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8.1 and Allison Trans

Discussion in 'The Garage' started by sjvcon, Apr 6, 2005.

  1. sjvcon

    sjvcon Registered Member

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    Anyone ever consider this swap in the '73 - '87 ('91) body styles? Are the mounts on the 8.1 the same as the 7.4? I had heard a rumor that GM cast two mounting locations on the block and that you could use the second set (matching the 7.4). Anyone know if that is true? Also, how would one mate the Allison to a doubler. Any suggestions would be most appreciated.
     
  2. BIG STEVE

    BIG STEVE Registered Member

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    Great combo got that set up in my 02 HD
    the electronics would be 2x the logistics problems
    they have been hashing that allison swap around since it came out
    havent seen one or read about one yet but maybe some one has?
     
  3. BigCountryx

    BigCountryx 1/2 ton status

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    It would be possible, cause I've seen it done to a Lil Red Wagon (Dodge Stepside) But it looked to be a nightmare from what I saw.
     
  4. Thuren

    Thuren Registered Member

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    The worst part would probably be getting the "fly by wire" gas pedal electronics in and working right.. The gas pedal response is horrible also, on my 01, but I'm not sure if they have updated the setup in the last few years.. From what I heard the block /bore is the same as the 7.4 but with a larger stroke crank... :)

    Don
     
  5. ZooMad75

    ZooMad75 1/2 ton status Premium Member GMOTM Winner

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    I've got an 8.1 sitting in my garage right now. I would have to do some comparisons to a 7.4 for mount location, but I don't have a 7.4 to compare too. 8.1's are not a stroked 7.4. They are all new including a different firing order. Only parts that could bolt up to an 8.1 to from a 7.4 is the valve covers and even then the 8.1's have the 4 coils per side mounted on them.

    I have not heard of anybody swapping a 8.1/allision combo into a 73-87 truck yet. The 8.1 should not be hard to fit, but the allision is huge and would need at least a 2" body lift or major floor surgery to make it fit. (the H1 Alpha needed a body lift for the allsion/dmax combo too) Not to mention the length. I would bet it would not be worth it on a short box or k5 because the rear d/shaft length would be so short. Long box or crew cab would work though.

    As for making it run, I don't see a problem with the throttle by wire. It works well in everything GM uses it on (which is about all the trucks, vettes, GTO/CTS-V). From the research I have done so far, you can run the Marine spec controller with a marine harness and gain HP/TQ over a standard truck calibration. Mercury Marine sells a ton of 8.1s for boats as well as get this, Volvo Marine. The Marine controller is similar to the ones used in the GM Ramjet FI crate engines.

    My plans for this 8.1 is to put it into a 64 Chevelle project I have with a 4L80e. Should run like a beast and still idle like a kitten.
     
  6. Thuren

    Thuren Registered Member

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    They are ALL new but the valve covers bolt up?? I have one in my 01 Silverado and the block sure looks like a 7.4L... I did hear somewhere that the 8.1 and 7.4 share the same bore diameter... :)

    Like I said before, I have one, and the throttle response is off. If they have fixed the problem great and if it's just my truck cool, but I've talked to a few people with the same problem... Daily driving you barely notice it but when creeping in 4LOW and trying to time the pedal with the huge amount of torque it gets you into problems... Kinda like this...


    "You nail the gas FAST trying to blip the throttle, and the engine is just a tad behind, you let off once the obstacle is cleared and the engine is still charging a bit." Really opens your eyeballs.... I hope it's just my truck actually but I don't think so. A way to convert it back to cable would be sweet... :D


    Don..
     
  7. 1977k5

    1977k5 3/4 ton status Vendor

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    I think they intentionally made the throttle less responsive while in 4 low. Supposedly GM thinks people like that better.
     
  8. ZooMad75

    ZooMad75 1/2 ton status Premium Member GMOTM Winner

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    Let's see you go bolt up a set of 8.1 heads on a 454. Its not going to work. The 8.1 is different, plain and simple. It shares some characteristics with the previous generation of big blocks as much as the new LS style small blocks have in common with the design that came from 1955. Which is to say some of the dimensions are the same but that's about it.

    Yes the valve covers are similar, why I don't know they just are. Heads are different in that they have tall/cathederal syle ports much in the same as the LS small blocks do. Coolent passages are in the head and the intake is dry with no coolant passing though it. The thermostat is located in a crossover tube that connects the cylinder heads together. Blocks are different as they have cast in provisions for Crank/cam sensors and so they match up to the revised cooling passages on the head.

    I'd love it if they were the same as I could have bolted a 4bbl intake on in and stuffed it into my K5 in a heartbeat. But running the fuel injection and all that comes with it would just be overkill for my dd k5. It will be sweet with everything in my little Chevelle though.

    As for the electronic throttle, minor changes to the calibrations have been made as far as I know, but all the 8.1s I deal with lack 4wd so I can't be for sure why yours does what it does in 4lo. Retrofitting the electronic throttle has another benifit in an older vehicle. Cruise control is all performed by the TAC (throttle actuation control) module. All I need to add is a the switch inputs. (I will have to use a stock style PCM instead of the marine unit though)
     
  9. Thuren

    Thuren Registered Member

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    I'm saying that the throttle response is off time, delayed, or wrong..... Not slow, and it's all the time... :)

    Don
     
  10. 75-K5

    75-K5 3/4 ton status

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    Yep, same deal with my '05. It's caused me to stall it out at lights more than once before.
     
  11. Thuren

    Thuren Registered Member

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    I see your point but I think the differences are more like comparing a 1992 small block and a 1996 Vortec small block... The heads wont swap but the ENGINE MOUNTS, starter, trans bolt pattern, crank bolt pattern, distributor, etc are interchangable..... with respect to the thread title.... The "engine swap" should be easy...

    The Vortec 5.7's have a changed block too... everything bottom end wise bolts up....


    good call on the benifit of the cruise... didn't think of that... :)

    Check this out.... 8100 info

    So the standard CHEVY trans pattern is there, the valve covers are the same, the water pump "LOOKS" to have the same chevy pattern, the bore centerlines are the same...... I'm thinkin the external bottom end is a good, easy swap... :)
     
  12. Thuren

    Thuren Registered Member

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    05??? I guess they didn't fix it.... :confused:
     
  13. sjvcon

    sjvcon Registered Member

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    Thanks for the input guys. I think that I will stick with my plan of a 454 in the Burb. 8.1 seems to be a bit of a stretch, and the fact that it has bugs is not too appealing. If I wanted to go through as much as I would have to for this combo, I might as well bite the bullet and go Cummins.
     
  14. BigOrange90Jimmy

    BigOrange90Jimmy 1/2 ton status Premium Member

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    Our 01' DMAX is like that also. But it only lacks response in 4-lo. I assume it's to keep from blowing the t-case or rear end up. 500 some odd ft. lbs. of torque is quite a large bomb to drop on an all aluminum t-case at a split second, much less the added reduction from the allison 1st gear.
     
  15. k20

    k20 3/4 ton status

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    I got an 8.1 in mine :D :D :D O wait, your talking about the new one, nevermind.
     
  16. thezentree

    thezentree 3/4 ton status

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    Whore...

    Why would swapping an 8.1/Allison cause more headaches than any other modern FI engine/computer controlled tranny combo?
     

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