That's the principle I was referring to... allowing the head to heat up to toasty temperatures before flooding it with cold water sounds like a recipe for CHG stress, especially when the, er, "thermovalve" starts to get a bit sticky.

Still, I'm sure the Formula 1 piston rings will kick in if that happens. Remind me... How many laps does an F1 engine have to do these days?

>I'd be tempted to try it on a iron block/iron head combination but alloy heads/iron block...

You've omitted a combination there; alloy block / iron head. As far as I am aware this arrangement was unique to Skoda who transitioned from all-iron to all-alloy via that route before they ditched their own engines for VW's. I used to own a Skoda 130 Rapid, I nearly wet myself with laughter the first time I had occasion to get stuck into the engine and noticed this. Heck of a fun car to drive, dead simple to maintain and parts were free with cornflakes too.

"

Another problematic engine was introduced for 1982. The 4.1 L HT-4100 was an in-house design that mated cast-iron heads to an aluminum block. HT-4100s failed in large numbers and many were replaced under warranty. [citation needed]"

http://en.wikipedia.org/wiki/Cadillac_Eldorado

A business partner of mine, a very experienced marketing consultant, always maintains that if your business sells spades and they cost ten pounds, then somewhere on the front of your website you need to say: "We sell spades, they are ten pounds".

Unfortunately, this rather pragmatic approach to marketing is not a view shared in marketing departments the world over, where the laws of nature simply do not apply!