13% mileage increase by relieving exhaust back-pressure?

[Peakster]

I was looking up my car on www.cartalk.com and found this to be interesting:

February 2005

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.) Thanks. -- Steve

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.

I took off my Fiero's catalytic converter way back in 2003 and found a noticeable increase in acceleration (with no noticeable noise increase), but I never would've thought that it could increase FE. However, if it would make my engine wear out faster maybe it's not such a good idea.

Still, it's something of interest for those of us who never have to pass an emissions test.