SMC Launch control

[Kelly2264]

In the document for SMC launch control it specifies a 100 ohm resistor to achieve 127 degrees celcius. I'm using a cylinder head temp sensor for my installation and was wondering if anyone knows what resistance value I would need to achieve 200 degrees celcius.

[Wroom]

First. If the coolant temp is 200 degrees celsius the SMC will be in limp home mode because that is the highest limp home coolant temp that can be set in the SMC. Default setting is 150 degrees in the SMC.

Looking at the Bosch sensors datasheet, the highest temperature charted is 160 degrees celsius, where its resistance is 46.88 Ohms. And the sensors guaranteed measurement range is -30 to +130 degrees celsius.

So i guess resistance at any higher temperature than 160 celsius must be estimated from the sensors NTC curve, or practically measured with a potentiometer and an ohmmeter.

Although the resistance will be somewhere around 23.44 Ohms. ;)

Simplistic extrapolation of the NTC curve follows:

Ohms from chart: 71.20@140C, 57.41@150C, 46.88@160C.

57.41/71.20=0.806
46.88/57.41=0.816
...

0.826*46.88=38.72 Ohms at 170C.
0.836*38.72=32.37 Ohms at 180C.
0.846*32.37=27.38 Ohms at 190C.
0.856*27.38=23.44 Ohms at 200C.

:D

[Kelly2264]

Thank you Johan!

But, now I'm curious. I'm trying to think this through a little further. My engine is aircooled, so the "new" default should be 175C, but I've changed my "Limp Home Coolant" table to go to 200C. Let's say that my CHT sensor sees 200C, briefly, near the end of the quarter mile run. My "Limp Home Coolant" table has no values in it right now, will the engine still run normally or what will the detriment be as far as the SMC is concerned? BTW, the CHT I'm using is mounted in the cylinder head behind the #3 cylinder combustion chamber and specs out the same as the Autronic coolant sensor.

[MRMRacing]

I did some testing yesterday in the tester with a 5 or 10 K pot, and I could newer get it to read more than 195 degrees so with limp home at 200 it never got into limp home.

So It may be my resistor not getting low enough but had not time to messure the resistance

[Kelly2264]

Thank you for checking this for me Robert.

I'm off to the track later today, so I'll find out whats what and if it's going to be an issue. I've temporarily remounted my standard Autronic coolant sensor as a back up.

[Kelly2264]

The CHT never went over 165C on the passes, so I think I'll be fine to setup the two step/launch control around 195C~198C. I'll try a 25 ohm 1% resistor and see how close I get.

Thanks again to Robert and Johan! :D