Al Gietzen wrote:

> SNIP<
Mazda's data indicates about

> a 4% increase in power with 9.7 vs 9.0 at 5000 rpm.  It may also
> improve some as the engine wears in - this is an engine with only a
> couple of hours of running since overhaul.
>
> I won't argue the case for/against the longer so-called 'tuned'
> induction runners, but I conclude that performance benefits are
> minimal. Tuning for a given range may give slightly higher power in
> that range, but I'll have to see the dyno results to believe it.  If
> you're looking for a flat torque curve (great for a car) this setup
> achieves it.

First off, thanks for the great data Al.  I didn't comment on the other list
because it's just too censored sometimes but FWIW, here's my take on it.

I agree with you that the RPM of interest is limited to around 6250 rpm max
but I reached very different conclusions on some things.  I'll have to dig
it up in my Mazda archives but power varied very little with compression
ratio from what I recall.  I was thinking it was in the range of 1% rather
than 4% for a 9.0 to 10.0 CR change.   The argument of what gear ratio to
use and what max rpm range to aim for in aircraft use is a different subject
so I'll just say that overall, 6000 - 6250 is the best choice for the
current generation of Mazda rotaries IMO.

On the other hand, I think the improvement at 6000 rpm to be had with
manifold tuning is much greater than you do.  Your dyno figures indicate a
HP of 255 at 6000 rpm.  This equates to a power of 169.8 for a two rotor
engine.  Good, but not great.  I have no dyno data but I'm making around 185
HP based on top speed comparisons with 180 HP Lycoming RVs.  I think 15 HP
is significant.  Other factors to consider in the comparison are my intake
ports which are small compared to yours and my manifold execution is crude
compared to the work of art you came up with.

But there is an even more significant factor which is not obvious.  I
certainly agree that your HP curve is still on the rise at 7000 rpm and that
the torque curve is amazingly flat.   This is exactly what I would expect
from a short runner manifold which would show almost no tuning effects below
7000.  As you pointed out, this is a desirable thing in a car.   Since the
engine will always opperate between 4500 and 6000 in-flight, why not tune it
for more power there?  Power lost at 7000 and above is of no consequence.

Now for the 'other factor'.   As a practical matter, most of us are stuck
with fixed pitch props.  If the prop is pitched for a maximum in-flight
engine rpm of around 6200, takeoff & climb rpm will be in the range of
5100 - 5300 rpm.  This is where significant gains or losses in aircraft
performance are to be found.   If the torque curve can be boosted at this
rpm it makes a big difference in takeoff & climb performance.   By tunning
the manifold at ~7500 rpm (just a guess as to the tuned length of your
manifold) I think you are giving up really significant power in takeoff &
climb power when using a fixed pitch prop.

The exact 'best' tunning point depends on you personal goals for the
airplane but tunning slightly above the climb rpm of the engine (~ 5600-5700
rpm) is probably a good compromise.  This also turns out to be the WOT
cruise RPM at altitude so it maximizes power at this condition as well.  If
you plan on racing, raise the tuned point to max operational RPM of  ~6200.

Tracy Crook