Re: Boost in a teapot.

[Bill Samaras DTN 223-7322 06-Sep-1994 0941 <samaras@wrksys.enet.dec.com>]

>All this discussion about overboost, computer mods, etc has popped several
>questions in my mind that I hope someone can answer:
>
>How does the amount of boost affect the operation of the computer?
>From Bill's excellent description, it is clear the computer controls
>the amount of boost via the frequency valve and wastegate. But besides
>this, does the computer need to change its output (ignition advance/retard,
>and fuel injection) based on the level of boost? Glen mentioned something
>about the IA stage II mods changing the timing based on boost.


The computer allows for increased boost if certain inputs say it's ok to do so. 
The computer gets at least these inputs from the engine to decide on the amount
of boost:

 -air temp at input to intercooler
 -knock sensor
 -current boost
 -RPM
 -engine temp

>
>What is a "Boost Map"? Is this the standard ignition/fuel injection
>parameter map stored in the PROM altered with new values as a function of
>(increased) boost?

Maps are tables of data stored in the computer ROM. The maps are just lookup
tables for various operating conditions. For example, for boost, the above
inputs together form an entry into the lookup table. The data stored in the
table (map) tells the computer how much boost to allow for those conditions.
There are also separate maps for ignition timing. Various conditions allow for
different timing curves. I believe the 86-87 turbo's have 3 distinct timing
maps to allow for better performance with higher octane fuel. 

>
>The reasons for my questions are to understand what happens when you
>mechanically or electronically fool the computer into allowing more boost.
>Besides the obvious danger of damaging the engine due to uncontrolled boost,
>or reducing the life expectancy of the engine, I would like to know if there
>are other hidden side-effects if the computer needs to alter the operating
>parameters based on increased boost but isn't.


Most of the boost modifications being discussed in this forum are simple minded
(not the folks doing them). All that is happening is fooling one sensor into
thinking that the current boost is lower than it actually is. This works
because the knock sensor, temp sensors, etc. are still connected and are still
correcting for the new boosted environment. For example, at higher boost, there
is more of a tendency for the engine to knock (detonate). The timing map has
enough range to force more timing retard and keep the engine from knocking
(detonating). If you are exceeding the limits of the timing map, the "check
engine" light will come on. There appears to be plenty of "range" in the engine
maps to allow for 12-15lbs of boost, even with a faked out boost sensor. Of
course, you MUST use premium fuel.

I know I get less turbo performance in hot weather than in cold weather for the
exact same boost pressure. I believe this is attributable to the extra timing
retard that automagically occurs to keep detonation from happening in hot
weather.

Some negative side effects of "reasonable" extra boost are:
- some extra engine wear (them extra horsies aren't free).
- higher oil consumption (more ring blow-by and/or more turbo bearing leakage)
- higher fuel consumption

Some negative side effects of "too much" extra boost are:
- detonation damage (melted pistons, melted head, melted/broken valves)
- stretched head bolts (leaks)
- rod bearing damage 
- your engine can just plain blow up

On the plus side, I've been running with a faked out boost setup for about 50K
miles. The car has over 250K on it now (86 5KCS TQ), and burns some oil if I
drive in high boost mode all the time. I wouldn't want this car the "old" way.
I run 12 lbs of boost max.

...bill