Urghhh. When autocraft put my heads on, they switched the sides. Now my temp sensor #2 is on cylinder #2 instead of #3. My memory is that temp sensor is a reverse relationship ( temp increases=resistance decreases). IS this significant? My thinking is that Cylinder #2 runs cooler less than #3. Thus, sensor at cylinder #2 would read more resistance than would be reflected by Cylinder #3. Thus, how do I compensate or will I need to switch the heads again?
Thanks,
L.McC

You are correct about the relationship. I suppose the wires might reach and it could be run that way. Not an optimum set-up, but it might work okay. Personally, I'd have the shop do the swap.

Well, this helps answer a question I had. I am new to 914 ownership and I have been learning about D-Jet. It looked to me that my CHT was on the #2 cylinder and I thought that at some point the heads must have been switched inadvertently after a head rebuild, but I did not know if they could be swapped side to side. Appartently they can.

I don't think the stock wiring reaches, because mine has a wire that was added and plugs into the existing connectors.

Since the CHT acts more like a switch, I don't think it will affect much. Once the cylinder reaches the right temperature, this mixture will be leaned out. If anything, you would run a little richer with the CHT on a cooler cylinder (which #2 is).

Since the shop screwed up, if I were you I would make them swap the heads anyway. I am in a different boat and probably won't bother until the heads need work.


Andrew

is this true any more ?

in VW antiquity, it was true of the upright engines because #3 sat downstream of the upright oil cooler.

the airflow in our flat engines isn't anything like that any more.

can someone who's REALLY instrumented these engines (Jake?) verify that in fact cylinder location makes a big difference on CHT ?

I'm sure Jake will speak for himself, but in the meantime, I have seen in more than one of his posts that #3 is hottest. I'm sure he'll pipe in and say by how much.

Andrew

That is what I remember with Type I is that Cylinder #3 ran hot. In the TIV, Cylinder #3 gets cooled air only after passing over the top of cyinder #4. I do not know if the heating of the air over cylinder #4 is significantly hotter.

Andrew, my understanding of temp sensor #2 is based on the principle that an excessively lean cylinder generates increased heat, measured by temp sensor #2 which results in the ECU enriching the mixture. If temp sensor #2 reads cooler, it would lean the mixture.

L. McC

Yes, I've moved a spark-plug CHT sensor around between cylinders (same day, same ambient temps, same loads applied), and cylinder 2 runs considerably (75dF) cooler than cylinder 3. According to Jake, the back cylinders run hotter than the front, and the right cylinders hotter than the left. Cylinder 2 is actually the coolest running cylinder, getting lots of cooling air right off the fan, with no oil cooler to pre-heat it.

The stock CHT is NOT just a switch. The warm-up enrichment is not just on or off. There is a threshold above which there is no warm-up enrichment, and it's likely that this threshold is below the normal operating temp of even the left side head. One can measure the CHT resistance when fully warmed up and see. I believe Brad Anders' site has data on where the warm-up threshold ends.

My understanding of the function of Temp sensor #2 is that you can place an in-line variable resistor (poteniometer) to increase the resistance and thus enrich the mixture. It functions throughout the temp range, including the higher temp.
Can someone confirm my understanding of the thermocoupler? That resistance increases as temp increases?

L. McC

Speaking of the stock CHT, if the temperature is low, the resistance is high and the ECU enrichens the mixture since in a cold engine the fuel does not stay atomized as well.

When the temperature goes higher, the resistance goes lower and the mixture gets leaned out.

The enrichening/leaning is to compensate during engine warmup, not to compenste for an overheating head.


You mention a themocouple. The CHT is not a thermocouple. A thermocouple generates a very small voltage based on temperature. The CHT is a thermistor, meaning its resistance varies with temperature. Specifically, it is a negative coefficient thermistor, meaning as temp goes up, resistance goes down.

I stand corrected on the CHT not being a switch. I was trying to say there was a resistance threshold that the ECU operated around. Lapuwali stated it better (correctly).


Andrew

I'm certain this has been discussed before, but...

The stock CHT is not a thermocouple, it's a thermistor. Resistance changes with temperature (goes down as it gets hotter). The VDO/Westach spark plug type CHT sender is a thermocouple, which generates a voltage (hotter = higher voltage). Think of a thermocouple as a weak battery that works better as it warms up. The stock CHT has a lower temperature range than the VDO/Westach CHT, which is OK since it's installed in a place on the head that doesn't get nearly as hot as the spark plug sender does.

The stock CHT is ONLY used for warm-up enrichment. Above a set temp, it provides no more enrichment. You can fool the ECU into thinking the engine is still cold by adding some resistance to the CHT output, and the ECU will stay in enrichment mode. This is something of a blunt instrument, but it can work to richen up the mixture in a crude fashion. Of course, you can also just up the fuel pressure and do the same thing. Enriching the mixture this way isn't all that useful unless you're doing other things to require a richer mixture (other engine changes). Since those other engine changes typically cost at least hundreds of dollars, cheapening out on the fuel control is not a wise move, esp. when a MegaSquirt ECU can be made and applied to the D-Jet hardware for under $200, and gives you complete, precision fuel control rather thanthe sledgehammer approach of dinking with the CHT.

With the CHT on the "wrong" head, it's possible that you'll be getting a richer mixture all the time because the CHT is never getting as hot as it does on the other head. This is what I meant earlier about checking Brad's site and measuring to see if this is so. I doubt it is, but it's certainly possible.

Thanks,
I went back to Brad's site to refresh my failing memory.
Temp Sensor # 1 and 2 are negative coefficient THERMISITORS and not thrermocouplers. As temp rises, resistance decreases. Therefore, adding resistance to Temp sensor #2 increases enrichment. I can't find information on Brad's site that there is a temp range in which Temp Sensor #2 alters the mixture. Does anyone have that knowledge?
L. McC