Bench mark
8,000 feet (density alt) is not magic, just an unwritten benchmark to compare one plane or mod to another. In theory in a Cessna or Piper you are assured 75% power or less. As Kevin points out RV's have a better air-box, exhaust and we're going faster, which adds a little more dynamic (ram), so we make more than 75% at 8k or even 8.5k. However not every RV is the same. Some RV's might only make 75% at 8k. Of course the VFR hemispherical cruising rule means we fly at 8.5 or 9.5.
I do think many still assume 8,000 feet = 75% power in their RV, since that was the case in their C-172. Still many RV's with fixed props don't have a MAP gauge, like many GA planes, so there is some blissful ignorance. The difference is the GA planes where flight tested with MAP gauges and documented. Now most new RV's have multi channel engine monitors, MAP, FF and C/S props. If you have a MAP gauge you have a better grip on actual power than without. The 8,000ft=75% rule is a good assumption in a Cessna 172, not so good in a fire-breathing RV7 or RV8.
What about leaning? Lyc says lean only at 75% or less, SOME RV's should not lean at 8,000 feet with wide open throttle, since it's over 75% power according to Lyc. Can you or is there any way to safely lean at 76% or 77%?
To be realistic most RV's will not be much over 75% power @ 2,500 rpm & 8,500 feet. Also fixed prop guys may be running away with +2,700 rpm with WOT, and may likely be over 75% even at 8.5k'. I am assuming NET manifold pressure (same as ambient) for that std day altitude (e.g., 21.92" @ 8,000'; 21.16" @ 9,000'). BTW, net MAP is excellent, since internal loss in the engine is about -1 (in-hg). Most Cessna's & Pipers run at 1.5 less MAP than typical RV's for all the reasons already mentioned, even with the same engine. That's good for us. Much credit goes to Van and his FAB air-box and the great exhaust pipes avaiable to us. The engine just breathes more and makes more power. The extra dynamic pressure from higher speeds does not hurt either. A Cessna at 100 mph v.160 mph of a RV, is about 0.55 in-hg more dynamic pressure. The FAB air-box we use have better pressure recovery, which makes the most of the extra pressure. The exhaust is the second part of the formula.
So at 8k' we may be at 77% power, may be 78% at WOT. So what? Can you lean? That is up to you. Officially approved by Lyc? No. The caveat leaning over 75% is not Lyc approved, but Lyc assumes we don't have 8-channel EGT/CHT and FF. Also I doubt a Cessna or Piper pilot with a Tach only can estimate the difference of 74% or 77% power anyway. Just food for thought.
You can always close the throttle to get 75% at any altitude, but WOT ops is more efficient than closing the throttle slightly. A partially closed throttle adds internal loss and inefficiencies compared to WOT. That is why climbing to at least the min altitude to run WOT at 75% power, leaned of course, is most efficient, wind allowing.
Constant speed prop guys have an easier time setting power, since an exact RPM can be set. Fixed prop guys have to take the RPM they get with WOT operations. The higher the rpm the more power you make. At least with a constant speed prop, you can lower the RPM to get below 75% power even with WOT. For example at 8.5k' @ 2,600 rpm & assuming 21.5", power is 77%. Dial the RPM down to 2,450 RPM, its 75%. For the fixed pitch guys, IT WOULD BE BEST (efficiency wise) TO JUST KEEP CLIMBING TO 9,500' or 10,500' TO ASSURE YOU'RE AT 75% OR LESS WITH WOT. On super cold dry days you may have to get into 5 digits, since the engine will make power higher with the dense air. I doubt leaning at 77% power will damage the engine, but than again I have no engineering evidence one way or another, just Lycs limit, which I respect. It's your engine.
Also, in my opinion, leaning is a function how much you lean. There's a big differences between 150F ROP verses running at 30F ROP EGT. The reason for not leaning at high power is to avoid detonation. A super lean mixture at high power can spontaneously and abruptly burn (explode or detonate) verses a controlled combustion or burn. When Lyc sets its 75% number it has some margin in it, worse case scenario rolled into it. Still a limit is a limit and there is no reason we can't respect it. There are some techniques to lean with out finding peak first, using target EGT and/or FF; we'll leave that for other discussion.
Fixed guys more than the constant speed prop guys need to make your own Power v Altitude charts. To do that you have to flight test with a MAP gauge. I'm all for a MAP gauge in all RV's, including fixed pitch prop RV's. Other wise you have no idea how much power you are making. Put those MAP gauges in and flight test. Altitude v power charts for a fixed pitch Cessna or Piper are flight tested for that particular airframe. No other plane can use them even if the engines are the same. Engine + Installation/fixed prop affects alt v pwr charts. Same with RV's, each one is different. The basic Lyc engine charts are a good starting point (see below).
Bottom line Density altitude + MP, RPM and TEMP (and to a small degree humidity) and airframe (drag) determine max power for a given altitude, not just altitude and RPM.