We have some awesome efficiency turbo engines floating around but just out of interest I’m wondering what the highest efficiency % of a Naturally Asperated engine is? Lets see who can come up with the highest figure 
I had an old I4 that I made for economy back when I only had the demo. A few tweaks later and it comes out at 40.69% (0.311lb/hph), although its now its only use would be as an oversized, underpowered generator because some of the modifications that gave a slight economy increase made it drastically worse in other areas. As a 1.9L 120bhp engine (with a rediculous build time) it was at 40.20%
edit
managed 40.76% with broadly the same settings by switching to an I6 and 40.85% with a flatplane V8. wont bother with screenies though
40% @ 1800rpm is pretty much the same as a modern deisel engine. With a decent cvt or something behind it would power a eco hatch nicely. Good job!
This is my cleanest engine
[quote=“lztd15”]This is my cleanest engine
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That’s amazingly efficient, but this thread is on Naturally Aspirated or N/A engines, and that engine is a turbo.
[quote=“theagentxero”][quote=“lztd15”]This is my cleanest engine
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That’s amazingly efficient, but this thread is on Naturally Aspirated or N/A engines, and that engine is a turbo.[/quote]
Here is my attempt. 10.9L DOHC DI with 40% eco and (almost) runs on pump gas. 
I was able to get it at 38% without touching any quality sliders, but to break 40% it had to be touched.
http://i.imgur.com/hn4ErXm.jpg Just finished tuning this little baby, only have them demo still but here you go. 46.14% @ .274
My inline 6 Turbo engine I decreased it to a 1.6L to fit in the current car bodies and it gets 63mpg in the hatch of course when you decrease the size the fuel efficiency goes down to like 25%
s24.postimg.org/bnixygj2t/1796472_10200937273107015_1808801028_n.jpg
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Congrats to the guy with 47%!
My attempt got me just over 46%.
Sam that is amazing! Have you fitted it to a car to check actual fuel use?
No, I haven’t. I don’t really build my own cars, so I don’t have one to use. 
That is a good idea though, maybe we should all get one model to base our MPG numbers on?
[quote=“SamSheepDoq”]
No, I haven’t. I don’t really build my own cars, so I don’t have one to use. That is a good idea though, maybe we should all get one model to base our MPG numbers on?[/quote]
That might work, I’m not too good at car building yet but I have a couple of test mules I run engines through. My 47% returns 64ish mpg (3.6-3.7L/100km) in a 2000lb fwd 5speed. Top speed of only 90mph though.