At 06:13 PM 11/30/2004, you wrote:
> I recently flowed an new stock1.0 inch manifold, 28" of water, manifold
> only, #3 port only, 85cfm.
> I also flowed a 1.25 under the same conditions, 105cfm.
Pardon my ignorance, but why would you use such high pressure differentials
to flow a head ? Wouldn't just a few inches of water be more similar to
actual conditions ?
Seems like you'd want the valve in there too
?.........................................
I didn't see a response to the question from those who know more than I, so
I'll give it a try.
When heads and manifolds are flowed, you can use any pressure you want. I
use 7" because it makes the math easier, and also because of the max
capabilities of my flow bench vacuum source. However, to compare to
readings taken by other flow benches or on other products, there must be a
common standard vacuum for comparison purposes. The 'industry' has settled
on 28" of water. That doesn't mean that you actually run your flow bench at
that vacuum, but it does mean that you've converted your readings to 28".
The conversion is easy or difficult depending on what you have for data
collection or conversion tables. A different standard exists for carburetors.
Flow measurements are usually made at .100" increments of valve openings,
usually up to .500 or .600. The highest lift TR cam of which I'm aware has
a lift of .565. It's interesting to me that I can make a change in a port
that significantly increases flow at .600 but has no effect at .500.
Of course, the flow bench doesn't really simulate what goes on in the
intake or exhaust port. It's just a measuring tool to get comparisons.
Since the engine is pulling in much more flow than you use on the flow
bench, and since it would seem to be a pulsating flow (although experts
debate that) it would seem that the mixture would act differently under
real life conditions. Certainly, tumble, eddy, laminar separation, and all
that stuff changes, but most of us don't know how to simulate those
conditions. The really high bucks flow benches come equipped not only with
data readouts, but also with probes to measure high- and low-pressure
areas, and some 'wet' flow benches provide for injection of a liquid dye so
you can see what's going on. Of course, then you need to have the expertise
to know what to do with all this information. Add to all that the analysis
of actual cylinder filling, and you get into really complicated stuff.
One of the biggest lessons I've learned is that those little air molecules
just do not behave like your 'common sense' expects them to. Some things
that are 'obvious' just simply aren't true. For example, everybody thinks
that port matching must improves flow. It does not, if it is improperly
done. Another misconception is that polishing the port walls helps. For
street engines, it does nothing at all except soak up time, and there is a
school of thought that supports the idea that slightly rough ports actually
increase the quality of the mixture. most porters put a nice glossy finish
in there because otherwise the customer thinks he's done a sloppy job --
it's only for cosmetics. What really matters is the shape of the port.
If anyone wants a hobby that will soak up hours and hours, this is it. Get
a flow bench.
uncle jack
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