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Date: Mon, 19 Mar 2007 22:11:25 -0400
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Subject: carbureted lean-of-peak
From: "bob mackey" <n103md@yahoo.com>
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Many have suggested a little carb heat. I have experimented
with carb heat and found that sometimes it seems to improve
the distribution, but not always. I suspect that outside
air temp has much to do with the evaporation rate of the
fuel and whether or not carb heat is going to help.

Chris Zavatson asks:
> How tight were you able to get the EGT spread?
> How tight is needed to good LOP operation?

I don't have a recording engine monitor, and have not made
a systematic study, so my answers are anecdotal at best. When
cruising at 10-12K feet, my economy settings are full-throttle
(about 20" MAP). Peak EGT is about 1500F at roughly 7.5 gal/hr.
I lean to about 6.5 gal/hr and see about 1430F. Call it
70 deg LOP, but take that with a huge grain of salt because I
spend very little time determining the exact peak EGT. It might
be 1470 or 1530 F. CHTs stabilize around 320-340 LOP compared to
390 at 50 ROP. Over the whole range, I rarely see the four EGTs
agree to much better than 50 F. The above numbers are a sort of
composite average of the four.

If I lean any further than 6.5 gal/hr, I feel engine roughness,
and one or more EGTs drop off sharply indicating misfires.
There is also a loss of airspeed / altitude with the onset
of lean roughness. This lean, the speed is down to about 150-155 kias
(~183-189 ktas at 11500).

If I had way too much time on my hands, I might build a
new induction system so I could watch the fuel distribution
in real time and adjust some vanes to improve the uniformity.
But I'm too busy flying places to go to that much trouble.
And my next Lancair will probably have a Subaru engine with
programmable fuel distribution. And knock detectors.

-bob