Rob,
A couple products that have worked well for me are "Pre" available through Eastwood. The other is "RM 900 Pre-Kleano" available through Industrial finishes. Both of these products work very well at removing dirt, wax, polish, grease, silicon, road tar, etc. and are used extensively as a metal "pre prep" prior to painting (no painter wants a client coming back to them complaining about the paint job they just paid $$$ for is discoloring and/or lifting due to a bit of silicon or grease left on the metal prior to painting). Make sure you follow the directions closely when using these products however, because they are very potent... You need to make sure you have adequate ventilation and proper PPE (Personal Protective Equipment)such as gloves, goggles, face mask, etc.- whatever the directions call for. Have lots of clean rags available because you'll want to continually refresh with a new clean rag after a couple wipes so you don't end up just moving the crud over to another place on your engine. I use a plastic spray bottle to spray the stuff on and then I start wiping. BTW, Acetone doesn't come close to holding up against either one of these products.
** I'm also interested in hearing what other people use for this application - so let's hear from you.
-Don