what is less damaging

[Lumps]

IO550/520 and we need to be at a non-screamingly noisy rpm as soon as possible due to a typically noise sensitive airport.

Which is less hard on the engine - reducing to 25/25 after take off or leaving the throttle where it is and reducing rpm to 2500rpm?

Since reducing the throttle on the Continentals turns off the 'extra rich' switch, the 25/25 may put the pots in as much stress as pulling the rpm back.

Have the APS guys done this - reducing RPM with throttle wide open and seeing what happens, when do things get nasty?

[barit1]

Do yourself a favor, look for someone else to be the guinea pig. Reducing throttle AND RPM has got to be less risk.

[Intruder]

What does the manual say? For a normally aspirated engine, unless there are specific limits stated, there should be no problem reducing RPM, even at max throttle. The engine power output is limited by RPM as much as it is by manifold pressure, so it cannot put out full power at reduced RPM.

Running slightly "oversquare" is not inherently a problem. It is actually more efficient to run at wide open throttle, with power limited by RPM. If within the mfgr's recommended envelope, then do it!

[Brian Abraham]

Don't have the books with me at the moment but the Bonanza A36 is limited in EU land, or certain places within, to a reduced RPM on takeoff for noise reasons. Think 2500, but don't quote me. Power reduced I think from 300 to 285.

[Lumps]

Barit 1 - We still want to climb as quick as possible, I do like everyone else, what the manual says (you don't have to read far on recip. engines to find this is not necessarily best practise) or arbitary figures like the old 25/25 handed down through the years (same goes for this)

just curious about the facts, if anyone has them.

[zerozero]

Years ago, flying out of Van Nuys, CA (VNY) we would routinely roll back the RPMs only just enough to stop the "buzz" of the tips going supersonic.

That's all you need, nothing more. Just stop the buzz with an very slight RPM reduction.

Disclaimer, this is anonymous internet advice, use at your own risk.

:8

[slacktide]

WOT-LOP-SOP. If you know of APS surely you know that you can run full throttle from takeoff through cruise and never touch that black knob.

[Desert185]

I have had a C-185 with IO-520 (with GAMIjectors) and an 86" Mac prop (noise) going on 14 years. During that time, I have left the throttle at max and reduced the RPM from 2850 to 2500.

Tests (including mine) show that the cylinder head temps tend to rise above 400 in the climb when reducing power to 25/2500. Leaving the throttle at WOT and leaning during the climb to maintain 1200-1250 EGT (which is my EGT at sea level just after takeoff) results in cool CHT's. This technique works quite well at sea level or at much higher elevations. I religiously keep my CHT's below 380.

As a side note, the cylinders are original Continental with over 1200 hours. Never removed. Good compressions and good oil analysis. WOTLOPSOP, indeed. :ok: