When I replaced my exhaust system with a set of headers I removed the catalytic as the system would not fit. I took the car up to a muffler shop to have a new muffler installed, I was told that the reason my mileage was so low was the computer was not getting the right readings due to not having the cat. installed. I think they are full of any thoughts on this.

May be full of it. I believe our o2 sensor is before the cat in the system so it shouldn’t make a difference if I’m recalling correctly. I ran mine with no cat for a few years and then moved to an area where emissions required me to install one and I never noticed a difference in mileage. I could be wrong but seems you may need to look for a different culprit

I know that I have a couple of the vacuum lines on wrong and they need to be changed so that may be the reason. The motor is new, so until it is broke in I expected the mileage to be bad, but not this bad. 7 to 8 mpg

im in the same boat. You would think mine has 2 750 hollys on it. I got the fuel mileage of a 427 with the power of a suzuki 1.3

I should be outside right now checking the vacuum lines, but we had snow this morning and its a little nippy, once I get that done and the gas consumption doesn't change I don't know what else to do, should have put a chevy in there like I was going to do.

but then your distributor would be in the wrong place

I think, since the cat is after the combustion, that readings from the cat don't effect the incoming fuel to air ratio one bit.

I could be wrong - but that's logical.

Yea there full of it. My 4runner doesn't have a cat and my mileage is pretty average. they just want your money

It can still be a problem, but probably not enough to drop your mileage to 8 mpg. Dang, I think ours is bad in the winter when we do short trips and only get 14.

The catalytic converter and muffler slow down the airflow some and make the whole system work well. You can muck with it some by putting high flow (low resistance) muffler/cat on it, and the O2 sensor should compensate, but I don't really know what the range is that it can make up for.

On a chainsaw, one trick to get more power is to "port" it, basically adding more venting to the muffler, thus less resistance. Then you adjust the carb to pump out a little more gas, juicing it for a little more power. That's all good, unless you have a non-pro relatively recent saw that has limiters on the carb adjustments. I never did get my two smaller saws (stihl and echo) running right after adding just a small amount of extra venting to them, even after overriding the limiters, but I have a couple Huskies as my go-to small and big saws. It's something to play with this winter, and at least one of them now needs a new fuel line, so it's hard to know if that was the reason I couldn't get it dialed in, or if it died in the last few years while sitting unused.

The short of it is, lacking a cat will affect things, but I don't really know if it affects things small enough that the sensors can get the engine to override/utilize the change.

Hopefully today I'll weld in my replacement muffler. This last one only made it 6 years, but it was the bottom of the line. The exhaust tool set you can use from AutoZone is pretty cool.