View Single Post
Old 08-12-2011, 09:16 AM
DanH's Avatar
DanH DanH is offline
Join Date: Oct 2005
Location: 08A
Posts: 10,452

Originally Posted by RV10inOz View Post
You guys are smarter than this, or is this a bait?
It's just a question. I think you'll find the real answer to be holistic, as in "whole engine". You don't actually believe Mr. Deakin, the LOP advocate, is recommending you abandon all the benefits of LOP (which, BTW, I think are real) at 9000 ft or so because he wants you to have more power? What about "cleaner" and "cooler" and "3% less speed for 15% less fuel" (or whatever is is)?

Seen this before anyone?
Why yes....yes we have. Let's look at reality. Your "No Fly Zone" list forbids a range from 200?F ROP to 60?F LOP at 80%.

The very first chart posted above is 26.8 MP and 2700 RPM....about 92% power. Peak EGT is 1480 on the hot cylinder, full rich is 1220 and the leanest reading is 1420. They pulled mixture from 260 ROP to 60LOP, with the engine temps at their limits, without detonation.

Let's go a little further. The "No Fly Zone" list is based on percent power. Again look at the real data:

26.8/2700 = no detonation at any 92%
28.6/2400 = 10%-12% detonation when leaner than best power 87%

The detonation limit is seen at the lower percent power. The same MP/RPM relationship exists as you move down the power scale, because detonation margin isn't a function of percent power. The simple, easy to remember truth is right there on the instructions that came with the engine......don't run it radically oversquare.

All right, a few notes.

I think very highly of George Braly and his people all. If I had a big bore turbo motor (routinely oversquare and higher induction air temperatures) I'd run it with the "Red Box" prohibitions in mind. If an RV owner insists on slavish adherence to the Deakin "forbidden power settings" list, well, fine. For a basic NA Lyc it is extremely conservative. You'll spend a lot of time and effort sweating details of EGT and power that don't matter, but it won't hurt. Just don't run into anything while your head is down.

The detonation survey charts are for a stock IO-360-A. They're here for illustration. The other plain vanilla low compression models won't be a lot different, but you be conservative as you like. As before, with high compression, more ignition advance, etc, you're an experimenter. You get to gather your detonation data with a boroscope.

Some recent engine operating charts don't have a limiting manifold pressure line. My IO-390 is an example. It's not because those engines don't have a limit. When I wrote Lycoming last year and asked, the response was (quote) "No we do not have anything as of yet for the IO-390 like you are requesting." Since then the 390 has become a certified model, so maybe the data is now available. Or maybe Lycoming has no intention of publishing it anymore. I don't know.
Dan Horton
Barrett IO-390
Reply With Quote