Re: Suddenly changing CHT/Oil Temps

Bob Holliston

I'm not a downdraft cooling fan but I'll bet raising and widening those inlets by 50 - 100% would make a big difference. Those oil flow tracks look awful. 

On Tue, Jul 27, 2021 at 6:50 AM Terry Schubert <jschuber@...> wrote:
Do these unusual high temps also occur at low altitude cruise?
You might look at cooling air pressure differential across the cylinders.  I don't know what engine you have but around 4"-5" of water column pressure is needed to cool a Lycoming at rated RPM.  Check back issue CSA Newsletters for delta P, cooling, expansion, etc.
Try a few drops of oil on the entry to the high pressure plenum to see what the air flow is going into or perhaps reversing flow at the cowl's air entrance.  See attached pics showing reverse flow.
Send me a few pics of your firewall aft installation and inlets.
If pressure is too low there will be inadequate cool air mass across, CH & O cooler fins and temps will increase.  A new air leak in the high pressure plenum could easily cause that drop in cooling air pressure and have an effect on all components requiring air for cooling.  
Terry Schubert
Central States Association Newsletter Editor

---------- Original Message ----------
From: "Drew Swenson via" <>
Cc: COZY Builders <cozy_builders@...>
Subject: Re: [c-a] Suddenly changing CHT/Oil Temps
Date: Sat, 24 Jul 2021 00:50:02 -0400

New sensors grounded all the way back to the Dynon?


On Jul 23, 2021, at 6:22 PM, Marc J. Zeitlin <marc.j.zeitlin@...> wrote:

I've got a customer with a Long-EZ with an O-320 H2AD engine. It's been running apparently fine (and still is) for a few hundred hours since it was installed on the plane. The current owner (not the builder) had me install a new Dynon EMS D10 engine monitoring system, to replace the ancient Rocky Mountain Instruments engine monitor that's been in there since the plane was first flown in 2014 or so. The plane has downdraft cooling with relatively standard (if not well shaped) air inlets above the strakes, and metal plenums.
The owner had always reported CHT's in the low 400's in climb and 360's - 380's in cruise, which is fine, and oil temps in the 190 - 210F range. Also fine.
We did nothing other than install a Dynon Autopilot and the Dynon EMS, and he changed the oil (not filter) with new Aeroshell 15W-50.
On the three flights that he's now attempted since the EMS install, he's seeing CHT's at 400F in cruise at 120 KIAS and 2400 RPM, 10,500 ft. (don't know OAT). The OT is running >250F (yeah, we know the redline is 245F, but I'm going on vacation for 5 weeks and he needs to get the plane home to an A&P that will have time to look at it). Obviously, these temps are both NOT what we've seen before and are too high at those power settings.
So. We did NOTHING to the engine itself, other than change the OT sender to the new Dynon single wire sender, and install the new Dynon EGT and CHT probes. After the second flight, we pulled the sender and Vernatherm and immersed them in a pot of hot oil with a candy thermometer for calibration. The Vernatherm extended fully and the OT sender read within 5F of the candy thermometer up to about 260F, so we believe that the OT's we're seeing with the new system are correct. As indicated, the engine's operation seems exactly the same to the owner as it did before the work.
My initial reaction is that the info we're seeing on the Dynon EMS is real, and that the info provided by the RMI system was not. It had homemade CHT senders and an unknown OT sender (2 wire, but unclear whether it was the RMI recommended one or another homemade sender). If this is true, then the engine's been running VERY hot for 7 years or so, both CHT's and OT's. Given the downdraft cooling (harder to make work correctly then updraft, except for a very few) and the homemade sensors and uncalibrated (confirmed by the builder) micromonitor, it's not impossible to believe that this is the case. However...
What's an alternative possibility here? Somehow, both CHT's and OT's happened to change substantially just at the same time as the engine monitor was replaced? Something else?
Looking for ideas on possible causes... Obviously, the owner would not like to continue to cook his engine if he can help it, and yet reconfiguring the cooling system is a LARGE job...
Marc J. Zeitlin                      marc_zeitlin@...
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