GXP-ify my sts?
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Northstar Engines and System Technical Discussion Discussion, GXP-ify my sts? in Cadillac Engine Technical Discussion; Howdy, Here are some questions to spur a theoretical debate. I am having some serious engine troubles with my 97 ...
  1. #1
    maydog's Avatar
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    GXP-ify my sts?

    Howdy,

    Here are some questions to spur a theoretical debate.

    I am having some serious engine troubles with my 97 STS. I found a guy who will source a used engine and install it for $1500 with a 1 year warranty. This was the best deal I could find.

    Anyway, the dangerous tinkering side of my brain has been active for the past few days. I have seen that there seem to be more 275hp SLS motors available than the STS motors. I have read at least 1 account of swapping a 275hp northstar into a deville with no other mods and the car ran well.

    Not that I would do it but...

    How would the stock computer handle the different engine? I have a low backpressure exhaust and could free up the intake some for the engine to breathe better at the high rpms.

    If the stock computer couldn't handle the engine, would a computer from a SLS be able to be swapped? My assumption is that the wiring is all the same.

    What could I expect from such a swap - better off the line performance, reduced top end performance, driveability problems, reduced MPG, explosions?

    What are your thoughts.

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    danbuc's Avatar
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    Re: GXP-ify my sts?

    The computer from an SLS, would not be able to properly control the STS transmission, since they are different. The final drive ratio is much shorter, and the shift programming is different too. Not sure exactly how you would get it to work, unless you purchased the PCM out of a Bonneville GXP. Even then, it wouldn't work with the rest of the car's various components, so it would be really difficult to get working correctly.

    What kind of trouble are you having with your engine? Is it a bad headgasket, or something more serious that cannot be fixed?

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    Re: GXP-ify my sts?

    I see no reason why you couldn't install an LD8 engine into your STS. They're the same except for the cam timing and the PCM only responds to the inputs from engine sensors so it shouldn't have a problem either. The transaxles are different (final drive ratio) but since you're keeping the original one, it will still be fine with your original PCM. Basically, you're going to be doing what Pontiac did with the GXP, right? I've always felt it would be a better combination than either the STS or SLS combination. The thing should be strong as hell off the line!

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    Randy_W is offline Banned
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    Re: GXP-ify my sts?

    Quote Originally Posted by Katshot
    I see no reason why you couldn't install an LD8 engine into your STS. They're the same except for the cam timing and the PCM only responds to the inputs from engine sensors so it shouldn't have a problem either. The transaxles are different (final drive ratio) but since you're keeping the original one, it will still be fine with your original PCM. Basically, you're going to be doing what Pontiac did with the GXP, right? I've always felt it would be a better combination than either the STS or SLS combination. The thing should be strong as hell off the line!
    Katshot is right, the GXP already shifts at about the same rpm as the old STS engine did, so obviously it won't hurt it. That should be one bad street ride, combining the torque of that engine with lower gearing.

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    Re: GXP-ify my sts?

    I purchased my car with 120K on the clock. The salesman told me than an older salesman purchased it new, all highway miles - yada yada.

    Anyway it has been smoking upon a hard deceleration for some time. Some folks on the board said that sounded like valve guides? I could live with it since it seemed the worst with a full oil resivor and subsided if the oil level was lower.

    Lately the car has been very rough upon startup. It started loosing coolant, the exhaust smelled like coolant. In the final days it appeared that coolant was getting into the oil and the car ran terrible.

    I thought about taking care of the car myself, but I just do not have the time with having a 1 1/2 year old, a wife, a full time job, and a graduate level class. This car is my daily driver and I would like to have it back soon. Right now I am driving my mother in laws 94 plymouth voyager wich clunks, whirrs and has bad struts.

    I will have to see if my mechanic has located an engine for the caddy yet. I was sort of asking about the possible swap, because the guy I hired seems to know the nuts and bolts of swapping northstars, he has done at least a handful - but I dont think he is really knowlegeable about the engine's differences. I figured there may be a remote possibility that I could get the wrong engine.

    In the other accounts I read about the 9 2 Y swap, the owners reported that the car ran well. Does anyone have any experience with such a thing? If 1 or 2 people post positive experiences maybe I will suggest it to my wrench. It may save me a few dollars and days locating an engine.

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    Re: GXP-ify my sts?

    Hell, ask Bbob. He'd probably be the best source of Northstar info. He also drives a GXP!

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    Re: GXP-ify my sts?

    I figured that he would chime in - but maybe being affiliated with cadillac prevents him for advocating such foolishness.

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    Re: GXP-ify my sts?

    There would be minimal issues with substituting a 275 HP Northstar in place of the 300 HP one. It should run fine. The only difference is the intake cams anyway and the difference is small enough that the calibration in the production PCM for your car will handle it ok. Yes, that is basically the GXP combination that works very well....

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    Re: GXP-ify my sts?

    Thanks for the response, the urge to tinker grows... but alas, the mechanic located an STS engine with 62K for a few hundred more $ than he originally quoted. I will just end up paying a bit more and have the engine intended for the car. Maybe someone else will try this swap?

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