I come up with lambda=1.23 using the same numbers as you. However, this presumes several things and ignores several other factors that have shown to make lean-burn SI engines not survive at high power outputs.
1. You're putting absolute faith in Renault's 3.5 bar MAP number, which I find to be rather high for an SI engine running spec'ed 98 RON gasoline knock free with a reasonable compression ratio for the desired high efficiency. 3.5 bar MAP might be the peak value, but counterintuitively is not necessarily the value operated at rated power. Furthermore, lean mixtures can also make engines more knock-prone, more on this in a moment.
2. Notwithstanding #1 above, the value of which is plausible given the following, methinks you overestimate the trapped VE. One of the problems with lean-burn engines is high EGT - how is this possible you may ask?? There is one common thread that connects 1. and 2. But to discuss only VE for a moment, one way the EGT might be addressed to protect the turbo is to run tons of valve overlap, in effect to scavenge the cylinders with pure charge air - the fuel is then injected only at a point after the exhaust valves close. Therefore, the VE one back-calculates out of the MAP includes a not-inconsequential portion that goes straight out the exhaust to protect the exhaust valves and turbine from limiting high temperatures. Edit: I wrote this before reading thoroughly through gruntguru's original post, which covered this subject. My apologies to you, gg.
Now, the connecting thread of both the above that has been so far ignored in the discussion is that lean mixtures in SI engines cause the flame velocity to decline. This is an irrefutable phenomenon observed from the days of Harry Ricardo. You have surely heard or read about engines overheating, burning exhaust valves or being more prone to detonation due to running lean - most spectacularly in dragsters and other race cars, often stories abetted by nitrous oxide, that ran "lean" (which were in fact still substantially rich of stoichiometric) that blew the engines to kingdom come. All this is because of the effects of diminished flame velocity and extending the combustion duration -- itself not an efficiency-promoting thing!
That said, the flame velocity thing in lean-burn engines that I just said above could be completely moot if F1 engines employ a stratified charge and all the brains and CFD power go into designing a combustion system that does an amazing job of stratifying a power-optimal fuel-air ratio core in an otherwise overall lean mixture in the cylinders (a problem that has vexed engine designers since Ricardo himself to this day). This is the subject of my PhD (so I'm not completely uninformed on this subject, albeit not specifically in an F1 context, unfortunately), but I would love to see the combustion chamber / piston top design and placement of the fuel injectors and spark plug of a state-of-the-art F1 engine; I can tell a lot just by looking at it.
P.S.: One tell-tale sign of a stratified charge engine, especially a poorly designed one, is smoke coming out the exhaust despite running allegedly lean global mixtures. This is that vexing problem to which I alluded earlier.
Edited by TDIMeister, 22 December 2014 - 13:20.