exhaust vs. gas mileage

[imdawrlus]

this is something that i posted in a different forum, i wanna hear you guy's oppinions:

"89 sierra 350...i just bought this truck and im thinking of doing dual exhaust with no cats and an X pipe...i hear that an easier breathing exhaust will DECREASE gas mileage, and still i hear it will INCREASE it....whos right?? also aside from the legality of it, is it a good idea to go without cats? feedback please..."

so lets hear what you think

[Sparky2263]

Here's my experience. Vehicles built since the late 80's don't gain more than .5 mpg with TRUE DUALS. Power, however, increases greatly. Due to the increase in power, fuel mileage generally drops because the fun factor increases.

So, if we were averaging everyone's experience with duals, we would find an overall decrease in fuel mileage. But it ain't got anything to do with engine efficiency ;).

[nomaboy]

personally i HATE cats. with a truck that old you won't have one problem running without a cat. it will increase mileage a little because it allows the engine to run more efficiently. plus, with a true dual system with a x-pipe and no restriction that thing will sound real mean. go for it!

[ZR1-S10]

MY experience is an incresse in both power and mileage. However I always have too much fun with my right foot so any improvment in efficiently helps. Overall I say go for it!

[Hammer]

personally i HATE cats. with a truck that old you won't have one problem running without a cat. it will increase mileage a little because it allows the engine to run more efficiently. plus, with a true dual system with a x-pipe and no restriction that thing will sound real mean. go for it!

actually if you have a good running engine with a functioning emissions components you will get better mileage with the cat than without, unless you simulate the O2 sensor.

even then you run richer unless you tune it out and waste fuel

it is against the law to remove emissions equipment if it came with it from the factory. I don't recommend removing the cat. replace it, yes, but remove it.... no. In many states you won't even pass visual if the cat is not inplace AND INTACT.

[Maximus]

I say go for the duals.The sound and power advantage is always worth it and it will help efficiency.The whole cat debate I will stay out of.

[Hammer]

yes i do agree though with the duals and x pipe. i like h pipes but i think x's are better. you could easily run two dual cats and pass emissions with that, atleast in theory.

i take that back, if you do anything infront of the cat, supposively it isn't legal.

[Supercharged-ZQ8]

Keep the cats (replace if necessary). Not having them is MUCH more of a pain than having them -- plus, removing the cats entirely won't gain you much unless the cats are clogged (providing that they are the "honeycomb" type cats and not the older "pellet" style cats).

Going with an X-pipe and duals will, theoretically INcrease gas mileage. The motor isn't working as hard to rid itself of spent gasses, so it will be more efficient. The more efficient it is, the better the fuel economy is. Of course, that is providing you drive it EXACTLY the same. As Sparky pointed out, there is the possibility of a DEcrease in economy due to stomping the Loud Pedal more often. . . But, overall, you will see an increase as long as you drive it that same as you did before.

Example: The mods listed for the Blazer in my sig have all been in place for at least a year except for the Dynomax cat-back. It's my wife's daily driver, and she isn't into hot-rodding or racing, so she drives it pretty much the same: conservatively. By installing just the cat-back, we've seen about a 1-2 mpg increase in fuel economy (because she doesn't drive it any differently than she did before and because it IS more efficient than before).

If you drove WOT all the time before, and drive WOT after, you'll still see a minor (read: hardly noticeable) increase. If you drove it conservatively before and go WOT more often after (whether because of the feel of extra power or because you want to hear the exhaust more), you'll see a definite decrease -- but because of your driving style and not because of the exhaust. Basically, it's all relative. I have a Cavalier as my daily driver. When I bought it, it was 100% stock. I've installed a cold-air intake, MSD coil packs, polyurethane upper and lower motor mounts, and a (very loud, very annoying) cat-back kit and a full tune-up. I drove it like a grandpa before and I still drive it like a grandpa now: no hard acceleration, not much more than 70-75 mph on the freeway, no revving (it's TOO annoying and ricey sounding), nothing different than before. It did get around 23-24 mpg (on average) -- I just did a check this week to see where it was sitting now: I managed to get 339.3 miles out of 11.93 gallons -- that equates to about 28.4 mpg. I improved the intake air flow and the exhaust flow, as well as improving the spark. All told, I gained about 4-5 mpg -- but that's because I drive it exactly the same. I've had a couple friends ask me why I put a Ricer exhaust on it -- seeing how I make fun of the Ricers with their fart cannons so often. . . GAS MILEAGE, is my answer. I don't think or pretend that it's fast -- I know better -- but the basics of flow don't lie: better intake, better exhaust, better economy. If I want to break the bank on gas, I'll drive my truck. . . WOT most of the time, 93 octane only, 2650 rpm at 70 mph in 5th gear. . . It SUCKS the gas down. . .

But, I digress. . . As long as you drive it that same as you always did, you will see better mileage.

[Sparky2263]

So in other words, women get better mileage than men? I'm calling the feds. on this one. Sexism at it's worst.

[imdawrlus]

now i'm thinking it would be a whole lot cheaper just to go with a single 3 inch...also i'd love to hear some more about the whole cat issue...

[Maximus]

You wont make as much power with a 3inch single as you would with true dual 2 1/2's or larger.And you won't get as good a sound.I recommend running a set of hooker ceramic coated long tube headers.2 1/2 inch dual pipes using good flowing cats.Then use the X tube out to Magnaflow stainless mufflers then exit from them out back or to the sides.You will have a great flowing exhaust that's emissions friendly and will sound great.

[Supercharged-ZQ8]

So in other words, women get better mileage than men? I'm calling the feds. on this one. Sexism at it's worst.
Well, almost. . . It's just good ole discrimination! First women, then old people (my reference to driving like a grandpa). . .

Who will I pick on next?!

How about those damn Germans? Sausage-eating, beer-swilling, sauerkraut-slurping, warlike, good-for-nothing. . . Um, wait. . . I'M GERMAN! Hmm. . . I'm feeling very discriminated against here!

Can I file a lawsuit against myself for discrimination? Where's my lawyer?!

Oh. . . Damn, I hijacked another thread. . .

Max, that DOES sound like a SERIOUS exhaust system. It would sound great, flow great AND be emissions compliant -- much as you said.

[imdawrlus]

well i just passed emissions today so i've got 2 years to do whatever i want with the truck before i have to pass emissions again...i'm thinking that i just want to run a single 3 inch exhaust without a cat...it wont be too much trouble 2 years down the road just to throw a cat on it assuming i still have the truck by then...i'm also going to be looking for a new muffler...any reccomendations?

[bvr775]

with an 89 a good high flow cat will help allot sence the prevortec engines did use a pellet type cat instead of the new free flow honeycomb cats(yes the new cats are true free flow). You want one of the round cats with a bigger outlet than inlet

I don't know what the big debate on exhaust duals vs singles is. For a everyday driver with stock enternals a single mandrel bent pipe if sized right will flow as well or better than duals. Here's the secret to a good flowing exhaust.....Find a good exhaust man and let him set it up.

[imdawrlus]

well today was my first day running without a cat and i can say that i dont see any drop in performance...if anything a slight increase...i notice that it deffinately sounds a little louder but not much, and it STINKS like a hotrod haha...but i didnt want to put a cat on because i dont have any money right now...i owe my dad a grand and i only have about 200 bucks in my name right now...haha and it isnt going to get any better until it stops raining up here in the northeast because i cant cut grass in the rain...also my other source of income, clam digging, is closed because of red tide so theres some tough times ahead haha but i'll make it...thanks for the help everyone...

[Matt 4.3 TBI]

Hey bvr: I'm not sure when the changeover to the honeycomb-style cats was, but my 91 came with one from the factory.