Magneto timing and engine temp

[Bill_Pyles]

I had the timing set on my J the other day and now the oil temp and CHT appear to run approximately 10 degrees hotter, all being equal.  Also, the engine does not like to idle as well.  Overall performance seems the same although I haven't done a speed test.

Is this temp increase likely because the timing was advanced slightly more than my earlier setting?  How accurately should timing be set in terms of degree BTDC?

[Becca]

Quote: Bill_Pyles

I had the timing set on my J the other day and now the oil temp and CHT appear to run approximately 10 degrees hotter, all being equal.  Also, the engine does not like to idle as well.  Overall performance seems the same although I haven't done a speed test.

Is this temp increase likely because the timing was advanced slightly more than my earlier setting?  How accurately should timing be set in terms of degree BTDC?

[Bill_Pyles]

Yeah, I was running maybe 25 MP and 2500RPM and seeing 210 oil temp and 400 CHT on the hottest cylinder, but if I trailed the cowl flaps I would lose 10-15 degrees of oil temp and 10-20 degrees of CHT.  I always run ROP.  I need to do the speed test NSEW and see what I am dealing with performance-wise.

[jetdriven]

 It needs to be set right dead on 25 degrees, there is no margin for a higher timing value. 

If you are running 10 degrees hotter and the CHT are 400 thats way too hot. Check the baffling and baffle seals. We realized a net reduction of 40 degrees from the new seals, but gave ~half of that back with the increased timing. Of course, you can lean to the same "pre timing change" FF and it runs around the same.  A sub-9 GPH fuel flow while 2400-2500 RPM and 27" gives 360 degrees.  50 LOP in cruise (~7.5 GOH) gives up some speed but the CHT are in the 300 range.    We always run it LOP, somewhere around 15-50 degree depending on what is what.

Per the service and maintenance manual you can also rig the cowl flaps to be open 1/4" at the trailing edge when closed.  That will help.

[Bill_Pyles]

The Mooney POH says max CHT is 475 and max oil is 245.  Why is it that 400 CHT is too hot?  I would prefer to see 380, because the strength of aluminum noses over significantly with higher temps, but why would the manual allow the higher temps?

[jetdriven]

There is endless debate on here and everywhere else, but the general consensus was 400 is the limit, 380 is watch it, and 360 is preferrable to get long service life, IE, make the 2,000 hour time on the engine.   This is suggested by John Deakin, Mike Busch,  and even Lycoming themselves say below 400 degrees.   Sure, will a quick trip to 475 degrees, and back,  kill your engine today? Probably not.  But regular CHTs in the 400s lead to an early top overhaul.  Ask the 231 pilots on here.  475 is a certification requirement. 

[Bill_Pyles]

OK, I gotta check the gap with the cowl flaps closed and adjust them to be open a little...when closed.

How in the heck do you evaluate the effectiveness of the baffles?  You close the cowl and hope they do their thing?

Now I am thinking I need to have the timing re-checked.

[danb35]

Quote: jetdriven

 It needs to be set right dead on 25 degrees, there is no margin for a higher timing value.

[jetdriven]

This was covered in a diffrent thread, but the -A1B6D and -A3B6D are the only ones not allowed to have the 20 degree optional timing, per the Type Certificate.  My data plate was stamped with a "0" over the "5", but it was done incorrectly. Its more than just a timing change, the magnetos were modified as well.

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/8DF482166B82DD2386256E75005A0248?OpenDocument