Mixture--science vs witchcraft?

"Jim Carter" wrote in message
news:000d01c7e206$d059b690$4b01a8c0@omnibook6100.. .

The engine won't generate 250 HP at altitude will it? Isn't that a sea
level rating based on the pressure of the air available at sea level? So
the 20-22 gph would be for sea level wouldn't it?

That is precisely what's at the root of this issue.


At FL180 the pressure is 1/2 of sea level so we can interpolate that at
9000' the pressure would be only 3/4 of sea level, then factor in the
temperature for the density altitude the engine will really breathe.


I don't think the relationship is that linear, is it? (going from SL to
1,000' is a bigger change than going from 9,000' to 10,000', AIUI) But I
agree with you in general.

Doesn't this mean that the normally aspirated engine in the OPs question
will produce significantly less than 250 HP? I'm not going to do the
math because I'm sure to get it wrong and there are many others on here
more qualified than I, so I'm only guessing that we might see 200 HP. If
that's the case then instead of 20-22 gph wouldn't we be looking for
around 16 gph?


And, as it happens, that 16 gph is pretty much right in the ballpark of what
I've been using. The climb chart tells me I should be seeing around 14 gph
in a climb through 9,000' DA, so including the 2 gph "enrichening factor,"
16 is what I'm seeing (numbers from memory, I do not have the chart in front
of me).

This sounds a LOT closer to the "science" I'm looking for here!! What's
this math that you don't want to do in public? If there's some equation I
can plug the variables into & come out with the right answer, I'll be a
happy camper! Is there "someone more qualified" than Jim (your words! to
show me the math? Thanks!
--
Doug
"Where am I to go/Now that I've gone too far?" -- Golden Earring, "Twilight
Zone"
(my email is spam-proofed; read the address and make the appropriate change
to contact me)

On Aug 19, 10:51 am, "Douglas Paterson"
wrote:
"Jim Carter" wrote in message

news:000d01c7e206$d059b690$4b01a8c0@omnibook6100.. .



The engine won't generate 250 HP at altitude will it? Isn't that a sea
level rating based on the pressure of the air available at sea level? So
the 20-22 gph would be for sea level wouldn't it?

That is precisely what's at the root of this issue.



At FL180 the pressure is 1/2 of sea level so we can interpolate that at
9000' the pressure would be only 3/4 of sea level, then factor in the
temperature for the density altitude the engine will really breathe.

I don't think the relationship is that linear, is it? (going from SL to
1,000' is a bigger change than going from 9,000' to 10,000', AIUI) But I
agree with you in general.

Doesn't this mean that the normally aspirated engine in the OPs question
will produce significantly less than 250 HP? I'm not going to do the
math because I'm sure to get it wrong and there are many others on here
more qualified than I, so I'm only guessing that we might see 200 HP. If
that's the case then instead of 20-22 gph wouldn't we be looking for
around 16 gph?

And, as it happens, that 16 gph is pretty much right in the ballpark of what
I've been using. The climb chart tells me I should be seeing around 14 gph
in a climb through 9,000' DA, so including the 2 gph "enrichening factor,"
16 is what I'm seeing (numbers from memory, I do not have the chart in front
of me).

This sounds a LOT closer to the "science" I'm looking for here!! What's
this math that you don't want to do in public? If there's some equation I
can plug the variables into & come out with the right answer, I'll be a
happy camper! Is there "someone more qualified" than Jim (your words! to
show me the math? Thanks!
--
Doug
"Where am I to go/Now that I've gone too far?" -- Golden Earring, "Twilight
Zone"
(my email is spam-proofed; read the address and make the appropriate change
to contact me)

According to my engr reference book the pressure at 9,000 would be
about 71% of the sea level pressure and the standard temperature is
only about 27 degF