So about those EGT and boost sensors. I'll make it a post as it may benefit some, but as am lazy and want some twitter likes first I will post stuff here. So then, what's the big deal about this EGT and what is she on about again?

(long thread that will last for days)

So the plan overall is to fit 6 gauges in the tt: oil temp and press, afr, egt, boost and voltmeter. For now I only have temporary space for 3 as I need to design a pod for 6 (a separate story), and cannot fit oil sensors atm anyway and am missing aft gauge.

So 3 will have to stick for a moment, and I'm testing the concept anyway: EGT, boost and voltmeter. https://twitter.com/notameadow/status/1251558884482957315?s=20

Voltmeter just works, duh, boost gauge has it's own sensor so it's a matter of fitting it and wiring it back, EGT, tho, that will be a tricky one!

What the hell is it anyway, I hear you ask? Well.

Exhaust gas temp sensor, it's placed in the exhaust close to the engine and measures you know what.

Why? In normal engines it is mostly irrelevant for you to know the value (but engines do monitor it). Exhaust temp is a measure of what's happening inside the engine -

- one of the factors, to be precise. What it gauges is the richness of air/fuel mixture which is crucial when you mess with an engine (and sometimes even if not, but then the stock ECU takes care of it, or should). Why it matters?
Hell, there are thick books on this but tl;dr:

In petrol engines the afr ideal value is 14.7:1, 14.7 air to 1 fuel. It's never that but that's the range. If the mixture is too rich - meaning too much fuel to air - the engine will do all kinds of stupid things, one of them is killing your o2 sensor :) but generally will live.

HOWEVER if the mixture is too lean, meaning too air - that is messed up, as contrary to popular belief, this will make the engine run hotter because fuel plays part in cooling the piston (the rest is not as fragile).

And, I kid you not, it may melt the piston.

There's a lot of physics, chemistry and math happening in the piston that makes it happen but the result is for example this, in extreme cases

(in diesels it's much worse as both too lean and too rich will make the temp jump, but I hate diesels anyway and wish them all the worst AND OMG I HATE THE SMELL SO MUCH )

So anyway, in a normal engine this may happen when an injector fails, for example, not totally but slightly. It most cases other sensors, esp. in modern engines will catch this (o2, EGT, but also knock sensors and other magic things), unless you're SOL which does happen.

In tuned engines it's a different ball game because we tune them by manipulating maps of air to fuel ratio vs speed, temp and other things. We also change fuel pressure, injectors, compression, everything. So there's plenty ops to fuck something up.

(this should already be a blog post but I cannot be bothered now, will happen!)

So then, I need an EGT. Actually I need 4, as this makes more sense to measure them individually, but that would require a custom manifold (which I will have made, but not now) https://www.ebay.com/itm/K04-023-Turbocharger-Exhaust-Manifold-for-Audi-S3-Audi-TT-1-8L-225-Seat-Leon-BAM/193014359326?autorefresh=true

My problem was that I'd have no way to even fit one sensor without a workshop which I don't have (but it's a simple enough job at any rate). However binging some stuff I tripped over someone mentioning spare EGT sensor for my engine... and, indeed! https://twitter.com/notameadow/status/1253406089732857856?s=20

But Meadow, how will you attach your gauge to the sensor?

That's the interesting bit! Now some boring things about sensors.

Basically there are 2 main types of temp sensors: resistive and thermocouple. Ackshually here's prolly like 11 but do your homework, it's out of scope :)

Resistive change resistance with temp, rising or falling. They're more expensive and more accurate but do not like high temps much. You measure temp as you'd measure a resistor (plus some magic with compensating the leads lengths, this makes it 3 or 4 wire installations).

The other type is a thermocouple , this is a pair of two metals welded together and by some magic exposing them to high temps makes them generate voltage in the range of tens of milivolts. There are several (10? or something) types of them, most common is K, chromel-alumel.

Who cares?

Well, my idea is that I will tap into the original EGT sensor in a way that will not disturb the original measurement and will keep my gauge in sync. And that will depend on what type of sensor is in the engine (thermocouple most likely),

what type of sensor my gauge expects and all that. So I will explore my gauge firsts (it came with a sensor) but I suppose I need to post this thread before I close the tab accidentally.