Quantcast

Running CHTs Too Cool

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

Daleandee

Well-Known Member
Joined
Sep 11, 2015
Messages
1,154
Location
SC
I thought there was a thread on this but a search found nothing of substance. With the collective wisdom on this forum I thought I might get a better understanding of the pro and cons of cylinder head temps that are quite cool.

Yes I'm referring to mine on the Corvair. Mike Busch claims that running cylinders too cool can/will cause valve sticking but I haven't had any issue. His article is here:


My CHT will be in the 255ºF range in summer and 225ºF in winter and sometimes on a descent in winter will dip to or slightly below 200ºF on a really cold day. After eight years and a bit over 230 hours the engine runs fantastic, burns no oil, and has shown no issue related to low CHTs.

I read somewhere (sometime ago and can't find it now) that Lycoming gave 150ºF as the minimal temperature at which a cylinder is ready to go to work. I've seen others that use 250ºF as a minimum. Seeing as how so many constantly fight high temps I find myself very blessed to have such a "problem." I asked a really knowledgeable Corvair airplane engine builder about this and he asked me what problem I was trying to solve. The Corvair engine in this particular air frame is famous for running exceptionally cool.

I'm certain that the answer to the question depends on the engine and the installation. I know that liquid cooled engines tend to run lower temps than air-cooled engines. So is there a minimum temp? What did you base your number on?

I seriously appreciate any input on this ...
 

proppastie

Well-Known Member
Log Member
Joined
Feb 19, 2012
Messages
4,714
Location
NJ
As to Mike Bush statement, my IO360 valves hang up slightly and I get rough engine on takeoff full power unless everything is in the green..I now do a full power check after the mag check to see if it is warm enough....sorry I do not know what the temperature range is but maybe the engine operators manual has it for aircraft engines.
 

N804RV

Well-Known Member
HBA Supporter
Joined
Jun 9, 2013
Messages
264
Location
Mount Vernon, WA
That seems REALLY cool. Where is your thermocouple attached?

Is this it? Do you have more than 1?



Here's the article I got the above pic from. Pretty interesting read on the subject. :)

 

Marc W

Well-Known Member
Joined
Mar 31, 2017
Messages
534
Location
Colorado
I have been wondering about this too. My VW runs about the same temps. It will get to about 310 F. in a climb and settles at about 270 or lower in cruise on a warm day. I have the sender under a center headbolt as recommended by Steve Bennett.
 

Daleandee

Well-Known Member
Joined
Sep 11, 2015
Messages
1,154
Location
SC
Where is your thermocouple attached?
I have my senders under the plugs and monitor the center cylinders. I've read a lot of the information on Fly Corvair and my engine builder was Dan Weseman (Sport Performance Aviation). Both Dan and Chris Smith (who built a clone of Dan's airplane also) gave similar numbers for cooling of the Corvair engine in this particular air frame. It hasn't been a problem so far.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
13,677
Location
Memphis, TN
Too cool and the parts are not getting to their desired stabilized expansion. It depends on what metal you are talking about. Exhaust caves because they are hot will expand quickly. The guide can be cold and the clearance tighten up. Rings like warm cylinder walls. The walls become compliant instead of the rings ridding in like cold. Piston to to cylinder gets the piston rattling because cold piston and warm walls have clearance open.

I doubt it’s too cold, but that’s the lucky side to be on. Local guy rebuilt his airplane but had an engine done somewhere else. Ran hot hot hot. He kept flying it hoping it would break in. Ends up in a tree because the engine gave out Everyone he asked said stop flying until you can get the temps down.

I would get more probes or at least another single probe and move it to each cylinder and map out all the temps.
 

Daleandee

Well-Known Member
Joined
Sep 11, 2015
Messages
1,154
Location
SC
Thanks for that ... I believe that's where I found the 150ºF before. At one tme I found some discussion about cylinder "choke" and that the 150ºF number was where the heat had effectively made the cylinder straight and expansion complete. Corvairs don't have any cylinder "choke" so that isn't part of the equation.
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
7,151
Location
Saline Michigan
I suspect that the temp anyone gets is a function of where the sensor is placed. Best place would be the hottest place in the head. Maybe someone knows where that is, but I do not. Since the OP indicates a good history at these temps, it sounds like a good temp for that location and level of cooling. Onward...
 

N804RV

Well-Known Member
HBA Supporter
Joined
Jun 9, 2013
Messages
264
Location
Mount Vernon, WA
I have my senders under the plugs and monitor the center cylinders. I've read a lot of the information on Fly Corvair and my engine builder was Dan Weseman (Sport Performance Aviation). Both Dan and Chris Smith (who built a clone of Dan's airplane also) gave similar numbers for cooling of the Corvair engine in this particular air frame. It hasn't been a problem so far.
Dan Weseman is the guy quoted in the article I linked to above. The author of that article discusses locating the CHT thermocouples. And it also references another story about a Zenith 750 with a Corvair engine that gets CHTs similar to yours.
 

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
8,406
Location
USA.
I have been wondering about this too. My VW runs about the same temps. It will get to about 310 F. in a climb and settles at about 270 or lower in cruise on a warm day. I have the sender under a center headbolt as recommended by Steve Bennett.
Highest CHT's in a long climb at WOT on a 100 deg day is 295. Normal cruise of 2650/2700 rpm the CHT is at 265/270. Cruise anything below 2500 rpm ( to stay back with my neighbore Piet) and the engine runs to cold. If the OAT's is under 70 , I unhook the air to the oil cooler. The hot oil box around the intake at the base of the carb is still cooling the oil 20 degrees.
Oil temps are always 100 degs above OAT.
 

Marc W

Well-Known Member
Joined
Mar 31, 2017
Messages
534
Location
Colorado
Your CHT's are similar to mine then. My oil temps run higher. I am not getting enough airflow through my oil cooler. They get to 210 in a long climb and settle at 180-190 on a warm day.

Mine also runs cold in cool weather. I keep thinking about adding cowl flaps for winter. I tape over the cooler inlet but it still runs cold with CHT's well below 200.
 

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
8,406
Location
USA.
Your CHT's are similar to mine then. My oil temps run higher. I am not getting enough airflow through my oil cooler. They get to 210 in a long climb and settle at 180-190 on a warm day.

Yes, very close.
Mine also runs cold in cool weather. I keep thinking about adding cowl flaps for winter. I tape over the cooler inlet but it still runs cold with CHT's well below 200.
I always thought about a sliding plate at the intake of the oil cooler box and use a choke cable to close the airflow on a cooler day.
 
Top