I recently recieved the "new" Vortech kit I purchased for my '05 GT. The price was fantastic and when it arrived, I found a big reason for that. Most of the boxes were dated 2008. The big issue there is this opened up the possibility I had received an S-trim rather than the Si the shop had told me I was getting.
I figured a quick Google search should settle it but that hope was quickly dashed. I e-mailed Vortech with my serial number but haven't heard back yet. I have my doubts that they've kept records that old anyway.
I'm posting the bits of info I did manage to find over several hours of searching. All of this assumes you've already ruled out the possibility you're looking at an SC / SCi which have an obviously different impeller, or a T / Ti which have a larger 3.75" inlet. Also, there are no V-3 S-Trims. Vortech introduced their self contained gear case after the S-trim was discontinued. Hopefully others can add to or correct what I've found and we'll have the answer.
This pic below is the only one I could find of both blowers side by side. Obvious visual differences here are removal of the "VORTECH" script from the volute, four small holes around the pulley shaft and some other small changes to the gear case. Of course, this doesn't mean that these changes necessarily happened at the same time as the S-trim ceased production. The removal of "VORTECH" from the volute would make sense. The Si-trim and Paxton Novi 1200 are twins so that would save them from making 2 different castings. I don't know if this applies to units with the 90 degree discharge.
Other info I found came from a couple of threads on Corvette forums.
Serial number 48614 was listed for sale described as an S-trim that had been sent back to Vortech for a rebuild and Si upgrade. The CARB EO number on that particular head unit is D-213-25. That document mentions the S-trim but not the Si so it seems very likely that any head unit with that CARB number was not originally built as an Si.
The second bit of info was from a thread where the owner of serial number 49057 mentioned they believed they had purchased an S-trim but contacted Vortech and was told it was an Si. The CARB EO number on that head unit is D-213-26. That document mentions both S and Si so any head unit with that EO number could be an Si-trim but it's not a sure thing. I assume some of these blowers left the factory without a CARB EO number so the above may not be helpful in every case.
I read several statements suggesting that S and Si production didn't overlap. If that's true (and we trust the info from the 'Vette forum) then any unit in question with a lower serial number than 48614 is an S-trim and higher than 49057 would be an Si. Anything in between would still be in question. Unfortunately, the head unit I recieved is one of those. It's serial number 48760 and has the D-213-26 CARB EO, leaving open the possibility it could be either one. It has the blank volute and gear case changes seen in the picture.
That's all I could find. If I hear back from Vortech, I'll add any info I can get from them.