Having been military jet trained, I am fairly new to pistons. But I do understand limits.
As I understand, the CHT limit is 500 with the recommendation to avoid continuous operation above 435 - I'll read that as max continuous. So what is it with this 400 thing?
Sure, I understand keep everything cool if possible. I am still running things in having only about 35 hours on the aircraft. In the climb, I hit about 415 on the hottest. Cruise is around 360. I am still adjusting things to get it lower but I really don't see why there should be an issue with numbers such as this.
Please feel free to educate me,i
Paul, there are a few things to consider.
500dF is a certification test limit, depending on who you want to believe at Lyc/TCM the recommended cruise limit is anywhere from 400-435dF. Lycoming publish the 400dF limit (which would read as 435 with a spark plug sensor and 400 with a bayonet) in one manual or bulletin and a different number in another.
Just to make matters worse, Continental publish a document for the TSIO-550-B,C & E. In that they list max cruise temps for the -B at 380dF and the -E at 420dF, yet the Max allowable is 460dF for both. Same HP engine, just a different oil pan.
We know that the aircraft OEM got sick of phone calls complaining about the red line CHT (due to baffles and poor mixture management ROP etc.) that they asked the engine OEM to help?..they did
Try getting a stroke of the pen these days to fix any of the bloopers though.
Given the metals are fundamentally the same, and we know from detonation testing that from 420dF things start getting serious especially for the non-intercooled turbo engines at or above 420, that a sensible alarm limit of 400 gives the pilot enough warning to deal with any emerging problem.
There is no big deal if you normally have an engine that climbs at 380, and on a hot day, heavy load etc you have to climb steeper and hit 400, even 405, but you should be alerted to the emerging issue.
More importantly is the far more common problem (not common as in everyone has this happen once a year common) is that a cracked ceramic leads to a preignition event. These are very damaging and do kill a cylinder in a matter of 1-3 minutes. If your alarm is set at 400dF you get that alert at the 1 minute mark or sooner. If you get it at 500dF you get that alert about the time significant damage is starting and if you fall for the ?.I wonder if it is a bad probe, and sit there watching for a minute longer
. Bad things follow.
The wise old folk who know their stuff all agree, a good installation flown properly will cruise under 380dF, often a fair bit lower. They will accept the odd excursion over 400, but they want to be doing something proactive by 410.
The strength of the alloy used in the heads varies from 95% of yield at around the 300dF range down to 80% at about 370dF and drops rapidly to 70% at 400dF and then to around 30% at the Lycoming red lines. At the 435dF mark it is around 50-55%.
It's your engine, armed with the facts, do what you will.
