So part of my quest on reducing rotational weight was the pesky shake the 2.8 RSRs had due to a harmonic problem that spit flywheels. My thought was to shed mass and shift the bad harmonic upwards away from actual operational speeds. So lighter is better :-).
However with respect to weight, I do realize that Ti has a cycle fatigue limit (lifetime)...so I didn't have crower go for the ultimate in lightness. I may have this incorrect, and Chris may be along to school me on my ignorant understanding, but like any material, if the induced stresses are below the critical energy, then that effect is suppressed. So in my discussion with the engineer, they did a comsol FEA and we 'set the mass of the rod' at a break even force to equate to 7000 rpm. In other words, made the rod beefy enough to try and minimize fatigue below that stress level. I figured that was a fair compromise between longevity and lightness.
So the pistons are Mahle 92 mm RSR. These are the older style that give you the ungodly compression with the 24-.7 stock 'small volume' heads. They have had the ring lands coated and the skirts. Gave the tops a quick polish. I still don't get how the thermal coatings are supposed to work given the thinness of the coating and thermal equilibrium...so I skipped that bit.
Got a piston on the block with a standard 0.25 mm shim and got 1.12 mm (0.044")deck height. Its not as tight as some run, but given that there really isn't any true squish band to induce turbulence (there is a hefty chamfer on the head to fit the larger piston), I'm ok with it. Actually considering going to an .5 mm shim.
Static compression ratio should be somewhere around 11.5-11.75:1. I am going to use the greased rings, assemble the whole stack, rotate to TDC, and liquid measure the whole assembly method. Maybe this weekend if I am lucky.
So you ask, WTF am I going to do with that compression ratio... the logic is that is static. The cams are a bit weird. They were Shrick steel cams equal to GE100s. Mr. Douguty was kind enough to regrind them to 3.8 RSR specs but on 102 lobe centers to better suit the smaller displacement. The idea is to have enough overlap to have a workable dynamic compression ratio a part throttle/lower engine speeds where turbulence is low and possibility of detonation is high. The hope is to be able to run on premium pump gas for three seasons and require mixing an extender for the hotter summer months (VP or toluene, I am a chemist
). Long term I would very much like to go EFI using the Porsche DFI injectors in the stock MFI injector location and run liquid propane injection. Its 110 octane rating, but it is temperature sensitive, so that is inject the liquid bit. Get a nice cool down from the latent heat of vaporization....but that is way up the road. Have to get it on the road first!
A few extraneous notes...
1. Since this is an early case, it doesn't have a thrust bearing on the IS shaft. It locates by using the helical thrust of the gear set balanced by oil pressure on a shim set plate. I wasn't happy with the float that was possible due to not having the helical thrust due to the straight cut gears... so did machine a cylinder of aluminum that fit inside the coupler between the IS and the oil pump to hard set the rearward travel. Its trapped inside, so it can't go anywhere and there is no relative motion, so that was my solution. I just wasn't happy with having the chains do all the locating.
2. I went with Curel T for the case halves rather the well used Loctite. Mostly this is because I have used Curel T for all my bikes over the years and I have not had any trouble with it. The devil you know...
3. I left off the O-rings from the thru bolts upon assembly. Got everything bolted and torqued quickly, then went back and added the washers later, one at a time, using lots of krytox so they can seat properly.
4. Went with AN to metric fittings for all the oil lines. The AN stuff seems to be a bit more robust to frequent assembly/disassembly than the choline stuff due to the threads being a bit more coarse.
5. Everything is getting redline assembly lube!
That's about it for now...
Next post... Steve Weiners CNC magic!