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Re: Safe Boost


As a bystander like you, this is what I think I've heard.

>It seems to me that I'm going to have a trade off between safety and
>performance.  Is this a correct interpretation?

There is a trade off in the sense that higher boost will stress the engine
more, and more advance will increase the chanes of pinging.  As I see it,
having read these and various other posts, the choices are:

-       use a Zener to clamp the output from the pressure transducer

        Adv:    - keeps timing stock, and flat above stock boost levels

        Disadv: - loses overboost protection

-       use a voltage divider to scale the output from the pressure transducer

        Adv:    - regains overboost protection, assuming scaling doesn't take
                  transducer out of its range
                - increased power from greater ignition advance under high

        Disadv: - increased risk of pinging due to greater advance than stock

-       use a relay to lock on the fuel pump when the engine is running

        Adv:    - doesn't mess with the ECU
                - retains safety features

        Disadv: - loses overboost protection

-       just lock on the fuel pump

        Adv:    - real simple

        Disadv: - loses safety features, e.g. in an accident with the engine
                  off, the fuel pump will still run

>The fix Peter Wales outlined seems quite easy to implement.  But others say
>that overboost protection is gone.  I'm not clear if this is sensor
>saturation or A/D saturation or something all together different.

Peter's fix clamps the transducer output, tricking the ECU into thinking
the pressure never rises above the stock boost limit.  Since the same
output is used to detect overboost and shut off the fuel pump (overboost
proction), this feature no longer operates.

In much earlier posts, there was a discussion regarding the use of external
pressure sensors to detect when some boost limit is reached, and shut off
the fuel pump.  If I recall correctly, Peter Wales claimed a source for
such sensors.

I haven't quite decided how to go im my urQ, but the most conservative
approach may be to use any design which keeps timing stock to minimize the
threat of pinging, and external sensors/relays to regain overboost

Choices, choices!
>If anyone knows the answer to these questions, please, for the good of the
>free world, post them on the q-list or e-mail them to me direct.  I would
>appreciate it greatly.  And from the quantity of posts generated from the
>electronic bag o' snakes I sent out last week, it appears many others on the
>list would also appreciate it.
>In closing, I want to thank all the people who took the time to reply to my
>request.  This truly is a great list and I'm glad to be back on it.  You all
>have my sincerest gratitude.
>Allan Tygert

Richard Funnell,
San Jose, California
'83 urQ
'87 560 SL