I can't speak for 16" Boxster wheels (I didn't know that Boxsters came w/16" wheels), but I can speak for 17" Boxster wheels....in my case on a '91 GT. Since '84s had different wheel/tire sizes, widths, and clearances you'd have to do a bit more research for your car.....but on my car, I find that the 17" Boxster S wheels work quite well.
As far as "100%" fitment is concerned, the rear fitment is very close
100% since the width, height & offset is almost identical to stock (at
least on a GT). The only physical hinderance, is the front wheel interface
(the thickness of the wheel at the bore). The Boxster interface is about
10mm less than those of the Design 90, and therefore the dust cap
protrudes through the bore & prevents the center caps from popping in.
This is an easy fix by reducing the cap protrusion slightly.
The wheels clear the big Brembos with no problem, and rubbing is a non-issue.
There are 2 other factors to keep in mind that I believe would be common with both Boxster & Boxster S wheels: The wheels are 1/2" narrower than the stock fronts on a GT; and although the same width for an '84, the stock tires are 205's. That isn't enough rubber up front for any 928...I'm sure everybody would agree with me on that one. The offset on a Boxster front wheel is +55 which is 10mm less than any 928, which presents no physical problems on my car, however it has been said that the steering geometry is very sensitive on 928s and 10mm plus or minus offset can have a negative effect on the car's handling. (I don't believe those who say that less offset can present caliper clearance problems. It's the amount of clearance from the interface or mounting surface of the wheel - regardless of the offset that matters there [IMO]).
I was a bit skeptical, thinking that 10mm couldn't make that much difference (it wouldn't on my Explorer), but the gurus on the list are right - I think that the tracking is off just a tad. My front end has a very mild 40k orig. on it, w/new alignment, and I still get a tiny bit of wandering off track....more than my oem D90s with V rated Comp TAs. I think that the guys on the list with the engineering & racing experience who say don't mess with the offset are absolutely right; but not-absolutely-true tracking isn't enough to concern me, even at posted highway + speeds; however I'd guess that it would be unacceptable at racing speeds.
That being said, if you find that the Boxster wheels will physically fit (without rubbing, etc.) you have to ask yourself, is the consolation of slightly incorrect geometric tolerances worth it for a good deal on a set of good looking winter wheels?
40k w/Boxster S wheels/Michelin SX (that work just fine, and look even better)
They will work but the 205 up front may cause some understeer issues. The rears will require spacers/adaptors.
On my '81, I bolted the fronts on W/O problems and used 16MM adaptors for the rear.
The wheels look good and the car is very smooth now.
'81 928 5-sp
Interesting. With the smaller width to balance the smaller offset, they (apparently) fit inside the wheel wells.
The conventional wisdom, which I relayed, was that the offset was not enough. Standing corrected.
a 15mm adapter can make a ET65 rimm fit on a ET50 intended setup. It will not make a ET50 rimm fit on a ET65 intended
setup.... unles you find adapters that have a thickness of -15mm (LOL) So: adding spacers will only make things worse.
The spacers come in two versions: simple insert-discs with the 5x130 hole pattern, and thicker discs that bolt onto the wheel hub (5 seperate nuts), and have 5 bolts that stick out and mount the wheel using the wheel lugs.
The ET stands for "Einpress Tiefe" which is the distance in
millimeter in which the hub mounting surface is out of the
centerline of the rimm. Pictures say more than words, so have a look at: 928 AUSTRALIA - http://www.landsharkoz.com
and look for the PDF that explains it. Some additional info on this subject is also on my web in the tech section.
'88 928s4 cherry red