Type IV Cylinder Head Temperatures

The following temps are based on the readings I have seen both on my dyno, in my test car as well as every car I own as I do 92 miles round trip daily, all aircooled. I have been able to see the effects of different temps on engines, MPG and come up with basic rules. These readings and parameters are ONLY accurate when your sender is the same place as mine, under the spark plug.

Less than 300F
Your engine is well configured, properly tuned and you probably are not driving it hard enough

300-340F
This is what I consider average for the Performance TIV engine that is well tuned and properly configured. With these temps, you'll be getting the best MPG and efficiency from the engine, it will live a long powerful life and you can do valve adjustments at the normal 6K mile interval, since heat isn't sinking your seats and stretching your valves.

350-375F
These temps are standard for a STOCK engine with a stock cam at cruise speed, if it is properly tuned. Generally, if you are cruising at these speeds, as soon as you hit a hill you'll see a spike to a higher level, approaching 400F. If you experience temps of 375 in 5th gear, you may be doing the engine an injustice by keeping the tranny in 5th, hitting a hill with a shift to 4th will keep load off the engine and that reduces engine heat and increases cooling fan speed- 4th gear is a friend to stock engines.

If you see temps in the 375 neighborhood constantly you'll be having to reduce the intervals between valve adjustments to maybe 3K miles due to the extreme heat cycling the engine and valve train parts are seeing. Engines that cruise at 375 are generally out of tune or may be misconfigured creating more heat.

375-400F
No engine should "cruise" at these temps, not for any reason. Cruising at these temps is a guarantee that under load up hills that you'll be over 400F and thats not acceptable.

400-420F
At these temps the rules change, 400 is the magic number that stretches valve train parts and really where cracking issues begin. 400F is OK for a few seconds if climbing to the top of a hill, but don't allow the engine to stay there long. In a perfect world as the needle starts to climb toward 400 the smart driver will grab a lower gear and remove load to cool the engine some, the extra blast from the fan also helps.

420F+
Well your engine has been damaged. Go home, do a valve adjustment and see how many valves have "tightened up" from your baking experience. Those valves are the ones that have seen the most damage, so make a note of it and upon teardown (that probably isn't far away) see how those chambers and exhaust ports look as cracks will more than likely be notable with the naked eye.

The other big issue is with guys that get their engine really hot and then immediately pull over and leave the engine running! THIS CRACKS HEADS!!!!!!!!
If you get the heads over 400F sustained, the best bet is to slow down and use the tranny to moderate the cool down period. If you do stop on the side of the road, shut the engine off and allow it to go through it's heat cycle naturally. This will keep the heads from cracking in most cases. The dumb guys pull over leave the engine running and watch the needle drop down to 250F and drive again, those guys cool the engine to quickly and crack heads.

So, there ya go... Thats a short summary of what I have seen for temps and how they effect the engine.
Once again THESE TEMPS ARE ONLY FOR GUYS WITH THE SENDER UNDER THE SPARK PLUG!!!
If your sender is there, you are not lazy and want the best readings for accurate monitoring of the engine. If your sensor is not there, put it there, OR remove the gauge and throw it away, because the readings you are getting are worthless.