A few weeks ago I threw out the cat in a Volvo V40 1.9D and combined with throwing out the EGR I went from about 33MPG to 47MPG average(that's right, no mistake here) and I'm pretty sure a little low-end torque gain also appeared. The Volvo is a '00 model year so I guess it was just clogged up. How about a Fiero? The cat in mine is probably also original so will I see an MPG gain in a petrol engine? It's an '85 2.5 auto by the way. Already removed the EGR(was causing the engine to run rough at about 3k RPM) but didn't notice any power or MPG gain whatsoever.

And lets put the legal issue of that aside. The Fiero is pretty much considered a classic car where I live so nobody will probably look up there.

[This message has been edited by cebix (edited 03-19-2012).]

Well, my '85 V6 gets right around 32-35 mpg with the egr installed and no cat. Prior to removing the cat I feel as though it was mid-high 20's

[This message has been edited by spc15tdime (edited 03-19-2012).]

A clogged cat or muffler will surely cause performance and/or efficiency issues. But they are like any other item on the car which needs maintenance at certain intervals. Removing a clean and functioning system will basically get you no change in the MPG department.

No where near stock anymore, but it was still getting high 20's prior to the rebuild, as long as you keep your foot out of it. Mods, ported polished, heads, intake, cam, thottlebody, exhaust, tuned, higher compression pistons. list goes on, but, thats the stuiff that relates ot this. Oh, its a 3.65 ratio, 4 spd manual

[This message has been edited by spc15tdime (edited 03-19-2012).]

quote Originally posted by spc15tdime: No where near stock anymore, but it was still getting high 20's prior to the rebuild, as long as you keep your foot out of it. Mods, ported polished, heads, intake, cam, thottlebody, exhaust, tuned, higher compression pistons. list goes on, but, thats the stuiff that relates ot this. Oh, its a 3.65 ratio, 4 spd manual

What octane are you running? Higher compression and higher octane adds improves the MPG because it's a more efficient fuel. With tune, higher compression, and more air coming in, you're probably also running a bit leaner than stock.

93 Octane, this started out as a performance build that was going to get a low psi turbo kit put on it, the cam is no overlap,...etc but with the price of gas this became a mega gas saver. But it DOES NOT like 87 fuel in any way shape or form lol.

I removed the cat on my 87 GT because it plugged up. I was planning on replacing it but after hearing what it sounds like without it I changed my mind. I do have to install one to get it through emissions (once every two years) but I’ve not seen any change in mileage nor would I have expected to. Cat’s are big noise reducers but there isn’t that much restriction under normal driving conditions. As far as the EGR valve is concerned, there isn’t any mileage or performance to be gained on a stock motor, intake and exhaust system. With that, even if you have added performance parts you would have to take it pretty far to justify removing the EGR and you will have already destroyed any mileage that you might have had. You do stand a chance of damaging the engine without it if you allow too much pinging.

So I guess it's just diesels that miserably drop on MPG if the cat and EGR get clogged up?

quote Originally posted by cebix: So I guess it's just diesels that miserably drop on MPG if the cat and EGR get clogged up?

This absolutely affects a gasoline engine in much the same manner. I think the discussion of it not making a huge difference is assuming they are both functioning normally. A faulty EGR system can cause several issues, from lean / rich problems, to idle and power issues. Depending on the manner of failure. A clogged cat CAN, severely restrict your emissions, resulting in a plume of dark sooty smoke, high EGT's (exhaust temp), low power, and generally a lot of "chugging" type behavior. But this assumes that it is in fact clogged not just deteriorated some. In any event, while removing the Cat will probably allow for gains with this motor, it is not the limiting factor in your exhaust system, and your gains in doing this would not be significant on their own (again assuming normal functioning). I would recomend leaving the factory EGR system in place if it is also functioning properly.

quote Originally posted by cebix: So I guess it's just diesels that miserably drop on MPG if the cat and EGR get clogged up?

There's a huge difference between clogged up, and a clean, new, fully functioning setup. A brand new cat these days has extremely low restriction of flow. Most of them you can look through and see out the other end just fine. And they do help reduce emissions.

Current design catalyst would not kill MPG IF the cat is clear.
Pebble-bed design used in Fiero would not kill MPG IF the cat is clear. Pebble design is easier to plug.

If any design is blocked may mean engine have problems, some additive (Oil and/or gas) mess w/ cat, etc...

5-10+mpg gain mean the cat is plugged, you did MPG method is wrong (How you get your data.), etc... A good cat will not be easy to measure MPG increase/decrease.

Edit to fix bold text. <> vs [] was working on my cave.
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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

[This message has been edited by theogre (edited 03-20-2012).]

Very well said!