Plastic is Fantastic: Part 6 – Assembling the Beast

Plastic is Fantastic:  Part 6 – Assembling the Beast

by Jonathan Spiegel

As usual, racecars rarely get finished on schedule when you’ve got a day job, but that hasn’t stopped us from burning up weekends (and a little midnight oil on weeknights as well). When we last looked at the Polystrand CRX GT-Lite car, we had just finished creating the new rear framework for the trick IRS assembly. For those of you who haven’t been following the project so far, you can click here to catch up on the first 5 installments of this crazy journey. Without much fanfare, we’re going to jump right back in, since we’ve made a lot of progress since then.

Once we had a stout foundation for the new rear suspension, it was time to line the module up and create the mounts. Basically, the suspension subframe hangs directly from crossmembers between the rear frame rails. We’ll also build some bracing that provides some fore and aft support.

Heres 'a shot of the IRS getting located in the rear so we can build the mounting brackets.

Once we bolted in the rear suspension, it was time to re-mount the fuel cell. If you remember, there wasn’t enough room to mount the rear suspension far enough forward to meet the wheelbase length required by the rules, since the fuel cell was in the way. Since the rectangular fuel cell was mounted longitudinally, we determined that we could gain enough clearance by rotating the fuel cell 90 degrees. A little rectangular tubing, some angle iron, and the trusty MIG welder was all it took (along with a little bit of time) and the fuel cell sits snugly in its new home.

The mounting is simple – six tabs with thru-bolts will support the system between two stout crossmembers.

Here’s a shot from above showing the placement of the crossmembers and mounting tabs. We’ve  “re-purposed” some of the old framework from the original configuration, so please excuse the funky paintwork!

Now that we've got the rear suspension nailed down, it's time to start tinkering with the powerplant. In our last installment, you saw the donor motor – a garden variety B18B1 motor out of a 1996 Integra LS. Since we have to run a tiny intake restrictor (22.5mm), the rpm range of the motor will be severely limited. Calculations show that a properly designed restrictor of this size on an engine with this displacement will reach choke flow at about 6200 rpm. Choke flow is the point at which the flow in the restrictor reaches the speed of sound, at which point the volume of air flowing through the restrictor can no longer increase. While the engine rpm can still increase, the volume of air available is now fixed, so the volumetric efficiency of the engine drops rapidly after that point, with a corresponding drop in torque and horsepower. Since we are limited by airflow, there was not much point in building a VTEC powerplant, since they make their power in a higher rpm range. LS motors are great at building mid-range torque, which is where we’ll spend most of our time anyway.

The fuel cell tucks neatly behind the driver and in front of the IRS. This puts the weight low and in the center, where it will least affect the handling as fuel load decreases
during a race.