Paul Yaw wrote:
Tracy Crook wrote:
At the risk of injuring some sacred cows I'll put in my $.02 here.
True, MSD (multiple spark discharge) is the only way to get long duration
with CD type ignition and MSD (the company) makes a fine one. This is as
compared to "regular" CD which gives a very short duration spark. The price
you pay is the weight of this system and high power consumption as well as
a pretty hefty price.
It should be pointed out that electronic ignition is not synonymous with CD.
Most all cars are equipped with "EI" these days but this just means that the
timing and switching of the coils is done with electronics instead of
mechanical points.
Inductive Coil ignition (as opposed to CD) inherently gives a nice long
duration spark which is important when doing lean burn combustion (what you
should be doing 95% of the time in an airplane). Magnetos give a longer
spark than straight CD but usually less than inductive coil ignition.
Another factor in ignition is energy in the spark. CD (especially msd type)
is potentially the highest here but there are exceptions. The Ducati CD
used on Rotax engines is so weak that its hard to see the spark in daylight.
I know people who have spent days trouble shooting this ignition system
(because they couldn't see the spark) when there was nothing wrong with it.
The energy in Inductive coil & magneto type systems depends on the
particular coil involved. Magnetos are usually at a disadvantage here
because of the small coil size. The 86 - 92 Mazda coils are relatively
high energy type (and this is reflected in their weight).
The spark energy form an inductive discharge system is also somewhat rpm
dependant. The '86-'92 have a very low primary resistance which helps
in this situation.
My advice to those using the Mazda rotary for aircraft use is to use the
stock coils unless you have a very compelling reason not to. In the RPM
ranges we are interested in (about 6500 rpm max), there is very little
advantage to using anything else. Those who claim that there is a world of
difference when using something else are either talking about operation at
the far end of the performance spectrum or they are dreaming.
When I say that an ignition upgrade improves fuel
economy, I don't mean that the BSFC was
better at 9000 rpm. I have yet to see anything but an improvement
across the powerband. Aside from full throttle operation, there are
gains to be had at low manifold pressures as well. If they didn't work
better, I would not recommend them. I do not sell them, and I do not
make any money when MSD sells them.
Gains due to
ignition changes are purely due to improvements in efficiency (which is
wonderful) so don't you think Mazda would have done this is the improvements
were this dramatic? They were so desperate for efficiency gains during the
design of the second generation RX7 (1986 - 1992) that the project was going
to be cancelled if they didn't find a way to reduce the car weight by ONE
GRAM per design team member. They sweat bullets and spend millions to do
that. Trust me, if a better ignition system would have done the job, they
would have done that instead.
First of all, we are ASSUMING that the engineers who
designed the rattle trap piece of shit second generation RX-7 did
everything right. Why ASSUME that? If these guys made efficiency their
number one priority, and then proceeded to do everything right, they
would have scrapped that heavy inefficient POS exhaust manifold. Major
manufacturers have many priorities that we will never understand.
Without knowing everything that was of importance to the design, it is
foolish to ASSUME that we know exactly what their priorities were, and
then ASSUME that they met all of their objectives based on what we THINK
they had in mind.
Aside from that, my buddy Sven Talo says that "trust me" is Swedish for
"#&$@ you"
True, there IS a big difference if you are going to run at 9000 rpm (like
the Racing Beat 3 rotor project)
The Racing Beat three rotor project makes peak horsepower at 7500 rpm.
but the average airplane builder would be
wasting their time (& money) going on a quest for the ultimate ignition
system. But, (as Dennis Miller says) that's just my opinion, I could be
wrong :-)
Without actually going on that quest, how do you know? Starting a fire
is not an easy thing to do in a rotary engine. Complete combustion
NEVER happens in a rotary, or a piston engine that runs on liquid fuel.
The situation is actually worse in a side port engine because the
"fresh" intake mixture is highly diluted with exhaust gasses. A good
ignition system certainly helps.
The turbo 13B that I just finished used a direct fire inductive
discharge ignition system. I did back to back passes with the inductive
system, and two MSD 6A's because I thought I heard a slight bit of
misfire above 5800 rpm. As it turns out, it was not misfiring, but it
did show an improvement in horsepower throughout the powerband. I
wasn't surprised. I will bring these runs home with me tomorrow so that
I can post them for you to look at.
Hope you didn't mind the "trust me" joke.
Paul Yaw
See... I told you Tracy you need a dyno :-)
Paul Y I think you ment "worse in a peripheral port engine".
Paul Lamar
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