Article by Warren Eames in
Dragster Australia magazine
 |
In Dragster
No.483 we published "Matters Of flow", an article that was written
by Michael Marriott. This article dealt with some of the priorities,
and complexities, of making an engine breathe. In his article
Marriott touched upon flow benches and Warren Eames from Flowbench
Manufacturing Australia sent us a letter which relates to Marriott's
article, as well as sharing some of his own experiences...... Please
read and enjoy......
|
|
|
I
read with agreement the article by
Michael Marriott. I certainly do agree with Marriott's opinion on
paper HP using the formula given, or by any other formula. I must
admit that I am also weary of calculations that tell you how much HP
it takes to push a certain weight of car to a certain ET. There are
too many variables to be able to say that an engine is going to
produce "X" amount of HP from CFM flow reading.
|
|
|
With our
FMA ( Flowbench Manufacturing Australia ) MK9-26
Flowbench we include a computer program to work out a customer's HP.
This was done at the request of customers but against my "better?"
judgment and our customers are advised of this by me. It can be used
as a rough guide only due to the many variables. The "Before
and After", plus the "Exhaust to Inlet Flow Ratio Charts" program
(also included) is a more worthwhile and useful tool. Your flow can
be plotted all the way through the lift range, both on inlet and
exhaust, not just at peak flow, which should help make cam choice
easier. |
|
|
Because
you are plotting your exhaust as well, this can help with
your final choice of exhaust system. The greater the
negative pressure (vacuum) that you can create by the
exhaust the greater the pull will be for the intake charge
when the intake valve opens. This sounds quite easy but l am
sure a cam designer will not agree as the timing has to be
just right (it will only be spot-on in a narrow RPM band),
so the inlet charge does not go out the exhaust (and
therefore wasted to a large degree, as it can be used to
cool the engine internals as well) as well as making the
best use of the inertia mentioned by Marriott.
|
|
|
This
may lead you to think that you need great flow readings on
the exhaust. I don't believe so. You must remember the
exhaust is going out under pressure. The only clue I can
give here is that it seems that the exhaust flow should be
between 60 and 80 percent of the inlet. This was proved many
years ago in the good old VW by placing a washer with a 5/8
in. hole in both exhaust pipes right back near the outlet on
a standard engine. A noticeable Increase in torque was
gained. This was done by a customer of mine after I told him
of my findings when working on Formula Vee engines which,
sometimes showed in excess of 100 percent flow of the inlet
flow, on the exhaust.
|
|
|
I
am a firm believer in flowing everything on the inlet side
together (ie: head, manifold, carburetor and air cleaner)
and the same on the exhaust, as they all influence one
another. Of course it's a matter of trial and error to get
the best combination within the rules of your class.
|
|
|
With
reference to pressure, the ideal seems to be to work towards
a funnel or "V" effect (which Marriott hinted at), with the
largest opening at the air entrance and the smallest at the
valve seat. This is the ideal but virtually impossible to
achieve, however if you think along those lines you can't go
too far wrong. |
|
|
The
"in-thing" at the moment seems to be air speed which is the
one thing that finally prompted me to write this article.
People seem to want to spend more money to measure their air
speed. If you have one of our FMA MK9 flowbenches, or any
other flowbench, you can see if you are getting a greater
air speed without any modifications at all. All you have to
do is remember, to get more air through a given hole, or
orifice, you increase the pressure, the air speed or both,
It's as simple as that. If you have an orifice of 50 mm and
a pressure of 10 in. of water which do not change (as when
flow testing), the only way to get more air through the hole
is to increase the air speed which will in turn give you
more air flow (cfm). Relate this to your engine by having
the valve seat slightly smaller than the port entry into the
head (remember the funnel or "V"). If you want to keep your
air speed up, work between these two openings. This is, of
course, when you get somewhere near your target airflow.
Remember, you can add material in the port if the rules
allow. If you do this without touching the seat area or the
air entrance area and your cfm increases (without
increasing pressure) you have increased your air speed. Some
ways to achieve this are:
1. To have a pocket under the seat
2. Use the funnel or "V" effect
3. Eliminate changes in port area ( as much as
possible)
4. Add material into the port area
|
|
|
Remember:
If you use a hot wire probe in your port to measure your
speed, it does not capture all the air and therefore will
not give you an accurate reading. If you use a vane
annometer it needs to be the same size as the port to
capture all the air. This creates a few problems:
1. Imagine all the different vane sizes you would
need
2. Whatever you put in the port will disrupt the
airflow characteristics
3. Whether it's a hot wire or vane, it has area and
volume so the air speed will be different when you take it
away anyway.
|
|
|
If
you increase your air exit area into the combustion chamber,
your air speed will go down. One very important fact to
remember is that everyone who knows anything at all about
flowing engine parts has and or uses a flowbench.
|
|
|
Do
yourself a favour, at least consider our Australian
manufactured FMA MK9 flowbench.
|
|
|
I hope this helps you in the science and art of flow bench
testing. |
|
|
Even
though I design and manufacture our FMA flow benches, I
am by no means an expert on flowing.
|
|
