The "P.S." part is interesting. Prolonged boost would only occur from
aggressive highway driving or street racing, correct. Normal usuage around
town or highway cruising would not cause the the turbo to be running high
boost levels. Am I correct in this thinking? I don't want to have to sit in
a parking lot with the engine running for a few minutes everytime I drive the
car. As I understood, the boost only kicks in as needed (aggressive
accelleration).
Dan
> From: gwishon@nd.edu
> To: shop-talk@autox.team.net
> Date: Wed, 9 Jun 2010 09:19:00 -0400
> Subject: Re: [Shop-talk] Premium for Turbos
>
> 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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