TDI to SDI downgrade

Greetings TDI Volks,

This is for a 2003 Jetta ALH, which allegedly uses the same models of Bosch VP & ECU on an engine having the same physical specifications as in the SDI version from the 90s. So instead of fighting an uphill battle against loss of compression ratio, is it feasible to downgrade to SDI by "chip tuning"? What else should be done in order to downgrade? Thanks, Milton

FWIW, the ALH uses the same block as the SDI/IDI engine from the same years. The difference was of course no turbo, and the rods and pistons are different. I actually got a brand new IDI/SDI long block several years ago and had it rebuilt using the ALH bolt ons with upgraded internals for a 2000 Golf. What you are thinking about would be the reverse but unless you were able to get all the necessary SDI/IDI bolt on hardware perhaps it would be possible. The question is of course the ECU and tweaking the software. That is the BIG question and is it worth it?

What do you mean by "uphill battle against loss of compression ratio?" And if you're concerned about losing compression, why would an SDI conversion help?

IndigoBlueWagon said:

What do you mean by "uphill battle against loss of compression ratio?" And if you're concerned about losing compression, why would an SDI conversion help?

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I figured that the worst-case scenario in "chip tuning" would be going into SDI emulation mode. But I wouldn't want to have to match the remaining hardware to do that exactly. A new SDI runs at 18.5:1 instead of the 19.5 for a new TDI, so as VW already has that figured out, that can be done "off the shelf".

I think I understand now, but I don't know why you'd want to do this. The turbo on ALHs is a huge plus in performance and efficiency.

I replaced the engine in my wagon two years ago with a new factory long block. My mechanic noticed the cam profile is significantly different than the ALH. We speculated that it might be an SDI long block. So there may be more differences than compression ratio.

IndigoBlueWagon said:

I think I understand now, but I don't know why you'd want to do this. . . . there may be more differences than compression ratio.

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but all minor differences, even for hardware, should have a software tweak to compensate for them. And even if I don't need to go all the way back to SDI mode, it would be a starting point from which to upgrade to my actual situation, being another data set to compare with that for a new TDI so that one can interpolate between the two sets, with less trial & error in "chip tuning". So if I were to buy a new ECU, it would be one that already has the SDI parameters in it. That's why I was wondering about "chip tuning" the TDI's ECU instead, by 1st programming it with the SDI's parameters.