Reading all the posts it appears to me that the entire question of optimum coolant temperature is undocumented by dyno testing etc. We know what Jaguar likes, and I suggest that Jaguar's decisions are determined by several factors besides optimum power, like emissions.
I did a little research. Checked Smokey Yunick's writings. He was pursuing the "abdiatic (sic)" motor with extremely high temperature - far beyond what is doable in our Jaguars.
My next source is A. Graham Bell, a Brit who now lives in Australia. Bell is a consummate tuner of both British and USA motors; and has written several books, all of which are very readable. In his latest "Four Stroke Performance Tuning" at page 575 he writes:

"A high-temperature thermostat, which maintains the water temperature at 88-90C is usually fitted to production cars, but that is not the temperature for best power. The 88C thermostat is fine if you want good heater efficiency in winter, and the higher temperature partly compensates for the stock manifold and carburetor being unable to vaporize the fuel properly, but apart from that there is no reason for its use.

Some feel that high engine coolant temperature in the range of 90-100C is good for power. The theory is that anything lower just takes away heat energy, which we should be using to push the pistons down and produce more hp. To an extent this is true at low engine speeds and small throttle opening, but generally in a full-throttle, high-rpm dyno test an engine will make more power at coolant temperatures in the 70-80C range. Only dyno testing will confirm what is ideal for a particular engine, but in my experience I rarely see an engine that makes more power running higher than 84C or less than 68C. I have seen engines gain 2-3% more hp when the temperature was dropped from 90C to 70C; conversely I have seen gains when the temperature was pushed up from 65C to 80C."

 

Did Bell do any testing with modern computer controlled EFI engines or just with carbs as suggested in his book.

Of course, none of this discussion has anything to do with the OP's goal of eliminating detonation.

 

True enough, nor to do with my OT questions LOL

If I had 2 cars I'd probably try it

 

I found my own thread now 3 years later
I am now going back to the idea of lower running thermostat temps for performance reasons as I am pushing more than a bit more power vs stock and still relying on a stock cooling system. Well mostly stock, my fan kick on temperature in the tune is set to turn on at lower temps, I added a higher flow coolant pump for the supercharger and I think lower thermostat opening point would help a bit with less heat soak at the track.
So back to the original question from 3 years ago, has anyone found a matching thermostat with lower opening temps?

 

so while you can do this, you need to talk to the tuner and see if they can turn off the P0128 code you will get for the temp change. It will be set and I have had to have these and other codes written out of the ecu so it wont turn it. on. This leads to a constant fault light and if you get a state insp. where you live it wont pass with this fault code on or stored in ecu. I have deleted p0128 for a 10* lower temp, p0300 for misfire at idle(cams with a slight lope), and p0430, p0420. Don't have cats

 

I just bought a 77c tstat to try and get cooler intake charge theu the intercooler and heat exchangers. Hope it isnt too low to throw a code.

This is an old racers trick to get more power and it really helps to be able to turn the fans on at a lower temp.

High temps arent for performance. They are for emissions to try and kill combustion byproducts. With all the sensors on the engine, the ecm will adjust the mixture and timing based on temp readings so hopefully it will give more power in the end. Heat soak is a common power power robber with supercharged motors

 

The F1 guys have proved this to be untrue a couple years ago...

Ford found out the same back in the 80's when they developed the EEC stuff. It's all about burn efficiency. Compare a Cummins 6bt to a powerstroke. The Cummins is much more thermally efficient and makes more power. I'm running a 302 radiator in front of mine and it's never overheated. The PS needs about 3x the fin space to do the same thing.

 

Lucky for me I dont have an F1 then

 

Check my signature. I do my own tuning on these cars and am able to delete cat codes, decrease fan kick on temps, so if there is a cel light for lower thermostat , im sure il find that one as well.

 

unfortunately this this thermostat does not fit 07-09 4.2 xkr's

 

Yes,. Bell did dyno low -temp thermostats on F.I. injected motors. Check with the Corvette folks, they have been using 160Fs for years with the direct injected LS motors.

 

Well theres “lower temp” and then theres “low temp” tstats.

The one I chose was 13 degrees lower which isnt to much. Its not going from a 195 degree to a 160 degree. Its only going down to 183 or so.

But yes. Too cold and things are not efficient and parts havent expanded to their optimum size for wear purposes and that robs hp. Plus theres the atomization of fuel issue. Too hot and there heat soak and detonation and timing gets pulled.

I will try my lower temp tstat and see where I am

 

Will you be able to report actual coolant temps once you have changed to a lower temp thermostat?

I am very interested as Leeper's explanation is 100% correct.

A lower opening thermostat simply sets a floor, not a ceiling on engine temperatures. Same category as "tuning chips".

Look here :http://www.tuneruniversity.com/blog/...the-advantage/

You can prove us, and the article above,both wrong if your engine coolant stays significantly below 195 when driving on a hot day or better still when stuck in traffic.

 

Heres how I see it. Without being able to adjust the fans to come on earlier, temps will rise in traffic up to the point the fan turns on. But I am not looking for lower temps necessarily at a traffic light (sure id like that but cant have it without adjust fans). I am doing this for some performance gain. Like when im driving and I FLOOR IT.

When driving the airflow should bring the temps down. Lower temps thru the intercooler will help performance. So yes, I will seee normal temps at a traffic light but once moving I should be cooler and thats my goal until I figure out how to adjust the fans turn on temp

 

What do you use to adjust your fan turn on temp?

 

I do not know what engine you have however if it like mine there is negligible coolant flow from the charge cooler side to the main engine coolant side.

They are essentially two separate cooling systems joined by a balancing pipe mainly used when you refill coolant. The lower opening thermostat will not gain you anything.

That is why I have an additional charge cooler radiator with two 6" fans that I set to come on at the temperature I dictate as well as an upgraded charge cooler pump.
A far bigger issue with the 4.2 SC engine is the fact that oil vapor from the sump thru the high load breather will cause clogging of the charge coolers after about 50k miles. Not so on the 5.0 as that engine has far better oil management.

 

I have full access to ecu controls. Fan at whatever temps I require ,Ecu tune for power and so on. Have the highest dyno numbers I know of for my mods due to my tune. Tune is available for purchase

 

Have you loged your intake temperature to see what aditional charge cooler helped with?
Im pushing nearly 100 extra hp out of my 4.2, no issues with clogging, but I guess i dont know what it looks like inside

 

I have not done that however I do not have a before comparison anyway.

I do know that my charge coolers were heavily clogged at 99,000 miles when I took them off when I upgraded my SC.

 

What are your temperatures now during normal driving and under hard acceleration? I am researching all options. So far i do have upgraded Bosch 001 pump