Pete Dehmler wrote:
>
> At the risk of severe flaming, I am tossing my 2 cents in.
>
> Two years ago, I tried running Mobil 1 15W50W (I'm pretty sure it was 15/50
> - may have been 20/50), in my G production 1600. It seemed to have
> significantly worse wear characteristics than my old standby, straight 50W
> Pennzoil racing oil. I had much greater wear on the cam followers, and I
> think that the oil also contributed to a valve guide dropping through the
> head on # 4 inlet, and several other guides pushed into the head but not all
> the way through. I also had several #3 rod bearing failures. I have gone
> back to the Pennzoil.
> Please keep several things in mind here:
>
> 1) This is a race engine - it spends >80% of it's time above 6000 rpm and
> under full throttle. Loading is much higher than in a street motor.
>
> 2) The forces on the valve train are much greater than stock due to much
> higher valve spring rates and much reduced cam root diameter.
>
> 3) Other factors are in play here - for example the rod bearings may have
> not had sufficient clearance or oil flow, and that may have caused the
> failures, not the oil. However, I have never had a bearing failure like the
> ones I had with Mobil 1, and even the times there was no failure, there
> appeared to be more wear on #3 and #2 rods. Also, the valve train was
> somewhat aged and tired, that may have been the factor there.
>
> 4) I may have been lulled into a false sense of security by the multi-vis
> oil and revved the engine higher when it was still cold. I am VERY careful
> with the 50W to wait until the oil temp comes up to at least 140 deg before
> I take it over the 4000 to 5000 range. I wasn't so careful with the Mobil 1.
>
> 5) A final factor in changing back was the statements of several racing
> acquaintances (who I respect based on their success and education), against
> synthetics in racing applications, and that my engine balancer (who is FAA
> certified) told me that synthetics are not allowed in aircraft engines. If
> someone who can speak authoritatively knows different, please comment.
>
> One last thing - I did notice a marked increase in engine output that showed
> up directly in lap times. I just can't afford to get horsepower that way
> except under extreme circumstances (like a particular race at Mid-Ohio in
> October).
>
> Once again, this is just my 2 cents, please don't anyone take it personally.
Boy I would have loved a chance to do a failure analysis on that engine! My
experience is that engine failures like this are due to clearances that are well
suited to the original oil but not to the new oil (be it synthetic or otherwise)
and to the the fact that the engine now revs so much more freely. On many "race"
engines the builder uses clearances that are at the top end of the limit (in
other words the engine already comes nearly "worn out" as far as bearing
clearances go). This is old school thinking but still very common. The idea is
to be able to get on the motor hard, right from the start. The problem with this
idea is that bearing clearances should not change over time, at least not
without the engine wearing out or failing. Unlike some clearances in an engine,
such as ring-to-bore or rocker-to-cam, where there is normally regular wear and
where the components are designed to "bed in" to one another, the crank/rods are
never supposed to wear on the metal of the bearings. In fact bearings are never
supposed to be in physical contact with the crank/rods in operation. A film of
oil should always be between the two. So bearings really shouldn't bed in or
wear during break in. Some bearings have three layers, a base, the bearing
material itself and a very soft top layer, which can burnish off, especially if
you do a lot of dry starts. With these types of bearings, if you start with a
large clearance and the top layer does get burnished off, the clearances will
then be too big so you need to set up engines with these types of bearing with
tighter initial clearances. Others have no top layer, just the relatively hard
bearing material on top of the base. These bearings are less forgiving of dirt,
improper assembly, and incorrect clearances, but will last much longer and can
be set with a bit more initial clearance.
But no matter what, if the pressure was good and the bearings weren't starving,
there should have been no way that *any* oil should have caused what you are
describing under normal operating conditions. Did you happen to notate what the
pressures were with each oil, and whether or not the pressures tended to
fluctuate more with the synthetic? Did you ever establish that the oil pump was
pumping enough oil? Inadequate oil supply could obviously cause all the symptoms
you are describing so that would be high on my list to check.
Last suggestion based on the info you've given. Most racing oils are
non-detergent because the detergents tend to cause detonation, which racers
consider the plague. Synthetics are *highly* detergent. If gunk built up in the
engine over time as you ran it with the racing oil, and when you switched to the
synthetic it did just what it was supposed to do, loosened all that gunk and
carried it to the filter, plugging the filter and causing the filter to start
bypassing. Then that gunk would have been pumped right to the bearings. Voilla,
supper accelerated bearing failure. I've seen this happen before. Its why they
say you can't switch from non-detergent to detergent oil. You can go the other
way from detergent to non-detergent, but not this way. One reason I think this
might be the case is the comment about cam follower wear. As long as there is
oil being supplied, cam follower wear essentially comes from only two sources,
dirt in the oil or oil that can't handle the shear loads. Synthetic oils can
handle much, much higher shear loads than petroleum oils, so this is not a
likely cause in your case. Dirt in the oil OTOH would explain the cam follower
wear and the bearing failures. I may be way off here, as I do not have all the
facts and did not get a chance to see the engine, but based on the info you've
given, this would be high on my list of potential causes.
--
Marc Sayer
82 280ZXT
71 FJ510
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