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kenikh kenikh is offline
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Join Date: Feb 2005
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Let me take a moment to explain the above technologies and the benefits they impart on the turbo motor and why:

Originally Posted by 911st View Post

You are right. After my comments above I did some more research and BMW's new motor variant is truly state of the art.

It is there first time combining variable valve timing, direct injection and turbo charging and some other trick technology's like controllable and variable oil pump delivery. It is able to run at over 10/1 CR on top of running a turbo.

In all, there efforts resulted in over 300# TQ by 1500rpm (apx 200rpm earlier than there twin turbo 6) and about a 7% increase in fuel economy.

Basically this little gem makes full boost by 1500rpm and pulls a 7000rpm red line.

It also makes more TQ and it makes it sooner than Mercedes E class diesel.

Cool stuff and it is interesting they did this by leaving the twin turbo behind for a divided turbine set up with a log exhaust system.
Direct Injection:
  • The 997 DFI injectors operate at 1750 psi. If MFI injectors used to be the holy grail in terms of atomization, injectors that operate at 5 times that pressure are a wonder.
  • Additionally, injection into the combustion chamber means that the fuel's full evaporative capacity is absorbed by the combustion gases, providing better cooling of the combustion process, as well as quenching rogue combustion elements.
  • With these injectors ability to control full directional trim control and withy the ability to fire more than once per cycle based on engine state, means that the fidelity of controlling warm up, knock and other variables is downright unbelievable in terms of tuning flexibility.

Variable valve timing:
  • Controlling overlap by varying lobe spacing is huge: you can go from zero overlap at idle to race overlap at high RPM. At idle, you can build pressure extremely quickly as you can optimize the intake tract's VE to the needs of an engine in terms of air requirements at level that is functionally infinitely variable. This is great for emissions and amazing for power production. No more compromises on overlap means that you can have your cake and eat it.
  • Controlling lift is even more valuable for turbos. By lift limiting the exhaust valve at low RPMs in relation to the intake valve, you can create a massive pressure differential on the exhaust side that creates significant dividends in the integrity of the exhaust wave heading toward the turbo. This translates into ridiculously low boost thresholds. You can basically use a full race cam, but make it pretend to be a cam that if static, would be useless above 3000 RPM, as it would choke off top end power for anything but a low revving diesel.

As far as the "log exhaust" goes:
This is an educated guess, but follow the reasoning: With variable cam timing and lift, at low RPM, the effects of a cam tuned for no-overlap and low lift can provide perfect exhaust wave resolution, then open things up higher in the RPM band for excellent HP production. In an engine like a 911 that has to compromise on a given, fixed cam timing spec, this resolution is not possible - any cam that would exhibit a good exhaust signature at low revs would run out of steam very quickly without being able to increase lift and overlap.

Thus, the reasoning is that at a low number of relative exhaust events over time (RPM), coupled with no overlap and high intake velocity (due to lower valve lift at low engine speed) allows the log type of exhaust to operate as well as a discrete, equal length system. BMW can make incredible low RPM power with a log style header that would not be possible in a fixed cam motor and by the time that RPMs rise, to make full power, the benefits of equal length exhaust tuning are gone (due to gas volume overcoming any synergistic effect that could be derived from pulse tuning/scavenging).
And that's the latest hail mary from an armchair ME wannabe...

- 1965 911
- 1969 911S
- 1980 911SC Targa
- 1979 930
Old 06-07-2010, 12:26 PM
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