Normally, testing fuel economy means filling your tank, driving somewhere really far away, filling your tank again, and dividing the number of miles you drove by the number of gallons you added. With the kind of fuel economy we’re already getting, we’d be back in Canada before we’ve run through two tankfulls. Instead we got a magic box called Scan Gauge II. It plugs into the diagnostic port, talks to the engine computer and calculates fuel economy as you drive.
When you do the road trip thing, your fuel economy represents an average of all the boring crap you did while you burned those 10 gallons of fuel. Instantaneous fuel economy, on the other hand, only tells you about the boring crap you’re doing right this second. If you happen to be accelerating, you might be getting 8 mpg. If you’re cruising downhill behind a truck, you might be getting 120 mpg.
If we want to see what kind of difference the Diesel Inside tune made, we need to eliminate every other driving variable and test the car in exactly the same instantaneous condition both with and without the new tune. That means no accelerating, no following trucks, no hills, no headwinds, etc.,
To eliminate wind and trucks, we wait until late at night. To eliminate acceleration and hills, we find a 2-mile stretch of flat public road bisecting the Seal Beach Naval Weapons Station. And in the name of science, we drive back and forth across this flat, dark, boring stretch with the cruise control set on precisely 60.3 mph, 3 times in each direction, and then calculate our average fuel economy from the six runs.
After a night of stalking the weapons depot, the entire Top Gear staff is now on the terrorist watchlist, but our sacrifice revealed a number almost as shocking as the dyno results. The same tune that just got us 22 hp and 72 lb-ft of torque also gained us 5.1 mpg. If the rest of the tuning adventure goes this well, we’ll be done tomorrow.
Just a tip for those of you new to the concept of reading: The point in any story where the author states “if all goes well” is precisely the point at which any story, regardless of subject, takes a dramatic turn for the worse. Observe:
This seems like the place where we’d put a picture of the flashzilla box, or the RS232 port mysteriously hacked into our Rabbit’s dash, or a fuel map, or something like that. All those pictures have been misplaced. Instead, here’s me looking up instructions on how to change diesel injector nozzles. Nothing on a diesel works the way a gas engine guy thinks it should.
If you want a Diesel to make more power, you just have to squirt in more fuel. If you’ve never tuned an engine before, this is the most obvious thing in the world. But if you’re used to tuning gas engines, this is the most preposterously naive and wrong-headed thing we’ve said in at least two paragraphs. Diesels, it seems, are a bit different.
Malone says our current tune is giving us all the power we can make with the stock fuel injectors, so he sends us a bigger set. At least we think that’s what he says. After 2 hours turning wrenches, we’re giddily expecting another 15 hp. Instead we get exactly the same power, but we’ve lost 40 lb-ft of torque. Worse, there’s black smoke belching out the tailpipe and the horrible noises coming from under the hood sound slightly different from the horrible noises we’ve grown used to.
Some poor Canadian insect dodged every bar on the grille and managed to snake past the radiator at 80 mph only to splat into the engine and fall down an injector hole to get crispy fried for the next 2,000 miles. Sucks to be a bug.