304 vs 345 mpg

Discussion in 'General IH Tech' started by WesV, Jul 24, 2002.


  1. WesV

    WesV Y-Block King

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    I am at the stage in my PU restoration where it is time to decide what engine to put in. I have a 304 and 345 in about the same condition and I was wondering what I should put in. I want plenty of power but decent gas mileage. Did anyone swap one for the other and see a significant change in their mpg? Just for kicks the truck has a 3 on the tree and 4.09 diffs w/ 31 inch tires.
     
    Last edited: Jul 24, 2002
  2. ttgchicago

    ttgchicago Farmall Cub

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    Dont know what you mean by "plenty" of power, but with my 2 barrel 304 Im getting about 11-15 MPG (depending on certain variables like highway/city, top on/off, AC, etc.) which Im REALLY happy with!!! :)

    Its strong enough to pull a 3000 lb boat and to cruise nicely on the highway at 75, but its certainly no racer!

    Ive heard some gas mileage horror stories from Scout owners with the 4 barrel 345's. Dont know if they are the exception or the norm though.
     
    Last edited: Jul 24, 2002
  3. Jay Tabor

    Jay Tabor Banned

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    Iget about the same w/304/345. the 345 has an auto trans=10-11 mpg. the 304 has the man t-19 =12/13 mpg. so take your pick.
     
  4. David Van Vorous

    David Van Vorous High Wheeler

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    Before it sneezed, the '73 304 was getting 16 on flat highway doing 65+ and around 12-14 in town w/ 4 spd tranny and 3.73s.
    The '75 304 is running rich and gets 11-12 with a 3 spd and 3.54s on the highway.

    D.
     
  5. Chas

    Chas Binder Driver

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    For the round trip to RMIHR it was 217 miles and used 14.373 gallons. That equates to 15.09 mpg. I didn't keep my foot out of it and there was some stop and go...due to a buddies engine problems. That's with a 345/auto 4bbl square bore and 3.07's. Checking for city milage this week, last week it was about 12 mpg city. Changed plug gap, timing, added an electric fuel pump, added diff fluid and changed oil to Mobil 1 Saturday night for the trip to Salida.
     
  6. Jay Tabor

    Jay Tabor Banned

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    of the three 345 engines on customers scouts two with q-jets one with carter 4bbl. they all get 12-15 mpg the qjets are auto trans the carter is 4spd man. I forget the ratios all have 31/32 tires. mine are 3;54 ratios auto and man.
     
  7. Allan E

    Allan E Lives in an IH Dealership

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    For my pickups and travelalls with stock height tires and 4.09/4.10 gears, the 345 gives the same economy, but more power and more economy in the mountains and/or towing. If you are on mostly flat terrain, the 304 is adequate.

    I just pulled a light 19 foot flatbed with a 304/4.10/31" tires and got 11 mpg for the trip, and I didn't take it easy. There was plenty of power except at low rpm on the grades, and I had to downshift on the slower roads because the engine could feel the trailer. With no trailer, I don't have to downshift on the grades, but accelleration is slow. With the 345, there is obviously more power. Then engine doesn't work as hard, so you can light foot and save gas.

    The carb makes a tremendous difference. I swapped the carb from my truck to the travelall and put in a different one, and the economy went from 15 to 11. The truck still runs well, but it simply uses more fuel to do it.

    HTH

    -Allan
     
  8. Gambit

    Gambit Farmall Cub

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    Post revival! I have a 304, soa lift, 38x13.5 tires, and a tiny 2 barrel carb made out of a quadrajet core. Getting 10mpg hwy. takes 2500 rpm to go 60. Does anyone have experience to guess how my mileage would change going from a 304 to 345? Especially considering the tiny 2 barrel carb if I tried it on a 345. Buddy has a good 345 and I'm trying to decide if we should hang on to it or I should encourage him to sell it. Good things are good things but only the things you use are good for you.
     
  9. jeff campbell

    jeff campbell Lives in an IH Dealership

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    304 for mpgs. 345 if your pulling a trailer or need the more power. 392 is the tits !
     
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  10. scoutboy74

    scoutboy74 Lives in an IH Dealership

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    With your tire size, if all the figures you provided are correct, that would make your axle ratio approximately 4.71 to 1, which seems unlikely. So what is your gear ratio? If you're taking your speedo readings from the stock speedo gauge with no correction for the ginormous tire size, your reading is way off. You're likely going quite a bit faster than the speedo says.
     
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  11. scout2000

    scout2000 Lives in an IH Dealership

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    no one, and I mean no one, is running an IH SV engine, even a 266 for MPG's.

    My personal observation/opinion, and you know what opinions are worth, is that the 345 is probably the finest SV engine that IH ever produced. I've seen them do amazing things, and come back from incredible amounts of neglect spanning decades.
     
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  12. winchested

    winchested Y-Block King

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    Yup I'd keep the 345. More torque to roll those giant 38s sure would help. What's your diff gearing.

    And I assume you didn't put 38s on your truck thinking you're getting good gas mileage lol...
     
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  13. gibby

    gibby Farmall Cub

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    Differences in fuel economy between the two will be negligible. Power depends on what year the motor is and if your going to do any head work. If i'm not mistaken, some of the early 304s (68-71) had more torque and HP than the later 345s. The later 345s were quite detuned due to emission laws coming into effect
     
    Last edited: Nov 20, 2019
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  14. RBS

    RBS Farmall Cub

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    The difference in fuel "diseconomy" between two properly running SV304 and SV345 engines in a pickup is going to be largely insignificant and I would always opt for more power in this situation. A 304 in great running condition with modern fuel injection will have more power and economy than a hurting and abused 345 so you are going to get all sorts of data points but basically you have a 41 CID difference with a rotating mass that isn't significantly greater in a vehicle that is less aerodynamic than you would find in a worn brick with rounded edges. In this situation, even dropping down to a 196 4 cylinder isn't going to provide a huge increase in fuel economy because of the basic engine design coupled with a vehicle and powertrain not designed for efficiency. If the front end would support it, an old IH 304 or 345 with an 8L90 behind it in a current Camaro would probably provide pretty decent fuel economy as the old IH iron would benefit from an optimized transmission mounted in a light and highly aerodynamic platform or alternatively a modern LS with its matching transmission would provide much better economy in an old IH pickup or Travelall but that would take the fun and uniqueness away.

    GM adds a lot of hardware (and additional failure points) with their active fuel management system that shuts down half of the cylinders in some of their engines at light load, the system shuts down the intake and exhaust valves for half of the cylinders and this removes the portion of the power consumed from the engine trying to pull its full displacement through a partially closed throttle plate. My 2016 Z06 has AFM except I never let it go active because I didn't buy a Corvette to run in V4 mode. Those who use AFM typically find around a 5% fuel economy advantage in highway cruising and even this is an unfair comparison because an engine optimized without the additional AFM hardware would be even closer in fuel economy to what an AFM equipped engine achieves in pseudo reduced displacement mode. With the similar design between the 304 and 345, the additional 41CID doesn't result in a big difference in operating economy.

    Rodger
     
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  15. Gambit

    Gambit Farmall Cub

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    4.56 gears
     
  16. Gambit

    Gambit Farmall Cub

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    I just don't want to go from 10 mpg to 6 or 7. Apparently I want to keep the option to drive across a state or two and have it cost me 1K instead of $1400 lol
     
  17. Gambit

    Gambit Farmall Cub

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    I may have to get this 345 and clean it up and keep it lubed future options. Thx guys. My buddy says it was the most powerful 345 engine he drove. He doesn't know why. Did not have a modern fuel system or transmission I don't think. just a 4 speed stick.
     
  18. scout2000

    scout2000 Lives in an IH Dealership

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    But we have so much more fuel now. The earth keeps producing oil at a constant rate, but we're using less and less with each passing year due to all these battery powered vehicles. :)
     
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  19. Sozzy

    Sozzy Farmall Cub

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    304 vs 345= bad mpg vs really bad mpg
     
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  20. Mark B 70SR2

    Mark B 70SR2 High Wheeler

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    Yeah, you are talking 11.3 vs 11.6 mpg.. I'd go with the 345 unless the 304 is already in place.
     

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