Tom believes in water heat....so he sells the tapped spacer between the thermo housing, and the block. I've seen some neat arrangements where water heat was used successfully. However, the Offy intake is not water-tight and requires welding up the two holes under the manifold. The stock manifold needs to be sealed water tight! He's used it for years and is comfortable with it. And you can't beat experience!
I went with the exhaust heat! The Fentons were tapped for the compression fitting and I made my own plate for the bottom of the stock intake. Used copper tubing, compression fittings and it was an EASY install. The lines become hot almost instantly, I know the plate is getting hot because I hit it with my infared thermometer.
So which one do you use? If you're good with a tubing bender, enjoy gathering brass fittings, don't mind springing a few bucks to get your Offy intake water-tight, and trust your work with the lifeblood of your cooling system, it's the way to go. It can be a "cleaner" install. If you mess up, I could see where chasing coolant leaks, could be frustrating.
On the other hand, with the exhaust heat, you screw in 4 compression fittings, hand bend the pipe, tighten the fittings and you're in business.
I don't think you'll find anyone who has a really valid reason to use one over the other. Pressure differentials and corrosion aren't really valid under real world conditions.
So, it's your call! Have at it!!!!!
