Re: EPROMs & mods, etc.

[Glen Powell/SMC <Glen.Powell@smc.com>]

Boost on all US-spec ur-qs is controlled 100% mechanically by the WG and can be 
changed directly by altering the stiffness of the WG spring and/or by altering 
the tension of the spring via external adjustment and/or by shimming, etc, up 
to the boost limit allowed by the stock CPU.

The 5000/200 series have additional controls and are not my area of expertise. 
This CPU can increase/decrease the boost by applying pressure/vaccuum to the WG 
to assist the tension of the spring to increase boost or subtract from the 
tension of the spring to reduce boost. I don't know fer sure but perhaps the 
"boost control frequency valve" others have referred to is the name of the 
device that controls the pressure/vaccuum to the WG to control the boost level 
on the 5000/200 turbos, under control of the CPU. These cars also have 
closed-loop ignition with knock detection to tell the CPU when to 
increase/reduce boost. The US-spec ur-qs do not have this feature either. The 
US-spec ur-qs are relatively primitive, but easier to control and modify for 
performance, IMFO.

I dunno about the tradeoffs with respect to performance levels possible with 
the earlier low-compression 5000s Vs the later higher-compression 5000/200. I 
would guess that if you are not seeking MAX HP and high boost that the later 
high-compression models might be the way to go. If you want MAX HP and high 
boost levels then the lower compression of the earlier models (pre 89?) would 
be the way to go. Others here can offer better opinions based on direct 
hands-on experience with respect to this issue.

Hope this helps,

-bob