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GM 6.2L Diesel or 4bt advice requested

Discussion in 'Diesel Tech and 24 volts Systems' started by 4BTFJ80, Jan 13, 2012.

  1. 4BTFJ80

    4BTFJ80

    Messages:
    123
    Location:
    Rosamond, CA
    I've decided to install a diesel into my '91 FJ80 and originally decided to go with a 4bt turbo diesel. I already have a 700r4 trans and the advanced adapter & FJ60 split-case.
    I'm wanting good mileage hopefully in the 22-25 range on the highway and to be reliable and not cost me a small fortune to complete the swap. I have read that the preferred 6.2L is from a '88-93 Chevy or GMC J-model.

    I found a 6.2L from a '84 Chevy pickup 20 series for $450 from a wrecking yard. I asked about the miles on it but they do not have that info but they said they did a compression check and is good, so what should be the range?

    Their description is: 8-379 (6.2L diesel), AT, w/o EGR, Vin "J", Atlas Turbo, Runs Great. Does this sound like a good deal and a good motor? Is this the J-series motor because of the "J" Vin code but it is a '84 so I'm confused.

    Please shed some light and thoughts about this motor (good or bad) and any other suggestions or advice so I can decide which diesel would give me what I'm looking for. Thanks.
  2. The early 6.2 had a two pc rear main seal. The last of the 6.2, and all the 6.5 had a one pc rear main seal. I don't remember what year this change took place, but it was in the very late 80s, maybe 90. The early 6.2 (82 and 83) and the late one pc rear seal 6.2 (90? to 92?) are all considered more desirable for rebuilders as they are more likely to have crack free main webs.

    The 84 you have located is in the area more likely to have main web cracks. But, it could be crack free, have them remove the pan and inspect it.

    What gear ratio is in the FJ80? The 6.2 likes 1800 to 2000 RPM at cruising speed (60 to 70 MPH?) to get good MPGs. Use the gear ratio calculator at grimmjeeper.com to plug in and play with your tire size and gearing to get your RPMs at the optimum range. With a turbo on there you are less likely to get the MPG you are looking for. The NA motors seem to be the ones that got the better MPGs.

    Don
  3. 4BTFJ80

    4BTFJ80

    Messages:
    123
    Location:
    Rosamond, CA
    gear ratio & tires size

    My '91 FJ80 has stork gears and should be 4:11's and the tires are 305/70/16 MTR's so are 33". The 700r4 was built for a diesel and has 10K miles behind a 4bt from PO who went to a MT. That is a cool calculator from grimmjeeper.com and looks like the stock gears with the tires I'm on will be ok. It looks like I'll to be able to run down the road at at 61 MPH @ 1750 RPM, 70 MPH @ 2007 RPM, 75 MPH @ 2250 RPM. I have no problem keeping it between 60-70 MPH for highway driving.
    Last edited: Jan 14, 2012
  4. FL cruiser

    FL cruiser

    Messages:
    1,304
    Location:
    St petersburg Florida
    The J code just means it's from a 3/4 or 1 ton truck with less emission requirements. The C code engine's intake has an EGR while the J code engine doesn't.

    In the description it shows a turbo. At that price with a turbo kit included, it's a good deal. It's probably an ATS kit.

    If the engine is junk, you could probably sell the turbo setup to recoup your money.
  5. 4BTFJ80

    4BTFJ80

    Messages:
    123
    Location:
    Rosamond, CA
    To be able to get my FJ80 registered in California with the 6.2L diesel do I need to find one with the "C" code or can I use a "J" code motor?
    Does anyone know what kind of mileage I can expect with a NA motor and what to expect with a turbo version?
  6. FL cruiser

    FL cruiser

    Messages:
    1,304
    Location:
    St petersburg Florida
    My NA 6.2 FJ60 got 18 MPG hwy. 15MPG city. When I turned down the fuel on the pump and if I kept the hwy speed under 65MPH and feather footed it, I got 19.5 hwy/17city.

    It's painfully slow if you adj the inj pump for mileage. I'd rather turn up the pump, lose an MPG or two and have some decent acceleration.

    The 6.2 only got 1.5 MPG less than my HJ60 or Isuzu 60's. Fairly good for an engine it's size. Power for engine size seemed down though.
  7. What tranny, gear ratio, and tire size were you working with? What was the engine RPM at 65 MPH?

    Don
  8. FL cruiser

    FL cruiser

    Messages:
    1,304
    Location:
    St petersburg Florida
    It had an NV4500, 3.70's and running 33" tires. At 70MPH it was turning 2050 RPM.
    Roof rack probably had an effect on mileage.
  9. I was curious as I'm wanting to put a diesel in my 40. It most likely will be a 6.2 as that is closest to my financial abilities. I already have a 203 doubler mated to a later 70s TC, plan to use an H42 tranny, and a Ranger OD to adapt the GM bellhousing to the Toyota tranny.

    What year motor is yours? I'm just wondering which precups you've got as that will make a bit of difference in mileage.

    The 84 motor that 4BTFJ80 has located may be early enough to still have the smaller precups that were better for mileage. I am thinking it may also be late enough to have the smaller valves that GM used to reduce the head cracking between valves that the first years had.

    Don
  10. FL cruiser

    FL cruiser

    Messages:
    1,304
    Location:
    St petersburg Florida
    Mine was an 82. Assuming the heads were never changed, it must have been a late 82 since heads had the (83 up) fine thread inj zozzle holes. The block had the very early rope style rear main seal which leads me to believe it was the original engine out of the 82 truck I puled it out of.
    Never had the heads off, so can't tell if they were small or large valve or know if they've ever been replaced.
  11. rtarh2o

    rtarh2o

    Messages:
    936
    Location:
    NE Texas
    I was getting around 18-20 on the highway with my NA 6.2 FZJ80. From what I have read the rpm range is critical, there is a substantial drop, (think it was from The Diesel Place Books) something like 1-2 mpg for every hundred rpm over 1,800 Can't remember the exact number but it was surprising.
    Seems like I also read that there really isn't much difference in mileage between the turbo and non.
    I agree with FL Cruiser, the fuel pump makes a big difference, it really comes to life when you turn the pump up a bit, it was worth a couple mpgs to feel the power.
    I have a 78 FJ40 project set up for a small block already so I am pretty sure I will be doing another soon.
    Good luck
    Rusty

  12. Thanks FL cruiser and Rusty. That's some good info to have, very helpful for me, and I hope also for 4BTFJ80.

    As far as I know my 83 is still stock. I'm surprised your block had the rope seal, not the newer style. I thought that had been stopped years ago.

    Don
  13. davegonz

    davegonz Keeping it Weird SILVER Star

    Messages:
    5,255
    Location:
    Austin, TX
    I'd go with the 4BT simply for it's size, simplicity, and dependability. The 6.2 might be cheaper to get, but I've had experience with a bud's 6.2 powered 80 and it just wasn't dependable. The upside to the 6.2 is that it's smoother and quieter than the 4BT.
  14. 4BTFJ80

    4BTFJ80

    Messages:
    123
    Location:
    Rosamond, CA
    4bt + 80 series in CA

    I'm aware of the noise and vibration with the 4bt but it should'nt bother me if I install sound proofing and hydraulic motor mounts. Money is one hurdle to get started so it depends on selling my 25' 5Th wheel travel trailer that I have'nt used as much as I'd had hoped, so it must go and the swap can begin.
    The second hurdle is how to get the 4bt legally registered into my '91 FJ80 in California. What years does the 4bt have to be out of and what about the weight class? Frito-Lay vans have a higher GVWR so does this create any problems? The GVWR of my '91 FJ80 is #6525. Has anyone done this swap in their 80 series in CA and if so what did you have to go thru? Thanks for everyones help.

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