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post #1 of (permalink) Old 11-01-2004, 01:43 PM
Bull
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91 Exhaust upgrade suggestions, remove cats or not

Hello all. I have been reading this board for a while but this is my first post. I have a question I hope you all can help with.
I want to put a performance exhaust on my 1991 Bronco (302 FI, E4OD, 4" superlift, 4.10 gears & lockers, 33x12.5x15 BFG AT KO's) because the old catalytic converters are plugged I believe. My question is: with the dual cats, will hollowing them out and goin with 2.5" Borla cat-back exhaust have any negative effects on my gas mileage or performance?
I have a 1992 F350 Crew Cab with FI 460, I removed the cat and welded in Flowmaster's Y-pipe, ran 3" pipe and a Flowmaster 70 series big block muffler (basically the same as sixliter's system on his Bronc'), it sounded great and really helped power, but my gas mileage sucked after removing the smog pump...does anyone know if I will have the same problem with my Bronco?
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post #2 of (permalink) Old 11-02-2004, 01:07 PM
 
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Re: 91 Exhaust upgrade suggestions, remove cats or not

I have a 91' f-150 with a EFI 300. I removed the cats and am running true duals after the H pipe 2" pipe and thrush glasspacks. Also have the smog pump running a loop and the timming advanced 5 deg over stock. I gained ALOT of low RPM tourque a bit more mid/high range, throttle response increased and mileage went up 2/3 mpg. I used to get 20 highway at best now i average 20 all the time and get 22/23 highway and i usualy drive 65/70mph average.
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post #3 of (permalink) Old 11-02-2004, 02:09 PM
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Re: 91 Exhaust upgrade suggestions, remove cats or not

I'd love for mine to be getting anywhere close to your mileage, but my Bronco gets about 12 mpg whether I'm cruising @ 55 or towing a fully loaded camp trailer. I know I have to replace the exhaust but I just don't want my gas mileage any worse than it is, this is my daily driver. [img]images/graemlins/scs.gif[/img]
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post #4 of (permalink) Old 11-02-2004, 05:53 PM
 
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Re: 91 Exhaust upgrade suggestions, remove cats or

Bull

I can't speak to 460s but the 302 went up in mileage after the new Y-pipe. The difference I can see is you may have put on a Borla with too little back pressure.

Other than that with your air pump not working I'm guessing your EGR isn't working either and that can knock off 2-5mpg just on it's own. A good working EGR is very crucial to mileage and doesn't hurt performance the way it did on older carbed motors.

If you have clogged or aged cats as it is, going to a newer higher flow cat or getting rid of the obstruction will definately help mileage.

IMO

Sixlitre

p.s./
Turning 33s, even with 4.10s ain't helping things
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post #5 of (permalink) Old 11-03-2004, 04:59 AM
 
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Re: 91 Exhaust upgrade suggestions, remove cats or

Pick you up a set of cheap wheels and some 31x10.50's or 235-75's for daily driving save your 33's for wheelin and save some $ at the gas pump!
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post #6 of (permalink) Old 11-03-2004, 10:43 AM
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Re: 91 Exhaust upgrade suggestions, remove cats or

I know the 33's ain't helping my mileage even regeared, but I just bought them and if I spend more money on tires right now, I will be getting divorced!
My air pump is working and I have no trouble codes from my engine (the egr is new also). My cats look plugged when I shined a flashlight through...the previous owner replaced the exhaust from the cats-back when the new motor was installed (about 6,000 miles on new motor), but he left the original cats.
Sixliter: Do you think I'll have problems with too-little backpressure if I hollow out the cats and put 2.5" pipe and a Flowmaster muffler on? Should I use 2.25" pipe?

p.s.: thanks alot for the replies, this is a great board!
post #7 of (permalink) Old 11-03-2004, 11:05 AM
 
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Re: 91 Exhaust upgrade suggestions, remove cats or

In a perfectly running engine, 33s with 4.10s will decrease the mileage a bit over stock. Your intown will be good but the highway will suffer. Voiding the cats by punching them out or eliminating them will get you a little IF they were plugged up, but if they were plugged up you would see a loss of power and a higher running temp.

the engine may still be a bit tight and causing a small loss of gas mileage but even that is only about 1 mpg tops.

I thing your main loss of gas the the gearing. 3.73s are about right for 32-33s for the best gas to power ratio, but that is about pointless for you to do.....
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post #8 of (permalink) Old 11-03-2004, 05:10 PM
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Re: 91 Exhaust upgrade suggestions, remove cats or

I'm assuming you meant 3.73 gears, not 2.73's? I changed my gears mainly because the locker I scored for my 8.8 already had new 4.10 gears with it. I figured for towing my trailers, it could not hurt...I found this on Fullsizebroncos.com forum (from Beerman) and also when I talked with Randy's Ring & Pinion:

Gear Ratio & Tire Size:
Stock 29" tire (235/75R15), with 3.55 gears, 90.7" is circumference of tire, thus for every turn of the drive shaft, you go forward 25.5" or so.

33" tire, 3.55 gears = 103.7" circ, forward 29.2", about 15% more than stock
33" tire, 4.10 gears = forward 25.3" basically the same as stock.
33" tire, 4.56 gears = forward 22.7", about 11% less than stock

35"s with 4.10s = 26.8, 5% over stock roughly.

35" tire, 4.56 gears, 110.0" circ, forward 24.1", about 5% less

37" tire, 4.56 gears, 116.2" circ, forward 25.5", identical to stock (within error)

38.5" tire, 4.56 gears, 121.0" circ, forward 26.5", about 4% more than stock

38.5" tire, 4.88 gears, 121.0" circ, forward 24.8", about 3% less than stock.

FYI, for those who are thinking they can run a 35" tire with 3.55 gears...

35" tire, 3.55 gears, 110.0" circ, forward 31", about 22% more than stock. That means that your motor is working (and your clutch or torque converter) quite a bit harder to move your truck. That's why it dogs so bad.

If it falls "under" your motor does not have to work as hard to turn the tires, but it does at a higher RPM. This means that as you go more "under" you lose top end. If it's "over" you'll have a higher top end, but less grunt.
post #9 of (permalink) Old 11-03-2004, 07:48 PM
 
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Re: 91 Exhaust upgrade suggestions, remove cats or

Sixliter: Do you think I'll have problems with too-little backpressure if I hollow out the cats and put 2.5" pipe and a Flowmaster muffler on? Should I use 2.25" pipe?


That's a two stage question

If you can get away with hollowing things out, why not try the free stock style muffler first, before risking divorce for a flowmaster ?

Tell you truthfully the biggest kick in the pants and mileage increase came after I replaced my Y-pipe even though I stuck the stock 2 1/4" muffler and kinked tailpipe back on (cause I couldn't afford a bigger muffler after the pipe bending bill).

Later when I went to the bigger muffler and mandrel bent tailpipe, not a measureable difference to be had, just sounded better.

Sixlitre

p.s./
as for tires point out to darling dearest that 235/75/15s are stock and the cheapest "Winter tires" money can buy.
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post #10 of (permalink) Old 11-03-2004, 07:50 PM
 
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Re: 91 Exhaust upgrade suggestions, remove cats or

Hey, what am I thinking ?

Check this out;

http://fullsizebronco.com/forum/showthread.php?t=28101

Sixlitre
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