Engine (shock) cooling what's the deal
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Engine (shock) cooling what's the deal
Last night I decided to climb up from my strip at 250 above MSL to 4000 to practice some stalls and minimum controllable airspeed etc, within a 4 miles radius above my strip, the temp at the field was about 12C on my departure at about 7pm, during the shallow turning cruise climb the CHT gauge was just over half way in the middle of the green which is where it normally is ( the engine is a lycoming 0360 c4f). Upon initiating my descent and heading back to the strip i noticed a cloud cover moving over at about 1500 to 2000 agl I decided to expedite my decent by basically pulling the power back a bit and pointing the nose down and dropping at about 1100 to 1300 FPM to get in under the approaching cloud cover. Levelling out at about 1200 agl to set up for landing i noticed the CHT was right at the very bottom of the green range which got my attention and I chastised myself for not doing a slower fpm circular decent to avoid such rapid cooling.
I understand that the type of decent I did was a dumb thing to to do even if the CHT never got outside the green arc. My question is that even if it did stay within the green arc can rapidly dropping from normal operating temperature to almost falling out of the bottom of the green operating threshold damage the engine?
I understand that the type of decent I did was a dumb thing to to do even if the CHT never got outside the green arc. My question is that even if it did stay within the green arc can rapidly dropping from normal operating temperature to almost falling out of the bottom of the green operating threshold damage the engine?
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For every person you hear talking about shock cooling being bad, there's an equal amount of people saying it's nonsense. Mike Busch, AOPA Pilots resident A&P, pilot and columnist thinks it's hogwash.
I don't know the answer. It's a bit like the rich or lean of peak argument - we will never get a definitive answer, I think.
I don't know the answer. It's a bit like the rich or lean of peak argument - we will never get a definitive answer, I think.
Last edited by AdamFrisch; 22nd Jun 2012 at 00:11.
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I am certain shock cooling is not a myth, but it is an issue only if the CHT is high enough to start with i.e. when the metal is not at its normal strength.
There is a well known article by a gliding tug operator who reported consistent cracked cylinders, until they did a bit of a low power section before the (rapid) descent, and it cured the problem.
A simple rate of change of temperature is unlikely to be a problem otherwise you would crack cylinders due to shock heating during takeoff.
Lyco recommend 60F per minute max rate of change, which is fair enough. The EDM700 warns if this is exceeded and it never is in normal ops at any significant value of CHT. It is easily exceeded just before landing but by then the engine is cool as a cucumber anyway.
When descending, I reduce MP by a few inches per minute at most.
The ROP v. LOP is no longer an argument, BTW
It is well established.
There is a well known article by a gliding tug operator who reported consistent cracked cylinders, until they did a bit of a low power section before the (rapid) descent, and it cured the problem.
A simple rate of change of temperature is unlikely to be a problem otherwise you would crack cylinders due to shock heating during takeoff.
Lyco recommend 60F per minute max rate of change, which is fair enough. The EDM700 warns if this is exceeded and it never is in normal ops at any significant value of CHT. It is easily exceeded just before landing but by then the engine is cool as a cucumber anyway.
When descending, I reduce MP by a few inches per minute at most.
The ROP v. LOP is no longer an argument, BTW
It is well established.
Last edited by peterh337; 21st Jun 2012 at 16:56.
Mixture settings:
ROP = Rich of Peak EGT
LOP Lean of Peak EGT
Source: Pelican's Perch #18: Mixture Magic
ROP = Rich of Peak EGT
LOP Lean of Peak EGT
Source: Pelican's Perch #18: Mixture Magic
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BGA research
Some years back the BGA did a lot of research into shock cooling, their work dramatically reduced the number of cracked cylinders within the glider tug fleet.
I highly recomend the people who doubt shock cooling is an issue take a look at the scientific report, it might change the way some view this issue.
I highly recomend the people who doubt shock cooling is an issue take a look at the scientific report, it might change the way some view this issue.
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many moon ago I worked for a place that flew bank checks and other high priority papers around.
we were told and checkridden using a procedure called: Stage Cooling. We were to reduce the manifold pressure one inch perthousand feet of descent, timed to arrive at the outermarker at approach power setting. ( lycoming engines on pipers)
now, in real practice sometimes it worked, sometimes it didn't...and many times we had to deal with the controllers strike in the USA and had to scud run (legally) using our wits. AND THAT MEANT cutting to idle and spiraling down through a big hole to a small airport or vice versa.
do your best, be nice to your engine and it will be nice to you...but you gotta fly it the way you see it and you can't SAVE the engine and lose the plane and pilot.
we were told and checkridden using a procedure called: Stage Cooling. We were to reduce the manifold pressure one inch perthousand feet of descent, timed to arrive at the outermarker at approach power setting. ( lycoming engines on pipers)
now, in real practice sometimes it worked, sometimes it didn't...and many times we had to deal with the controllers strike in the USA and had to scud run (legally) using our wits. AND THAT MEANT cutting to idle and spiraling down through a big hole to a small airport or vice versa.
do your best, be nice to your engine and it will be nice to you...but you gotta fly it the way you see it and you can't SAVE the engine and lose the plane and pilot.
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LOP or ROP?
WRT the picture from Pelican's Perch #18: Mixture Magic shown above, a bit further below the same article states that
So actually at least the poor armchairflyer is solidly confused concerning the proper mixture setting (except for taxiing, where AFAIK any degree of leaning does no harm in that almost idle regime).
This second and much more serious red area holds true through the entire spectrum of power settings, so it is much more serious. Virtually all factory big bore engines suffer from this uneven power distribution, which sets an artificial limit on just how lean we can run. Uneven mixture distribution causes the entire lean-of-peak-EGT region to become a red zone, not available for use!
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A simple approach, which yields nearly all the benefits, is to forget LOP and just use peak EGT.
Around peak EGT, the combustion is stochiometric i.e. optimal, and since power comes from burning juice (gosh, really?) once you are stochiometric, there is no way (to a first degree approximation, anyway) of getting more power for the same fuel flow.
I fly at peak EGT all the time, and don't bother with LOP. LOP is very sensitive to the exact balance of intake air distribution and other factors.
The exact setting is not critical because the power curve is quite flat around the peak EGT point.
Lyco authorise all (I think all) their engines for peak EGT at 75% of max rated power or less. So there is nothing dodgy about this.
Obviously the over-riding thing is all this is cylinder temperature management so one can use peak EGT only in cruise, at cruise speeds.
I wrote this up a while ago...
The benefit of LOP relies on second order factors... the lean mixture burns slower, which suits low RPM (less friction losses in the engine) because one is stuck with the fixed ignition timing in our engines. One gets about 5-10% more MPG at say 2200rpm than at say 2500rpm. But if one can fly peak EGT at 2200rpm then the gain is really small. I did very careful flight tests, where the TAS was held constant (i.e. constant thrust) and could never achieve any MPG improvement using LOP, over peak EGT. The measurement resolution was about 1%. Most people who fly LOP are getting great MPG gains because they end up flying slower
Around peak EGT, the combustion is stochiometric i.e. optimal, and since power comes from burning juice (gosh, really?) once you are stochiometric, there is no way (to a first degree approximation, anyway) of getting more power for the same fuel flow.
I fly at peak EGT all the time, and don't bother with LOP. LOP is very sensitive to the exact balance of intake air distribution and other factors.
The exact setting is not critical because the power curve is quite flat around the peak EGT point.
Lyco authorise all (I think all) their engines for peak EGT at 75% of max rated power or less. So there is nothing dodgy about this.
Obviously the over-riding thing is all this is cylinder temperature management so one can use peak EGT only in cruise, at cruise speeds.
I wrote this up a while ago...
The benefit of LOP relies on second order factors... the lean mixture burns slower, which suits low RPM (less friction losses in the engine) because one is stuck with the fixed ignition timing in our engines. One gets about 5-10% more MPG at say 2200rpm than at say 2500rpm. But if one can fly peak EGT at 2200rpm then the gain is really small. I did very careful flight tests, where the TAS was held constant (i.e. constant thrust) and could never achieve any MPG improvement using LOP, over peak EGT. The measurement resolution was about 1%. Most people who fly LOP are getting great MPG gains because they end up flying slower
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Not on a Continetal
Graphic engine monitors, EGT/CHT analyzers, fuel-flow transducers and digital tachometers give exact readings for every engine parameter measured and can be great aids when trying to troubleshoot engine problems. However, a single-point EGT is the only tool needed to sufficiently lean your carbureted 0-470 engine. This is because fuel and air distribution in the engine's induction system is grossly inefficient and monitoring of single-degree changes and measuring fuel in ounces rather than gallons is unnecessary. Adjusting the fuel mixture for a particular power setting can be easily accomplished with a single-point EGT using TCM Service Bulletin #M89-18 as a guideline. Percent power and knowing where you are in relation to peak EGT is then all that's necessary for adjusting the mixture on an 0-470 engine. (Keep in mind that too much fuel is as destructive as too little fuel.)
That's Continental for you
That's Continental for you
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Piperpoy,
I worked for an engine overhaul shop for many years. Shock cooling damage is real, and occurs quickly with careless engine operation. As has been said, 1 inch of manifold per minute, or thousand feet maximum rate to reduce power. Yes, the engines will stand more abuse, but not forever, and it only takes once to badly crack a cylinder or 6. Glider tow and paradrop ops usually have a good grasp of these things.
Think of it this way: You have a cast part with many intricate features, in particular variances in the thickness of the casting where there are fins, heavy areas, and thinner areas (between exhaust and intake ports) These all reach their intended operating temperature during normal cruise, and run hotter during a climb, where the engine is working harder, and airspeed (= cooling) is less. During the time available for the engine to heat up after start, these temperature even out. Normal operation keeps them normal.
You're cruising along, and decide to go down. You carelessly snap the throttle closed. The generation of heat pretty well stops, as the engine is no longer developing power. The cooling, however remains the same, 'cause you did not slow down first. So the effect of the cooling is unchanged, but no heat is being generated to balance it. The areas of the cylinder subject to the greatest effect of cooling, will cool very quickly. Other thicker or more shielded areas take longer to cool. Now you have vastly differing temps in the adjacent areas of the same casting, with some shrinking much faster than others (we agree that cooling aluminum causes it to shrink right?). If a whole bunch of crystals of cast aluminum shrink at different rates, some are going to pull away from others, and you get a crack.
I have seen lots of these cracks, and with some cylinders, you just know where to look. You can tell how well a pilot treated that engine. If you must pull power off quickly, pull the nose up lots first, and slow the plane down. At least the slower plane will not cool the engine so fast, and reduce the risk of damage.
I worked for an engine overhaul shop for many years. Shock cooling damage is real, and occurs quickly with careless engine operation. As has been said, 1 inch of manifold per minute, or thousand feet maximum rate to reduce power. Yes, the engines will stand more abuse, but not forever, and it only takes once to badly crack a cylinder or 6. Glider tow and paradrop ops usually have a good grasp of these things.
Think of it this way: You have a cast part with many intricate features, in particular variances in the thickness of the casting where there are fins, heavy areas, and thinner areas (between exhaust and intake ports) These all reach their intended operating temperature during normal cruise, and run hotter during a climb, where the engine is working harder, and airspeed (= cooling) is less. During the time available for the engine to heat up after start, these temperature even out. Normal operation keeps them normal.
You're cruising along, and decide to go down. You carelessly snap the throttle closed. The generation of heat pretty well stops, as the engine is no longer developing power. The cooling, however remains the same, 'cause you did not slow down first. So the effect of the cooling is unchanged, but no heat is being generated to balance it. The areas of the cylinder subject to the greatest effect of cooling, will cool very quickly. Other thicker or more shielded areas take longer to cool. Now you have vastly differing temps in the adjacent areas of the same casting, with some shrinking much faster than others (we agree that cooling aluminum causes it to shrink right?). If a whole bunch of crystals of cast aluminum shrink at different rates, some are going to pull away from others, and you get a crack.
I have seen lots of these cracks, and with some cylinders, you just know where to look. You can tell how well a pilot treated that engine. If you must pull power off quickly, pull the nose up lots first, and slow the plane down. At least the slower plane will not cool the engine so fast, and reduce the risk of damage.
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On a Lycoming 360 the magic number is 380F, below this temp research shows that power changes and sudden cooling does not harm the engine.
Above 380F is the danger area, only change one of anything that cools the cylinder heads. i.e. Do not reduce power, richen the mixture and increase airspeed together, will crack the heads over time.
Forget Lycomings max 480F cylinder head temp, this is an absolute red line
it may run, but wont last long.
Cruise temps should be 380f or less for long life and above 400f in climb are into the danger area. Best if in all modes of operation temps stay 380f or less.
Point of LOP is not just saving fuel, but keeping cylinder head temps low using air rather than expensive fuel. Matched fuel flow for each cyl is required to do this see GAMI injectors. Difficult to do with a carb.
Lycoming did issue a service bulletin to reduce timing on 360 from 25 to 20 degres this reduces cyl head temps and does not affect power as over timed anyway.
Note Running at Max EGT is not the max power point, it is 125f down on the rich side of peak.
Lycoming say at 65% power no damage can done by any cooling or mixture changes. But Lycoming do say that any damage to one of their engines is down to the operator not their design and manufacturing faults which are many.
Above 380F is the danger area, only change one of anything that cools the cylinder heads. i.e. Do not reduce power, richen the mixture and increase airspeed together, will crack the heads over time.
Forget Lycomings max 480F cylinder head temp, this is an absolute red line
it may run, but wont last long.
Cruise temps should be 380f or less for long life and above 400f in climb are into the danger area. Best if in all modes of operation temps stay 380f or less.
Point of LOP is not just saving fuel, but keeping cylinder head temps low using air rather than expensive fuel. Matched fuel flow for each cyl is required to do this see GAMI injectors. Difficult to do with a carb.
Lycoming did issue a service bulletin to reduce timing on 360 from 25 to 20 degres this reduces cyl head temps and does not affect power as over timed anyway.
Note Running at Max EGT is not the max power point, it is 125f down on the rich side of peak.
Lycoming say at 65% power no damage can done by any cooling or mixture changes. But Lycoming do say that any damage to one of their engines is down to the operator not their design and manufacturing faults which are many.
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Note Running at Max EGT is not the max power point, it is 125f down on the rich side of peak.
Lycoming say at 65% power no damage can done by any cooling or mixture changes.
I don't like the old "cruise climb" institution; it doesn't achieve anything at all, while delivering high CHTs which are hard to avoid.
I did flight tests, climbing from ~1000ft to ~7000ft, using various cruise climb profiles, using the constant-EGT profile (the best way), and using a full-rich profile. The fuel burnt during the climb was pretty similar for all of them, but the cruise climbs took longest to get there and produced the highest CHTs (actually pretty high values; manageable only with an EDM700 or similar). The constant-EGT climb performance was quite similar to the full-rich climb, but the latter would not work for high altitudes e.g. over 10000ft because the engine would be way over-rich.
I think "cruise climb" is an artefact of the goode olde days when you had no instruments and when also "over square" was not allowed because nobody understood that it is/was a meaningless concept in engineering terms
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My plane is a fixed prop 0360 (vy is 90mph) so what is the best method of climb up to 5k do you think? i.e. cruise 2300rpm at 100mph or full power,full rich at VY
One of the problems with this discussion is the question seems to be binary
Yes shock cooling is a big deal or No it is nothing to worry about.
My personal opinion is for the simple 4 cylinder engines fitted to your typical club trainer/tourer it is pretty much a non issue. The flying school I help out at has permission to go to 3500 hours before overhauls on the C 172 engines (0320 D2J with 2000 hr manufacturers recommended TBO). They live a hard life with lots of idle power approaches frequent power changes in the circuit etc etc and in general no procedures to prevent shock cooling) Most engines make it to 3500 hours with no cylinder work at all.
However I also fly a Cessna C 421 with the 375 hp Geared turbocharged Continental GTSIO 520 engine. The engine manual cautions going abruptly from cruise power to idle power in flight will reduce the engine life by 50 hours
These I think represent the extremes. I think you can divide engines into 4 broad groups
1) Simple 4 cylinder less than 200 hp where precautions to prevent shock cooling while nice to practice are not required.
2) Non turbocharged 6 cylinder more then 200 hp engines where prevention of shock cooling is desirable and will likely extend cylinder life
3) Turbocharged engines making up to 350 Hp where prevention of shock cooling will definitely extend cylinder life and is essential to make TBO
4) High power turbocharged engines of 350 hp or more where prevention of shock cooling is absolutely essential to prevent cylinder damage from occurring over quite short time frames and the engine has no hope of making TBO if regularly shock cooled.
However even for the simple engines my philosophy is to be nice to the thing that is keeping me in the air. So I let it warn up before applying power, watch the oil temp and CHT (if fitted) and especially on warm days, avoid high power low airspeed operations to avoid overheating and try to make all power changes slowly and smoothly.
Yes shock cooling is a big deal or No it is nothing to worry about.
My personal opinion is for the simple 4 cylinder engines fitted to your typical club trainer/tourer it is pretty much a non issue. The flying school I help out at has permission to go to 3500 hours before overhauls on the C 172 engines (0320 D2J with 2000 hr manufacturers recommended TBO). They live a hard life with lots of idle power approaches frequent power changes in the circuit etc etc and in general no procedures to prevent shock cooling) Most engines make it to 3500 hours with no cylinder work at all.
However I also fly a Cessna C 421 with the 375 hp Geared turbocharged Continental GTSIO 520 engine. The engine manual cautions going abruptly from cruise power to idle power in flight will reduce the engine life by 50 hours
These I think represent the extremes. I think you can divide engines into 4 broad groups
1) Simple 4 cylinder less than 200 hp where precautions to prevent shock cooling while nice to practice are not required.
2) Non turbocharged 6 cylinder more then 200 hp engines where prevention of shock cooling is desirable and will likely extend cylinder life
3) Turbocharged engines making up to 350 Hp where prevention of shock cooling will definitely extend cylinder life and is essential to make TBO
4) High power turbocharged engines of 350 hp or more where prevention of shock cooling is absolutely essential to prevent cylinder damage from occurring over quite short time frames and the engine has no hope of making TBO if regularly shock cooled.
However even for the simple engines my philosophy is to be nice to the thing that is keeping me in the air. So I let it warn up before applying power, watch the oil temp and CHT (if fitted) and especially on warm days, avoid high power low airspeed operations to avoid overheating and try to make all power changes slowly and smoothly.
Last edited by Big Pistons Forever; 22nd Jun 2012 at 00:43.
Something that's always bothered me is the effectiveness of short engine 'warms' during pfls and descents.
I can see the point of regularly making certain that the engine isn't icing up and will provide power when required, but does a 4 second burst of power really keep the engine significantly warmer than doing nothing at all? Or perhaps descending with half an inch of throttle (rather than idle) and some sideslip?
I can see the point of regularly making certain that the engine isn't icing up and will provide power when required, but does a 4 second burst of power really keep the engine significantly warmer than doing nothing at all? Or perhaps descending with half an inch of throttle (rather than idle) and some sideslip?
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I think "cruise climb" is anartefact of the goode olde days
A relic possibly of radial engines? The power reduction after takeoff in a flat 4/6 is pretty well established in the industry, but have yet to hear a reason other than avague “to be kind to the engine”. I’ve yet to see temperature figures that suggest it is.
I’ve seen quitea few replies to this question that are similar to Pilot DAR’s, an engineer who has found cracked cylinder, and attributed it to shock cooling.
I personally think that shock cooling has been massively overplayed, and that the 1” per minute or the like is silly over complication. However, I’d very much like tosee some real data (I’m dreaming I know) relating engine cooling rates (downloaded from an engine monitor) compared to cylinder life. If there is a connection, I would suggest it is related more to engines being cooled rapidly from a temperature they shouldn’t have been operating at, rather than engines being cooled rapidly from a sensible starting temperature.
Without numbers,the arguments could continue forever. The problem is that cylinders crack, because unless you park you plane in the shed the engine is going through hea tcycles, but how the nature of those heat cycle affect this… that’s the argument. There are well known clusters of cylinder cracking between certain serial numbers on some brands. I think pilots take the rap too often.
Last edited by Aerozepplin; 22nd Jun 2012 at 01:49.
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Big Pistons - I also fly geared engines, but the GTSIO are especially highly strung. I always quiz my old Commander mechanic (he knows serial numbers by heart after 30 years of only doing Commanders) if the old GTSIO-powered 685 would be something I should step up to at some point, fully knowing it's his least favourite of them all. Always gets him going. He says they rarely make TBO and often have to have some top work done before hitting 1000hrs. Sluggish take-off as well, even though they have all that power. They do perform like turbines up high, though.
I think the part of the recommendation in the manual you talk about is probably due to gearbox limitations rather than a cooling thing, no? The gearbox chatters and doesn't like to be pushed by air. I always try to fly mine with positive drive and have been cautioned to never pull to idle ever, except in the flare. It sometimes means she's little harder to slow down, but if you dump gear early, it's normally not a problem.
I think the part of the recommendation in the manual you talk about is probably due to gearbox limitations rather than a cooling thing, no? The gearbox chatters and doesn't like to be pushed by air. I always try to fly mine with positive drive and have been cautioned to never pull to idle ever, except in the flare. It sometimes means she's little harder to slow down, but if you dump gear early, it's normally not a problem.
I manage a PA31-325 Navajo with the Panther upgrade (so a Lycoming TIO-540 350HP engines instead of 325HP). I've also flown C421s (Continental GITSO-520 with 375hp) & a PA31P (also geared, but a Lycoming GTIO-540 with 425HP).
In all cases I never slam anything open or closed. As a rule of thumb my pax. shouldn't hear much of an obvious change of engine sound in flight from one moment to moment. Descents are done with cruise MP/RPM until 'x' number of minutes prior to arriving in the circuit or at the FAF. 'x' is the difference between cruise MP and the MP appropriate in the circuit or at the FAF. The MP difference, if you like. From the 'x' point on I reduce the MP difference to match the minutes to go to the circuit or FAF. For example, in the Navajo, I need 25" in the circuit/FAF but usually cruise at 33" MP LOP. I descend while maintaining 33" so at 7 minutes prior to ETA I reduce MP to 32", at ETA-6 minutes I reduce to 31", at -5 minutes MP is reduced to 30", and so on. RPM stays at cruise setting. On short final, when the prop is on its low pitch stop I set the RPM levers for max RPM.
I had the owner of the Navajo install a JPI EDM when I took over managing it. As far as I'm concerned an EDM should be standard equipment. Using it I can watch every cylinder. For example, the aircraft's POH specifies to lean at climb power to between 28-32GPH. The CHT on a couple of cylinders rises to an unacceptable level at anything less than 32GPH but that's not shown on the factory gauge. Still within the factory limit 460 or 480 deg F. but damned if I ever want to see CHTs that high. Using the EDM, I've found that 32GPH per engine works well to keep CHTs around 400 in the climb.
Leveling off I leave the cowl flaps open while accelerating, then to half then closed, then set cruise power. I use the CHTs as a guide for this.
Cruise at 65% ROP most CHTs run over 400 deg F, a couple as high as 430. LOP most CHTs are under 400 deg, and only one or two at 400-410. I know I'm comparing equal power production because the TAS is the same. I gain on fuel consumption. I'd rather use excess air (ie LOP) to keep the engine cool instead of excess fuel (ROP).
In descents I can see the CHTs very gradually reduce until by the time I'm in the circuit they're quite cool. I do constant power approaches at reducing speed, adding drag as necessary so there is little in the way of major power changes on the approach and by the time of the last power reduction to flare I'm at threshold speed ie slow.
I taxi in at, or near, idle power which gives the last bit of cooling and lets the turbos spool down still with oil pressure. The couple of minutes taxing in corresponds with an EDM TIT 700. Using that as a cluebat, for short taxi ins I wait at idle for the TIT to reduce to 700 deg. In the same vein, if I have to use a higher power settings to manoeuvre then the cool down period restarts. It's really easy with an EDM - all the CHT & TIT information is right there in front of you.
In all cases I never slam anything open or closed. As a rule of thumb my pax. shouldn't hear much of an obvious change of engine sound in flight from one moment to moment. Descents are done with cruise MP/RPM until 'x' number of minutes prior to arriving in the circuit or at the FAF. 'x' is the difference between cruise MP and the MP appropriate in the circuit or at the FAF. The MP difference, if you like. From the 'x' point on I reduce the MP difference to match the minutes to go to the circuit or FAF. For example, in the Navajo, I need 25" in the circuit/FAF but usually cruise at 33" MP LOP. I descend while maintaining 33" so at 7 minutes prior to ETA I reduce MP to 32", at ETA-6 minutes I reduce to 31", at -5 minutes MP is reduced to 30", and so on. RPM stays at cruise setting. On short final, when the prop is on its low pitch stop I set the RPM levers for max RPM.
I had the owner of the Navajo install a JPI EDM when I took over managing it. As far as I'm concerned an EDM should be standard equipment. Using it I can watch every cylinder. For example, the aircraft's POH specifies to lean at climb power to between 28-32GPH. The CHT on a couple of cylinders rises to an unacceptable level at anything less than 32GPH but that's not shown on the factory gauge. Still within the factory limit 460 or 480 deg F. but damned if I ever want to see CHTs that high. Using the EDM, I've found that 32GPH per engine works well to keep CHTs around 400 in the climb.
Leveling off I leave the cowl flaps open while accelerating, then to half then closed, then set cruise power. I use the CHTs as a guide for this.
Cruise at 65% ROP most CHTs run over 400 deg F, a couple as high as 430. LOP most CHTs are under 400 deg, and only one or two at 400-410. I know I'm comparing equal power production because the TAS is the same. I gain on fuel consumption. I'd rather use excess air (ie LOP) to keep the engine cool instead of excess fuel (ROP).
In descents I can see the CHTs very gradually reduce until by the time I'm in the circuit they're quite cool. I do constant power approaches at reducing speed, adding drag as necessary so there is little in the way of major power changes on the approach and by the time of the last power reduction to flare I'm at threshold speed ie slow.
I taxi in at, or near, idle power which gives the last bit of cooling and lets the turbos spool down still with oil pressure. The couple of minutes taxing in corresponds with an EDM TIT 700. Using that as a cluebat, for short taxi ins I wait at idle for the TIT to reduce to 700 deg. In the same vein, if I have to use a higher power settings to manoeuvre then the cool down period restarts. It's really easy with an EDM - all the CHT & TIT information is right there in front of you.