Catalytic converters effect on MPG?
 

So at about 200K miles in my VX the cat clogged dropping my MPG 50%! So the immediate affordable solution (since I don't have to pass emissions here) was to gut the cat (hollowed it out with screw driver and a hammer). This immediately fixed the problem and the engine felt stronger than ever! This was before I knew anything about lean burn or hypermiling. So my question is, does this help or hurt my MPG? The thought would be that a better breathing motor would increase mpg (thus the cat being clogged reduced my mpg). So is there any negative effect? Loss of low end torque from lack of back pressure maybe? Sorry I can't a-b-a it, a little pricey for an experiment :)
Any thoughts?

Honestly, a properly flowing and working cat provides very little backpressure, and therefore will make very little (if any) difference in both power and mpg.

Flow speed makes torque, not backpressure, so hollowing out the cat won't hurt low end torque at all.

assuming you only care about the fuel economy of the motor and not the increased emissions a cat removal will create then...

it's still not a cut and dry answer on whether the removal or gutting a cat will help or hurt your fuel economy.

your theory on reducing back pressure to increase economy is sound, but the velocity in which the exhaust gas travels also plays a part, so pipe size, converter size, location etc all play a part in figuring out whether or not a particular car will benefit from a cat removal.

Thanks, good to know. I guess I was basing it on my experience with my 98 Formula (LS1), with an aggressive cam that makes its power in the upper rpm range. When I ran off road y-pipe (no cats) I felt a little loss in throttle response and torque in the lower rpm range. I have been happier with the high flow cats back on

The other thought I had was concerning the VX motor specifically, how it may effect lean burn, vacuum, etc. . .
I think on the highway a hollow cat would breath better and get better mpg provided running 2500rpm or so. But what about city driving, where much of it may be done at or below 1500rpm, I know there has been talk about the benfits of low end torque there. Just a theory, I'm certainly not about to buy another cat to try out

if your trying to compare the efficiency of a normal cat (clean not clogged) compared to a hollowed out cat then from what i have seen there will be little to no difference.
I looked at this myself some time ago, took the cat off the car, i was going to hollow it out then looked into it and noticed i could see straight through the cat, there was a grid of metal to provide a large amount of surface area but it was constructed in a way to provide little to no impedance to air flow at that point i didn't bother hollowing it out.

If however your looking at the difference of a clogged cat compared to hollowed out then i am sure there will be a big difference as the engine works of a pressure differential between the pressure generated by the expanded gasses and the pressure of the exhaust.

You probably already know, but you can get a universal cat for a much more reasonable price than an OEM or direct-fit. If you're concerned about flow, you can get a high-flow cat, or one rated for a heavier vehicle (they seem to rate cats by vehicle weight). Assuming your car has 1 3/4" exhaust (which would need verification) this list, for instance, should have something that fits.

We don't have emissions testing either, but my adult-onset asthma has made me appreciate working catalytic converters.

Get a new cat.
MPG isn't everything.

Creating and operating the new cat probably causes fat more pollution then the cat will ever clean up in its entire service life.

That's an interesting question. Not really sure how to find out an answer though.