I'm in need of some advice on a specific engine build, a mix/match engine based on BMW components - below is some background on the engine and details on the question. My question involves the selection of rings as it relates to the block/cylinder walls.
BMW engines have regularly been rebuilt using a mix and match of stock components to satisfy varying needs. My last engine build was a 1988 2.7 liter eta engine (otherwise known as the super eta) combined with components of the 2.5l to maximize the 2.7l potential (mainly head/intake and engine management changes). There are other mongrel engines now involving the M42 engine (1.8l) - using modified M47 or M44 cranks, boring, and the use of M3 pistons to get a 2.0l stroker.
I'm in the process of building a Locost 7 - based on Ron Champion's designs - my chosen engine for the build is the M42 1.8l. In stock form, it produces about 138hp/175ft/lb torque, but I'm modifying the engine so it runs low boost using the supercharger from a mini cooper, and increasing stroke. The car will be used exclusively for track days on road courses (not true racing, just fun driving events).
Stock compression for the M42 is 10:1 so running boost requires a bit less compression (stroke 81mm). A regularly performed modification to the M42 is the use of the M44 crank and rods, which has a stroke of 83.5mm to increase stroke (but retain existing pistons since the M44 is 85mm bore versus 84mm of the M42).
Ultimately, the compression ratio after performing the above remains 10:1, so this alone doesn't help much if boost is going to be used. After several hours of researching I found that the M54B30 pistons can be swapped into the M42 - the compression height, flat top design, bore, and other measurements yeilds a compression of 9.6:1 - lower if I slightly increase head gasket size (and increase quality for boost). Here's the problem/question.
The piston rings from the M54 are 1.2mm (nitrided), 1.5mm, and 2mm (material not listed in specs). The M42 rings are 1.5mm (chrome), 1.5mm and 2mm (chrome). I've been unable to find a set of rings that fit the M54 piston, but match the chrome materials of the M42 rings. I've read and researched that it is always best to match piston rings to cylinder wall materials - so I'm assuming the chrome materials of the M42 rings where used for a reason. So I seem to have 2 options:
1 - use M54 rings and assuume the metalurgy differences won't have an impact (if I can find them - seem difficult to source and are very expensive)
2 - increase the ring gap of the M54 pistons to accept the M42 rings - is there a downside to doing this?
I could use some help in determining if my assumption about metal qualities of the rings and cylinder walls has merit, and what detrimental effects will result from increasing the height of the compression ring in the piston.
Obviously other comments on the above are welcomed. The engine is broken down to a bare block, pistons and rods have been cleaned in an ultrasonic bath, and I have all the parts necessary to start rebuilding with the exception of the rings and gasket sets.
The below pic was taken for testing purposes to confirm measurements taken before the pistons had been received.
You do not have the required permissions to view the files attached to this post.