From: mchaffee@ux4.cso.uiuc.edu (Real Life?!?! HA!!)
Subject: Re: Using Brakes....a Clarification
Organization: University of Illinois at Urbana
Date: Thu, 21 Jan 1993 21:49:58 GMT

andyh@chaos.cs.brandeis.edu (Andrew J. Huang) writes:

>In article <1jkorvINNpch@usenet.INS.CWRU.Edu> aas7@po.CWRU.Edu (Andrew A. Spencer) writes:
>>
>>In a previous article, bew@brahms.udel.edu (Ben Williams) says:
>>[nifty summary deleted]
>>I have heard that when "engine braking" is used, there is actually
>>somewhat of a "negative" effect on the piston/ring assembly, in that
>>they are, bye useing them to dissipate power from drivetrain, rather
>>than intended use of transferring power TO the drivetrain, can actually
>>cause an excessive amount of wear.  In fact, i have been driving down

>The "negative" effect is that the combustion chambers, instead of
>being under pressure from combustion, is under vacuum from the car
>forcing the revs higher.  (Actually, this is only relative since the
>same is true under acceleration but to a lesser degree).  The net
>effect is to suck more during the intake stroke, which effect can be
>quite dramatic.  My brother and I were driving somewhere (pre-gas
>crisis, and -environmental consciousness days, when we BOTH would
>drive separate cars) once and he downshifted to catch up.  At 65mph he
>downshifted into 3rd, and all I saw behind me was a mushroom cloud of
>blue smoke from bad rings and intake valve seals.

Not really rings, necessarily; I've always heard that smoke under engine
braking is a symptom of worn guides or bad stem seals.  Smoke under power in-
dicates rings.  An interesting anecdote:  In the '70s, BMW had not quite 
figured out the right valve guide hardness.  As a result, on their large six-
cyl engines, the guides would wear quite a bit, but not enough to merit re-
placement.  On the Autobahn, where these cars could typically run at about
120-130 mph (5700-6000 rpm) constantly, whenever one had to slow down, the
driver behind him would be warned by the incredible cloud.  Not harmful; in
fact very little oil is burned under these conditions\.  It is, however, more
than a little unnerving till you get used to it.

>>the road with people who claim "that guy downshifts too hard", meaning,
>>that they had seen enough instances of this particular(on/off/whatever
>>throttle) burning of oil to believe that the wear which facilitated this
>>bluing of the exhaust was caused by downshifting and the "negative force"
>>on the pistons/rings.

>I don't put much stock on it causing damage.  Especially it will not
>cause intake valve guide or seal wear.  Ring wear?  I don't think so
>since there is going to be vacuum on the rings no matter what.  Its
>just a matter of degree.

Agreed.  The rings are operating under a hell of a lot less pressure (vacuum,
whatever, it's all just pressure) under engine braking to any degree than they
are at full throttle.  As for wear, the thing that causes the car to suck
oil under engine braking is high-rpm running, which wears out the guides (and
everything else, too) much faster.

$0.02

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Michael T. Chaffee       | mchaffee@ux4.cso.uiuc.edu  |  ______    
"It has been said - mostly by illiterates and conserv-|  \    /   HATE
atives - that the belief in God is at the base of the |   \  /  is not a
American system, and that the United States is a prod-|    \/  FAMILY VALUE
uct of Christian piety."  -  Leonard Peikoff          |
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