Crystal change (Discard my first post!)

[Marc Alexander]

	Oops, I made an error here. 
I got some of the richer/leaner comments backwards, please discard
the previous post, and use this one. I hope I've got it the right way around
this time!

--------------------
 
I don't know the details of the Motronic ECU, but I can say with 99% certainty
DON'T change the crystal unless you know what may happen!!!!!
If you do, pay very careful attention to the results.

If you put in a faster crystal, usually the following will happen. All the
system timers usually run from this crystal, and it changes them all.
Injection timing pulse width will be reduced. Will run leaner.
Ignition timing will probably go more advanced. Engine destruction risk.
The ECU will think the RPM is lower than it actually is, raising the
rev limit, and changing the map values for that engine condition.
The map values used now may well be leaner too.
Any serial port/diagnostic link baud rate will change, meaning you may not
be able to communicate with the ECU diagnostics. ECU communications with other
in-car devices (like ABS / suspension control, etc) may well still work OK,
as the ECU often uses a 'clocked' serial port (SPI) for this.

The reverse will happen if you lower the crystal value.

Actually, raising the crystal value by the percentage rev limit increase
desired may work ok sometimes. Especially if the ECU has knock control,
and re-retards the ignition timing to suit. The other risk is that you will
get slightly leaner fuelling, also made leaner as the ECU thinks the 
RPM is lower, and probably looks up a leaner fuel value from the map.
This would be pretty bad. Some method of making the ECU deliver more fuel
would probably be required. Maybe an increase in fuel pressure or injector
size would do the trick.
The ignition timing would advance some, but an ECU with knock control may
compensate.  The ignition timing advance is the most risky result, as
the engine may start pinging/detonating more, and melt the pistons/blow up/etc