Re: CAFE standard and BMW's wimpy engines. - Saab 9-5 Bulletin Board - Saabnet.com
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Re: CAFE standard and BMW's wimpy engines.
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Posted by Dean (more from Dean) on Mon, 3 Feb 2003 10:29:14 Share Post by Email
In Reply to: CAFE standard and BMW's wimpy engines., J, Mon, 3 Feb 2003 00:03:37
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I would not assume that a N/A engine is automatically a friendlier lub situtation than a turbo engine.

Take BMW: Saab turbo engines are 9.1 compression ratio. BMW's compression ratio is much higher. When you get on power with a normally asperated engine, you are going to be in a different gear and the RPMs are much higher. At high RPMs you will get inertial loads from the reciprocating parts which are very high. With the turbo one usually goes into boost and does not gear down. Inertial loads from reciprocating parts are lower. N/A engines can have more aggressive valve trains to get power running at higher RPMs, with turbo's and forced induction, much of this is not needed. Valve overlaps are less and you are not after exhaust tuning with a tubine a few inches away from the head and high turbine back pressures while under load. A turbo engine when under load is running rich and the timing is relatively retarded. A HP N/A engine with its agressive ignition timing generates some very high peak loads near top dead center.

So for HP N/A engines, I think that you will find that the engineering is pushing some interesting limits of technology, or at least endurance of the technology. Many different issues that the technology limits for the turbo engines.



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