What has been said here about turbo'd engines running at higher effective
compression ratios (when on-boost) being at risk of detonation is correct. I
have owned turbo'd cars for a number of years now (currently a '98 TT Supra),
and every manufacturer warns against running lower octane fuels. Each will
list a recommended minimum.
Most (all?) turbo cars will have a boost limiter that will pull timing or dump
boost beyond a certain factory pre-set limit. Modifications (electronic
and/or mechanical) can be made to increase the limit at which the boost/timing
is pulled. Increasing the limit further increases the risk of detonation and
above a certain level, engine builders/mod'ers will add an electronic fuel
management system to gain more precise control over fuel delivery.
Some manufacturers of newer turbo cars have added octane sensors (in addition
to traditional boost sensors). I can't speak to how accurate these are but
they are intended to work in concert with the boost sensors to limit risk of
engine damage from detonation.
I *never* run with fuel with less than the prescribed octane rating. In fact,
I've found a convenient source of racing fuel nearby (I occasionally run at
significantly higher than factory boost levels).
Gordon
P.S. Another common failing of turbocharged cars turbo failure from 'coking'
of the turbocharger's bearings. This occurs from shutting the engine off
immediately after running for any length of time at high boost levels.
Shutting the car off causes engine oil to stop circulating through the
turbocharger's bearings and become subject to the intense heat of the
turbocharger, the result of spinning well in excess of 100,000 rpms. The oil
will literally turn to coke, with bearing failure occurring soon after.
To combat this, another commonly added aftermarket 'safety' feature is a turbo
timer. The timer will keep the engine running (and circulating oil through
the turbo's bearings) for up to several minutes after the key is turned to the
'off' position.
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