Lower Cowl closure mod and CHTs

[Theo]

The CHTs have always been boarderline high on my M20C. Enough so to require step climbs for cooling after a fuel stop, but not really an issue on the first take off of the day. I have sealed the dog house up and had several mechanics look at it - plus it has done this on three different sets of cylinders! - so I am thinking about adding the cowl closure mod.

 

Has anyone on the forum done this and noticed a difference in CHT reading?

[piperpainter]

I've never had a problem with my CHT. I have the cowl closer and it looks great. But I can't tell you what it did before cause I had it when I got the airplane.  But I know it looks better than the big hole they had originally!

[GeorgePerry]

My 74 has the cowl closure mod and I have no problems with CHT's...OBTW I used a EDM-700 to measure CHT, so it's pretty accurate

If your running hot the first thing I would look at is the baffling and the cowl flap rigging.  If your baffling is tight and your cowl flaps open the correct amount then you might want to have your mixture checked to see if the engine is running "pre-leaned" i.e. there's not enough fuel getting to the cyclinders.

[N207LS]

With what device/gauge are you checking your CHTs.  My UBG-16 with CHT probes on all 4 cylinders shows a descrepancy with the original CHT gauge.  Prior to the UBG-16 I had the EI 8A analyzer and it had similar descrepancies (although only had 2 CHT probes).  I think that the gauge may have as much to do with the higher CHTs as the actual cowling.  With that said, I have the complete closure (ARI, see my photos in my gallery) and get high temperatures on the original gauge during initial climb out, but am showing at least 30-40 degrees cooler on the UBG-16 accross all of the cylinders.

Aaron

[Theo]

Thanks for the suggestion. I will add the mixture check to the squawk list for annual this winter. We have gone over the baffling and cowl flaps with the local mechanic and at Joey Coles a couple of years ago when he did a new top, but that is worth another look too I suspect.

I am useing a JPI EDM-800. It does read a bit higher than the factory guage on #3 where there are two probes. It is #4 that usually wants to get too hot though. If all I had was the factory guage I wouldn't even know about it.

I have always thought it was interesting that in Bob Kromer's test flight article from 2000 he had near red line CHT for a some of his climb. This sort of led me to think it was the nature of the beast in some respects. I assume he was just using the stock guage on #3. Makes me wonder what the other three cylinders were doing.

Bob's test data:

Continuous Climb Data 1966 M20C N3411X
Full throttle, 2700 rpm, mixture leaned to 100-125 degrees
rich of peak, cowl flaps open, airspeed as noted

Elapsed Time	Altitude	MIAS	MP	RPM	Oil Temp	CHT	OAT	Rate of climb*
Climb#1 Airspeed at Constant 100MIAS
0:00	1000'	100	27.8	2700	200	400	97	---
1:02	2000'	100	26.9	2700	200	440	92	968
2:40	3000'	100	25.9	2700	205	440	89	612
3:57	4000'	100	24.9	2700	205	430	84	779
5:08	5000'	100	23.9	2700	205	415	81	845
6:30	6000'	100	23.0	2700	200	415	79	732
8:04	7000'	100	22.1	2700	200	420	73	638
9:47	8000'	100	21.2	2700	200	415	69	582
11:37	9000'	100	20.4	2700	200	415	64	545
13:42	10,000'	100	20.0	2700	200	415	59	521

Elapsed Time	Altitude	MIAS	MP	RPM	Oil Temp	CHT	OAT	Rate of climb*
Climb#2 Airspeed at Constant 120MIAS
0:00	1000'	120	27.8	2700	205	410	98	---
1:00	1800'	120	27.0	2700	205	425	96	800
2:00	2550'	120	26.2	2700	205	430	92	750
3:00	3480'	120	25.2	2700	205	425	88	930
4:00	4080'	120	24.7	2700	205	425	82	600
5:00	4860'	120	24.1	2700	205	425	79	780
6:00	5570'	120	23.2	2700	205	420	74	710
7:00	6250'	120	22.9	2700	205	415	72	680
8:00	6680'	120	22.4	2700	205	410	70	630
10:00	8680'	120	21.7	2700	205	405	65	500
12:00	8950'	120	21.9	2700	205	405	60	270
14:00	9320'	120	20.1	2700	200	405	56	335
15:46	10,000'	120	19.7	2700	200	405	54	385
* Under perfect flight test conditions, rate of climb should decrease smoothly as altitude increases. However, the air was very turbulent on our day of the test, so that is the reason why the numbers don't decrease evenly. Nevertheless, this test was a good one for 'real world' conditions and is pretty representative of what a C model will do on a hot day.

[Lood]

I'm having the lower cowl closure mod done within the next month. Partly becuase of the cosmetic improvement but mainly because of the improved cooling and small speed advantage it is supposed to offer. I don't really a have high CHT problem, but rather a somewhat high oil temperature. My Mooney still uses the factory style single cylinder analogue CHT gauge but I am also upgrading to a JPI 700. I manage to avoid the CHT to ever go higher than 400 deg F by playing around with the mixture and cowlflaps.

My oil temperature bothers me some days. Although it stays in the green, it stays too close to the 200 deg mark. I plan to do the oil cooler relocation mod next. Apparently this mod has a huge improvement in oil temperature and overall engine cooling.

[The-sky-captain]

I've got the JPI 700 with fuel flow, cht temps... I would like to add oil temp. Any idea on the cost to do this?

[VanH]

Just got the oil temp probe, it was around $300.

 

V

[Flybeech21]

WOW!  $300.  I just priced an oil temp probe for a little over $100.  Maybe you have a different probe than me since I have a M20C.  I would bet the probe is the same though..