Increase in mpg w/ bigger throttle body

Discussion in 'Fuel Economy' started by Gageraid, Aug 6, 2014.

  1. Gageraid

    Gageraid Well-Known Member

    I recently changed out my throttle body. My engine is a 3.7L and I slapped on a 4.7L throttle body. I also have a cold air intake, exhaust and programmer.
    I'm currently on my second tank of gas with the mod. My first tank was 20.02mpg (hand calculate).
    That first tank was 85 percent highway at very fast speeds. Prior, I was getting 17 city and 18.5-19 highway. Looks like the gain will be at least 1 mpg. I'll keep an eye on it over the next few tanks.
    A little more info about the throttle body... The stock one is 2.5 inches. The new one is 3 inches. The new throttle body pairs very nicely with the cold air intake since the pipe is 3 inches wide.
    The Jeep's performance has increased and not by a little, it's very dramatic!
  2. phoebeisis

    phoebeisis Well-Known Member

    What is the WHY behind this improved FE?
    Bigger diameter TB means the throttle plate has to be closed a bit more for a given load??
    Why would that help part throttle FE?
  3. Gageraid

    Gageraid Well-Known Member

    I haven't figured it out yet. I think the air is just less restrictive coming in now.
    I can feel it with my foot that I don't nearly need the gas pedal as I used to.
    Been able to coast further and it holds speed better when off the gas.
  4. brick

    brick Answers to "that guy."

    If the car is driven hard then eliminating a restrictive intake (if in fact that's the case) could conceivably help. But for normal street driving, especially if you're trying to be efficient, I don't see it helping. Most of us operate our engines well under WOT and at relatively low revs where airflow really isn't an issue. Other stuff could be at play, though. Performing a mod that you think will make your car more efficient often causes you to drive more efficiently, in which case you're still winning.
  5. RoadWarrior222

    RoadWarrior222 rockit serjun

    IMO, performance mods are often FE mods, as long as you don't USE the performance part.

    However, it's also possible you've inadvertently fixed something like a flaky TPS or leaky vacuum line, or a throttle plate that didn't seal well.
  6. phoebeisis

    phoebeisis Well-Known Member

    Road warrior might have a point
    Accidentally "fixing something"
    The old "something not working right"
    usually a carb- you take it apart-fool with it
    but don't really find a problem
    put it back together-and it "works better"
    Sometimes you never know what you "fixed"

    Just stumble around and get lucky
    It has happened to me

    And of course-just changing something-and expecting an improvement
    especially if you put some $$ into it
    You will unconsciously drive to get better FE- just like brick suggests.

    I am old-tried all sort of FE and performance mods in the past
    I have now come to the conclusion that GM( 98 suburban)
    and Toyota OEM engineers are "better" at tuning for good FE than I am
    Now I"tune" me
    and pull seats out of the suburban- 130 lbs or so-for weight loss which always improves city FE(not so much that I could easily measure it-but I know it does-so it makes me happy)

    Weight loss-tire pressure-maybe aero mods-are about it for DIY FE mods
  7. RoadWarrior222

    RoadWarrior222 rockit serjun

    Yah, but they tune for worst case scenario, and bean counting for mass production harms top potential.

    For example, you can probably drive across the low parts of Arizona midsummer on "mountain gas" and it will cope... meaning that in all other situations you could be running about 3* more base ignition advance if you can figure a way to get it in there.

    Similarly production tolerances will have cams as much as +/- 5* off their spec timing. In some cases that advance or retard will be better than spec for MPG, in other cases not...

    Then cores shifts in manifolds to head fitment may help or hinder things... it all averagely works, and 10 cars driven on EPA test regime will probably average stated EPA mpg.

    Anyway, I wouldn't say you'd easily do better than the works team at a WRC or Touring car event, but you can definitely improve what rolls off the regular production line.
  8. Gageraid

    Gageraid Well-Known Member

    That would be pretty funny if I just simply fixed something.
    A little more info in case you'd like to know.
    Like I said my engine is the 3.7 and the new throttle body is off a 4.7.
    The 4.7 is identical, just has two more cylinders. Makes sense that it would work better IMO.
  9. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    Don't see why it would help? Sure possibly more power, but better mpg? More power takes more fuel.
  10. RoadWarrior222

    RoadWarrior222 rockit serjun

    More power after you hit peak efficiency takes more fuel. ... usually though you have to do something pretty drastic to be able to use more fuel, like have longer duration cams or beef up the valvegear so you can rev to 7000 RPM or something.
  11. Gageraid

    Gageraid Well-Known Member

    More power so I can coast longer... that's my theory for city driving and maybe the more air is making the air/fuel mixture leaner at highway speeds.
  12. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    That's why I said "possibly" more power. I doubt even that. Yet Gageraid is claiming big power gains.
  13. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    Higher speed beginning coast would allow you to coast longer. How would more power change that?
  14. RedylC94

    RedylC94 Well-Known Member

    That's the most plausible explanation so far.
  15. phoebeisis

    phoebeisis Well-Known Member

    Wish I could take credit for it-and and I certainly would if I could delete Roadwarriers earlier post

    But- I clearly ripped Road off
    However, it's also possible you've inadvertently fixed something like a flaky Tps or leaky vacuum line, or a throttle plate that didn't seal well

    And I most certainly have bumbled around taken "something" off stared at it for a bit
    Found NOTHING wrong
    Put it back on
    and Voile' damn thing was "fixed"
    Yeah my staring at a part I pulled off
    sometimes has magical properties

    But my guess is this argument is the old
    "Volumetric efficiency vs energy efficiency"
    Volumetric efficiency can be just "how much you fill a cylinder-90% 110%"
    And it usually is more applicable to PEAK POWER WOT

    Absolute energy efficiency is energy in-energy out-I think Toyota claims 37% with the Prius
    Good TDs maybe 40%
    Good spark engined cars maybe 30%
    These are from memory figures-and subject to my sneakily changing them if you guys say otherwise.

    Anyway PART THROTTLE FE- doesn't usually have much to do with volumetric efficiency- more to do with pumping loss and friction losses which are more or less directionally proportional to engine size number of pistons
    small engines with power outputs well matched to load-get best FE
    AND TURBOS- can get more for less
    fewer RPMS less ring to cylinder "contact" per HP output(lower friction loss)

    Sometimes you can use a BIGGER displacement engine-and turn it fewer RPMs-
    Toyota did it when they switched from the 1.5 to the 1.8(probably because they KNEW they were going to be building the heavier V model
    yeah probably could have maxed the 1.5 and improved FE more

    Anyway-always interesting stuff

    But I have gotten old-given up on trying to beat the OEM engineers FE wise
    except with weight loss-tires -tire pressure-and "someday" a Basjoosing of whatever suburban I have as a road tip second car pet evacuating vehicle (4-5 cats 1-2 dogs usually-3 adults+Prius)
    and various hypermiling modifications to me.
  16. Gageraid

    Gageraid Well-Known Member

    Thanks for keeping this thread full of information.
    For what it's worth, I just filled up for the second time for 19.03mpg.
    Mostly city, but still higher than usual. Hand calculated.
  17. some_other_dave

    some_other_dave Well-Known Member

    I figure that there are only a very few ways that "bolt-ons" can help MPG.

    1- If the original part was very inefficiently designed, often for cost-saving reasons. Many manufacturers, especially in the years before FE became important and CAFE standards were in place, would use a cheap part that caused the engine to be less efficient than it could be.

    2- The part that was replaced actually had something wrong with it in some way. Most power and/or efficiency gains for fancy expensive spark plugs are probably in this category--the old plugs are worn and not performing correctly, the fancy new plugs are new and perform exactly as well as new standard plugs would, but they are being compared to worn-out plugs.

    3- The new part increases emissions while at the same time helping efficiency a little bit. A home-brew lean-burn system would fall into this category, because you can cruise using less fuel but you produce much more oxides of nitrogen in lean burn. Some diesel mods also are this way. And, in the cases where removing a catalytic converter actually helps FE, it definitely falls into this category. (Sometimes there are other constraints, like noise restrictions, that are violated rather than emissions regs.)

    4- The new part will not last as long as we expect our engines to last, or will cause some other engine system not to last as long as most people feel it should last. Removing an air filter, for instance, might help FE in some cases but it would tend to lead to the engine wearing out relatively early.

    There may be other ways, but these are by far the most common ones. And modern vehicles generally don't try to save money by getting worse fuel economy, because people now buy in part for economy, and because every MPG they can squeeze out means more pollution-heavy and profit-heavy vehicles they can sell otherwise. (Read: Luxo-barge SUVs, super high-end sports cars, etc.)

    I believe that it is very unlikely that any car made in the last 2-3 decades would leave easy fuel economy on the table, unless you are willing to compromise other things.

  18. RoadWarrior222

    RoadWarrior222 rockit serjun

    Well I only see it as last 5 years that one the whole they've really picked up their game across the board, various manufacturers with various models had spates of "lets do fuel economy" but if they weren't losing sales to it, they didn't bother much.

    Even then, as important as it may be, manufacturers are NOT going to hand fit and finish every individual part to wring out the last few percent.

    An example of close fitment parts that have now become production items though, due to improvements in tolerances, are crank scrapers and windage trays. Wanna burn 3% of your fuel throwing oil around? Nope, me either, modern engines have much better windage control than 5 or 10 years ago, it is still absent on many older designs though.

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