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Date: Thu, 3 Mar 2005 16:52:03 -0500
From: "The Malt Hound" <DaveHinznospamcop.net> wrote in message 
news:38pd13F5pfgtcU1nospamvidual.net...
> On 3 Mar 2005 12:36:35 -0800, R. Frist <fristnospamt.edu> wrote:
>> Just a thought:  Is it not true that in order for the knock sensor 
>> to
>> work it must sense a knock?  If so, it follows that running lower
>> octane in a turbo must cause a little more wear and tear than high
>> octane fuel.  I suppose it depends on how often the knock sensor 
>> lets
>> the ignition and boost advance until another knock is detected.
>
> Makes sense, but no.  No matter where the knock point is, it'll find
> it and set ignition and boost just short of there.
>
>> Another consideration may be carbon deposits.  A friend of mine has 
>> a
>> Cadillac with signs "Use only premium fuel" all over it.   He 
>> claims
>> that using premium is a waste and has only used 87 for the last 
>> 10,000
>> miles.  He now complains that the car keeps running on roughly 
>> after
>> turning off the ignition.   Granted, the Cadillac is not a turbo 
>> and
>> has a higher compression ratio then turbo engines at atmospheric
>> pressure.  Maybe this is not a problem with turbo engines.   Does
>> anyone have any data?
>
> If he runs mostly city miles, no good long runs to burn the crap out
> of the engine, it'll build up.  I don't know of that being related 
> to
> octane, more of how the car is driven.  People generally will drive
> a Cadillac differently than a Saab owner will drive a Saab.
>

To elabarate, my understanding is that the knock sensors in most EFI 
set-ups are able to sense the very early stages of detonation and 
thereby prevent the damaging effects of pre-ignition.

The SAAB system of sensing using the DI unit is not clear to me, but 
may offer an even better ability than the acoustic methods normally 
used.

-Fred W 

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