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RACER.com Diary 5: Power to the Ground

Although the course at Bonneville is over five miles long, it’s still useful to think of it as a drag race. As the Challenger II accelerates, our return on expended power diminishes exponentially. In other words, going faster gets harder, so if our setup requires 50% of the course to achieve 50% of our maximum speed, we’re sunk. Every car builder has some version of this problem, but salt flat racing presents a few unique challenges.

Our runs take place on what is essentially a dried lake bed, and track conditions can resemble anything from hard packed snow to slush. Maintaining traction is always an issue, so we place a lot of faith in our tires. We’ll be using a 16 inch set manufactured by Mickey Thompson Tires, which is something I’m very proud of. They’ve been producing high speed salt flat tires for a number of years with great success, and most of the other cars you see on the salt will be similarly equipped. When my dad first got to Bonneville, he couldn’t find a set of tires that were resilient and consistent, so he made some. If you can’t find what you need, build what you need. That was M/T. 

The tires themselves are an experimental design rated for up to 575 mph. They are 4.5 inches wide, banded with steel wire, and extensively cross woven with nylon. The rubber coating is only 1/32 of an inch thick, and feels almost like it’s painted on. There are two reasons for that. First, expansion. As the tires close in on maximum speed, they will grow by nearly an inch. If they were caked in heavy rubber, they would grow by even more, interfering with traction. Second is heat. More rubber causes a higher temperature buildup, and a hotter tire is less reliable. 

Thanks for following along. See you next week. 

RACER.com Diary 4: Balancing the Power

How much power does it take to beat the world land speed record? When my dad ran the Challenger II in 1968, he had roughly 1800 horsepower at his disposal. The car had a supercharged 427 Ford SOHC in the rear and a second fuel injected SOHC in the front. The power output of the engines was not balanced, so he stabilized it manually by rolling his toe back and forth over a split gas pedal. 

He was forced to perform that foot-operated witchcraft because the low slung driver’s compartment meant that that there was no space for a front blower. That hasn’t changed, but we’ve decided to fuel inject both engines in the updated car in order to balance power output. We’re using dual billet aluminum Hemi dry blocks running on 50% nitromethane. They are modified versions of what you see in top fuel dragsters, and will produce roughly 2000 horsepower each. That should give us more than twice the power output of the original Challenger II. 

If you’re not familiar with dry blocks, they don’t require radiators or a water system. All of the cooling is provided by the massive amounts of fuel flowing to the engine. In our case, approximately 10 gallons of alcohol and nitro per mile. The hardest part of this project is working within the confines of the existing space, so being able to eliminate those components was a big advantage. 

Still, packaging is a major challenge. We had to fabricate a special low profile intake manifold for the front engine to maintain cockpit visibility, while moving the throttle bodies out in front over the clutch can. Then comes the fuel system, oil system and all the plumbing that goes with it. 

Jerry Darien is supervising our engine combination and is working with RC Performance on the build. The rear engine is being assembled this week, with the front engine to follow shortly. Thanks for following along. See you next week. 

RACER.com Diary 3: In the Cockpit

In my last post I mentioned our commitment to maintaining the authenticity of the Challenger II during the update. My least favorite part of that decision has been the cockpit, which started out small and has been steadily shrinking. Given the difference in girth between M/T and myself (sorry Dad) I assumed I wouldn't run into any trouble, but preserving the driver's area has become a tricky problem. 

The streamliner's original roll cage was probably adequate, but rule changes concerning minimum tubing size meant that the whole unit had to be cut out of the substructure and refabricated out of thicker material. The pivoting canopy that covers the driver's area has a maximum aerodynamic height, so the changes to the cage had to be absorbed internally. By the time I added a HANS device, I found that my head could no longer comfortably fit inside, mostly because modern helmets are about 25% larger than they were 50 years ago. 

I've modified the aluminum driver's seat with specific indentations for the rear fuel pump, and sacrificed part of the footwell to house the larger dual magnetos demanded by the new engines. This problem is exacerbated by the Challenger II's unique four-wheel-drive system, which requires the front engine to sit backward in the streamliner. By the time I added two fuel shutoff valves, two parachute buttons, two air shifters, three fire bottles and, oh yeah, the steering wheel, I started to feel all kinds of sympathy for watchmakers and the guys that design the iPhone.  

The key to the whole thing is that I have to be able to exit the streamliner by myself in under 20 seconds, preferably faster, in case of a fire. My obstacles are two canopy latches, a 7-point harness, arm restraints, a HANS device, and the tight cockpit. People laugh when they see me practicing, but I do it for the same reason I work out everyday and eat unpleasant salads. Space is tight, and I need all the room I can get. 

Thanks for following along. Next week, engines.

RACER.com Diary 2: Chute First

This week I'm going to a talk about the modifications we've made to the tail section of the streamliner. Cumulatively, they represent the largest external change to the Challenger II, so we've been approaching them cautiously. Although we've been updating the car for over a year, almost all of the changes have been beneath the hood. The body is still old school, and we want to maintain the integrity of the original while making sure that the final product is fast and safe.

The Challenger II originally deployed its parachutes by ejecting a fiberglass subsection of the rear wing with compressed gas. That would never fly under current safety regulations, so our aero man Tim Gibson devised a plan to extend the original tail section of the car to accommodate integrated parachute tubes. This has two advantages. First, it moves the car's center of pressure away from its center of gravity, which improves overall stability in adverse (windy) conditions. This is especially important to me after a tumble I took a few years ago while we were running the world's fastest Ford Mustang. Second, the larger undertray will take advantage of the air moving beneath the streamliner to increase net downforce without a significant increase in drag. That's a rare win-win.

The reason we've been able to accomplish this without dramatically changing the exterior of the car comes down to improvements in parachute technology. In 1968, the Challenger II required chutes with 12ft blossoms. Today, we can get identical stopping power with 4ft chutes. We've performed an extensive retrofit to support the additional tail length – approximately three feet – which has involved a new substructure, as well as more mundane things like new cable mounts and brackets. We've also added a fixed Replay XD camera, which will allow us to visually monitor parachute deployment.

Next week we'll take about body shape and how we're blending it with the bottom of the streamliner. Thanks for following along!

Announcing our RACER.com Project Diary

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We've teamed up with RACER.com to provide you with a weekly progress diary. The first entry can be found here goo.gl/10SpL. In the coming weeks we'll cover changes that are being made to update the chassis, aerodynamics, and power train. We hope you follow along!