Smaller? Well, yeah, if you only measure across the lens face, which is indeed 9cm (3.5"). But these lamps are also deeper (5.6" for the halogen, 6.4" for the Xenon) which on a 240 means either they protrude rearward into space that's already taken up with stuff like batteries and airboxes, or they protrude forward and look dorky. The BiXenon is an enormous big bruiser: 7-3/4" deep! No way it'll fit and not look thrown-onto the car. Remember also, it's not enough just to make it fit the space and stay still, you have to pay attention to center of gravity when you mount long/heavy projector lamps, otherwise they'll rock and roll no matter how firmly you mount them, and you'll shake bulbs to death quickly and often.

These lamps come in a lot of varieties. E-code halogen low beam projector w/H7 bulb, E-code halogen reflector high beam w/H7 bulb, E-code Xenon projector low beam w/D2S bulb, E-code Xenon reflector high beam w/D2S bulb, E-code BiXenon projector high/low beam w/D2S bulb, DOT halogen projector low beam w/H9 bulb, DOT halogen reflector high beam w/H9 bulb, DOT Xenon projector low beam w/D2S bulb, DOT Xenon reflector high beam w/D2S bulb, DOT BiXenon projector high/low beam w/D2S bulb. Of these two of them are identical except for lens markings (the E-code and DOT BiXenon high/low beam). With the single-beam units, all of them are pretty good, but the E-code varieties produce better focused beams, and there are several good options for upgrading H7 bulbs (Osram makes a really cool 65w 2100 lumen H7). The BiXenon version is nifty, but its performance on low beam isn't as super amazing as you would (reasonably) expect for nearly $600 apiece. (There are also the smaller 50mm versions of the single-beam lamps, "available soon" from Hella. They're small, but that's about all that recommends them. As the originator of this thread points out correctly, size matters to performance.)

Naw, if space behind the rad support panel were no issue (move the battery and/or airbox, or maybe on a V6 car) and I wanted projectors, it would be easier for me to keep the rectangular lamp setup and put in Bosch or Hella rectangular-housing projector low beams ( http://dastern.torque.net/BoschDE.jpg , the one on the left) together with a good reflector high beam. But here again, we're looking at a big bruiser of a projector lamp

But, I'm not sure what we're going to accomplish with this swap. The 90mm front lens is too small to look right on the front end of a 240. It would look, to repeat myself, dorky. Kind of like the angel eye headlights Hella makes for the '92-'98 BMW 3-series...the lenses are too small for the overall design of the front end of that car, and it jest don't look right! And even if you didn't mind the "tiny eyes" appearance, it would look schlock to put the 90mm lamps on a 240 unless you were able to come up with a custom-fabbed bezel to fair them into the front end (Easier and less silly-looking, perhaps, to put the 90mm units behind the lens from a new stock headlamp on an '86-up car, as long as you polish the optics off the inside of the lens to make a "window" for the 90mm lamps to look through)

I like the quad round look as long as the right grill is used. Black turbo or flathood grill (w/rectangle "criss cross" lines) rules, standard grill (black or chrome w/only vertical lines) drools. And yeah, there is one particular set of 5.75" round headlamps that beats the snot out of pretty much anything else that can readily be made to fit a 240 without radical modification (Cibie CSR H1/H1). But, some people just can't cope with round lamps.

One other possible swap for those who gotta have rectangular lamps but want something that works better than small inherently weak rectangular sealed beams or H4s is a set of '87-'88 Chevrolet Caprice headlamps (composite replaceable-bulb sized and shaped to replace the quad-rectangular lamps that had been used up to '86 on the Caprice) slotted into place where the quad rectangulars went, after moderately minor modifications to the stock quad-rectangular bezel (to eliminate the center divider bar). They're not E-code, so no sharp cutoff on low beam, but they're not too bad for upward glare light and they're a better performer for actual seeing than sealed beam or H4 small rectangular lights. They take 9005-9006 bulbs, for which some nifty upgrades exist ( http://bmwz.org/articles/lighting/0506trick/ ). As long as you use the '87-'88 lights, which have a low beam bulb shield, you can use the high-zoot 9012 low beam bulbs. The same size/shape lamp was used on the Caprice from '87-'90, but the bulb shield was deleted for cost savings for 1989 (typical GM), performance went down, glare went up, and I'm not sure but I imagine you can only get the '89-'90 lamps from a dealer or aftermarket, so you'd have to find good used ones or New Old Stock -- check for the bulb shield! Those lamps look like this: http://tinyurl.com/7kwqb

Then there's the fun that can be had with interesting nonstandard lights for the setup that takes one large round light per side of the car...but that's another post.