From: rrusk@ssd.csd.harris.com (Bob Rusk)
Subject: Re: Mopar 360 V-8 question
Reply-To: rrusk@ssd.csd.harris.com
Organization: Harris Computer Systems Division
Date: Mon, 21 Dec 1992 18:36:06 GMT

On 21 Dec 92 17:01:31 GMT, Harry.Johnson@launchpad.unc.edu (Harry Johnson) wrote:

>I recently bought a very large vehicle equipped with the Chrysler 360 V-8. 
>My 1-ton Tradesman van doesn't have much original about it, but the engine
>and transmission are strong and (so far) dependable. 

>I kind of have the willies about driving the van at highway speeds,
>though.  I don't know what a reasonable top running speed for prolonged
>(120 miles or more) trips is and don't want to wear out the engine really
>fast.  I have rather arbitrarily set a limit for myself of 2700-2800 rpm
>for continuous freeway driving.

>Do you think that this is a reasonable speed at which to run this engine? 
>Am I underestimating the durability of the engine at high speeds (i.e.,
>could I run at 3500 rpm without problems)?  Am I really saving any wear
>and tear with this sort of limit?  

A lot of people seem to consider 3500 rpm to be the upper cruise limit
for longevity in a boat engine, with 3000 rpm considered more
desirable.  (The boat engines to which I refer are essentially
American light truck engines with different accessories on them.)  I
wouldn't be concerned about cruising it at 3100-3200 rpm all day long,
probided that it's in good condition.

>The engine has good oil pressure and I
>tuned it up immediately after buying it, and I'm not worried about
>overloading the cooling system or knocking/pinging problems. 

Then it sounds like it's in good condition.

>On a more general front, what is the best speed at which to run an engine
>so that it lasts the longest possible?  50% of redline?  Idle?  By lasting
>the longest, I don't mean time-wise but rather number of rotations of
>crankshaft before some sort of rod/crank/valvetrain failure.  

Depends on the load you place on the engine.  If the engine is heavily
loaded (towing, carrying heavy loads), you want to run it in its
torque band.  Otherwise, the lower the better.  I know a taxi driver
in Arica, Chile that got over 500,000 km out of each of his two 1.8L
Isuzu Geminis (rear-wheel-drive I-Mark's in the USA) by changing his
oil frequently, driving gently, and shifting at such low speeds that
the engine probably never saw 3000 rpm (and the Isuzu's power band and
red line are considerably higher on the rpm scale than they are on
your 360).  While this could be considered to be lugging the engine,
it's not if you don't request a lot of power at those low engine
speeds.

Also depends on how your engine is set up.  Some engines don't take
well to prolonged use at low rpm's, others are designed for it.  My
'92 Dodge Caravan (3.3L V6) cruises at 1800 rpm at 60 mph.  While this
is too low for many engines to be cruised, my step-father has one of
these with more than 100K miles on it and it shows no ill effects from
the low rpm's.  On the other hand, our '75 Buick Regal with the 3.8L
V6 carboned up readily unless you took it out and drove it hard
periodically.

--
 Bob Rusk
 rrusk@ssd.csd.harris.com
 My thoughts, probably not Harris'.