When we last left off with Project S13, we just started getting ambitious with our second-hand project car. The base platform for cheap speed was already in place when we started - SR20DET swap, 198whp, new brakes, and a TEIN suspension. Shaking loose nineteen years worth of age was as simple as new Energy Suspension urethane bushings, brake pads, tires, and a comprehensive fluid and water pump swap. Almost.
Built in 1989 and older than some of our readers, Project S13's fastback chassis is the softest of all 240SXs. The back hatch glass, which might as well be made out of cast iron, leaves a massive non-structural void in the rear and thus the fastback is substantially less rigid than the coupe model S13. With Super Lap Battle Street Class time attack rules stipulating that all factory glass must remain in place, there wasn't much that we could do about the hatch. But chassis stiffness can be addressed.
Although allowed by class rules, we didn't want a full roll cage in the car. Tying together the chassis with a custom welded cage would increase chassis stiffness by a large amount but this car sees more street time than track time and cracking our skulls open on a metal tube doesn't exactly sound like a great Friday night. This left us with the options of stitch welding, bolt-on chassis bracing bars, a 4-point roll bar, and urethane foam. We may try out all these methods eventually, there's no such thing as a S13 that's too stiff, but the urethane foam is one of the most intriguing and so we started there first.
We've seen expanding urethane foam in automotive applications before. Mike Kojima tried it with success on Project Nissan 300ZX and it's used in such OEM applications as the Acura TL and the Mazda RX-8 R3. The stiffer a chassis is, the less it will flex against forces applied to it, i.e. suspension movement. This means that forces from the road surface will be contained and absorbed by the suspension rather than being allowed to beat up the body shell. As the chassis is stiffened, ride quality will improve and even stiffer springs can be used before the chassis is upset again. Foam filling a chassis is a cheap and light method with which to dramatically increase chassis stiffness.
Keep in mind though that the foam you want to put into your car isn't the cheap spray insulation that you can buy at your local Home Depot. Many of those foams have a density in the range of 0.5 pounds per cubic foot. We were after the 2 pounds per cubic foot injectable kits for the upper pillars (made by Handi-Foam) and the 8 pounds per cubic foot stuff for the rockers. Foams in the 8 pounds per cubic foot density are commonly used in marine applications and we've never come across one that was an easy to use two part injectable kit. Our expanding urethane foam pour kit was sourced from US Composites and came in two unassuming metal cans. The liquid in the two cans is mixed in a 1:1 ratio and has a claimed 45-second pour time. In actual use, the realistic pour time seems to be closer to half that. We mixed up about 20-ounces at a time and a paint mixer and power drill came in handy for us as we zapped the mixture for a mere 15-seconds before beginning the pour into our rocker panels. Any longer and the foam would begin to harden in the funnel as we poured, clogging any more of the mixture from entering Project S13.