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Old 04-22-2014, 07:33 PM   #1
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Default What's the difference between the 98 4.2 and the '02+ 4.2 (Swap question)

Okay, so I'm looking to replace my old motor (it's got like 300k miles on it).

My options are:

- Buy used 1997-1999 4.2 and just replace my motor, after doing the t-belt and a few other things.

- Buy used 2002+ S8 motor and swap it in.

My question is basically, what's the difference between the motors.
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Old 04-23-2014, 04:22 AM   #2
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Pre 99.5 4.2's are 4 valve engines.
Post 99.5 4.2's are 5 valve engines.
There will be a meriad of detail changes, though from a basic perspective there's probably no reason why you couldn't swap it in.

The additional power of a later S8 engine is as much the ECU mapping as the engine hardware.
If you use your existing ECU, you may see some improved power (those high lift cams and S8 specific injectors must do something ).
If you're expecting to swap the ECU as well, I hope you have some time!

Once you go past 2005, the 4.2 goes back to 4 valves but adds direct injection (FSI). I doubt your ECU could drive a FSI engine...
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Last edited by twentysevenlitres; 04-23-2014 at 04:44 AM.
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Old 04-23-2014, 10:28 AM   #3
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Thank you for your input. I'll source the motor and have my ECU flashed.
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Old 04-23-2014, 08:06 PM   #4
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Here's my other question, if I can't afford the whole S8 motor, can I swap the heads only and get some benefit, or do I need the S8 short block also.

Thanks
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Old 04-23-2014, 08:06 PM
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