We have a 05 Bonneville GXP and I was telling my wife that I don't really have a lot of confidence in the northstar and would like to get rid of that car. I have said before that I wish we would've bought a 5.3 car because that is a motor that I have a lot of confidence in, and she goes "why can't we just put a 5.3 in our car?" So the question is would that work with my tranny and would it be hard to do?

 

You would likely need a LS4 and the trans-axle already mated to it (transaxle mounted starter), wire harness, and computers(ECM, TCM). If you had all that, it would probably be down to fabricating some mounts...

It would be relatively easy if you had the two cradles out and next to each other on the garage floor...

 

Dosent the Bonne have a different (and better) trans?

 

The N* uses the 4T80 transmission. It is much larger/heavier and the LS4 will not fit alongside it. It also doesn't have a starter mount for the LS4.

It should be fairly simple to just swap over a LS4/4T65 combo in place of the N*/4T80.

 

Thanks for input guys. Do you think it would be worth doing? Is the Northstar something that I should maybe have a little more confidence in? I just like LS motors better. Besides the LS4 would be faster and proly get better mileage.

 

Things to consider:

Factoring in an additional $2500-$3000 to get the LS4's 4T65-HD trans built by Triple Edge Performance or some other knowledgeable builder. If you don't, you're most likely going to find yourself rebuilding it again eventually.

LS4s typically can get very good highway mileage, especially with a tune. I've seen as high as a 31 MPG calculated average on a 430 mile trip. On the flip side, city mileage is usually abysmal, with some folks getting 12-15 MPG regularly. This is mostly due to our very loose torque converters. If you go the route of building the trans, you can do what I did and get a lower stall converter. It gained me back 1-2 city MPG.

The good thing is, dropout 40k mile LS4/4T65 combos can be had for under $1000 in running condition. It's just a matter of getting the work done.

As far as your Northstar is concerned, pretty much all of the bugs had been worked out by the time your 2005 Bonne was built. The head gasket failures and coolant leaks should all be things of the past, while that trans is basically bullet-proof.

It's your car, so it all depends upon what you want and what you're willing to do.

 

theres no such thing as a stupid question.