Dear Tom and Ray:
I have a '95 Geo Metro with a 1.0-liter engine and a five-speed transmission. I recently took my car four-wheeling and broke a few things. OK, OK, I broke a lot of things -- the catalytic converter being one of them. I have no idea where it is now; it's just gone. Anyhow, when the converter was there, I got 52 mpg, but now I'm getting just over 60 mpg! I want to know why I'm getting better mileage. And would I be evil if I just never replaced the converter? (I have a straight pipe in there now, all the way to the back.) The muffler is gone, too, so the car sounds like a suffering cow, but the mileage is so good that I'm willing to live with that. If I'm caught doing this, do I get sent to Guantanamo Bay? Thanks. -- Steve
TOM: Steve, Don Rumsfeld is on his way to your house as we speak. But on the plus side, we hear Guantanamo Bay is quite balmy this time of year.
RAY: The reason you get better mileage is because you've eliminated most of the "back pressure" from your exhaust system. In addition to protecting us from air pollution (converter) and noise pollution (muffler), those two devices also restrict your exhaust to a certain degree. And that decreases your mileage. It's the price we pay for what we call civilization, Steve. You should consider joining it sometime.
TOM: But the back pressure also protects your valves. Without sufficient back pressure, the hot exhaust will blow past your engine's valves so quickly that it'll burn them up, and eventually you'll need to replace them. Let's do the math.
RAY: You're getting an extra 8 miles per gallon. Which, over the course of 12,000 miles, saves you about 30 gallons of gas. So, without the converter and muffler, you save $60 a year.
TOM: A valve job on this car would probably run you about $600. But don't forget that while you're at Guantanamo Bay, you won't have to pay rent on your apartment. Factor that in, too, and then do what you think is best, Steve.