3800 series II or III swap into 79 deville? - Page 4
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RWD 19xx-1984 DeVille and Fleetwood,
1985-1996 Fleetwood and Brougham Forum Discussion, 3800 series II or III swap into 79 deville? in Past Cadillac Vehicle Discussion; Yeah, those 3800's will get 30-32 mpg all day long.....in a modern, 3400 lb, aerodynamic FWD Grand Prix....not a '79 ...
  1. #46
    I~LUV~Caddys8792's Avatar
    I~LUV~Caddys8792 is offline Moderator
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    Re: 3800 series II or III swap into 79 deville?

    Yeah, those 3800's will get 30-32 mpg all day long.....in a modern, 3400 lb, aerodynamic FWD Grand Prix....not a '79 Cadillac. You might see 24-25, if you can get it installed and wired up.

    I'd just save the time and effort and buy some old late '80s era beater with the 3800.
    "They call Alabama the Crimson Tide, call me Deacon Blues."

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  2. #47
    ferrisworld is offline Cadillac Owners Fanatic
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    Re: 3800 series II or III swap into 79 deville?

    Well, even 24-25 if a huge improvement over the 16-17 I typically see.

  3. #48
    brougham is offline Cadillac Owners Master
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    Re: 3800 series II or III swap into 79 deville?

    You probably won't even get that from it, and if you do it'll never pay back for the work involved.
    There's no point to northstar it. It wouldn't be a huge jump if any for fuel mileage and then you'd also be stuck with a whole pile of other problems.

  4. #49
    ferrisworld is offline Cadillac Owners Fanatic
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    Re: 3800 series II or III swap into 79 deville?

    Yeah, these things are all potentially true, but it's also true that payback isn't always monetary.

  5. #50
    csbuckn is offline Cadillac Owners Master
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    Re: 3800 series II or III swap into 79 deville?

    I still think you should do it. Then we can have some hard evidence of mileage instead of educated guesses. Either way, I think your gonna have a decent upgrade in mileage, the reliabilty of a 3800 and overdrive. Just imagine overdrive with 2.28 rear gears...sounds like 30+mpg on the highway to me. 65 mph at 1700 rpm would be nice to have.

  6. #51
    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
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    Re: 3800 series II or III swap into 79 deville?

    OD with 2.28s would be useless. The engine would be spinning way too slow to be efficient.

  7. #52
    csbuckn is offline Cadillac Owners Master
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    You think? Lower then 1700? I don't think it would be too low. What speed do you think it would run at? Isn't there a calculation to do to figure out final drive rpm? Or you could cruise at 85 if the rpm is too low for 65.

  8. #53
    jayoldschool's Avatar
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    Re: 3800 series II or III swap into 79 deville?

    The highest gears that ever came with an OD/RWD that I know of are 2.56s with a TH700R4/4L60(E). They are marginal, and cause the trans to constantly shift in and out of OD with any type of acceleration or hills. My best friend has this combo in his Caprice wagon, and driving it is a big difference than my identical wagon that has the trailer package 2.93s. My car stays locked in OD unless I hammer it. Mileage is the same between the two cars because my car is closer to peak torque at operating speeds.

  9. #54
    Bro-Ham's Avatar
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    Re: 3800 series II or III swap into 79 deville?

    Quote Originally Posted by ferrisworld View Post
    Well, even 24-25 if a huge improvement over the 16-17 I typically see.
    You're lucky, I have never been able to get over 15 mpg in any 425 Cadillac I've ever owned.

  10. #55
    outsider is offline Cadillac Owners Connoisseur
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    Re: 3800 series II or III swap into 79 deville?

    those MPG sound better than what my tired old 307 gets :$

  11. #56
    ferrisworld is offline Cadillac Owners Fanatic
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    Re: 3800 series II or III swap into 79 deville?

    My dad used to have an 89 lesabre wagon with the 307. We got 23 highway.

  12. #57
    Stingroo's Avatar
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    Re: 3800 series II or III swap into 79 deville?

    On what planet? Did you have like, the best tuned 307 GM ever produced?

  13. #58
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    Re: 3800 series II or III swap into 79 deville?

    I used to get good mileage in my 87 Caprice with the 307...

  14. #59
    ferrisworld is offline Cadillac Owners Fanatic
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    Re: 3800 series II or III swap into 79 deville?

    We were probably only doing 60 - 65 in it.

  15. #60
    csbuckn is offline Cadillac Owners Master
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    Re: 3800 series II or III swap into 79 deville?

    So put the 3.8 in with new rear gears and see what happens. I've had plenty of 3.8s and I think they would have no problem pulling the Caddy around. Definitely one of the best daily driver motors ever made.

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