basically, you're wrong.
Well I'm glad that was cleared out :p
Measure them. They look like they'll give a respectable CR, but there's lots of dish in those valve reliefs, and sharp edges on those unnecesarily large valve reliefs will mean hot spots and more chance of det. they're not a great choice for your car.
Wössner says on their documentation that piston dome has volume of +5 cm3. Stock piston has depression on center of piston so I would imagine it has negative volume.
I don't think valve reliefs are unneccessarily large, they are very useful for oversize intake valve conversion, 35 mm (M42 33 mm stock) should give no trouble at all since it is very close to OEM S50 size which those pistons surely are able to accomodate. On previous engine those reliefs were more than enough for valve lift of 12,7 mm!
Those are forged pistons as said and they are more dense material than cast pistons so they will generally run cooler. Only really thin and sharp corners are on edge of piston near valve reliefs but that is not a problem because heat is transferred to bore walls from that area. Wössner pistons have won BMW WTCC and ETCC championships so I am quite optimistic that they know what they are doing
your car has a 10:1 compression ratio, not 10.5:1. I've told you this before. Compression height between the pistons is not identical, it's marginally shorter.
You are actually right, I checked from BMW papers this morning. It's just that most of the internet sources say E36 M42 has CR of 10.5 :eek:
I think lower stock CR of 10:1 and slightly lower compressiong height of forged piston is actually better for me. That means when fitted the static CR will be closer to 12:1 then 12.6:1 which should suit cams with little overlap better because they do not lower dynamic CR that much.
But to be sure I have to measure the combustion chamber/piston dome volumes as you said, too much guesswork right now.
those figures are miles apart
But does it really matter as long as it works? I am not building an competition engine where every last % counts. I want it to be higher then stock and I do not want it to be too high to cause eccessive retardation of ignition. That's why the bracket is so large.
Compression ratio can be adjusted by machining either piston dome or cylinder head or fitting thicker/thinner head gasket. So there is possibility to adjust it when the design progresses.
no. the bushes are OVERSIZE. Please learn before contradicting me, you've done this several times before and it's insulting.
If you bore engine to have larger bore diameter is it undersize or oversize? It is oversize.
On this case, inside bore of bushing is made to have smaller diameter. Logically it should be undersize. Or is there some different logic that applies to bushings?
If you feel insulted, then stop reading my posts, or atleast replying to them. Nobody is forcing you...
as per my original post, just buy the right pistons for the job. Sure pistons that actually fit your car will cost you 375euro extra, what will a rebuild when the small end bushes give out cost you?
Actually 750€ pistons are 450€ more expensive then what I paid from mine. Plus I have 2 as spares.
To fit M42 forged piston I would have to most propably hone the bores anyway so no savings there. With these pistons I atleast get bit of extra displacement (5% or such) and lots of room for larger valves.
Stock M42 con rod small end bush costs 4€. It is simple interference fit to con rod so old one is pressed out and new one is pressed in. Then you have to hone it to have correct clearance with wrist pin. Work should not cost much because it is standard procedure.
Only question mark is to find/manufacture suitable bronze bush to reduce the wrist pin diameter by 1 mm compared to stock
I had the choice of having these forged pistons or having no forged pistons at all. The real question here is, are these better then stock cast pistons. And I think they surely are.
