That graph shows detonation on the affected cylinder. I have done this testing, and indeed, that’s what the temps do. If you catch it in time, there will be no damage, but left to continue for a minute or so, you’ll have a hole melted through the piston (which I have also seen) and then the engine is in sorry shape.
To reduce the risk: Faster engine speeds, then application of power – detonation takes time to occur. If RPM is high, time per stroke is short, and detonation is much less likely to develop. Keep temperatures low with an effective oil cooler, good cylinder cooling and a more rich mixture.
If you suspect this, power as low as you can manage, for as long as you can. If it starts to clatter, shut it down to save what you can, you’ve put a hole in a piston. Once you land, have the mechanic remove all the top plugs, and look at the piston tops. If they are black and oily, no harm done. If they are clean and shiny aluminum, you’re still safe, but it was close. If they look like the surface of the moon, or have a hole, the engine has to come apart.
any CHT goes above 500F (too high really but OK for a very short time)
With the newer ones (730/ 830), you can set your own alarms. As long as the instrument is not primary. If it is primary, you are stuck with factory setup (like 500 °F for CHT).
You need to know the aircraft systems
Exactly. It’s a nice tool to have. But it’s rather useless if you can’t make any use of it and have no idea what to do when you encounter something like this.
I reckon you can change them anyway. This is from the EDM700 IM
Yes, you can change the JPI alarms as long as the instrument isn’t your primary engine display, in which case it has to be set to the POH limits.
Mine is set to alarm at 400F CHT.
as long as the instrument isn’t your primary engine display, in which case it has to be set to the POH limits.
If my EDM was my primary instrument, I would still not set it to the 500°F from the POH but the 380°F I have it now at. If the POH has incorrect and unsafe information, I do not care about what the regulations say. Neither would my maintenance shop…
What I was getting at is that you can change it as you wish. It’s just that if you do, you are no longer compliant with the STC 
My CHTs were hard to keep below 420F departing from Turkey in +35C… Now, with the reworked baffle seals (two different materials – written up elsewhere here) I rarely see 360F. So, yeah, 400F is probably a good value to aim for – on a TB20 with non-shagged baffle seals.
I reckon you can change them anyway.
I think 700 is not primary. 711 is. And 730/ 830 are not primary either, that’s why you can change the alarms. 900/ 930 are primary and I don’t think you can change the factory alarms for primary “fields”. I’d have to check the manuals. Of course, if they are selling one unit that can be used as both primary and secondary and ask no questions about it, you can. I mean, it wouldn’t the first botched installation in GA. And there might be a way around it.
I do not care about what the regulations say
Frankly, If you set it up on the cautious side, I don’t see how someone could nail you for it. The issue might be that units STC’d to be primary might not have that option.
Can you climb at Vy at ISA+10 and keep your CHT below 400°F? I’ve done baffle work, sealed every gap with RTV etc but I am still not completely satisfied. It’s really not easy to get it completely tight and unfortunately there are very few people in Europe that how this stuff for a given airframe.
On the TB20, the EDM700 is on the factory TC as primary, and there are no other CHT/EGT instruments.
On the TB21, the EDM700 is not on the TC as primary so the analog CHT/EGT instrument has to be retained.
Of course, if they are selling one unit that can be used as both primary and secondary and ask no questions about it, you can
That may be what has happened. What a shame
Can you climb at Vy at ISA+10 and keep your CHT below 400°F?
Of course not for more than a minute or two 
but almost no GA SEP with a reasonably large engine can.
Climbing at Vy or Vx is bad anyway because you can’t see any traffic anywhere near in front of you, yet the place where you are most likely to meet it is near an aerodrome…
Well, since you say 700 was a factory installation, Socata might not have cared about it being offered as only secondary. And they got away with it and got it certified. But I don’t think JPI has STC’d it as primary. I’m assuming 711 is basically the same unit with different firmware (and maybe higher price tag). But why is TB21 different? Which primary instrument they needed that 700 wouldn’t offer?
