The SL-C originally came with a set of fiberglass seats that were molded from the Porsche 962 race cars. These are lightweight, grippy, and are easy to mount, especially with the new aluminum mounts included with the newer kits. In addition, there is a no-cost option at order time to allow for a "gentleman's seat" version of the same seat that is two inches wider. The car can optionally be ordered with two regular, two wide or one of each.
The factory seats can also be ordered in different materials, specifically Kevlar, for increased strength. This is an extra-cost option.
As of 2013, a new seat design was introduced that is somewhat similar to the Tillet seats in that it is of a two-piece manufacture and has built-in shoulder harness openings. There is an inner and outer shell that are joined together to form a very rigid seat that has built-in ports for seatbelts. These seats are relatively narrow in the shoulder area (a critical dimension for seats in the SL-C) and so fit well in the car, while offering a better appearance, stronger construction, and less need for modification for harnesses. These are now shipping as standard on the car. The original seats are still available on request.
Both seats will need to be modified with appropriate slots for lap and anti-submarine belts. It's a good idea to wait to cut these holes until the seats have had an initial fit, and you are otherwise happy with the mounting of the seat, and the proposed location for the belts. Then open the holes for the belts. Be sure to line the openings with a grommet of some sort, either purchased or made from split hose or an appropriate trim that is glued to the opening. Failure to make appropriate grommets will cause wear to the belts, and worn belts are a very common failure at tech inspection.
Good discussion thread here.
However, some builders have used other seats. The reasons are as different as the builders, but some of the popular reasons are that other seats could be stronger, even lighter, offer more padding, might offer more headroom, provide different upholstery solutions, be made of alternate materials, permit a wider range of recline angles, etc.
The SL-C isn't the only narrow car that could use other seats- follow the link to a chart from Flyin Miata that shows seat sizes- these are directly relevant to our cars- not that every seat in the chart fits, but that it shows sizes for a a wide range of seats.
Another possible solution is to take a seat that you like, but which doesn't quite fit, and modify it to fit. Typically, you would cut down the shoulder wings (if present), lower the seat bottom, and remove foam where needed to make the seat narrower.
At least one builder who is a bit wider than others decided to make seats using poured foam. This allowed him to conform the foam to the exact shape he needed to fit in the car, while at the same time still fitting on the confines of the car. This is common race car practice, and often times you can see drivers carrying their custom seat insert to the car when there is a driver change in endurance racing. Kits to make these seats are available from many sources, including this one at Pegasys.
Another similar idea is to adhere pink foam insulation boards from Lowes or Home Depot into the seat area and carve the shape you want in a seat. A sharp knife followed by successively finer sandpaper will yield any shape you need. At least one builder has done this as well, because he couldn't find a regular seat that fit his body type. This is a bit of work, and you need to plan the carving to allow for any upholstery you want to use (and how you want that to be attached), but the results can be effective and attractive if done properly.
Doc Kaler has built a spreadsheet with seat models, widths, etc that he used during his seat selection process. It is available for download here: Seat Dimensions.xls. This is a useful way to quickly compare seat sizes, in order to gauge whether they will work for you, in your car. The spreadsheet also includes details as to the mounting method, an important factor to ponder when considering adjustable seats on sliders (side mount seats can use sliders with no loss in headroom in most cases, but sliders on bottom-mounts are normally additive- the sliding rails add an inch or so to the installed height, and to the seat bottom height.)
Also, the Lotus Elise/Exige series of cars has a similar seating constraint, and aftermarket seats for those cars will often fit the SL-C. The stock Lotus eats are an alternative, but they are as expensive as the aftermarket seats, and seem to offer little benefit over a bespoke seat at the same or similar price. The Elise seats are too tall in their un-modified state and the headrest has to be shortened to fit.
While the legroom for our cars is exceedingly good (the car has actually had a 7' driver fit in it), headroom is where the limits are reached first. Most people solve that by reclining the seat back, and for many, the optimal angle is about 45 degrees, paralleling that of the of the aluminum panel that separates the fuel cell from the passenger compartment.
Several builders use seats from Tillett Seats because they offer a very low seating position, are lightweight, strong and narrow. These seats come from a company that also makes FIA-certified seats (one of which, the B6F, fits in the SL-C and would be a good race seat), and all of the seats are made using a proprietary process using carbon fiber. Some seats also have fiberglass components, though they can usually be ordered in all-carbon at extra cost. The Tillett B5 makes a great seat for street and street/track day cars, as it is low weight, allows one of the lowest possible seating position, can be mounted an an angle to optimize recline and head room, and - important for a street seat- has low sides that facilitate ingress and egress. The B5 has become one of the most popular seats for the SL-C.
An additional benefit of the Tillett range is that they don't have to be upholstered-- or if you decide to, they are extremely easy and as such, inexpensive, to upholster. Here's a picture of a completed car with the B5s; these seats have a carbon fiber front, a FG back and have been painted body color. This car has had custom upholstery added in the same design and material as the optional Tillett covers
Another seat that works in the SL-C is the B4. Note the higher sides, and the design for a more reclined driving position.
The absence of a requirement to spend a lot of money on upholstery makes their price point easier to accept. They can be ordered in all carbon fiber, or with a carbon front and a FG back- great if you want to paint the back in body color.
Another possible option is the Saker seat from RaceTech, a New Zealand company. This seat is a laydown model, and is designed to be used at the angle most builders find they need to run their seatback- about 45 degrees, paralleling the fuel cell bulkhead.
Alternatively, for the lowest seat possible, custom seats may be fabricated to meet specific needs. The image on above and right shows an SL-C with a 6' driver sitting on a custom aluminum seat shell based on the RCR GT40 seat. Note the available headroom in this configuration- a 6' 5" driver with all the height in the torso could fit in such a seat. Or a normal 6' 7" driver at the same recline angle.
Several builders have tried the Cypher (or Cipher) seats available from eBay and other sources. These are inexpensive, have a reclining mechanism, and look good, but still have the normally fatal flaw of a too-high seat bottom. Unless you are quite short, save your money. Some have tried to make the seat bottom more shallow by carving foam, usually with unsatisfactory results.
The seats can be fixed (i.e., bolted to the floor) or can be mounted on sliders. A sliding seat may offer more possible seating positions that work for different drivers, and the sliders are inexpensive and easy to source. Many sources, including Pegasys sell the popular OMP slider set, which seems to be ubiquitous.
It should be noted that the use of sliders does not have to cause a loss of headroom, at least with side-mount seats, as the brackets can be fabricated or adjusted so that the seat is as low as it would be if bolted to the floor. If you do use sliders, make sure they mount to the sides, not under the seat- most drivers can't afford to lose the headroom that under-seat sliders cost.
However you mount your seats (on sliders, or directly to the floor with fixed brackets), be sure to remove any Dynamat, insulation, carpet, or any other material from the floor where they are mounted. Placing the brackets or sliders over such material will cause the seats to rock, and not be rigidly mounted to the chassis as they are intended. This is a safety as well as a comfort issue.
My seats are still too wide to fit!
Some builders with the optional "gentleman" seat, or even the stock seats, find that there is still not enough clearance between the body and the seat shoulders, or the console, etc., on initial fit-up.
Usually, the fitment problem arises in the passenger area. This is so because the center spine in the chassis is always offset 2" to the inside, relative to the driving side. In other words, in a LHD car, the center spine is installed 2" to the right relative the the actual centerline of the car. The purpose of this is to optimize for the driver, at the possible expense of the passenger.
In other cases, it happens with the driver seat, when the builder has not carefully aligned the dash, steering column, pedals and wheel to optimize the seating position. The SL-C isn't the type of car where any old seat, or even the stock seats, can just be bolted up anywhere without careful measuring and thought. Everything needs to be considered, and careful seat placement, along with all the other factors mentioned here, should be carefully planned before holes are drilled.
Once you have the basics in place, you may find that the seats are still too close to the body or doors. There are several solutions a builder can employ. The first is to mount the seats as far inboard as possible, consistent with everything else. If you can put your fingers between the seat and the console, you should consider moving the seat all the way in so that it is just missing the console. You only need enough room for the lap belt to come out (you did make the adjusters pull-downss, didn't you?) Another solution is to shave off material at the shoulder area of the seat, where most fitment problems occur. Many factory and aftermarket seats are modified this way, especially ones that are the least bit wide at the shoulders.
Finally, use of the optional door panels exacerbates clearance issues; if your selected seats still hit the doors, consider modifying the optional door panels or seats, or consider just not using the panels at all, if that is enough to get the clearance you need at the shoulders.
It should be noted that builders who have used the popular Tillet B5 seats have been able to fit them, unmodified, in cars with complete interiors and door panels, even in older cars with consoles that are wider than the current console. These seats are great if you can fit in them, but they won't fit all drivers- so check fit before you buy them.
Finally, it bears repeating that the stock seats do fit, though you may need to shave a bit off the shoulders, depending on how you mount them.