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The Black Box - Techno Babble - Dave Coleman

Dave Coleman
Feb 26, 2007
0403_sccp_01_z+technobabble+dave_coleman Photo 1/2   |   The Black Box - Techno Babble - Dave Coleman

I can't honestly remember which class it was, but somewhere in the blur of high school I remember a science class where we did a black box experiment. We were given a small black box with three rods jammed through each side of it. The rods crossed inside the box and we were told there were washers stuck on some of the rods. Our task was to figure out which rods had washers, how many there were, and where they were sitting relative to the rods crossing from the other side of the box.

The point of the exercise was to demonstrate how the combination of knowledge and observation can help you understand what you can't see. While that's the basic premise of science and the very foundation of our understanding of how the world works, what it showed me is how you can use your brain to make your car better.

2017 Nissan Sentra
$24,990 Base Model (MSRP) 25/31 MPG Fuel Economy

Every fun part of a car is essentially a black box. You can't see pistons and valves in action, and you can't watch the suspension when you go around a corner. You have to take what you know about how things work, combine them with your observations and figure out what's happening inside.

The trick with the box was to put your fingers on the rods and tilt the box. When the washers slid around, you could feel which rods the washers were sliding on, and which they were running into. So figuring out what was happening in the box was simple; it was figuring out how to figure it out that was so difficult. The fact that we already knew there were washers inside was also critical. The same is true when you try to think your way inside your car. If you just stared at an engine making strange noises, you would never guess what was wrong if you didn't already know there were pistons and valves and all that nonsense bouncing around in there.

This jumble of experiment and guesswork can easily go wrong if you interpret your experiment wrong. I have a bizarrely mixed-up suspension on my SE-R Spec V rally car. Up front there are massive DMS rally struts, custom-built with WRX inserts valved to work on a Nissan Pulsar. (Note that Sentra was never mentioned there.) In the back, I have Progress shocks that were also custom-built and have been revalved on a nearly race-by-race basis as I return to Progress with my usually conflicting feedback. I have two sets of these rear shocks, just in case I rip my rear suspension off sailing over a tree stump or something.

So last May, when I was trying to figure out how to mix DMS and Progress dampers, it came time to install the rear shocks and I had to decide which pair of rear shocks to use. It had been five months since my last race, and I'd long since forgotten the meaning of the arbitrary code words I had scratched into the shocks to explain their valving. Desperate, I tried compressing each shock to feel the damping by hand.

Easy. Two shocks extended on their own; two did not. With your standard black box model of a shock, this means the two that didn't extend had blown a seal and no longer had any pressure in them to push the shaft out. I installed the non-blown shocks.

It took one-and-a-half rallies and three months to figure out I was using the wrong mental model. Halfway through the Gorman Ridge rally in August, I was lamenting the fact that the rear suspension was bottoming out too often, and the car's balance was basically flawed. Then it clicked.

This car has never blown a rear shock. The shocks sitting in the back of my van were probably the ones I had taken back for revalving the last several times, asking for more low-speed damping every time. They now had so much rebound damping it took an hour for them to extend themselves. Grabbing some stiffer rear springs at the same time (so the whole car didn't take an hour to extend after a jump), I frantically swapped to my supposedly blown shocks.

0403_sccp_02_z+technobabble+black_box Photo 2/2   |   The Black Box - Techno Babble - Dave Coleman

It worked. Suddenly DMS and Progress bits were working in harmony, the car was balanced, took jumps well, slid where and when I wanted it to, and went faster than ever before. By the last stage, I was running a shocking third overall behind two EVOs.

Then I hit a tree.

Sometimes you don't even have to have the proper understanding of what's happening to stumble into the right answer. Take prom night. I was trying to borrow my dad's RX-7, since it smelled so much better than my 510, but it wouldn't start. No sputtering, no cranking, just a loud click every time I turned the key. In the black box of a rotary engine, turning the key is supposed to cause cool whirring noises and lots of bad smells. Experience suggested the only reason this wouldn't happen was a lack of electricity, but jump-starting and even swapping batteries didn't help.

Then we tried push-starting it. I pushed it down the street and my dad put it in second gear and popped the clutch. The car screeched to a stop with the rear tires locked. Normally, a rotary engine is so easy to turn over you can push an RX-7 while it's in gear. Something in the box was jammed.

Using my best mental model of what was happening inside, I guessed the starter gear had somehow jammed in the flywheel and wasn't letting it turn. Since it was stuck in the flywheel, this was also why the clicking noise was so loud-the solenoid was slamming into the back of the jammed starter gear. Now, how to get it out?

We put it in fifth gear, to give the wheels the most torque multiplication, stepped on the clutch, and pushed the car... backward. It worked! When my dad popped the clutch, the engine turned over backward, the gear dislodged, and the car started on the next turn of the key.

Brilliant. But my diagnosis was probably wrong. More likely, my dad's sedate driving in the year or so since he had stopped autocrossing had caused carbon buildup on the faces of the rotors. This is apparently a common problem with old RX-7s that are treated gently. Eventually, a big chunk of carbon fell off and wedged in the corner of the combustion chamber, refusing to be crushed. When we kicked it back, it landed somewhere else and the engine turned over.

So in the end, it doesn't really matter if you understand what's in the black box or not, as long as you guess right.

By Dave Coleman
94 Articles

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