5.3/T56 swap MPG?

Discussion in 'High Tech Retrofits' started by rumblebox, Aug 3, 2018.

  1. rumblebox

    rumblebox New Member

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    I googled first and found nothing. I'm going to be using the 79 Z28 as a commuter. So I'm considering a 5.3 instead of 6.0 or 6.2 since it would still make double the power it came with. What are you 5.3 guys getting for hwy gas mileage with an OD?
     
  2. badazz81z28

    badazz81z28 Veteran Member

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    A little more to It then that. Rear gear? Tire diameter? Stock 5.3? What 5th and 6th ratios in the T56?

    You could easily get 20-22mpg
     
  3. rumblebox

    rumblebox New Member

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    Turning 2k @80 on 315 35 17's. 5.3 with a 216/220ish @.50 cam.
     
  4. rumblebox

    rumblebox New Member

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    But im really just looking for experiences with whatever setup people have
     
  5. redriderjf87

    redriderjf87 Veteran Member

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    I calculated 22.5 mpg out of mine, which is a SBC with a hot-ish cam, turning 2500 rpm at 75 mph (4.11's and a TKO 5-spd). Even at 80, with a 5.3 and lower rpm's you should be able to exceed that.
     
  6. jeff swisher

    jeff swisher Veteran Member

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    You can actually turn too low of an rpm for a given engine size and vehicle weight/drag and hurt the MPG.

    I have seen it a few times 1700rpm at 70 MPH could be lugging the engine and getting up around 2200 rpm could result in less throttle opening and better MPG.
    Carburetors respond very positively to less throttle opening because you could be cruising on the highway in the idle circuit instead of the mains.

    Another big issue is say 1700rpm cruise only allows 22 total timing. you could see better MPG if you had 35 or more degrees at that cruise speed.
    Many variables.
    With the 268H comp cam 218@ .050 on a 110LSA I have got 18+ MPG many times with it and NO Overdrive.
    Old school carbed 350's

    I have not been very impressed with the fuel injected LS in the MPG department.. but could be the ones I have dealt with were still in the heavy trucks 2012 and 2013 year.
    They get 13 and 15 MPG on a good day. Not impressive.

    I feel it may be due to cool fuel and poor velocity or atomization.
    It is what it is.
     
  7. rumblebox

    rumblebox New Member

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    That's the issue I'm having with my Denali. The L92 heads are too big to be useable below 2500, so i actually get better mileage in 5th gear doing 2500@80 than in 6th doing 2000@80. So I'm about to just do a 4.56 gear swap.

    Cathedral ports tend to get a lot better MPG from my experience. My 99 silverado with a 5.3 got almost 20 on the highway, and my 99 formula got almost 28 with headers.
     
  8. redriderjf87

    redriderjf87 Veteran Member

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    I agree you can go too tall, but I wouldn't have guessed that would've been too tall for a stock-ish cam to handle, in a car at least. That's in the ballpark of stock LS1 4th gen gearing which I'm guessing would handle 80 all right.
     
  9. rumblebox

    rumblebox New Member

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    The problem really is the wind resistance. The extra weight matters a little, but not as much as the drag
     
  10. redriderjf87

    redriderjf87 Veteran Member

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    Agreed, that's what I mean. Sorry I don't really have a good gauge on how much low end torque a 5.3 with a street-ish cam would be making in that situation at 2000 rpm to hold the car at 80.
     
    Last edited: Aug 15, 2018

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