Longevity of WRXs...?

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

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.

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

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

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)

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

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.

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

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

Edward Hayes wrote:

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.

...

Your posts are my proof. IE No real evidence!

HTH

Edward Hayes wrote:

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. :)

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

cheers

"k. ote" wrote: