I've only played with a few FI engines but was curious as to what is considered the ideal water temp for these engines. I understand that my 2012 5.0l S/C engine thermostat opens at 195*F and is fully open at 205*F. I also have a 5.3L 2016 Chevy truck that runs at 210*F while my V6 3.5L Lexus runs at 195*F . Being I have a scan gauge running all the time on the Jag I watch the WT run from 198*F to 207*F during normal driving regardless of the outside temps or traffic conditions. Then again my old Ferrari 3.0L with 4 webers ran at 180*F. So with the ECM control aimed at finding the ideal F/A ratio what would be the normal temp range and does a few degrees either way make much of a difference.

 

From the X150 Workshop Manual, Section 303-03A:

'On both naturally aspirated and supercharger vehicles, the thermostat begins to open at 88 - 90 °C (190 - 194 °F) and is
fully open at 102 °C (216 °F).'

Depending on emissions and to prevent sludge build up, each manufacturer uses a temperature range for optimal engine performance. Thus do GM and Toyota use different temperature specifications.

 

OK sorry I just checked the manual and it says starts to open at 190*F (88*C) and fully open at 205*F.(96*C). This was for the 5.0L S/C engine.

 

From X150 Workshop Manual (From VIN B32753 to VIN B44939, 5.0 litre, NA and SC), Section 303-03A, page 1161.

 

My XK temp stays between 197 and 200 degrees F at all times once warmed up, even on hot days in North Carolina.

 

Haha this is funny as page 1156 has the numbers I posted and 1161 has the numbers you posted so they must be both right.

 

Thermal efficiency and cylinder bore wear were the primary reasons dating back to my late '60's Fords. Hot engines need less timing and fuel to ignite a given fuel charge plus the resulting emissions are cleaner because it burns more thoroughly. Getting them to warm up quicker through coolant passage restriction helped get the cats working earlier to satisfy the EPA cold start pollution limits in the late '80's. The '70's were a train wreck of low compression, vacuum lines and thermal switches that wasted a lot of energy.

One interesting thing I learned from my Cummins swap was their 6bt engine is very thermally efficient in that it does not require a large radiator like the I/H 6.9 and 7.3 designs do. The more heat energy you turn into motion, the less fuel you need to burn in order to do it. The Cummins needs about one half the cooling capacity and makes much more power versus the higher displacement binder engine.