|Location: ||Colorado USA|
|I had originally tacked this on to the end of an old thread but it didn't show up in the Recents, and is really a different topic than the thread title (912 vs 907 cam towers), anyway, so...
Judson Manning wrote:
Your theoretical analysis of bearing dynamics mirrors my initial work. What opened my eyes was a technical article I read from Clevitte analyzing the performance of 180^, 270^ and 360^ bearing designs. Then I noticed the progression Lotus' made in main bearing combinations in later iterations.
The S1 Esprit essentially used JH bearings, but in subsequent years the middle bottom bearing changed to a plain bearing, then the top bearing changed to a plain bearing and finally the cross-drilled crank of the SE changed every bottom bearing to plain. Lotus' reasoning mirrors exactly what Clevitte said in that article and what every other major manufacturer was doing.
The progression of main bearing design further supports the prototypical nature of the 907 and why given the choice (and funds) it's a good idea to up-grade to later specs. To answer your question a 912LC "should" have iron liners (easily machinable) and a non-cross-drilled crank. Iron liners are attractive and if 912SE plain bearings are available at a reasonable cost the crank can be cross-drilled easily.
In the quest to run up to 8000rpm the problem is the mains holding enough pressure to feed the rod bearings. The stock main bearings have a wide and shallow groove with a big hole in the middle - just like a rain tire! To spin that high it's necessary to upgrade to the plain bottom bearings (Esprit or Chrysler) with a cross-drilled crank so that oil film can work undisturbed.
Anybody still on this board up on these things by chance?
I agree with the idea of a plain lower bearing and grooved upper (after reading the Clevite article http://www.stealth316.com/misc/clevite-main-bearing-grooving.pdf), but current thought is that simple* cross-drilling the crank is generally not a good idea for high-rpm engines (approaching 8,000 rpms and up) because the oil to the rod aerates or is starved completely due to centrifugal force pulling the oil away from the rod bearing supply passages. Figures I've seen indicate the oil pump would have to overcome over 40 psi even at 7,000 rpms. This concept is borne out by research done by Toyota http://www.tytlabs.co.jp/english/review/rev383epdf/e383_044suzuki.pdf
Is there anything different about the 907/912 crank where cross-drilling is indeed necessary for high rpm running and/or to allow a plain (not grooved) lower bearing? Or is the plain lower/grooved upper Clevite's paper discusses, with non-cross-drilled crank, the best way to go?
I'm more curious than anything else; not sure I'll be confident enough in my build to take it to 8,000 rpms.
*there are apparently sophisticated "cross-drilling" designs that apparently work better than traditional, 180^ cross drilling, but I don't think that's what Judson et al were talking about.