Longevity of WRXs...?

[k. ote]

It is not the clutch & brake wear but the extra reverse loading on
engine bearings and of course valve train wear due to higher than
necessary rpms. Unless one is trying to apply additional braking when
racing I think it's not a good thing.

Engine bearings?

Huh? Accelerating puts a much greater strain on those parts than rev-match
downshifting does.. it's not like there's a hard force being applied the
way there is with (for example) 3/4 throttle in 1st gear.

So the remaining possibility is that you think that the reverse pressure is
by itself somehow bad for the engine..? Does this mean I should never let
up on the accelerator when I'm driving at > 4000 rpm? If I rev-match to
even 4000 rpm, aren't I doing essentially exactly the same thing as: just
letting up on the accelerator?

The only difference is that I'm not doing a full double-clutch.

 

[Edward Hayes]

The bearing stress when using engine braking from very high rpms put
reverse loading on the connecting rods and bearings which is not good.
Rods have been known to break due to being stressed beyond their yield
point i.e pulled apart. Acceleration put the rods in compression.

 

[Carl 1 Lucky Texan]

The bearing stress when using engine braking from very high rpms put
reverse loading on the connecting rods and bearings which is not good.
Rods have been known to break due to being stressed beyond their yield
point i.e pulled apart. Acceleration put the rods in compression.

Isn't there still gonna be compression? Otherwise, where would the load
come from to cause deceleration? There's still compression,etc. just no
(or limited) fuel and spark.

Carl

 

[Edward Hayes]

There is no compressive forces as there will be high vacuum in the
induction system. That negative pressure is what pulls oil into the
combustion chamber and tries to stretch the connecting rods and tries
to pull the rod halves apart.

 

[Michael Pardee]

The bearing stress when using engine braking from very high rpms put
reverse loading on the connecting rods and bearings which is not good.
Rods have been known to break due to being stressed beyond their yield
point i.e pulled apart. Acceleration put the rods in compression.

I haven't heard of that happening. It would be an odd thing, since although
the compressive forces can be very high indeed the limit of pull on the
piston is atmospheric pressure (about 15 psi) times the area of the piston.
For an 85 mm bore, the roughly 9 sq inch area could exert about 130 lbs of
pull at sea level. (The figure was obtained with conversions, not with the
rounded numbers.) I'd hate to think my conn rods will pull apart with forces
I could exert by simply grabbing each end with my hands and pulling.

In my experience, broken conn rods are either from sucking water (or oil)
into the cylinder and trying to compress it or more commonly from loss of
lubrication to the main bearings.

As k.ote says, the stresses in engine braking are identical to those in
lift-throttle conditions. I've used engine braking for nearly 40 years now
and done my own maintenance pretty much as long. I've never seen any sign of
adverse effects as long as the driver uses the clutch as intelligently on
downshifting as on upshifting.

Mike

 

[Carl 1 Lucky Texan]

There is no compressive forces as there will be high vacuum in the
induction system. That negative pressure is what pulls oil into the
combustion chamber and tries to stretch the connecting rods and tries
to pull the rod halves apart.

Yeah, you're right. With the throttle closed the vacuum is highest when
decelerating.
I don't think its as detrimental perhaps as you paint it, but that its a
technique better suited to the track. Not saying that being in or
prepared to engage the 'proper' gear for unexpected manuevering isn't a
good idea in a lot of normal driving, but 'downshifting and rev matching
to engine brake' is likely not needed under normal situations.

Carl

 

[Edward Hayes]

It is allot more force than 15 ppsi. To that add the reciprocating
mass of the rod, piston and pin. Often time the failure will not show
up as a broken rod but a spun bearing. A spun bearing will close off
the oil feed hole which at first thought is loss of oil as the
primary cause when it was the secondary cause.

 

[Michael Pardee]

It is allot more force than 15 ppsi. To that add the reciprocating mass of
the rod, piston and pin. Often time the failure will not show up as a
broken rod but a spun bearing. A spun bearing will close off the oil feed
hole which at first thought is loss of oil as the primary cause when it
was the secondary cause.

Only the 15 psi changes when engine braking. The rest is always there. As we
say, exactly the same as lift-throttle.

Spun bearings have nothing to do with cylinder pressures and everything to
do with improper installation of the bearings or failure of lubrication. The
installation problem occurs when the bearing is not seated in the cap and in
the conn rod end when the rod is bolted down, so the locater tabs are
mashed. Sooner or later the lack of clearance caused by the deformation
ruins the indium coating and the crank is pressed against the steel base
metal. The locater tabs are unable to hold against that forever and the
bearing spins. Everything happens very quickly after that. Of course, oil
starvation does the very same thing but it just rips the locater tabs off as
the bearing seizes.

Mike (who has rebuilt one engine and sat in on several others)

 

[Edward Hayes]

You of course have one of the explanations for spun bearings and I
agree. However: I stand by my edit that one contributor of spun
bearing is overloaded bearings. Ed (who has rebuilt ~17 Porsche
engines, air & water cooled & some with spun bearings)

 

[Michael Pardee]

You of course have one of the explanations for spun bearings and I agree.
However: I stand by my edit that one contributor of spun bearing is
overloaded bearings. Ed (who has rebuilt ~17 Porsche engines, air & water
cooled & some with spun bearings)

You are talking about loads sufficient to exceed the film strength of motor
oil? That can only occur under very high throttle settings, never under
lift-throttle or engine braking conditions. How could the forces of
lift-throttle, which can never exceed those of a 2:1 compression ratio
engine that isn't firing, overload a bearing that is designed to withstand
the forces of the power stroke?

I admit there are a number of areas of automotive controversy I don't know
the answer to (I'll try not to revive the "pulsating brakes" hoopla, for
example) but this is cut and dried. Engine braking is not capable of
applying significant forces to any engine, and the forces are exactly the
same as those of closing the throttle while cruising. Has anyone, anywhere,
ever said, "I took my foot off the gas and *bang* there was a hole in my
engine"?

Mike

 

[Ron N.]

As k.ote says, the stresses in engine braking are identical to those in
lift-throttle conditions. I've used engine braking for nearly 40 years now
and done my own maintenance pretty much as long. I've never seen any sign of
adverse effects as long as the driver uses the clutch as intelligently on
downshifting as on upshifting.

One probably needs a large amount of statistical evidence to
analyze the effects of downshifting, since the difference in
failure rates, if any, are likely small. I can only add one
anecdotal data point. I used downshifting extensively on my '83
Celica, and after over 325,000 miles needed no major engine work,
and about 5 brake jobs to 1 clutch job. (I did need to have the
transmission linkage repaired once; but that was a few weeks
after I had learned to rev match well enough to shift without
using the clutch!)

Now I've got a new Legacy sedan with an automatic, and I'm
wondering whether I should manually downshift using the sportshift
mode or not.


IMHO. YMMV.

 

[Phil]

I know we are talking different type motors and conditions here but "engine
braking" seems very popular with truck manufacturers and truck drivers....

 

[Edward Hayes]

I don't remember when this thread started but, I stand by my original
statement as far as I can remember. Unnecessary engine braking from
high RPM (down shifting) causes more engine wear and the possibility
of major engine damage over the long haul. I quit

 

[Tony Burns (permanent dismisser of Andy, AJS and ]]

As you should, you have not supplied any real evidence to support your
theory.

 

[Edward Hayes]

Nor have you.
"Tony Burns (permanent dismisser of Andy, AJS and ]v[etaphoid" <"The
Young Ones"@wrbike.com> wrote in message

 

[Ron N.]

Nor have you.

Yes he did. Several examples of engines that were not
damaged in any manner attributable to engine braking over the
long haul.

Also, I think truck and bus drivers have used engine braking
extensively, and these engines get far more mileage on them
than a typical passenger car.

"Tony Burns (permanent dismisser of Andy, AJS and ]v[etaphoid" <"The
Young Ones"@wrbike.com> wrote in message

As you should, you have not supplied any real evidence to support
your
theory. ....

 

[Tony Burns (permanent dismisser of Andy, AJS and ]]

Your posts are my proof. IE No real evidence!

HTH

Nor have you.
"Tony Burns (permanent dismisser of Andy, AJS and ]v[etaphoid" <"The
Young Ones"@wrbike.com> wrote in message

As you should, you have not supplied any real evidence to support
your
theory.

 

[Tony Burns (permanent dismisser of Andy, AJS and ]]

Sorry Ron but I can't see any, perhaps you could repost them.

Nor have you.

Yes he did. Several examples of engines that were not
damaged in any manner attributable to engine braking over the
long haul.

Also, I think truck and bus drivers have used engine braking
extensively, and these engines get far more mileage on them
than a typical passenger car.

"Tony Burns (permanent dismisser of Andy, AJS and ]v[etaphoid" <"The
Young Ones"@wrbike.com> wrote in message

As you should, you have not supplied any real evidence to support
your
theory. ...
Edward Hayes wrote:

I don't remember when this thread started but, I stand by my
original
statement as far as I can remember. Unnecessary engine braking from
high RPM (down shifting) causes more engine wear and the
possibility
of major engine damage over the long haul.

 

[k. ote]

I don't remember when this thread started but, I stand by my original
statement as far as I can remember. Unnecessary engine braking from
high RPM (down shifting) causes more engine wear and the possibility
of major engine damage over the long haul. I quit

Ah.. that's high RPM, but you never actually defined what you consider to
be "high" RPM. I don't mean to sound like we're attacking you: this kind of
discussion is only ever beneficial to both "sides," I just fail to see why
fiddling around and rev-matching at a maximum 4K RPM in, for example, a WRX
STi is more harmful than simply lifting up my foot off the throttle. In
fact, wouldn't it be safer, because I'm lifting it up gradually as I lift
my other foot off the clutch? Then it's not a sudden force, but a carefully
modulated one.

Sure, beating the hell out of the car at redline RPMs can be very hard on
the car: just listen to it, it even sounds like it's labouring. But doing
it under normal driving conditions where 4K is a rearely-touched top-end
and 2K-3K is the norm?

My dealer's staffed with a Subaru master mechanic, and their advice was to
feel free to engine brake in normal driving. They've been godlike in the
past with their service and abilities, so I don't see any reason to doubt
them now.

 

[Tony Burns (permanent dismisser of Andy, AJS and ]]

Thank you for your post, I agree in total with you, as for the others
nothing but hot air.

cheers