I took the engine out of the BMW about 3,5 weeks ago, and disassembled it completely to check if there's anything wrong.. and had to weld the body (rust) and do some painting so was a good moment for doing something for it; and currently have a Volvo V40 1.9d manual because the BMW had too many problems to be fixed and fuel consumption got very high for some reason.
Every oil seal and gasket were leaking, and a few valves (intake and exhaust) had hit the piston, but the valves were straight or straight enough not to cause problems. Probably valve spring's seat/open pressure reduced or worn timing chain / guides / tensioner has caused the valve timing to go too advanced or retarted plus a bit too much valve guide clearance.
Only marks on the pistons, but not on the valves so they've "just touched".
I was very surprised noticing the exhaust valves or seats hadn't burnt; actually in very good condition compared to other similar engines I've disassembled (and heads)!
Pistons were also good, no marks of melting or detonation or anything.. quite clean because of E85.
This is a good sign, because I had ran the engine around lambda=1.20..1.25 mixture and even at lambda=1.40 when trying the lean burn limits and highest advance was 52 degrees BTDC (which only reduced power -> worse fuel economy).
Anyways, I've got now 2.8 liter crankshaft and pistons (which need modifying) so I'm able to reach a lot higher compression ratio.
Sadly I've had a lot of problems with my mira valve seat cutting machine (very old), and haven't been able to finish my cylinder head.. and without those very frustating probelms I would already be driving the BMW.
Maybe some day..
I did start thinking about gearing (for next year), and its benefits.
I have noticed many newer 2 liter diesels fitted with 6 to 9 speed automatic transmission run around 1500-1800rpm @ 62mph.
As an example, one version of BMW N47D20 engine produces about 200nm@1500nm.
The petrol engine; M52B25TU has ~180nm@1500rpm if the engine is in good condition and there are no faults active.
2.8 liter, high compression and E85 should have no problem producing ~220nm at 1500rpm.
A 3 liter straight six engine with 14:1 compression ratio, itb, small cams (stock) and E85 produced 220-230nm@1500rpm with a lot later intake cam timing ; allowing it to breathe at higher rpm's .. peak hp at 7000rpm, but reducing peak torque and power below 3000...4000rpm.
So in my opinion aiming to around 1900rpm@60mph (from ~2700rpm) should have a very dramatic effect on fuel economy on the BMW.. which is possible with 6spd gearbox and 2.93 vs. 3.23 final drive.
And still be fun to drive.
Here's an interesting article about lean burn and port injection strategies:
https://www.speed-talk.com/forum/vie...hp?f=1&t=63866
(there's another link on the site, that opens the pdf)
If I could cause the engine ignite with just compression->heat aided with spark plug for longer time, it should be possible to run with very lean mixtures; like lambda=1.40 or leaner and improving fuel economy.
I'm quite confident on saying the very high compression ratio helps with the pre-ignition mentioned earlier on this thread, caused by too much ignition advance and maybe enough heat in combustion chamber?
Or something else that caused sudden acceleration, reducing also a lot of the momentary fuel consumption because it was possible to reduce throttle and still keep up the speed.
And because there were no signs on the engine of detonation or anything that could have had potential causing an engine failure, I don't see any problem trying to causing auto-ignition while cruising.
Only potential problem is, how to control it.
On gasoline it would be very hard and high risk of detonation or melting pistons or burning valves, but E85 might have the possibility on being "sort of controllable."