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Re: [turbobike] Compression Ratio

Dave Williams (dave.williams@chaos.lrk.ar.us)
Wed, 10 Nov 1999 01:18:00 -0500

-> the coating to stay there over the long haul was difficult, although
-> it has mad its way into some production programs.

 Suzuki started coating pistons in some of its motorcycles in the early
'80s.  Jeeps have done it for years, and many of the new Chrysler
products use coated pistons.  Federal-Mogul replacement pistons for the
454 Chevy also come coated right out of the box now.

 I've been using the Tech Line coatings on racing engines.  It stays on
the piston skirts just fine if you apply it per instructions.  I spent
this morning sandblasting a set of $750 Wiseco and a set of $650 Venolia
pistons.  They weren't nearly so pretty after meeting Mr. Sandblaster...


-> Gimmick or true benefit, I'm
-> not really sure.  Any opinion/data on this?

 The major benefit is you can reduce the cylinder wall clearance.  This
helps reduce cold start noise and idle noise, which is why OEMs do it.
Racers who use large piston clearances can close things up some without
risk of seizing the piston.  The tighter clearance keeps the piston
straighter in the bore (which isn't always easy with short skirt
pistons!) thereby aiding the ring seal somewhat.

 No big magic there, just a technique.


-> In my opinion getting the ovality and profile of the skirt right are
-> more important.  Since the skirt/cylinder interface operates under
-> hydrodynamic lubrication once the engine is running, frictional loss
-> is a function of the oi viscosity (shearing forces) as long as there
-> is no hard contact due to a profil

 Well stated and true.  The rotating assembly slings off plenty of oil
and the pistons slide on it.  Unfortunately the people who sell coatings
seem to be stuck on trying to sell low friction, which doesn't really
apply in that situation.


-> problem.  Therefore the only benefit of the skirt coating should be
-> on startup, or if the shape of the skirt is marginal to begin with.

 I'm not a big believer in startup wear.  However, many pistons *are*
marginal on skirt shape, judging from the wear marks on some used
pistons.  One nice thing about coated skirts is the coating, by filling
in some of the low spots, helps support the skirt a bit better.  Any
high spots are worn down by direct contact with the cylinder wall, so
the result is a skirt that conforms more closely with the cylinder.

 I've not yet encountered a piston maker who will do custom ovality on
the skirts.  There have been a few times I'd have preferred to make a
direct adjustment to piston shape instead of working around it with
coatings.

==dave.williams@chaos.lrk.ar.us======================================
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