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