# How Much Back Pressure before it's too much



## TripleStripeD21 (Dec 17, 2009)

Im thinking about messing with my exhaust because I've tried everything to get my fuel economy better, but nothing seems to work. my friend tells me that maybe i dont have enough back pressure in the exhaust, but i think there is plenty because the stock muffler and pipes are on it. is there anyway to get more backpressure without doing harm to the engine


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## zanegrey (Dec 5, 2005)

the ka on a stock truck will get 20 to 26 mpg..what are you getting and have you checked the codes..?


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## rbo1577186 (Jan 16, 2005)

This should not be an issue with OEM exhaust.


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## zanegrey (Dec 5, 2005)

some times when i think the question is non sense (even though there is not one question mark) i forget to answer it..


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## CMax03 (Jan 17, 2009)

??? Added more backpressure....huh??? How about adding more internal engine friction while you're at it......


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## TripleStripeD21 (Dec 17, 2009)

just checked this morning no codes and its getting around 12 mpg, also my alternator belt just snapped rolling out the driveway


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## rbo1577186 (Jan 16, 2005)

4WD?

Auto or Manual Trans?

Extended cab?

Any mods to engine, tire size, gearing, etc??

What climate are you driving in right now? Typically cars get less MPG in colder weather. In addition to some areas use additives in fuel during certain months that will slightly hinder fuel economy.



Back pressure is something you want to leave alone unless you know what you're doing or at least have some means to measure your changes. The factory exhaust should be fine. Too much back pressure will normally cause the engine to work harder to push the exhaust out.


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## zanegrey (Dec 5, 2005)

12 is not good ..w/ no codes ..
check brakes for dragging wheel bearings for freeness..

check alignment ..
consider a tune up..


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## TripleStripeD21 (Dec 17, 2009)

its a 93 2.4L 4x4 short bed, no mods, standard tires, and i checked everything and it all seems to be good, i drive probably about 25 miles a day during the week, and its all cruising around 50-55 never drive above 60 cause then i get like 8 mpg, 3 stop signs to work on the way there, most of the time i drive economy style, like no shifting above what i believe is 3000, i have no tach, also i tuned it up before winter in like november, and the avg temp is about 20 give er take


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## SPEEDO (Jun 9, 2003)

bad 02 sensor, plugged up cat, injector dumping fuel, air filter dirty....
a couple of ideas anyway...
(If anything, you would want to open up the exhaust, not close it up.)


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## TripleStripeD21 (Dec 17, 2009)

well i checked the o2 sensors, they checked out the injectors are all fine, the air filter only has like 500 miles on it so it aint that (its clean too), im thinking it might be my cat, would it be fine to just rip it off and just using piping, cause i dont need emissions anymore


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## zanegrey (Dec 5, 2005)

maybe make sure the fresh air supply to the cat is working b4 u remove it..


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## dentedsub (Oct 23, 2007)

have you checked for fuel leaks?


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## TripleStripeD21 (Dec 17, 2009)

that was the first thing i checked for


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## zanegrey (Dec 5, 2005)

redouble ur tune up..

check plugs for being correct plugs and gap..

how old is dizzy cap and rotor/wires ?

check all belts for being to tight.

check air filter and pcv..

how old id t-stat. is the truck running at the correct temp ?

how old is oil and what type of oil are you using..go to 5 w 30..

check trans oil/fluid .check gear oil ..

what tires are you running ? are they inflated properly ..consider pumping up the pressure to 40 psi..

maybe it is the way u drive ..are u a lead foot..

racing to the stop sign and taking off aggresively ?

what gas are you using ?


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## TripleStripeD21 (Dec 17, 2009)

plugs are right, dizzy stuff is all brand new (1.5 month old), Belts are all good, filter was just replaced, dont know how old the thermostat is but its running at the right temp, need to get an oil change in about 1000 miles (so in about two weeks) ive been using some mobil 1 10w 30 like i used on my last hb, gear oil is fine, stock tires running at about 37 psi, i know its not the way i drive cause its always wet out so if im hard on the gas at all ill just spin tires, and im using regular 87 octane gas... funny thing about this so far, my muffler rusted off a couple days ago while i was driving...luckly it was at about 1 in the morning so no one was on the road, i went back to pick it up off the road and there was A LOT of crap in the muffler... i dont know if that was the problem but im guessing it was cause after it fell off on the road i heard something shoot out of the pipe... like it was from the cat... but anyways im going to fill up again tomorrow and if my mileage stays around 20 im keeping the muffler off


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## SPEEDO (Jun 9, 2003)

might pull the cat and check it out.. see if its getting plugged or if you still have one! Might have broke up and went into the muff


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## TripleStripeD21 (Dec 17, 2009)

ya im going to look as soon as i get out of school


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## zanegrey (Dec 5, 2005)

u were on the right track with the back pressure idea ..

just 180 degrees off...

on one of my trucks i do not run a muffler at all ..iit has a short pipe after the cat ..

it is the z24 engine and i run 235/75/15s and the truck gets very close to 30 mpg..

i drive like an old man (cause i am one ) ..

run another tank thru ..sounds like you fixed it...


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## zanegrey (Dec 5, 2005)

lol.........


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## rbo1577186 (Jan 16, 2005)

After you check your cat, I still wouldn't imagine the 2.4 would get much better than the low 20's for mileage. It's pushing around more weight and has a greater rolling resistance than the 2WD. Also less aerodynamic.


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## TripleStripeD21 (Dec 17, 2009)

thats what i did with mine after the muffler rusted off, and it would appear im getting about 27 mpg i rounded up from 26.6, i take a different way to school and work now and i go an average of about 45-50 mph, i plan on stacking my truck just for s***s and giggles, also my cat seems to be fine so i kept it on


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## CMax03 (Jan 17, 2009)

zanegrey said:


> u were on the right track with the back pressure idea ..
> 
> just 180 degrees off...
> 
> ...


Your speedometer is off running that size tire bud so your MPG calculations are way off too....


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## TripleStripeD21 (Dec 17, 2009)

My speedo is right on dude... my neighbor is a cop and has a laser, i had him clock me and my speedo is spot on, just filled up this morning. 25.9 mpg its official im keeping the muffler off


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## zanegrey (Dec 5, 2005)

your truck may have california standards ..that is they use a smaller muffler with tighter tolerances for the outlet..

which was giving you too much back pressure..

u could go with a hi flow muff or a cherry bomb to quieten it and still not loose mpgs...

thank for the update and it has to be like getting the monkey off your back by knowing what the trouble was..


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## TripleStripeD21 (Dec 17, 2009)

yes it was, now i have to figure a quick fix for my frame, im probably just going to put 2 pieces of half inch plate steel and bolt it through the rusted areas because u can almost fit your fist through the holes


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## SPEEDO (Jun 9, 2003)

I think I would weld it... not bolt it


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## TripleStripeD21 (Dec 17, 2009)

Cant find anyone thats willing to weld it.. its too much of a liability, and i dont know how to weld all that great, i know how to spot weld... thats about it


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## zanegrey (Dec 5, 2005)

just tell the shop you will sign a wavier or disclaimer ..

or put an ad on ur local cl for some one who will weld it for you ..


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## CMax03 (Jan 17, 2009)

Go see the difference between the stock size tire overall diameter and yours @ discount tires, ain't no way in hell 235/75/15 is the correct diameter.....I upgraded (plus2) back in 1989 to a 225/60/15 which is the correct tire size upgrade....All the overall tire diameter is the same....Yours are way off dude....Tell me what your stock size wheels/tire were it's printed on the inside door jam....please!


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## TripleStripeD21 (Dec 17, 2009)

The inside does say 235/75-15... i also looked it up on tirerack just to be sure


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## CMax03 (Jan 17, 2009)

What year is your truck????? HB's never had that much meat from the factory....unless they were 4x4's or Pathfinders (Sorry, dude!!!!!) I just read page 1 of this thread, you stated it's a 93 4x4.....awesome mileage but unbelieveable unless you're travel down hill constantly! I'm getting 22 mpg combo highway/city when driving spiritedly and maybe 26-30 if I'd drive it like an old lady......THAT'S HARD!!!!! I'm running a cam/ported head/custom exhaust/hi flow cat/Ram CAI/Ignition...but possible if, I can stay out it!


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## TripleStripeD21 (Dec 17, 2009)

I drive pretty much a constant 40-50 all the way to school and work now... i get enough speed going down hills in neutral to keep it going by putting it in 5th when im almost to the bottom, also it is a 1993 HB 4x4 2.4L 5 speed... also i kind of have to drive like an old lady cause every morning a couple of cops sit on the side of the road on my way to school (Dumbasses that street race in the morning... i've had to pull out like 5 of those asses cause they're dumb enough to race when snows on the ground) and my loud stacked exhaust... yes i wasted money on like 5 dollars worth of flex tubing, and got the rest of the piping from some buds, its more for show and being loud then anything... anyways thats why i have to drive like an old lady... but even when i do drive normaly i still get at least 23 mpg... its all country roads


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## AtomCzar (Dec 28, 2010)

Hey do you have pics of your "stacked exhaust"?? I have holes in the front corners of my box and wondered about doing that.


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## CMax03 (Jan 17, 2009)

I have a custom T409 2.25" crush bent exhaust sytem with Pacesetter Headers, Magnaflow High flow cat, and a Magnaflow 6" round 14" long muffler with a resonated tip!
Really mellow and nice sound, during idle your pants leg will be pushed against your leg!


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## jizzinyoface (Jan 28, 2010)

i got horrible gas milage too it ended up being my calipers were sticking n not releasing its easy to check just jack the front of the truck up n spin the tires if they dont turn easy or if they only turn like 1/4 of a turn its probably that...i get like 25 now with 31 inch mud terrains with a 4 cylinder 5 speed tho


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## jizzinyoface (Jan 28, 2010)

oh and i recently just put a 2 inch flowmaster delta flow 40 series muffler on my truck n thats all the exhaust it has n i love the sound it sounds great n not much noise inside the truck n i dont even have carpet in my truck so theres no noise dampening


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