No worries Bob.
Not sure where your oil temp is picked off from but suffice to say that sudden jump from an oil temp of 77 to 120 at T.O. is likely an indication that your vernatherm is doing its thing and recirculating oil and bypassing the cooler. I would not think the pick off is at the coldest point it is usually at the end point of its journey through the working internals.
For those wondering what this strange worded device the vernatherm is, it essentially a thermostat for your oil circulation system that regulates the flow through the oil cooler. I can't give you much more than that as I am not a mechanic.
Bob you mentioned no real airflow through the engine when the cowl flaps are closed on the ground. I disagree there is still considerable airflow just not enough for proper cooling in most normal conditions on the ground. My personal observation on some types, Shrike is one I recall is that with the cowl flap open on a frosty morning the oil temp can take an age to get to the minimum for take off (75f from memory).
My personal technique on the Shrike was close them up tight for the first 1 or 2 minutes then open to half after that and full open 1 or 2 minutes later. But this is for sub zero OAT starts. All other times open before start, that way I can hear their little electric motors running and get a feel on how they're doing out there. They have a tough life and get little to no attention, cowls were always parked closed after shutdown.
But closed can lead to issues during the warm up if not done with some thought and understanding.
Back in the late 90's early 2000's John Deakin (as I recall) equipped his Bonanza's cylinder barrels with some temp probes at various locations around the circumference. What he discovered, and this was only on his Bonza was that there was poor cooling airflow around the cylinders and a wide range of temps were recorded.
He also stated and again on his Bonza that the under cowl footage showed the cotton tufting flew best when run ups were done bum to wind. I would like to see a lot more on that before I bought in. I would strongly caution against this unless you own shares in a prop company or hate your boss and can blame another pilot for the fact the blade leading edges look like steak knives.
Over to John Schwanner's book The Sacramento Skyranch Engineering Manual which from memory has some photos of cylinder walls with 4 equally spaced vertical scoring marks attributed to strong differential temperatures around the cylinder barrel. The marks are a result of piston to cylinder contact due to warpage attributed to the cylinder base nut configuration under unneven temps. Note no present barrel temp instrumentation for this just one probe per head if your lucky. Once the cylinder/piston reaches operating temps the clearance tolerance fall into place, particularly as the top of the cylinder is choked (smaller). However when still cool the tolerances are tighter.
Have a look at your inter-cylinder baffling on your particular aircraft, on the Shrikes I flew the inlet opening of the wrap around cylinder baffle might be 30mm and the outlet 5 to 10mm. Almost nothing so a strong pressure gradient is required to get anything to pass through here and in light of the only airflow being from a prop creating the high and low pressure and the inherent gaps and design flaws in the baffling system we really are up against. Having said that the first 30 seconds you could near still hold your hand on the barrels so in icy conditions you are pretty safe for a few minutes, in fact the sooner you can get to operating temperature (with in reason) the better and the piston gets a head start on expanding as its alloy and at the coal face, the barrels lag being steel and not at the blast face.
The key to it is to not exacerbate the already crudely designed cooling system with poor cooling air distribution and pressure gradient unless there is good reason. Frosty mornings are the only good reason and it doesn't stay frosty for long under there.
I think it was John Scwanners book that stated you need 6" of water column pressure diferential to adequately cool a 300hp aircooled engine. It takes 160kph of airflow in the front and most importantly around the outlet lip to create that differential. A prop won't come close
As they say warm up fast cool down quick they do.
The above is all from my memory and posted on a forum for aviation nutters by another nutter who likewise likes the sound of our own opinions, seek clarification before adopting.
Last edited by Obidiah; 2nd Apr 2015 at 06:42.