contactpatch

as the pressure difference, for cylinder head flow
measurements. 28 inches of water, is close to 1 PSI.
.
One psi does not seem nearly enough, to get
the air moving Mach 0.5 , where the interesting
stuff happens.
.
I would guess that the partial vacuum in the cylinder
would be a lot more than 1 psi.
I'm not saying the 1 psi measurement is useless,
only that real pressure differences are higher, and
that things don't scale when you get to Mach 0.5 .

OldSStroker

An engine with a very good, unrestricted intake will have an intake manifold vacuum of about 1.0 in. Hg, or approx. 0.5 psi. at power peak. 4 bbl carbs and throttle body flows are rated at 1.5 in. Hg (about 0.75 psi). This may be one of the reasons 28 in. H2O has become more or less the defacto standard for comparing head flow. Some folks use 25 in. H2O, some use less and some considerably more.

Read some of the posts from maxracesoftware. He (Larry) does a lot of flow work at 28, but also checks the ports at higher depressions to see what happens.

Mach Index is made up of five terms combined into one value and helps predict volumetric efficiency. These terms are: (1) average intake flow coefficient, (2) intake valve flow area, (3) cylinder size, (4) speed of sound in air and (5) rpm. In simple terms Mach Index relates the average velocity of the intake charge past the valve to the speed of sound. This is based on work by C.F.Taylor and friends from MIT 60+ years ago (when I was just a pup). It applies most directly to engines with fairly conservative cam timing, which is still fairly close for engines in the 1.0 to 1.5 fwhp/cubic inch.

Average intake tract velocity at power peak in a good strong 350 is around 340 ft/sec, which is about .33 Mach, but the Mach Index is in the high .4 range, so you can achieve the .5 Mach at the valve with low manifold vacuum.

jimlab

Inches of water (in. H20) is a pressure measurement based on the height of a column of water that a given pressure will support. The conversion from in. H20 to psi is 14.7 psi (one atmosphere) / 406.88 in. H20, or 0.036128. 28" of H20 is therefore approximately 1.0 psi.

A flow bench measures air flow at a constant test pressure. The pressure drop is measured across the test piece (a cylinder head, for example). Using the ratio of the pressure drop to the pressure drop across a known calibrated orifice, you can determine the volume of the air flowing through the test piece in cfm.

As the valve in a cylinder head being tested is opened in 0.10" increments, the flow increases and the test pressure will drop if you don't adjust the bench. A control **** allows you to recalibrate to 28" of H20 and observe the change in volume of air (cfm) on the flow meter at the new valve position.

Basically, you can use 25" of H20 or 28" of H20 or whatever you want as the test pressure, as long as you recalibrate between changes to the test piece. However, the test pressure being used makes the results comparable to those obtained from other flow benches (or not), so 28" of H20 (~ 1 psi) has became the default "industry standard" for comparison.

At various test pressures, there is a maximum flow capacity for each valve diameter. The sensitivity (and accuracy) of the bench is better at higher pressures, so 28" is also used to show a wider change in flow rates between valve opening increments.

arnie

Since flow data acquired on a flow bench, does not automatically translate into superior (read: producers of most torque/hp) flowing port during actual engine operation, I question the use of 25" or 28", as being the better all around depression to use for reference, to simulate actual port flow thru an operating engine. IOW, use a depression that would prevent the high bench flowing 'duds' from falling thru the cracks. 48 or even 60" may actually be more representative. How many commercially built flow benches would actually qualify?

Demon SS

I am sure you guys understand that flow is not the alpha and omega of cylinder heads, velocity is just as important if not more so in some cases. Our SF-600 will support flow testing up to 48" of H2O and the SF-1020 will support testing up to 65" of flow. And while I could say it in my own words this is easier
http://www.superflow.com/support/sup...t-pressure.htm

arnie

This helps explain why some configure their benches to check in the 3 figure range of test pressures, although they don't use it exclusively.

OldSStroker

Had to laugh.

I can't remember what I paid, maybe about $10. My copy was new in '64 and was the textbook we used when I was taking Automotive Engneering. It was the newest edition then.

C. Fayette would probably not believe the power and rpm folks get nowadays with the SBC, and with F1 engines. They all obey the same principles; we've just gotten better at applying them.

Watching WinterNationals qualifying last nite I still marvel at ProStock engines. It appears that they are accelerating at over 3000 rpm per second in the middle gears. I was hoping Erica Enders would qualify in Pro Stock, but she missed by .02. I was more surprised at Anderson in 6th.

contactpatch

Demon, I somewhat disagree with your comment about
velocity. To me, velocity is a sideline parameter.
To me, the important issue is, how much air
is retained in the cylinder when the intake valve closes.
{affected by cam timing, and other}

My speculation is:, two different cyl heads could 'flow'
similiar at 1 psi, but be different at 8.7 psi, 240 inches.
Of course, no one tells the customers.
Who publishes stuff like that?, I want to know.

OldSS, peak velocity will be higher than average velovity.
My understanding is, flow hits a brick wall at
about Mach 0.6.

OldSStroker

Hmmm...

To retain air in the cylinder at IVC, you first need to get it flowing in there. There is lots of evidence, and a lot of discussion here and elsewhere about the importance of velocity AND flow. If only flow was important, HUGE ports would be the only thing anyone used, and most good head porters would employed elsewhere.

I believe it was Warren Johnson who said something like using the smallest port which gets you the CFM you need.This equates to high velocity.

Yes, around M.55 at the valve flow basically stops increasing. As far a peak velocity vs. average velocity at the valve in a cycling engine, that is more complex.

Some folks DO tell their customers about how heads flow at higher depressions, as I suggested above. I suspect many "customers" haven't a clue as to the relavance of higher depression flows, so they are not routinely told unless they ask. Think about those folks who only concentrate on peak flow, or peak hp or peak torque and don't look at the entire flow curve, hp curve or torque curve. Area under the curve is really the more important parameter, but peak numbers sell. Lots of things in nature are that way.

Too much information without understanding can be as dangerous as too little information...or too little understanding.

In the case of an NA engine with good intake tuning, you might see 5 psi at the valve just as it is closing. I'm not sure you'd need to test @ .050 and .100 valve lift at 140 inches, but depressions above 28, like 36, 48, etc. could give you an idea of how good your porting is.

My $.02

contactpatch

IMO, the best intake 'valve' would be,
at wot, the cylinder head is removed during the
intake stroke. velocity would be low, VE would be good.
.
Keep in mind that any 'porting' benefit will be limited
by the valve area and circumference of the existing valving.
I could attach a gallon jug with a tube to the port,
increasing volume, but that would not help.
{it might, you never know}
.
any 'tuning', always seems to have a very sharp dropoff,
just beyond optimum rpm.
That is what I don't like.
I don't like it when they claim they are doing me a favor
when they do so.

OldSStroker

?????????????

Geoff Chadwick

Wow I feel young and ignorant after reading this thread...

I've read many of the same things, saying "get the smallest port you can for the CFM you want". I understand the pressure changes and am grasping many of the details mentioned here (though some of this is still over my head).

The subject of the original post has been hit pretty hard, but I wonder how things go in FI castings. Obviously you dont have to worry about as much about maintaining velocity, but I know it's still important. Is it just a case of the changing fluid dynamics with different pressures, or is there more concern then that? I know some velocity is needed, but when guys tweak intake manifolds and head ports to maintain an incredible velocity with an immense flow (I'm also now remineded of variable length intake manifold runners) where is the point when FI setups stop "gaining ground"?

And then there are Wankel engines that dont even have valves...

THEMADTYPH00N

As long as you guys are on the topic of books. What are some other good books to read on engine theory like C.F. Taylors works.

OldSStroker

Oh, lordy, there are dozens, hundreds, even. Look at the bibliography of Taylor's books.

An interesting book, discussed in Advance Tech occasionally, is Scientific Design of Exhaust & Intake Systems by Philip H. Smith and John C. Morrison. Bentley Publishers. In soft cover it's about $24.00.
It should keep you occupied for a while.

Loony

I didn't see the answer in here, I only skimmed through it...

But 28"s came from the fact that, that's all the GM could pull at the time...just turned into the industry standard.

I think it was GM