----- Original Message -----
Sent: Saturday, August 21, 2004 3:39
PM
Subject: [FlyRotary] Re: EM-2 MAP
readings
Hmm, Weber was pretty much upfront with the fact that their TB
ports
were only for balancing idle and shouldn't be relied upon for
accurate
readings at any sort of power setting. Perhaps the TWM port
is placed
differently than the Weber TB.
I'm still skeptical,
but I can't think of any way to prove whether they're correct or
not.
IMHO, I think your 42s should do just
fine
at the revs you're turning. In fact, I think the intake charge
velocity
created by the 42mm tract may help you. I have info here (not
in the
attic!) reference intake VNE as well as a link to a calculator
for gas
velocity in a pipe. The link is
http://not2fast.com/gasflow/velocity.shtml
- you'll have to do the
metric/English conversion thing but the math is
sound. I'll post the gas
velocity thing later when I can get to the
info. I want to say 600 f/s
but will confirm.
Neat calculator, but
let's see if I'm using this correctly.
I'm looking at one
rotor, since each TB bore and intake runner feeds only
one rotor. The displacement of a 13B is 1.3 liters, so one
rotor is .65 liters. My handy converter turns this into .023 cubic
feet per rotor, per revolution. Multiply this by 7000 rpm,
and I get 161 cubic feet per minute. I enter this into the gas
flow program, leave the temp at 100F, and put in the runner ID of
1.625". That gives me a mean velocity of 186 ft/sec.
From the notes at the bottom of
the program screen, and your recollection of 600 fps, my 186 fps figure
doesn't appear to be stressing my intake size at all. The real
question is- did I screw something up in the way I figured
this?
Feeling better about my intake all the
time. Pity the area is covered with thunderstorms today. I really
don't want to test my new XM weather that bad :-)
Thanks,
Rusty (replacing home security system
destroyed by lightning)
Rusty, for airflow/ displacement purposes,
expecially when using calculators for reciprociating engines, you can simply
treat it as a 160 CID 4 cyclinder, 4 stroke engine, each cylinder
of 40 CID.
But for a
sanity check, taking your inputs we have 160*7000/(1728*2) = 324 CFM or
since you are only taking 1 runner (1/2 the intake) we divided that by 2 = 162
CFM divide by area of runner with ID 1.625" A = 2.07 sq inches
converting to feet =
0.014402 ft^2 and
(162ft^3/min)/.0144 ft^2 = 11248 ft/min / 60 = 187.4691 ft/sec which I
say is close to your figure. Now this is your average intake velocity
over a revolution.
The 600 fps velocity figure is
frequently mentioned in lituerature on intake tuning. However, it turns
out that is the MAXMIUM allowable (recommended?) velocity in the
tubes. Above this velocity, air flow losses start to become
prohibitive. Your average velocity is considerably lower as you have to
taken into consideration the pulsating nature of the flow which could
mean the lowest velocity you will see is considerably lower than your
average. But, none of those conditions exists but for a
fleeting moment.
So the fact that your average
velocity is 187 fps really tells you little about your peak
velocity. To put it into perspective your little 1.625 ID tube has
air flow an average of 127MPH! The peak goes above this
but can't say how much So the air is not exactly sitting still. I would
say considerably stronger than your average vacuum cleaner {:>).
But, again it does not tell you
whether you are exceeding the peak recommended velocity of 600 fps and
possibly incurring substantial losses. I personally doubt that you
are. Your intake size is not significantly different from the
Rotary stock intake (if you add area of a primary and secondary) perhpas a bit
larger so, I believe that they would have paid carefully attention to high rpm
airflow and avoided tubes that would have caused serious airflow
problems. But, thats not a fact, just my opinion.
You would need a fairly sophisticated
simulation to arrive at anything close to an accurate peak velocity.
There's a good one on the market for about $600 if you are interested
{:>)
No, have not got to the airport yet,
tomorrow if it goes better than the rest of the week {>)
FWIW
Ed