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Of course you're right about residual boost with wastegate open. My
30" take off would have to be done at partial throttle. Am I correct in my
inference that gradually closing the wastegate as I go higher and higher will
minimize intercooler requirements and wear and tear on the turbo?
I'm counting
on not needing an intercooler using this same logic, so I guess we'll find out
if my engine blows up. It would also seem like it would
save some wear on the turbo bearings by bypassing as much exhaust
as you can, but I'm not sure how significant this would
be.
Additionally, I have
put a 36" or so limit on MAP to safeguard the engine. Does the turbo also
have a boost limit? If the turbo has, say, a 10" boost limit, then as
ambient pressure dropped off, I would reach my turbo normalize limit at about
20" ambient or around 10k'. What would the boost limits be on the Mazda
stock turbo? Are they a factor in normalizing to 15k' - 18k'?
Take it away,
Ed :-)
Yes, turbos
have limits, but this is the part of the turbo class that I'm not qualified to
teach. As I understand, each
turbo unit has a "compressor map" that shows it's efficiency range as a function
of absolute pressure, and airflow. Absolute pressure is basically the
boost that the turbo produces above ambient pressure, which is why I maintain
that a traditional boost gauge is handy to have.
My
(over)simplified way of looking at this is- I'm using the same turbo as
Mazda sized for this engine, so airflow, and boost needs to be kept in the same
range that's been proven to be durable in the cars.
The difference is the duration of the boost, and the extremely free intake
and exhaust systems that we have compared to a stock car, though many modified
cars are in about the same situation. I figure that guys can easily run 8
psi of boost on the street, so I should be able to do the same in the
air. 8 psi gives me about 16 inches of mercury, which is 15-16
thousand feet of normalization. That's plenty for
me.
Another thing
to consider is just how much normalization you really need. I
don't think too many people are planning to cruise at full sea level
power. If you do, I hope you're flying a fuel tanker :-) Let's just
say that we'll use 75% like you would with a Lycosaur. A NA
Lycosaur can still produce 75% power up to 8k ft, so our turbo only
needs to make up the difference between 8k and whatever our max cruising
altitude is. Even at 18k ft, that's only 10k ft of normalization for
cruise power, or 5psi (roughly). I really wish I
knew how much boost the 9.7 rotors would tolerate. If 6 psi is safe,
then I'd be tempted to just set the relief valve there, and try to adjust the
throttle for 5 psi. I suspect this will still be with a fully open
wastegate. Did I mention that I can't wait to actually try some of this
stuff :-)
Rusty
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