K&N filter on old TBI Chevy 5.7L engines to increase MPGs

Discussion in 'Performance & Fuel' started by Crawdaddy, Oct 29, 2013.

  1. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    My 1991 Suburban has the 5.7L engine with a TBI intake. I checked on what solutions K&N has for my generation truck and engine, and all they have is a replacement air filter assembly and filter (http://www.knfilters.com/search/product.aspx?prod=63-1007-1)

    They claim a whopping 6HP increase at 4100 RPMS which is really not what I'm interested in. I'm after getting better MPGs. I know the traditional cold air intakes can help with fuel mileage, but will this one help me out any? I've read that K&N's filter for these engines actually hurts since it's pulling in hot air from around the engine instead of slightly cooler air from outside. Is this true?
  2. buckmeister2

    buckmeister2 Rockstar 4 Years 100 Posts

    Christopher, I hate to break your bubble, but there is very little improvement with either K&N, or CAI install. The posted gains have been refuted over and over. Since you are talking about a '91, you might see very slight improvement (3-5HP) from a CAI, since yours does not have the factory CAI (I think those came standard with Vortec, in '96, and all years forward). You can make you own CAI with tubing and a flange pulling air from under the bumper, or in a fender well. Pre-96 5.7's can benefit from other upgrades, but I am not too familiar with what is best. Do a search on the 'net for "early 5.7 gm upgrades" and see what comes up.
  3. dobey

    dobey Epic Member 5+ Years 500 Posts

    You're not going to get better MPG with an air filter change (unless your current filter is bad enough that it needs changed anyway).

    If you want better MPG, a good place to look at on that engine is to find a good set of shorty headers, and replace your probably worn out cat with a higher flowing new aftermarket unit. Depending on the condition of the exhaust you might also want to look at a cat-back system as well as the headers, with a good muffler. If you have true dual exhaust on already, get an X-pipe put in.

    Check the O2 sensor, and other sensors, to make sure they are relatively clean and operating in their optimal ranges. Replace any that aren't. A good fuel injector cleaning might help as well.
  4. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    For the air filter, it has less than 10K on it, but I was wondering if the factory air cleaner assembly was restrictive. Personally, I know nothing about air intake systems.

    Headers are in the future for the truck, whether they be shorties or long tubes. I need torque and MPG more than HP, which seems to lead towards long tubes. Recently I had a muffler shop replace the exhaust with true duals and an x-pipe from the downtubes back. He did a really shoddy job and didn't put cats back in like I wanted. I will most likely end up rebuilding the exhaust from scratch myself.

    O2 sensor also has less than 10K on it. A complete TBI rebuild is in the near future which from what I've read will make a huge difference. The engine and intake itself (minus water pumps, alternators, etc) are still 100% factory original and untouched.
  5. stchman

    stchman Active Member 2 Years 1000 Posts

    A K&N may give an engine a slight MPG(up to 0.5MPG) or HP(up to 5HP) gain, but it will be very minimal. Don't expect your gas hog to become a gas sipping Geo Metro with a K&N.
  6. dobey

    dobey Epic Member 5+ Years 500 Posts

    Then you want shorties. Long tubes are for better for top end performance, and will cause a slight decrease in low end torque and MPG. They're great for NASCAR trucks that spend all day at 150 MPH on a track, in high RPM ranges, but they are horrible for real trucks in real world driving.

    City MPG depends more on torque vs. weight (more torque will move the weight of the truck faster), while highway MPG depends more on aerodynamics and overcoming drag. For city, you want to get up to speed, and in the highest resonable gear, as quickly as possible. For highway, you want to reduce drag as much as possible. Weight reduction and improved low end torque, and will give better bumps to city MPG. Reducing drag, and improved gear ratios will help highway MPG. Unfortunately, there isn't really a whole lot to be done on the trucks to reduce drag. Lowering helps, but not a huge amount. If you don't tow often, replacing the large flat-backed side mirrors with more aerodynamic mirrors is an option. A kit that would let you swap between the two, for when you do need to tow, would be optimal. I don't know if any exist though.

    And large studded off-road tires will kill your MPG no matter where you are. So avoid them.

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