Hi guys. I have a 2003 A8 with the 3.7L engine and GQE gearbox. Now the gearbox is troubling me (I have another thread on this) and I've been offered the 4.2 engine and gearbox for quite cheap. I've been searching but it seems no one else had done this.
Firstly what parts would i need in order to do this swap other than engine and gearbox? (For a start, the guy selling it suggests i won't need the ECU which seems odd to me. He says i just need the wiring loom and i can plug that into my existing ecu. If that's somehow correct, i would prefer this). From initial thoughts, would i need the keys and ignition system from that car? And are the radiator and cooling system, brakes, suspension, etc. All the same? And the main drive shaft, diffs, etc. (My car is a Quattro, so that might be something to bare in mind).
And secondly what would i be getting into with the swap?
Thanks in advance.

 

If it is the same engine code/trans, your ECU/TCU should work with it. Everything else mechanically (drive shafts, mounts, coolant system, etc) will also work. However, doing the engine/trans swap is a fairly significant undertaking.

 

Well i want to swap my 3.7 for the 4.2. I'm not sure if the engine code would be the same....and I'm pretty sure the gearbox code isn't the same. Which is why i questioned the ecu and tcu working with the replacement engine (by engine and gearbox codes, I'm referring to the codes as stated on the label in the spare wheel well....unless there are different codes you're referring to). And i do get that it would be a lot of work. But for the price I'm getting the 4.2 engine and gearbox for (provided they are in good nick), i thought its worth seeing what else I'd need and how much i can get a local garage to do the labour for. I have friends who've had engine swaps and rebuilds by local reasonably priced garages without issues, who i can ask. So although i know making this financially feasible is a long shot, there is hope. Plus if i know I'm getting the bigger engine for my money, I'd be happier investing in it (again, only if it works out to be feasible). As for the gearbox issue, i am noticing slight changes in the gearbox behaviour with the more i drive it. But I'll update that thread again after a bit more driving so that i can explain that in detail. And again, thanks for the input.

 

If you go to the 4.2 and a different transmission you will definitely need the control units and wiring harnesses for both. Everything should bolt in since the chassis is identical. It's certainly not as simple as the seller would lead you to believe. Trust me.

 

Oh yeah the seller's intentions were well understood...simply by his "why are you asking me these dumb questions....just but my sh*t" attitude. Lol. Ok so he didn't exactly say it with that tone of voice but his attitude was enough to know to take everything he says with a pinch of salt. And I'm pretty sure he didn't have any of the additional parts such as the control units (will double check this though). Plus i know with Mercedes if you change the ecu you then need the keys and ignition switch and a couple of little parts next to the ignition switch, to go with the ecu, since both the keys on an old e320 cdi failed to work and i got all the parts with the new keys for around £80...compared to around £300 Mercedes wanted for one key. Plus their cost to come out to code the new key to the car (since without any keys i couldn't move the car and get it to the dealers). One problem i have with that car now is that I can't communicate with any of its control units anymore. I'm guessing it's something to do with the vin number in the ecu being different....but anyway, point being, let's suppose the seller does provide the control units as well (long shot, but worth double checking with him), I'm afraid of having the same issue. Which is why when he said i can use my existing control units, i did hope he'd somehow be right. Anyway, I'm going to speak to one mechanic tomorrow. If he can guarantee the work and it all works out within the price of a transmission overhaul, and the engine and gearbox I've been offered check out ok (as well as i can check them, anyway), I'll consider going ahead with it. Otherwise now, I may actually be done frying my brain on this issue for the time being to be honest.