Power Settings Piper Seneca 3 PA34

If those numbers are correct, I would be confident that 26" 2400rpm or 27" 2300rpm will work also.

I would pick the setting giving the least vibration, if there is any notable difference.

2200rpm comes with the least internal friction and therefore should give the best sfc, but other factors could change that. Sometimes, but not always, lower rpm is also easier on the ears.
For the engine longevity, internal pressures and temperatures, I doubt that rpm within that range has any significance at all with only 55% power.

According to the Advanced Pilot Seminars, high RPM is better for longevity, though I forget the reason.

I imagine it is to do with timing and reduced ICP (most of their suggestions arise from that) but am not sure.

Logotivity as defined by TBO is calculated based on max RPM for normally aspirated I doubt running lower RPM makes it any higher? but I think FF and cooling are the main concerns why we do not run on high rpm…

Lower RPM advances ignition (not in degrees, of course but the timing). Therefore it’s a bit more efficient, however, it results in higher internal cylinder pressure and therefore higher CHT. Anyway, we have a TSIO360 as well (if only one ) and we always run it at 2200 RPM and 65% and we reached TBO +20% with the last engine and it still was in perfect shape. So I would say, as long as you can keep CHTs below 380F 2200 RPM is good.

At lower RPM, there is lower internal friction loss and the prop is more efficient.

Running LOP decreases the flamefront propagation speed, and (in case of fixed ignition timing) effectively retards the point of maximum pressure later in the downward cycle of the piston, resulting in more mechanical advantage.

Running Wide Open Throttle (if allowed in the engine for cruise), combined with LOP and low RPM is most efficient for cruise. The airframe will also be more efficient due to being closer to best L/D.

On the other hand, on certain engine/prop combinations such as on my IO520, there is an SB to avoid prolonged running at 2100 RPM due to internal harmonics peak. 2200 or even 2000 RPM is fine. Watch the CHT temps, vibration and your time/speed/fuel priorities.

The peak Internal combustion pressure (ICP in @Timothy’s post) is something we have no indication of on any engine instruments. Reducing RPM usually increases ICP without changing CHT, if power and mixture are kept the same. But slowing down the combustion speed would have the same effect on ICP as increasing RPM, and that can be done with the mixture lever. The fastest combustion is obtained with a mixture around that for best power, while richer or leaner will slow down the flame front. So, a low RPM and a best power mixture would be the worst you could do in terms of ICP. If we had an ICP-meter, we could reduce RPM, maintain power with the throttle and counteract the ICP increase by leaning the mixture. And if we also had a target ICP and a knowledge about how important ICP really is for engine longevity, we could develop an “ICP SOP”.

I cannot know for sure, but I still think that for conventional piston Lycomings and Continentals, 2200 RPM is not a very low RPM, and 55% is a quite modest power, and the engine will be very happy with that setting. Looking at engine manuals for a range of engines, many have limitations for how much continuous power is permitted at a given RPM, and you will not find a single engine where 2200 RPM and 55% power is not far away from any limit. I don’t have the manual for the Seneca 3 engine (TSIO-360-KB), but the Continental TSIO-360-RB manual has data for 2000 RPM and 38" (79% power), and the equivalent Lycoming TSIO-360-A and -C for 2200 RPM and 34" (75% power).

From my own experience with these turbo’s, the practical limits on low RPM is sometimes determined by bootstrapping, i.e. unstable RPM because the engine has a fixed wastegate and so any variation in RPM immediately changes manifold pressure.

Edit: agree with @dirkdj above.

Although we don’t have a measure of ICP (arguably the most important parameter in engine health), APS taught that CHT is a close analogue.

Does that relationship break down?

The integrated schools operate 2200RPM and 28in in the cruise, leaned to 11USGPH aside. CHTs tend to be around mid 300’s and TIT around 1350-1400.

They have 100,000’s hours on the Seneca 200T/220T and presumably this approach leads to least maintenance issues.

terbang wrote:

we have a TSIO360 as well (if only one ) and we always run it at 2200 RPM and 65%

What MP and FF? Around 30" and 11gph at 8000ft? Any tips for a -231?