Sunday, January 25, 2015
Engine Vibration Update
Since I posted my last entry several readers emailed to suggest that failed engine mounts could explain the vibration. While I am aware of that failure mode and I suppose anything is possible, I'm having a hard time believing that's the cause for several reasons:
- The BMW OE mounts are quite stout and I have never heard of a failure of these parts in all of my reading about these cars. If it occurs it's an isolated problem rather than a hot-button issue like the water pump. The mounts usually outlast the engine.
- I replaced the mounts in 2012 during my front suspension overhaul and they have just over 30K miles on them at this point. This does not preclude the possibility of infant mortality but it seems highly unlikely.
- I first noticed the engine vibration long ago, around the 150K mile mark and it's grown steadily worse in the 100K miles since. Only very recently has it become much worse.
I had plans to replace the mounts again when I pulled the engine for overhaul so the cost of the mounts is "baked in the cake" so to speak. The issue now is when to replace them -- before or after I pull the engine. If I replace them prior to pulling the engine and the vibration goes away that would only delay the inevitable.
Engine Overhaul Parts Research
I spent some time this week researching my options for the overhaul. While my plan is to reuse as many parts as possible I have to be in a position to suggest or provide replacement parts for those that are unserviceable. So I began by looking at rods, pistons, crank, and cams.
The factory M52 connecting rods are forged and quite strong -- reportedly capable of 500 ft*lbs, so they're totally acceptable for a stock rebuild or even a mildly boosted one. As long as they're in spec I have little financial incentive to replace them but most aftermarket rods are lighter than the factory rods. A reduction in reciprocating mass translates to a reduction in losses and thus an effective increase in horsepower. I doubt it's much, but it's something worth considering.
Although rods for BMW engines are readily available in the aftermarket two manufacturers stood out in my research: Molnar and Pauter. Molnar produces a quality H-beam rod for a reasonable price (about $650 for a matched set) but they only do QA in the US; their manufacturing is done in...you guessed it...China. Pauter has a great rep in the industry and despite the considerably higher price ($1350/set) their cross-beam rods are a low windage design and are made entirely in the US. If you're wondering why I didn't consider Carillo it came down to price ($1800+ a set). I'm not saying Carillo rods aren't good pieces. I just think this is a clear case of the juice not being worth the squeeze.
When it comes to pistons for a stock overhaul at least one well-recognized rebuilder, Metric Mechanic, recommends the factory cast aluminum pieces. While cast pistons are not as resilient as forged pistons (i.e. they do not support high horsepower builds and tend to fracture when driven hard), cast parts do not expand nearly as much as their forged counterparts. Forged pistons therefore require a slightly larger bore to accommodate the increased rate of expansion relative to the cylinder. The wider the clearance between the bore and piston, however, the more likely the ring grooves will wear out prematurely as the rings flex up and down and the piston skirts will tend to "slap" the cylinders, especially when when the engine is cold.
When you're building engines for the track the weight savings of a forged piston may mean the difference between a win or a loss, but for a street car I see this translating into nothing more than a shorter TBO or Time Between Overhaul as we say in the aviation business. While I honestly don't expect to put anywhere near another 250K on this car before one of us is sent to the great big Nürburgring in the sky, reliability is paramount -- this is the last time I want to pull the engine out of the car.
Interestingly, Metric Mechanic came up with what appears to be a unique solution -- high (13%) silicon content forged aluminum (alusil) pistons that expand about as much as the factory parts. These pistons are also hard-anodized for reduced wear which gives them the characteristic dark gray / black appearance. The problem is they only appear to be available in 87mm bores (required in the S52) rather than the 84mm required in my M52. And I can't just take someone else's pistons and hard anodize them because anodizing not only penetrates about 1 mil below the surface of the metal but adds about 1 mil to the overall dimensions. This means I would have to start with a piston that is slightly undersized so the anodizing brings it up to spec. Needless to say I have no plans for custom pistons so that's out.
BMW provides pistons in four sizes, all manufactured by Mahle:
- Original spec: 83.980mm
- Oversize (3 mil): 84.060mm
- Oversize (10 mil): 84.230mm
- Oversize (20 mil): 84.480mm
Since many have reported that the cast iron blocks wear like, well...iron, I do not expect to have to do much machining to bring them back to spec. This likely means I'll wind up using the 10 over pistons and bore the block to match. At nearly $300 each with my discount they're not cheap but I don't see any other cast alternatives. In any case I can't order anything until the bores are evaluated because if we have to bore beyond 84.230 to correct the cylinder dimensions I'll need to buy 20 over (84.480mm) parts.
My concern about bearing wear turned my attention to the crankshaft. Fortunately it appears that crankshafts for the M52 (and S52 for that matter) are still available new from BMW and that's a good thing since I'm not dumpster diver, but even if the bearings have gouged the crankshaft journals it's pretty amazing what a little bit of grinding, polishing and balancing can do to bring the crank back up to speed. As long as it's mated with oversize bearings to compensate for the metal removed it should be good to go.
During my research on the topic of crankshafts I also learned that the S52 crank is the same part as the one used in the M54B30 that powers my E46. The M54B30 combines this crank with slightly shorter pistons to produce a 3.0 liter displacement. The S52B32 achieves a full 3.2 liters by increasing the cylinder bore to 87mm (3mm larger than the M52 or M54) and using what I'll call "normal height" pistons. Particularly interesting is that despite its lower displacement, the M54 manages, with dual VANOS and some DME firmware tweaks, to produce the same power as the S52.
I must admit I briefly considered converting to the S52/M54 crank and M54 pistons to grab some horsepower but when I realized this would require a custom tune I abandoned the idea. I also considered the possibility of simply converting to the S52 parts and boring out the block to 87mm but I was stopped by the realization that most of the off-the-shelf tunes are for manual transmission-equipped cars. I don't think I could apply those tunes to my car and retain the automatic transmission. If I find that it doesn't matter the S52 build may become a reality. If not I'll stick with M52 parts and the stock tune.
My guess is that even if the camshafts are not visibly gouged they are likely worn below spec at this point and therefore will need to be replaced. Regardless of what I do to the bottom end I'd love to use this opportunity to upgrade to the M3 cams but the only tune available for this particular collection of parts requires use of the M50 manifold. While many consider the M50 manifold a bolt-on upgrade I don't think it's so easy on an OBD2 car. With just a few minutes' research I discovered it requires a customized intake air temperature sensor and some accomodations for the vacuum lines that attach to nipples that do not exist on the M50 manifold, and that's more than I care to do. I could also move to Schrick's Sport cams (264/256 degrees) but they require require custom tuning as well. I am also aware that a similar duration cam used on my E46 ZHP results in a slightly lumpy idle and one of the main goals of this rebuild is to get this engine back to the way it ran when new -- so smooth I simply couldn't tell the engine was running!
Clarion FZ102E Curiosity
I recently noticed a post on bimmerforums that showed a Clarion headunit (model FZ102E) that closely matched the style and color of the factory headunit. Unfortunately research revealed that the unit was only available (officially, anyway) in the Euro market and I could find no US sellers carrying it. That's understandable, of course, as import of electronics designed for other markets is usually frowned upon. But in this case the only real difference I can decipher is the FM tuner, which tunes in 100Khz steps rather than 200Khz steps. That is a non-issue here. And then there's the argument that no one listens to terrestrial radio anymore, so who cares how the tuner works?
I asked Crutchfield to use their influence with Clarion to suggest they bring a version of the FZ102E to the US but it's anyone's guess as to whether Crutchfield will forward the message as promised or if Clarion will do anything about it. If the unit does show up on US shores you know who to thank.