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1. 05:42 PM - Stratus Subaru seasonal mixture (LarryMcFarland)
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| Subject: | Stratus Subaru seasonal mixture |
Hi guys,
Still in test mode, (mentally,) I thought it best to confess my latest
flub so others might learn or gain insight.
September 29 I was at the airport at 7:00 a.m. Larry Dorgan (a friend
and 701 Builder) arrived shortly after and we took off toward Burlington
into a decent headwind. A thin overcast with sunshine provided a soft
light patchwork of brown and green fields and we had smooth 75-degree
air. At 2500 feet and 20 minutes out, water and oil temps crept to my
EIS safe set points 220-coolant and 250-oil. We got a warning light. I
let off 4900 rpm and temps subsided temporarily. The heat muff was
moving cool air, so that wasnt the problem. We were both disappointed,
but decided to safely return to MLI and forego this years Open House at
Zenith. At home, I re-visited the Bing Manual and realized Id noted
178 and 2.78 jets are for summer. I was still flying with the leaner
176 and 2.76 winter jets. These are intended for a winter-air-soaked
engine that could tolerate a warmer mixture. My summer jets should
have been 178 and 2.78 for a richer mixture to reduce engine heat so
coolant and oil can do their part. After lunch, I drove back to the
airport and re-jetted the Bings to 178 and 2.78. Id mixed assumptions
about thin hot air versus thick cold air, which are less a concern. The
percentage of engine heat output reduced by a richer mixture in summer
is important. EGTS reduced from 1425 by 50 degrees should allow coolant
and oil temps go lower by similar numbers. The afternoon wind was
blowing pretty hard so confirmation of this would be left to the next
good day.
This good day, I took flying gear to the hangar to check numbers for the
178 and 2.78 jets. I took off from 27, climbed to 4000 ft and maintained
4900 rpm for 15 minutes. This was repeated at 2500 feet. Coolant
temperatures never went above 197-deg F, Oil remained at 230-deg F, CHTs
read 203-deg F and EGTs stayed under 1380. After the test flight, I
landed on 27, taxied back satisfied and reassured by the results. Its
true that correct seasonal mixture per one jet size can have everything
to do with cooling the Subaru engine. In my haste to be ready for Open
Hangar Day, I missed the symptoms of an engine that had been running
near safe EIS set points most of the summer. Nothing harmed, but its a
point that should be made and will have to be remembered for the next
trip. We probably missed a good Saturday too.
Larry McFarland 601HDS at www.macsmachine.com
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