which is unfortunatelly what the I5 suffer from. However if I switch on an extra motor and close the valve slightly to bring pressure back to 10" it stabilises although reading a tad higher at high lift until the port chokes which I establish from the pitot readings. These are the readings I tend to prefer to go with as they are the more stable ones.

Using that method I find the results matching upto 80-85% of published results then there is the other ones. There is another factor involved though which is rarely taken into account which is sea level. It sounds strange that it can have such a drastic effect however it doesn't matter where rods/pistons come from in the US all three sets of digital scales always weigh them in 2.5-3gramms higher than the manufacturers published figure on the data sheets. I'm wondering whether flow bench results can be effected by air density aswell as air temp. May have to have a look at that using a barometer in the future for personal curiousity more than anything.

Anyway it looks like the moral of this story is don't try and compare things before they are up and running in the car and then let the race tell the story. Because if you really want to throw a spanner in the works, porting by port velocity is as important if not more than by cfm especially on FI motors. Much like a lot of mappers now map by EGT aswell as or instead of AFR ratio's.

Here's an intersting article highlighting this http://www.enginebuildermag.com/Arti...selection.aspx

[zlobo]

i am satisfied wit my flow test i can se the diffrance on my cylhead vs orig. i cant tell if my flow chart is 100%

[S2central.net]

AT end of the day zlobo - you've done before / after tests yourself on the same bench with same config - so even a novice in this area like me can see you made some nice gains in flow.
Its when we start to compare your figures with someone else that it gets complicated... Then we will have the potential for some tuners swaying the results on customer cars by bench config for hyped numbers after some mild headwork... not saying anyone is doing that here - I just bet that its fairly common practise in certain circles due to the black art of the subject.