Is it worth updating to a 120 pin ECU?

My search skills aren't that great so fogive me if this has been answered elsewhere in a different form.

I recently aquired a 99.5 Jetta. It's a reasonably solid car that needs some love. Most note-worthy is the engine is coming out for a rebuild. Low compression, a nasty RPM dependent vibration (bent rods I think), probably a failing DMF, and it's filthy. I saved the wiring from an '01 TDI front clip I bought some years ago. I've been bitten by the Go-fast bug so I know I'll be tweaking this one too. Right now I'm looking at an 11mm pump (on hand already), PP520s (on hand already),RC tuning, possibly a larger turbo (if needed), SMF and VR6 or larger clutch package, bigger downpipe and exhaust (I'm leaning towards 2.5"- have a new cat already), at some point a PD manifold and an FMIC. For a ballpark I think 250-300ft/lbs would make me smile. My current GTD is less than that (I think) and I giggle like a little school girl when I lay into it. Given the extra weight of an A4 I think that area should make me squeal just as loudly.

I'm no stranger to swaps/frankenwagens. I've built 2L 16V vanagons, Vanarus, An A2 TDI, and currently drive a souped up A3 GTDI. Bentleys, soldering irons, and heatshrink are my friends.

That all being said, is there any real benefit to upgrading to the later model ECU? If it won't provide me with additional power/useful flexibilty I wont bother. Thanks for your time.
-Jake

The main benefit to the later ECU is the ability to flash the ECU over OBD. The trade off (aside from the added wiring) is that you'll have DTC's unless you put in the cluster and key from the '01 (of course the immobilizer would also have to be deleted). You'd better ask Jeff if that is something that he can tackle if you intend to take this on.

You're probably better off leaving it alone unless you REALLY like your soldering iron. At that point maybe "like" isn't the right word. Besides rumor has it that stock that ECM will hold 3 tunes...

Yes, it's worth it. Drive a '99.5 and an '00 or later car side-by-side and you'll feel the difference. I've got a 121 pin harness waiting to go in my track day Golf.

Souzafone said:

You're probably better off leaving it alone unless you REALLY like your soldering iron. At that point maybe "like" isn't the right word. Besides rumor has it that stock that ECM will hold 3 tunes...

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That can be done with most ECU's. The '00-02's have three coding options. '03's have two coding options but you could also switch to '02 software.

Perhaps I'm missing something (new to A4 modding), but I thought removing the IMMO would negate the need for the ign. collar and the cluster. At which point the only major hurdle (which would probably be minor for Jeff) would be disabling the IMMO on the ECU.

You're right.

tdibeliever said:

Perhaps I'm missing something (new to A4 modding), but I thought removing the IMMO would negate the need for the ign. collar and the cluster. At which point the only major hurdle (which would probably be minor for Jeff) would be disabling the IMMO on the ECU.

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It will work but you'll have permanent DTC's stored for the can-bus stuff that isn't there.

Are these DTC the sort of things that cause E-check failures, or more on the same level as radio missing, window motor failure (oversimplified).

tdibeliever said:

Are these DTC the sort of things that cause E-check failures, or more on the same level as radio missing, window motor failure (oversimplified).

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My car used to have those codes stored permanently because I'm using a mk3 instrument cluster....but I can't recall if they were EOBD related. If they are then they'd probably fail you.