Date: Thu, 13 May 2004 09:55:56 -0500 From: "Walt Kienzle" <wkienzlenospam.net> Subject: Re: Timing Belts (was Re: Cost of repair Audi BMW Saab...(crossposting))
"eBob.com" <eBob.comnospamllybogus.com> wrote in message news:NwLoc.45839$CC4.16822079nospam4.srv.hcvlny.cv.net... > That's interesting. Since all of the brands of cars which I have owned have > had them, I just assumed ... > > Anyway, how 'bout educating me on two things. Just exactly what do timing > belts do? (Yes, I am pretty dumb in this area.) And what do the BMWs w/o > timing belts have which perform that function? > > Thanks, Bob > Bob, Timing belts operate the "overhead cams", which are the things that open and close the valves that let fuel and air into each cylinder of the engine (the "intake valves") and let the exhaust out of the cylinders ("exhaust valves"). The valves need to open and close in sync with the operation of the pistons and the rest of what is going on in the engine. In some cars this is done using a belt. Belts are quieter and (usually) less expensive, but less durable than the alternative of using a chain. Some engines with a timing chain can be almost as quiet as those with a belt, but it has to be carefully engineered. Chains can wear too, and when that happens they tend to get very noisy. Belts usually give no notice that they are about to break like a wearing chain does. The problem is when the belt or chain breaks, the engine stops. In some engines, the pistons keep moving for a few seconds - just long enough to crash into the now stopped valves which potentially destroys the engine (this is known as an "interference" engine). Other engines are designed so that there is still enough room so that the valves and pistons don't crash together when this happens (this is a "non-interference" engine). Some engines based on older designs don't use belts or chains, but they usually only have two valves per cylinder (one intake and one exhaust) instead of the typical four valves per cylinder that modern (and usually more efficient), overhead cam design engines allow. (Some of the preceding is personal opinion.) Walt Kienzle
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