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Thread: 6 cylinder vs. 8 cylinder

  1. #1
    Join Date
    Mar 2009
    Location
    Western Washington
    Posts
    194

    Default 6 cylinder vs. 8 cylinder

    Is there a big difference between in the mpg of a 6 cylinder ( in a 1966 IH truck not sure the size ) and a 345? I have my pick of the engines and was leaning towards the 6 cylinder so I could afford to drive this truck. Then I thought there may not be as big of difference as I think. Also where the 6 cylinders decent motors? It looks like both of the 6 cylinders in IH trucks where made by AMC and I'm not sure if that is good or bad or neither.

  2. #2
    Join Date
    Feb 2009
    Location
    Minot, ND
    Posts
    2,421

    Default

    Quote Originally Posted by damnesia View Post
    Is there a big difference between in the mpg of a 6 cylinder ( in a 1966 IH truck not sure the size ) and a 345? I have my pick of the engines and was leaning towards the 6 cylinder so I could afford to drive this truck. Then I thought there may not be as big of difference as I think. Also where the 6 cylinders decent motors? It looks like both of the 6 cylinders in IH trucks where made by AMC and I'm not sure if that is good or bad or neither.
    The 6 cylinder engines were made by IH until the switch in 69 to the AMC engines. In a D series (square) pickup or travelall, it's an AMC 6. In 68 or earlier, it's a 220, 240, or 265 IH, and those are 650 lb engines. They get about 10-20% better fuel economy in my experience, have a very flat torrque curve, and will only turn up to 3400 rpm continuous, so you lose some top end. I have a 68 travelall that gets 17 when properly tuned and the speed is kept below 55. Without the right carb, it drops dramatically.

    In your truck, speed is the key. It's not the rpms, but rather the wind resistance that makes the difference. If you have a running V8 already, keep it. For slightly better economy, fuel injection would be easier and cheaper.

    A properly tuned 304 can get a consistent 14-15 mpg without much effort at 50 mph. At 65, figure 11-12 mpg. That's my personal experience.
    Allan E.
    Curmudgeon Extraordinaire
    Charter Member, Old Hippie IH Club
    Old fashioned binder freak

  3. #3
    Join Date
    Oct 2010
    Posts
    7

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    While we are on the subject, I have a '68 scout with a 304 out of a 65 3/4 ton pick up and a pertronix ign. I have found that with the scout 800 'sleek' body the rpms seem to make all the difference and that is why I would so love to be able to put an overdrive in for daily drive use. But with a T-18 four speed I find that a consistent 55mph at 2500 rpm does OK. Also, with four drum brakes you probably don't really want to go much faster than that anyway.

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