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I think that you have some things confused. At at steady highway speed in these speed ranges, the Saab engines are not on boost at all, and are running perhaps a 20% vacuum. So the Saab engine under these conditions is runing like a NA engine with a compression ratio of 9.3 which is not very high.
The Honda engine may have a higher compression ratio. So it will have more specific power than the Saab engine in off boost conditions.
The best MPGs would be for an engine which is near wide open throttle at cruising speeds. This reduces or eliminates pumping losses from having to pull air through a restrictive throttle (not wide open). But running an engine at WOT at cruise has disadvantages. You have to downshift to maintain speed on small hills or to overtake. With an automatic trany this disadvantage can be largely overcome. With a CVT (continuiously variable transmission) getting close to WOT would be a design objective.
One of the reasons that a diesel engine has real world MPG advanages over a spark ignition engine, is that the diesel does not have a throttle. It is WOT all of the time. So it does not suffer from any throttle related pumping losses as gasoline engines which are rarely on WOT. And yes, 22:1 compression ratios also help. (The theroretical efficiencies of spark ignition engines at WOT is higher that diesels, for the same compression ratios. But theory does not account for the fact that you can't run a spark gaser at 22:1 and diesel won't work at 10:1.)
With a manual gearbox, crusing near WOT would make the vehicle feel very sluggish. The petal response is very poor unless you change gears. That bogged down feeling of being in the wrong gear. To run near WOT continously and efficiently requires high octane fuel, good fuel sprays and mixtures, good combustion chamber shapes. The engine needs to be able to run for long periods of time near WOT without fuel enrichment or dropping ignition timing to an extent that lowers efficiency. Also, running near WOT also creates a greater amount of NOX that the catalytic converter needs to deal with.
So there are many compromises in such design. Optimization of any single aspect is often does not create a desired result.
posted by 207.43.195...
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