Forgotten Field wrote:
Glenn and B29 Flt Eng,
I am ready to abandon anything that is OWT, with right physics and chemistry. I am definitely on board with the excess fuel for engine cooling at take off power. But why all the converts to the lower power settings on take off, and the lower maintenance? I can see that if just one less cylinder is changed in a year, that is going to be looked at by an operator as sterling performance, and might be a fluke, but aren't several people doing this? The 91/98 octane stateside training explanation is a very good one for alternative minimum power settings, but doesn't this apply in the new formulation process, if 100 LL is more like 91/98 octane? Thanks to Randy H for reminding me, but aren't the more modern octane rating techniques actually making us run gasoline with lower octane ratings- what I mean is that older fuels naturally had a higher octane rating due to inaccuracy in the cracking process and octane calculation? Again, I am not the expert, only repeating what may be OWT, but am very curious about the chemistry/physics of the whole subject. So here are some follow up questions that may spur further discussion:
1. How does modern octane rating formulation and burn data on gasoline correlate with older octane rating formulation and older burn data?
2. How are you altering the actual chemistry/heat cycle of the engines (R2600 especially) by employing Alternative Minimum Power Settings (AMPS) when using modern 100LL?
3. How, considering the modern scarcity of running engines, would you be able to counterpoise these two methods of operation in order to lend credibility to either method?
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Have not seen any evidence of lower maintenance with reduced power takeoffs. Actually, I've seen more. And it is usually seen in cylinder changes as a result of hotter running cylinders. I know some out there swear the opposite. However, I know of one operator who has so many bad exhaust valves due to overheating, but still won't believe the data. Don't know why so many are ignoring all that has been written, by the manufaturers and documented in tests for the sake of OWT. 100LL is the SAME as 100/130. Get a hold of ASTM D910 and see for yourself. The method of measuring octane rating in avgas has not changed. 100 is still 100.
Ok, to answer your specific questions. 1) there is no difference. The fuel flow data in the manuals is pretty accurate. Was then, and is now. 2) lets use the R-2600-35 in a B-25. By using the 91/98 numbers you aren't getting the power enrichment the engine was designed to have for adequate cooling. As a result you will have higher CHT's on takeoff. Been proven time after time. Heat is a cylinders enemy, especially if it can not be removed as it was designed to. As far as I am concerned, the lower you keep the temps, the better. Gas is still cheaper than cylinders. <<older fuels naturally had a higher octane rating due to inaccuracy in the cracking process << you are assuming that the fuel ALWAYS came out with a higher than advetettised rating. Could have easily gone the other way too. 3) the manufacturers had all the high priced help to figure this out. The military and airlines flew them thousands and thousands of hours. If a better way exists I'm happy to review the data. But so far I haven't seen any. Why re-invent the wheel.
My 2-cents after 25 years of maintaing and flying these things.
Glenn