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Old 07-29-2002, 04:05 PM   #1
Scooby Newbie
Member#: 4507
Join Date: Feb 2001
Location: Portland, OR USA
05 STI
Aspen White- No wing

Default Link users-What are good cruise egt's?

I have been trying to adjust my fuel levels in the typical cruise zones using egt's as an indicator. For now I have kept egt's around 700-720C (1300F).As I lean out particular zones the egt's climb to around 750C (1380F) Is this too high for regular freeway type cruising?
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Old 07-29-2002, 04:36 PM   #2
Scooby Specialist
Member#: 5994
Join Date: Apr 2001
Location: Fort Collins, CO
2004 Impreza WRX STi
Java Black


I honestly don't know about the Link and cruising yet. But I can tell you from LOTS of observation (about 15,000 miles worth) what the stock ECU and unichip used to do.

vacuum up to about 5psi of boost = 14.5-14.7:1 stoich A/F. The EGT's really depended on what speed you were traveling.

70mph = ~1300-1325F
80mph = ~1350-1400F
90mph = ~1400-1450F

Those are rough ideas. My EGT probe is mounted off the passenger side manifold right before the collector for all 4 cyl's. These numbers are also based off of one hi-flow cat in the downpipe as well.
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Old 07-29-2002, 04:46 PM   #3
Scooby Specialist
Member#: 8221
Join Date: Jul 2001
Chapter/Region: NESIC
Location: Belchertown, MA
2011 Legacy 2.5i Ltd


What both of you report is pretty much what I see with my stock ecu, unichip, and avc-r.. so you should be ok with your link Paul.

Tapped off of cylinder 3.
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Old 07-29-2002, 05:07 PM   #4
Scooby Specialist
Member#: 12976
Join Date: Dec 2001
Chapter/Region: TXIC
Location: Huntsville, Alabamy
2002 WRX
grocery getter

Default Textron Lycoming leaning procedures

NOTE: this is airplane stuff, but the principles of controlling the mixture are the same:

"The manual recommends that you lean to 50 degrees rich of peak temperature on the EGT. But that peak is hard to find, so for a first approximation I lean till I see a drop in RPM and enrich to peak RPM. This gets you close to best power. I do this as soon as I get above 3000 or 4000 feet and then again when I reach cruise altitude. When in cruise configuration I wait for the EGT to stabilize and then sneak up on peak EGT. Once I find the peak I note the number and then enrich by 50 degrees. I have found that the peak is pretty consistent across a range of altitudes so as I go up and down I re-lean to that peak minus 50 number. If the power is changed the peak needs to be checked again. The whole process can take 15 or 20 minutes but the first approximation is quick and will work even on short and up/down IFR training flights."

Some other relevant things about Lycoming motors-- they are horizotally opposed 4 and 6 cylinder engines just like our Subarus. Just like our cars, Lycomings run rougher on the lean side of peak EGT. Strangely enough, Lycoming actually recommended running on the lean side:

"When operating on the lean side of the power curve, the pilot may observe that airspeed and power are less. If you desire to regain lost airspeed and continue to fly on the lean side of the curve, two steps are important. If sufficient throttle is available at the lower altitudes; first add two inches of manifold pressure to the standard cruise setting and then lean 25 to 50 degrees, (lean of peak). Occasionally, some pilots prefer to fly on the rich side of the power curve; this is permissible. Adjust the mixture control until EGT peaks and then enrich mixture until you get 25 to 50 degree drop on the EGT gauge. Acceptable continuous (cylinder) head temperature is an important reference here."

Why? Because "lean misfire" is voodoo.

"The engines run rough because on the lean side of peak, most engines do not have uniform cylinder-to-cylinder horsepower output. This cylinder-to-cylinder horsepower imbalance causes vibration, which has been often and wrongly characterized as lean misfire. We know this is pure nonsense because if one does precisely balance the fuel/air ratios of all of the cylinders, one can lean the engine to more than 100 degrees lean of peak even 120 to 140F lean of peak and the engine will just smoothly lose horsepower until it finally, and gently, dies."

So there you have it. During economical cruise, when you're going for max economy and you have one eye on the EGT gauge, it's okay to lean. As someone who's never messed with standalone control on a WRX, I would not go any farther than 20 degrees lean of peak EGT until I was sure everything was ok, and even then only during leisurely cruising. Real tuners probably know better
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