Jerry Hey wrote:

Paul, in response to  my inquiry, I received the following analysis
from Rolf Pfeiffer.  If this is studied in conjunction with P Port
drawing I posted a couple of days ago and also the insert drawing that
I sent last week, it is very useful.   I think just about everyone
would be interested.       Jerry

Begin forwarded message:

From: Rolf Pfeiffer

Subject: P- Porting


Jerry,

Based on my calculations, the timing in your drawing is as
follows:

IO, 31.24 mm is close to 101° BTDC, (32.59mm = -100°,  30.59 =
-102°)
IC, 80.76 mm is close to  73° ABDC,  (80.47mm =    73°,   81.09
=   74° )

EO, 77.92 mm is close to  69° BBDC, (77.39mm =  - 68°,   78.01 =
- 69°)
EC, 53.10 mm is close to  60° ATDC, (53.81mm =     60°,   52.61
=   62°)


The difference on the inlet port is 49.52 mm = 1.95 " width.
For the inlet, the maximum air velocity should not exceed 0.5
Mach, otherwise choking starts. Best is to stay lower. My German
book recommends about 120 m/s average. (Since the intake is near
a sine curve the maximum speed at peak is quite higher.)

At 6000 rpm each rotor sucks in 654 cc over 270 degrees shaft
rotation. The time for 270 degrees at 6000 rpm is 0.0075 sec.
Average velocity is 0.654 l / 0.0075 s = 87.2 l/s.

Area = Vol/sec / velocity = 87.2 l/s / (120 m/s *10) = 0.073 dm2
= 7.3 cm2 = 3.04 cm diameter or 30.4 mm = 1.2 in dia. That would
be the absolute smallest. So your  1.5" dia is ok. It is 38.1 mm
or 11.4 cm2.

The average gas velocity is then: vel = Vol/s / area = 87.2 l/s /
11.4 cm2 *10 = 76.5 m/s which is a good number. Remember that the
peak flow at the maximum opening rate is higher. You have some
leeway to go up to 8000 rpm. 8/6 * 76.5 = 102 m/s. But this is
cutting it close to the limit.

In order to improve things, the port should open later than 100°
BTDC. The closing at 73° is about right (or it could be a little
earlier.) That means that the port should be more rectangular in
nature. You could make it a bit wider, say 45 to 50 mm and a bit
shorter circumferentially to say 25 mm. If you leave the IC, the
IO could be at about 44 to 45 mm. This is close to IO = 75° BTDC.
It improves things. A little less exhaust will enter the intake
and less exhaust will be recycled. But don't worry, if you leave
it as is, it will certainty work ok.

On the Exhaust side there is less you can do. The timing is about
right for the space availability. Here too, the velocity should
not be too high, say below Mach 1. The air volume is also higher
due to combustion and heat up. So the port should be large. Rough
calcs: Volume 87* 3 = 260 l/s. Vel = 250 m/s. Area = 260 / (250
*10) = ~ 10.4 cm2. 1040 mm2 / 24.8mm = 42 mm wide, plus some
allowance for the radii at the four corners. If it is a bit
smaller it will still be ok.

All ports should have at least a light radius across the housing
width, so the seal is not suddenly engaging the full surface
again.

Hope that helps.

I am attaching again the calculations that I made for the port
timing. Note that EO and IC are on one measurement scale, and EC
and IO and the other dimension scale. You have to read right or
left of the dimension columns.


Best regards,
Rolf

BTW here is the HP curve for the Power Sport P-port engine.
Note the flattening out of the HP curve at 2500 prop RPM 6000 engine
RPM.
(PSRU 2.4 as I recall) Sorry about the picture. I was never able
to get a copy of the original data.

Paul Lamar

Wish we knew the size of PowerSports' p ports.  The flattening looks,
to my untrained eye,  like the engine is starting to choke as Rolf
predicted.  Isn't this engine red lined at 5400 rpm?    215 hp at 5400
rpm  ain't bad.    Jerry

No that is 6400 RPM. My guess is they are 1-3/8 or so. Perhaps 1.5.

Paul Lamar
 
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