Quote:
Originally Posted by Mike@VAC
Some comments on sleeves:
http://www.vacmotorsports.com/blog/a...ur-bmw-engine/
We have the resources to offer any S65 solution, but re-coating the bores can get expensive. Dropping in Mahle pistons is an easy solution as well.
Quick tidbit: Alusil is a brand owned by Kolbenschmidt Pierburg AG. Nikasil is a brand owned by Mahle.
Our client requested sleeves since he will be beating this engine HARD and they offered some extra security. (The engine was in good shape btw) We have had 100% success with CP Pistons and either Darton or LA Sleeve solutions, so its a no brainer. |
Their seems to be some confusion on what Alusil and Nikasil actually are.
Nikasil is indeed a silicon-carbide coating that is electrostaticly plated to aluminium cylinder bores to make for a wearing surface that an aluminium piston can run on with no liner required. Nikasil is an extremely tough and reliable coating for a cylinder bore but many manufacturers went away from using it in North American markets during the 90s due to the fact that it didn't deal well with our high sulphur fuels and would prematurely wear as a result of it. Nikasil is a trademark of MAHLE!
Alusil is
NOT a coating, it is a hypereutectic Aluminium-Silicon Alloy (Al17Si4CuMg or A390). Alusil is composed mostly of Aluminium (78%) and Silicon (17%) with the remainder of the alloy being copper and magnesium. When an Alusil block is cast the silicon crystals precipitate evenly through the alloy upon solidification.
ALUSIL is a trademark of Kolbenschmidt Pierburg AG when spelt with all capital letters, however the acronym Alusil refers to any eutectic or hypereutectic aluminium silicon alloy with 16-18% silicon.
When boring/honing an Alusil block a three step process is used.
1): The cylinders are bored using a boring bar to within a few thousandths of an inch.
2): The cylinders are honed with a hone using a traditional style hard abrasive stone and taken to a smooth finish (~50% of the material is removed in this nature).
3): The cylinders are honed again using a soft honing stone. The abrasive is generally either SiC or corundum bonded to the backing plate in an elastic medium (There are also felt honing stones available that are meant to be used in conjunction with an abrasive compound similar to a valve lapping compound, the compound is to be applied to both the bore and the felt hone. These felt hones are generally marketed as a lower cost alternative for machine shops that see only a limited number of alusil engines per year). This elastic honing stone wipes/cleans the aluminium off of the faces of the silicon crystals and exposes them. As the soft hone exposes the silicon it also slightly abrades the aluminium in between the silicon crystals essentially making a surface comprised of peaks/plateaus (silicon) and valleys (aluminium). These peaks/plateaus and valleys make for a good surface for oil retention and have adequate roughness to allow rings to seat to the bores, when done to correct specs of roughness that is. This third and final honing stage leaves the bores at their final specifications and ready to accept pistons and rings and it is absolutely critical that the surface be at the proper finish of about 0.5-1 microns of silicon protrusion. Getting the surface at the proper finish requires using the proper abrasive used at the proper speed and for the proper amount of time per bore, then checking the finish with a profilometer(the remaining ~50% of the material is removed in this process).
I can understand people's thoughts on sleeves as being a better alternative to going with a proper Alusil honing process, which many shops can not do to any degree of confidence. People and machine shops are familiar with sleeves and feel more comfortable working with them. Most (but not all) veteran machinists are not used to working with these newer alloy's since they have only recently caught on to the degree that people are bringing them to machine shops for modification/repair. Couple all of this with the fact that people generally think of iron as strong and aluminium as weak and the consensus is to sleeve the block as soon as any engine work is done.
One major drawback I see with sleeves in the S65 is heat transfer is greatly reduced when going from an aluminium bore to a cast iron liner (Alusil conducts heat at a rate 400% higher than that of cast iron). Since the S65 is very sensitive to knock, and knock is usually a result of higher cylinder temperatures, any means a person can take to get rid of heat will benefit the engine by means of additional timing and reduced detonation risk. Sleeves may be good, but I have yet to see the S65 block itself fail without a rod first failing and going through the side of it. Personally I don't think anyone has gotten to the HP celling of the block casting yet and I would like to see it pushed harder before people just start doing sleeves out of fear.
Thanks for the reply with the link. By looking at VAC's shop pictures and technical info on the website I can tell that it is an extremely reputable shop that has a good staff and excellent equipment. You guys may very well be building stronger S65's for high boost applications by installing sleeves, but, some people like myself would like the option of keeping the Alusil block intact. I am very interested in building a stroker for my car much along the lines of what you guys have done with this one (Stroker crank, larger bore, cams, valve springs & retainers and possibly some head work) but I won't want the block sleeved. If you guys have the ability to properly bore and hone the Alusil block to factory tolerances and get me the MAHLE Alusil pistons in the proper dimensions we may be able to do business in the near future.