Degrees ROP (With Poll)

[takair]

The large opening on the older models actually provides less cooling, not more.  You're quoting 300 deg CHTs while running ROP at 75%.  Is that with a multi-probe CHT gauge or with the single factory gauge?  If it's the latter, I suspect you have an instrument error.

The large mouth may not be aerodynamically efficient or good for cruise speed, but it does a good job of cylinder cooling. I have a 4 probe CHT and the original single probe factory CHT. All temps are within approx 20 degrees of each other, even here in the south running 100 ROP and 75%. It is unlikely that I have 5 bad probes and has run this way for the 15 years I've owned and maintained the plane. I have seen many other folks running these CHTs. The old Mooney has a doghouse baffle. When in good shape it does a great job of directing the airflow. It is a real pain in the next for engine maintenance. Small holes will cause CHTs to go way up. I don't think the flex baffle is as effective, especially when worn.

[jetdriven]

I did some testing in that region and with a 75%, 80 ROP,  11.2 GPH power setting at 2500 RPM,  at 7000 feet with an OAT of 80F, the CHT would go over 400.  Of course adding another gallon or so woud keep it in check, but now you got yourself a 13 NMPG airplane instead of a 17.5 NMPG one.  I'll take the 30% improvement in efficiency.

[Hank]

Byron--

 

That's part of the tradeoff between my 180-hp, carbureted plane with a doghouse, and your 200-hp, fuel injected engine with a fancy set of baffles.

 

Where ICP peaks is of interest, and certainly it's desireable to keep ICP below any pressure values that will cause problems [overstressing connecting rods, engine case bolts, cylinder bolts, etc.]. Running power settings straight from the OM should have been considered by both Mooney and Lycoming, even "back in the day." I've never seen any published values for Internal Cylinder Pressure, nor how much it takes to overstress the bolts; as for valves and camshaft, pressure on them is timing-related, and you know better than most just how adjustable timing is and is not.

 

I don't have an engine monitor, just the factory single-point gages. My operating limit tends to be oil temp getting to the end of the green stripe rather than CHT. Numbers on those stripes are a bit of a joke, there may be two on each one. If I could run LOP, I would certainly like it, but I can't so I run how I am able and I watch three needles:  EGT, Oil Temp and sometimes CHT. Temp is usually only an issue during the summer. I'll look the next time I'm out, since I don't remember them from yesterday.

[Hank]

P.S.--at 7500 msl, standard day, my OM shows the following:

 

WOT = 22.5"; 32ºF; +10ºF --> -1%; -10ºF, --> +1%

2500 RPM, 22.5", 78.8%, 14.0 gph, 167 mph

2500 RPM, 22.0", 76.6%, 13.5 gph, 166 mph

2500 RPM, 21.0", 72.1%, 9.1 gph, 162 mph

 

So I happily trade 5 gph in fuel savings for a loss of 5 mph. One I notice, the other one I will not. Yes, I usually come very close to book speed and fuel burn, even with my 3-blade prop--must be that 201 windshield and wingtips making up for it.

[jetdriven]

Hank, I dont think it is possible at normal power settings to get a 360 Lycoming to exceed a safe ICP or detonate in stock form, short of 50 ROP at 100% power. I have ran 75% power at all settings, I have been to LOP at 85% power, and to 150 ROP at 95-100% power.   The rotating assembly is a lot stronger than required as well.  The cylinders and pistons take all the abuse and sometimes do fail. 

 

Mike Busch says that if the CHT isnt running over 380 there is not enough ICP to do damage.  APS-Braly-Deakin-Atkinson says thats not true on cold days.   You can exceed safe ICP, (say, 90% power ROP) and it wont exceed 380 CHT. Especially for TN or turbocharged engines.  I side with APS on that one. 

[201er]

Maybe 380 is a good number for hot days or standard temp days but I agree, on a cold day it could mask what would have been 400+ on a warm day. But what about LOP? Is it bad to be over 380 while LOP?

[aaronk25]

My plane got new cylinders before I bought it because the previous owner no knowingly was cooking the heads by cruising at what he though was the right place; 50 rop on a single probe egt. Said it seemed like every 200 hours he was having a cylinder fail compression check or started using higher rates of oil.

Guess where he cruised at about 11.2gph which as Byron and me have found is 400+.

Another thought In the winter I adjust my cowl flaps so there flush withe the bottoms when closed and in the summer leave the open 3/4 of a inch when closed. Generates similar chts.

I don't think exceeding the engines rated power due to cold temps is as bad as getting it over 380cht. Heat hurts them more than high power setting in cold temps ECT.... But that is purely my opinion.

[jetdriven]

Heat is one thing, it will ruin cylinders of continuously operated at high temperatures. Exceeding safe internal cylinder pressure is a purely mechanical thing. It manifests itself  usually with mechanical failure of the cylinder, piston or connecting rod.  Such as the classic "blown jug" with the cylinder head separating from the barrel, or the entire cylinder departing the case. Too much pressure (from preigntion or a too high power setting while leaned, or both) overstresses the cylinder and it gives.  In the summertime, you can use CHT to gauge cylinder pressure. Anything over 400 CHT might be indicative of a power setting with too much cylinder pressure, IE, too close to peak at a high power setting.  The APS crowd thinks that cold weather results in such good cooling, that unsafe ICP can be masked by the low CHT. 

 

To avoid that, don't lean to 380 CHT automatically in cold weather at low altitude.   Lean to the same setting you would use in the summer.  If you limit yourself to 10.0 GPH LOP thats 75% power. ROP is more like 25 square.  I can only see this as a problem below 4000-5000' with temperatures less than 30-40F in a naturally aspirated airplane. Turbonormalized Bonanzas can do it at any altitude. "Heck it's only running 330 CHT, so lets give it more MP and fuel,  and let it run."  Hence the stern warning from APS.

[201er]

How much of a role does ambient temp play on CHT? Is the correlation direct? At same power setting 80F ambient leads to 380CHT, so is it 360F on 60F day? I don't think its so direct or I'd have like 330CHT in winter being 50 degrees colder but I don't. Is there a way to translate the 380 mark based on OAT to determine the equivalent for that day? I don't mean to put too much importance on numbers but it would make it easier to have a ballpark of what to look for.

[aaronk25]

Before i started adjusting my cowl flaps to stay open 3/4 of a inch in summer I would say 20-30 degrees is in the ball park of cht difference summer to winter conparison.

Keep in mind IAS plays a role to is that is how much air is going into cowl. I normally can run harder at 4-5k than at 8k and still show same or lower cht temp even though the engine puts out less power at 8k. Remember at 4k you might be running at 70% power but are 50 degrees lop and at 8k might 63% power but at peak or 10lop and have higher cht due to less indicated airspeed but also less air going into cylinders.

Cylinders get cooled by outside airflow over them as well as excess air in combustion chamber.l and oil of course.