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Thread: MPG with a turbo Gen 3 truck?

  1. #1
    Tuner in Training V8 Supra Builder's Avatar
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    MPG with a turbo Gen 3 truck?

    I'm doing some research for my current swap, a Gen 3 V8 into my 1998 Durango (originally a 5.9 Mopar Magnum) as to what engine to buy for the swap. I figured this would be a great place to get some real life experiences from those with a gen 3 turbo.

    I'm rebuilding a 4L80 due to the weight (about 5100 #) of the truck, and for when I may do some towing with it. The truck has 3.92 diffs and is 4WD. I am leaning towards buying a 2003-04 LM4 (aluminum block 5.3) out of a Trailblazer due to local availability along with low cost and weight but am still open to other options (4.8 or 6.0). Since I'll be commuting in the truck 60 miles a day, I want to build it for decent MPG. Unlike my prior car swaps, I'm not as interested in going fast.

    There seems to be some debate over MPGs for NA vs turbo trucks, so I thought I'd ask here for anyone that has some real life numbers for a turbo truck.

    I'd like to get the following data:
    Year/model of truck

    Engine size & mods (such as cam, heads, injectors, single/twin turbo, etc)

    MPG city/highway or average

    Thanks for any input.
    My prior projects include Chevy Gen 1 V8 into an FB RX-7, Gen 3 V8s into an FC RX-7, a MK3 Supra, a BMW E34, and an LT1 into a 280Z. Still have the Datsun, a 383 LT4 Trans Am, and some bikes.

  2. #2
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    Turbochargers add thermal efficiency to an engine so they improve economy.

    That said, its only about 1mpg or less usually.

    Other than that, MPG is a test done at 100% Cruise situation using cruise control. It has nothing to do with power output or other modifications unless they disturb the engines natural range for comfortable economical cruise continuous RPM which may be around 2500rpm for most V8 engines.

    MPG is mostly controlled by WEIGHT so if you are looking to consume less fuel then weigh less is priority #1 over all else it will have the most drastic effect. F=ma
    At 3000lbs I get 23-25mpg highway using 5.3 + 4l80e + turbo (not a truck weight) and I expect to be able to get higher with weight reduction.


    A 3000lb car with 2.0L Had 400 horsepower and 32mpg, and they swap in a 2002 5.7L six speed swap and it balanced the four corner of the cars better, weighed about the same, same power output, and also got 32mpg.
    Example: https://ls1tech.com/forums/conversio...ce-inside.html

    But even if the 2.0L only had 200 horsepower it wouldn't have made a difference. Actually now that I think about it, economy probably would be worse with the natural aspirated 2.0L, since the auto manufacturer would raise the differential numerically to make the engine more peppy and drivable that would increase highway RPM and lower MPG, also with no turbo there is no positive pressure intake tract forcing air against the throttle body because the only other really big thing for MPG is having too high a constant rotating speed and having to work hard to draw breath as with filthy air filters and long intake plumbing, are costs which a compressor wheel covers by using wasted exhaust gas energy.

    I'm right there with you about the mpg, I wouldn't have done the V8 swap without a turbocharger both because I know what a 2.0L turbo can do (more than a N/A V8 in that car) and because it allows the engine to stay extremely stock, reliable, and driveable while putting out more(double)-than-2L-turbo-power.
    Last edited by kingtal0n; 07-12-2018 at 11:01 PM.

  3. #3
    Tuner in Training V8 Supra Builder's Avatar
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    Thanks for your reply, it makes sense. My wife has a 2009 GMC Acadia (about 4,700 # curb weight) and it has a 3.6L V6. It only gets around 17 MPG average (mostly rural driving) and it's my belief that had GMC put a larger engine in it that it would get better fuel economy. Being a front wheel drive, there is not a lot of room to do so, and the V6 is a nightmare to work on. I had a similar year Dodge Challenger (about 4,000 # curb weight) with a 5.7 Hemi and manual transmission and it easily got 25 MPG on the highway. One telling thing to me is that the GMC cannot hold a set speed while on cruise control on rolling hills- it varies from +/- 5 MPH, while the 5.7L Challenger and the 5.9L Durango would hold within 1 MPH.

    I'm looking forward to reading more input on this topic.
    My prior projects include Chevy Gen 1 V8 into an FB RX-7, Gen 3 V8s into an FC RX-7, a MK3 Supra, a BMW E34, and an LT1 into a 280Z. Still have the Datsun, a 383 LT4 Trans Am, and some bikes.