Alternative location for Cylinder Head Temp Sender, Considering one of the #3 Cylinder exhaust studs Options

[bobhasissues]

I have some new 914 2.0 repro heads from DRD. The new heads use a 12mm spark plug rather than the stock 14mm. I want to use a ring terminal type Cylinder Head Temperature Sender under the #3 spark plug just as I did on my stock heads. The problem is the outside diameter of the ring terminal of my 12mm CHT sender is larger than the bore of the spark plug hole so I can't put the sender under the #3 plug as designed. As many of you probably do, I had the 14mm ring terminal sender on my stock heads and that took a lot of manipulating of the terminal stem to get it to fit. This is way worse, the OD of the 12mm ring will not even fit into the plug bore.

I asked Darren at DRD what to do and he suggested to use the fuel injection CHT location out on a fin by the intake manifold. I don't like that idea because it seems too far away from the chambers to pick up any major temp fluctuations.

I'm considering changing to an 8mm ring terminal on the sender and double nutting it onto one of the #3 exhaust studs.

Anyone have a better location?

[Frank S]

I run a thermocouple under spark plug #3 and a BMW CHT sensor in the stock CHT sensor location with MS.
I'm using Lens heads with the ceramic inside and the black outsidecoating.

Both Sensors report the same temperature with a little advantage in reaction time on the under the spark plug sender.
However temps are always within 2% of eachothers reading, which is neglectable.

Will take the car for a spin today and can post a pic later...

[GregAmy]

I run a thermocouple under spark plug #3 and a BMW CHT sensor in the stock CHT sensor location with MS.

Would love to see that correlation! It would thus be an excellent alternative (and easier to install) location for monitoring engine safety. I've got the correlation data for the under-the-tin-screw versus spark plug and it is decisively not a good spot for The Raby Recommendations.

But if your correlation between CHT boss and spark plug proves strong, then let me offer a secondary reason why this is not a good place for using that boss for Mega/Microsquirt (or any EFI) warm-up: because it's historically shown to not be a good place for measuring D-Jet warm-up.

D-Jet has a problem with warm ups. The prevailing thought is that the CHT sensor warms up quickly and causes the the D-Jet to lean out too fast, well before the engine itself is ready for it. As a result, we have this "hole" where the engine fires up nice and rich, but soon leans out because the CHT is telling it to; idle and driveability suffer. VW/Porsche tried to fix it with a resistor and a retune, and even today we battle it by putting in steel spacers to reduce the rate of thermal rise.

The DubShop sensor in an engine tin screw near the intake manifold solves this. This location warms up much more gradually and slowly, and offers a much wider band of time to fine-tune the engine warm-up. I can start the engine foot-off with it dead cold with ambient temps in the 30s (I don't use drive the car colder than that) and it'll immediately settle into a solid idle, and core that idle all the way to full engine warmup (I use ignition timing to manage it).

My "full warm" point is 160F, and it takes about 3-5 minutes of light local driving to get there, depending on ambient. That's plenty of time to get the long block warmed up, the oil flowing nicely, and about the time I get to the interstate on-ramp. And given this wide range of time/temps to get there, I have a lot of flexibility in tuning to improve driveability.

For reference, on a typical highway drive the engine tin screw "CLT" (how it's logged in Microsquirt) is about 75-100F lower than the spark plug CHT. Whereas the CHT at cruise is ~330, the CLT is ~240.

I've read where some people use oil temps for warm-up enrichment management, but I suspect that this rise is a little TOO slow. I do want some compromise where I can reasonably inferentially measure the combustion chamber temps and still have a wider band of time than the spark plug. Plus, where would you measure it? If you kept your stock oil temp gauge you'd need to tap in somewhere to get oil sump temps.

So for all these reasons, I prefer the engine tin screw for EFI input, and the spark plug (or maybe the CHT boss?) for true CHT monitoring.

GA