Jerry Hey wrote:
I just got of the phone with Jim Mederer at Racing Beat. I had
called inquiring about using one of his exhaust port templates with
my P Ported engine. He said the "Street" template would be
appropriate, however he went into great lengths to tell me why in his
opinion we were going in the wrong direction with the 6000 rpm p.
port engine. One point he made was that p ports are most efficient
above 8500 rpm. At that rpm p-ports are generating over 300 hp. I
told him that Power Sport was getting 215hp at 5300 rpm and he said
he believed it but that side ported engines would deliver the same hp
much better.
He was not talking about 6 ports. He thought the right way to
build an aircraft engine was to take the side housings from a Turbo
four port and use them with the rotor and housing from a NA 6 port
which has higher compression. Only a small amount of die grinder work
is then necessary to achieve 200 plus hp.
Jim went onto say that if we do build a p port, that we should keep
the ports small to maintain good velocity. I tried to get a
recommended size from him but all he would say was that 1.75 inch
diameter was too big for our target rpm. He felt that the port
timing of a p port if transferred from the side ports per Lynn's
suggestion would have too much overlap.
Another point he made surprised me because he said it would not be
sufficient to secure the sleeve with DevCon only. He recommended
threading the sleeve in and also using DevCon.
Even though Jim Mederer has not built rotary engines for aircraft,
his experience and expertise in building rotary race engines, make
his words carry considerable weight.
Right now, I am still committed to p porting my engine, but am
worried about port timing and size. Paul, I have the feeling that if
you were to talk to Jim and to Lynn and gather all the data you could
design an aircraft p port engine that we could build with confidence.
This is way beyond my level.
I wish we could read Everett Hatch's notes on the subject.
Jerry
I am rather surprised that Jim feels that way. I am willing to bet
he never tried an NA P-port with long intake runners as used on the
Leman engine and the Power Sport engine. He has had no reason to
do so unless he has been commissioned by somebody to build a P-port
engine that would never see more than 7500 RPM for some strange
reason (aircraft use). Most race car situations put no such limits
on RPM. Everett Hatch at Power Sport on the other hand had every
reason to do so. Everett had two dyno's. A conventional
al dyno and a prop type dyno. The current Power Sport also has a dyno.
I don't think Jim was directly connected with the Leman effort. The
vast majority of racing car engines use short runners to get peak
power at 8500 to 10,000 RPM. Variable length runners are now outlawed
in road racing. Why? Because they work that's why. We never ever
run the engine over 7500 RPM so longer runners are called for
such as used on the Leman engine at 6000 RPM.
I get the impression that Jim reads very few Mazda SAE technical papers.
For example, according to this chart, the Leman P-port engines
spent a good 20% of the total time between 6000 and 7000 RPM.
No way would a road race driver tolerate an engine that bogged down
at 6000 RPM. The Leman engine had about the biggest P-ports
you can imagine but it also had very very long intake runners
at 6000 RPM by rotary race motor standards.
My contention is the length of the runners is the most important
factor. Not the size of the p-port. Bigger is better in almost
every case. Rotary engine ports have been getting bigger and bigger
over the years. The RX8 motor is the last example with 20% larger
ports.
In this picture the length of the intakes pipes are twice as
long at 6000 RPM.
Paul Lamar
The side port engine that Jim described is typical of the engines a lot
of us have. 4 port housings with n.a. high compression rotors and street
porting. That's what Bruce Turrentine built for me and that's what Tracy is
currently flying. I think Tracy estimates his HP at about 185 - 190 and
thats what I hope to get from mine as well.
As I've mentioned in years past, Racing Beat has built a 4 port aircraft
engine. A member of my EAA chapter had it built to his specifications and
hopes someday to market it. I've sent pictures to the list in the past.
Jerry, if you'd like to see some pics or want further info let me know and
I'll email you off list. BTW, there was extensive dyno testing done on this
engine and the result was 190HP @ 5900 RPM as I recall.
The first year I attended the Copperstate fly-in (I think 95) Everett Hatch
did a presentation on the Powersport engine. The gist of the presentation
was that much experimentation and dyno time was expended in coming up with
port timing, runner size, port size and shape, and runner length in order
to optimize the engine which at the time was rated at 210HP. I dont
remember the details but the timing requirements were considerably
different from the typical PP timing used by the racing crowd. And as Jim
mentioned to you the optimum runner size (and port size) was also smaller
than is typically seen on PP race engines. Keep in mind Paul that I'm just
the messenger but my recall of Everett's presentation is that bigger is
better definitely did not apply in his testing. There were significant
benefits in optimizing port size, runner size, and particularly port timing.
In addition to HP potential, it seems to me the real advantage of the PP
is the simplification (and decreased weight) of the intake manifold. As
with many things I suspect its not as simple to arrive at an optimized set
up as it appears on the surface. Jerry, I hope the homebrewed PP engine
works out for you. It's an upgrade I'd be interested in pursuing in the future.
Mike Wills
Everett Hatch was not entirely infallible. Look at his incorrect position
on the torsional vibration issue. Neither is Jim Mederer.
Compared to the engineering resources of Mazda they are fleas on
the back of an elephant.
It always amazes me how people are in complete denial on the Lemans
engine. It is a big mystery to me. Do you think Mazda was lying
about 240 HP at 6000 RPM? Do you think the drawing Mazda published
in the SAE paper showing the P-port to be at least two inches in diameter
to be a cartoon? Have you read what I wrote and looked at all the pictures
of the Lemans engine? Do you not believe that the 3rd gen ports are larger
than the 2nd gen ports and that the RX8 ports are larger than the 3rd gen
ports by 20%? Do you not believe the bigger the pipe the more the flow
at the same velocity? What's up? I don't get it. Am I to be completely
discounted? Are you people being contrary just be contrary and create
a controversy? :-) As my wife is found of saying; "what does logic and
reason have to do with it"?
I'll tell you what is probably fooling more than one cottage industry
rotary engine tuner. And that is this. The diameter of the pipe also affects
the tuned length. Surprise! Have you been to church lately and seen an organ?
If you are not trying all combinations of pipe
length and diameter on your dyno you are fooling yourself. See
the chart below. Lets say you want to try only 3 different diameters
and 3 different pipe lengths and 3 different timings. That is 27 different
combinations and dyno runs. Did Everett or Jim do this? I don't think
so. OK lets say you make three different pipe diameter variable length tuned
intake manifolds. Or start small and tear the engine
down each time you increase the size of the p-port.
You will also need six sacrificial rotor housings with different
p-port timings. Did Everett or Jim do that? I don't think so.
This sort of trial and error applied research is economically
beyond the average cottage industry or shade tree rotary engine tuner.
Just copy the Leman engine for *&^%$#@ sakes!
Paul Lamar
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