This tube-chassis Pontiac Firebird might be the coolest build of the year based on the headers alone.
If you were paying attention to the cars of SEMA this year, you would have seen Riley Stair’s Poncho in pictures. But even then you probably didn’t get to see and hear him do a walkaround of the build himself, which Hoonigan just dropped today:
Riley finally gives a clear explanation of why this car has such a wonderful bundle of snakes in front of the engine.
It’s all a mix of engineering and packaging.
To get good weight distribution, Riley moved the engine a good 20 inches back from where it sat originally. Good weight distribution means good handling.
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For good power, you want equal-length headers. They’re when each cylinder gets the same length of tubing directing exhaust gases out to the exhaust. This is for good tuning, as Hot Rod explained in an article last year:
When an engine’s exhaust valve opens, there are two distinct movements within the exhaust pipe. The first is the pressure movement of the shock wave that is generated by the violent expansion of hot exhaust gas past the valve. This shock wave propagates through the gas in the pipe at a speed of 1,300–2,000 ft/sec. The second movement is that of the exhaust-gas slug itself as it travels from the cylinder to the end of the pipe. The speed at which this slug travels is determined by the exhaust pipe’s cross-sectional area and the engine’s piston speed, but is usually about 200–300 ft/sec at the engine speed where maximum power is produced.
When the shock wave reaches the pipe’s open end and passes into the atmosphere, a rarefaction, or low-pressure wave, is reflected back up the pipe. If the pipe length is correctly adjusted, at a given engine speed this reflected low-pressure wave will arrive back at the exhaust valve during the valve overlap period when both the intake and exhaust valves are open, in theory completely scavenging the cylinder of any residual gasses.
So equal-length headers help you run your engine as efficiently as possible, which you want when you’ve got a high-RPM naturally-aspirated engine like Riley is using. That’s a 400 cubic-inch V8, meant to top 10,000 RPM. Even idling it sounds like a beast:
You see exhaust setups like this particularly from the days when high-revving naturally-aspirated V8s were the peak of performance in racing, 40-50 years ago. Ford GT40s, Lotus Grand Prix cars, Quad Cam Indy Cars, all of those would run these bundle of snakes exhausts to get as much power as possible.
Riley also wanted the car low. Real low. It’s got to look good, but if it’s to be as low as it is, there’s no room to run a huge exhaust setup back out the whole car. It’d cook Riley in the driver’s seat.
So if the exhaust is to have equal length headers on a rearward-set V8, you’re going to need a lot of wiggly tubing to make the headers for the front cylinders meet up at the same spot as the headers for the cylinders set way at the back. And if those headers aren’t running under the car, they have to run somewhere, and that’s going to be in front of the engine, then winding back to the passenger side, again to reduce how much Riley gets cooked in the driver’s seat.
So that’s why this Pontiac has what looks like Indiana Jones’ worst nightmare under the hood.
Now I just have to wait to hear it run out to redline. It’s got to sound wonderful.