+1 on the S100 - just make sure you get the right stuff. There are a bunch of S100 products... you want the Engine Brightener. Another option is Adam's Undercarriage Spray. It's not meant for the purpose, but it holds up to high temps and works well.
My next test is the S100. I have the Pig Spit right now and while it does work, I'm not impressed by the overspray. The S100 has an 'applicator stick' and you can control the spray. Pig Spit also doesn't do that great of a job in my trials.
+1 on the S100 - just make sure you get the right stuff. There are a bunch of S100 products... you want the Engine Brightener. Another option is Adam's Undercarriage Spray. It's not meant for the purpose, but it holds up to high temps and works well.