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Old 06-23-2022, 06:31 AM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 7,472
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Originally Posted by Teal View Post
Ok, On this mornings flight I tried to duplicate some of the other RV9 cruise flight numbers.
At 8k ft and 150KTAS it was indicating 8.9GPH. at best RPM and manifold pressure which was 6300 RPM and 35.7INHg.
At 10k ft and 150KTAS it was indicating 8.5 GPH and 6300 , 35.3INHg.
Important to note that at these cruise numbers the AFR is still only 12.9-13. so there is much leaning to go in the tune at these manifold pressures. Also we are still tweaking on the fuel flow computer calcs each flight.
Perhaps of some importance is the above numbers represents about 20MPG (no winds). At the end of the last few flights I have drained the fuel and looked at total miles covered and it averages about 21MPG. That seems pretty good considering on the 1 hour flight I climbed to at least 12kft. Yes, I get some back on the glide descent but not equal.
In the end though my goal wasnt to try to get better fuel economy than the standard aircraft engine I think they do a great job of already.
With the big bore Lyc's, we are getting good fuel economy by running lean. These engines are turning low RPMs, so easy to get very lean. In the 10, I am cruising mostly at 50-75* LOP, which is probably about 17:1 or a bit less. Given you are turning higher RPMs and possibly can't get as lean, there may be better economy by running 15:1 with the higher speed. Not sure how lean you can get with 35" of MAP though. On my porsche boxer engine with turbo, I go quite lean in the tables with the closed loop mode, but it is in cruising condition at lower RPMs with no real boost involved. Never tried getting less than 12.5:1 on the 8 PSI of boost. Too scarry and not worth it for me as, unlike the plane in cruise, I don't spend great deals of time there. I would be quite cautious about anything below 13:1 when boosted. That said, I have no real experience to offer. Do you have an electronically controllable waste gate? Maybe an idea is to drop the boost way down and try lean settings at cruise. Lower RPMs with a coarser pitch may be a benefit given the limited blade area you referenced.

One thing I do know is that each combustion chamber design has different characteristics that limit AFR and advance, so be sure your tuner has experience with this engine if he is recommending AFRs lower than 13:1 on 35" of MAP. leaner AFR's can get things pretty hot at high MAPs. When speaking of pistons and rings, heat kills. Then again, many engines designed for high boost applications have pretty low dynamic compression ratios and therefore aren't producing a lot of heat at low boost levels and therefore don't need excess fuel there. Devil is in the details.

Larry
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Last edited by lr172 : 06-23-2022 at 07:13 AM.
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