I have an inline 6, 4 liter and I am trying to squeeze the greatest gas milage I can out of it that is why I need your peoples opinion on my exhaust. I have stock headers with the stock head pipe next I removed the cat converter and put strait pipe in its place followed by my flowmaster 40 series delta and then a mandrel bent tailpipe. Is this too free flowing for good gas milage? Is there some general rule in relation to gas milage and exhaust flow resistance?
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How free flowing should the XJ exhaust be?
- Thread starter Quills
- Start date
PurpleCherokee
NAXJA Forum User
- Location
- MO
Oh shit... you did it now buddy...
Go ahead and edit the part about the straight-pipe in place of the cat while you still can.
Hasta
And to answer your question, no, that set-up's not too free flowing for good gas mileage. In fact, free flowing is what you're shootin for for good MPG's and power. Especially w/stock manifold and down pipe you're fine. But don't get your hopes up. Maybe +1MPG or so. But as it sits now, you're gunna get some shiddy mileage w/out a downstream O2 sensor. Have you compensated for that?
Go ahead and edit the part about the straight-pipe in place of the cat while you still can.
Hasta
And to answer your question, no, that set-up's not too free flowing for good gas mileage. In fact, free flowing is what you're shootin for for good MPG's and power. Especially w/stock manifold and down pipe you're fine. But don't get your hopes up. Maybe +1MPG or so. But as it sits now, you're gunna get some shiddy mileage w/out a downstream O2 sensor. Have you compensated for that?
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Beej
NAXJA Forum User
- Location
- The Wet Coast of BC
Do a search on cats to find out why removing them is not only illegal and irresponsible, but worse than leaving them in. I think the ensuing flame war around cat removal is probably what Purple is getting at.
Free-flowing is good, but you can accomplish that better by upgrading your headers, downpipe and exhaust to larger diameter units...
Free-flowing is good, but you can accomplish that better by upgrading your headers, downpipe and exhaust to larger diameter units...
j99xj
NAXJA Forum User
- Location
- Colorado Springs, CO, USA
Most cats from the mid 80's to now are very good units. (honeycomb type)
It was the old fashioned pellet types from the 70s that were very restrictive.
It was the old fashioned pellet types from the 70s that were very restrictive.
Jester99
NAXJA Forum User
- Location
- Chattanooga, Tn
Put the ****** cat back on!!! You're ruining my air!
gradon said:
Why did you get a cat with the O2 bungs in it already? What year is your XJ?
I am asking because i wanted to change the cat on my "01" for mileage reasons too. I have 124k on the motor and only get about 220 miles a tank of gas. All of my O2 bungs are in the exhuast and not the cat. Sorry for the Hijack.
A 96 is obd2 and has 2 o2 sensors, the main one in the collector of the header and a second one in the back of the cat that checks to make sure the first one did its job. I got a 2-bung cat so I could put a wide band o2 sensor in the bung in the front of the cat to monitor the AFRs and eventually use a piggy-back ecu to tune it right(or how I'll like it). Oh yeah, the cat I took off was a perfectly fine magnaflow cat that I had on my 94 and then had a bung welded into it for my 96. I got my little brother hooked up with a nice 93 2dr 4.0 ax15 w/140K for $600 from a friend. The exhaust from the cat and back were done. So my brother bought a Dynomax cat back and currently has no cat. He needs to pass emissions on Wednesday, so between that and me wanting a 2-bung cat led me to get one and give him my <3 yr old one.
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Slo-Sho
NAXJA Forum User
- Location
- The 2nd most regulated state.
96+ ZJ 4.0 have the 2.5" down pipe.
xjallday!
NAXJA Forum User
- Location
- Foothills of the Adirondacks
exhaust can never be too free flowing, it can only lack exhaust gas velocity, which hampers the scavenging effect of escaping gas pulling exhaust out of the combustion chamber. Mandrel bent pipes are good=free flowing, headers are good=free flowing, a quality muffler is good=free flowing. Where you run into problems is the diameter and length of the exhaust system. Too narrow a pipe, or long a pipe=too restrictive, too large a pipe=slow moving, quick cooling gases that sit in the system and create restriction. So, what we're looking for is the straightest path with the correct dia. pipe. I believe 2.5 is optimal for the 4.0 I6, I also think that you shouldn't put too much thought or $$$ into exhaust unless you're racing. Spend that time and effort in lift, lockers, and winch!!
WaXJ_Skier
NAXJA Forum User
- Location
- Bellevue, Wa
I'm pretty sure removing the cat has a $10k fine.
wolfpackjeeper
NAXJA Forum User
- Location
- 500' and 500KCAS, RAF lakenheath
the 10k fine is to the shop for the shop removing a cat, or passing a car in an inspection if it is removed. It does not go to the owner. You fail inspection and have to buy a new cat. This is what my buddy who owns a Meineke told me a year or so ago.
seanyb505
NAXJA Forum User
- Location
- Simpsonville, SC
I think its a 2k fine in Florida
i considered removing the cat, but i couldn't even find a damn shop to fab up a straight pipe with 2 flanges for me, they all refused. i don't think there's a law against doing that necessarily, they just didn't have the stones. i suspect there are licenses at risk in addition to the fines, but maybe they were just being lame. this was in cali.
alex22
NAXJA Forum User
- Location
- CT: waterbury area
Does anybody else here think there should be a sticky bulliten in this section saying that the catalytic converters are an insignificant impedance to horse power & exhaust flow and that federal laws prohibit their removal. This must be the 5th thread about a removed cat i've seen in the past few weeks. It always gets the same responce.
~Alex
~Alex
alex22 said:
do you have a dyno that supports your claim for the XJ specifically?
just because something is repeated several times over, does not make it fact. their is quite a large market for hiflow cats and that is for a good reason, they have been proven in many applications.
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wolfpackjeeper
NAXJA Forum User
- Location
- 500' and 500KCAS, RAF lakenheath
high flows just have less stuff in them, still not much of a gain. Most people on the F-body boards go with them because they are cheaper than the correct ones, and the car runs better with them than without them. Camaros have goofy cats that cannot be reused after a LT install.
bjoehandley
NAXJA Forum User
- Location
- WeGo, Chi-Town, Ill
It is illegal to remove if it's in good working order under 80k miles ($10k fine sounds about right too), but don't you just need a straight piece of 2.25" pipe or gut the OE cat (and maybe combine the two)?
OK now for the dumb question, will a cat thief get fined in the same manner if caught in the act of stealing a working cat besides the theft charge?
OK now for the dumb question, will a cat thief get fined in the same manner if caught in the act of stealing a working cat besides the theft charge?
PurpleCherokee
NAXJA Forum User
- Location
- MO
Pretty soon (in about a week) I'm gunna have an actual dyno sheet that I can use to stop this debate once and for all! Those who think that cats aren't a performance bottleneck beware :firedevil lol.
