Actually, according to the "gurus" on this forum, all internals of the silvertop 20v are the same as the GZE, minus the pistons. I have seen forged pistons for the silvertop floating around on eBay every once in awhile, but have never really found a retailer that carries them. You could also have them custom made. Economical? Probably not, although some of the prices I've seen for 20V Silvertops in Canada are pretty damn cheap, even in CAN dollars. GZE's will be just fine in FWD, since all of them are transverse, wether mounted in a Corolla or MR2. The GZE is obviously built for boost right out of the gate, but the abilities of the ECU are greatly contested, I've heard anywhere from 10 to 15 psi. Transmission's are a whole other story, I would assume the stock GZE transmission will hold up, but that's for someone else to answer. I'll tell you this much, if someone would develop some mounts for the 3sgte into our cars, I would say that's your winner right there. At least going by Toysport, the GZE and 3S are the same price for a cut of the same generation, maybe you should look into getting mounts made up, and find someone good at wiring. People won't be passing you with a touch of the gas, that's for sure. You could try this site
www.phoenixtuning.com and get ahold of Dr. Tweak. He has done a 3S swap into an A engined Celica, and says our cars are perfect canidates. Group buy on mounts?
Either way, hopefully someone will give you some more information, but be prepared for directions to the search button.
