Turbo vs. N/A city fuel economy

Some passenger cars offer gasoline engine options for the same displacement but with a higher compression ratio normally aspirated- and a lower CR turbo mill. The turbo has higher max power output of course, but I am wondering the likelihood it’s lower CR will lead to higher city fuel consumption (where the turbo isn’t boosting as much) compared to the gas model?

As far as I can tell from EPA mileage ratings (for what they are worth) the mileages are the same.


When comparing vehicles things get confusing, because on a vehicle level there are so many things that effect fuel economy, such as transmissions gear ratios and/or final drive ratios, or the transmission driveline overall efficiency. Also bear in mind that a lot of the Brake Specific Fuel Consumption (BSFC) improvements you get under laboratory conditions on an engine dyno won’t necessarily all be seen on a vehicle when the engine is calibrated, as refinement, driveability, emissions and other criteria have to be met.

All other things being equal- which in the real world- they rarely are- using 8:1 compression ratio as a starting point, for every unit raise in Compression ratio- you gain about 3 % in part load fuel economy until about 10:1 when the gains become progressively less. Again this is assuming the cam timing and duration are the same, which they rarely are on a Turbo engine versus a naturally aspirated (cam timing and duration will effect pumping losses and combustion-mostly via EGR).

Power output for power output- again-all other things being equal- when considering a 5.8 litre naturally aspirated engine versus a 3.6 litre twin turbo charged engine (both producing about 400 Bhp)- despite the drop in compression ratio- the smaller engine will have better fuel economy. This is because it is throttled less under part load use- being a smaller engine- and also because under full load conditions this engine is making effective use of otherwise potentially wasted enthalpy out of the exhaust. However it’s not always clear cut, I know a lot down sized Turbo charged engine packages that are running as rich as Lambda 0.6 or 9:1 AFR under full load full speed conditions just to keep exhaust components cool!

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