View Single Post
Old 10-11-2023, 04:39 PM   #4 (permalink)
Ecky
Master EcoModder
 
Ecky's Avatar
 
Join Date: Dec 2011
Location: New Zealand
Posts: 5,002

ND Miata - '15 Mazda MX-5 Special Package
90 day: 42.54 mpg (US)
Thanks: 2,863
Thanked 2,495 Times in 1,544 Posts
I'll first state that, on an individual level, I'm not overly concerned with emissions. Don't take this as me advocating for not tuning from a pollution standpoint - I'm not.

I'm skeptical that pulling 10% fuel, which would also result in an approximate 10% reduction in torque/power and require more throttle under the same conditions, would result in a 20% improvement in fuel economy.

The G1 Insight had a second catalytic converter with a NOx trap, specifically to capture the pollutants created by lean burn. When cruising on the highway, it would drop out of lean burn for maybe ~5 seconds every 5 minutes, to purge the second catalyst. When it dropped out of lean burn, you'd see instant fuel economy drop from 100mpg to ~75mpg. However, you'd also start to gain speed, so you would need to pull the throttle back. Ultimately, at the same speed, non-lean cruise would get 90-95mpg where the lean cruise mode was 95-100mpg.

Similarly, I spent around a year fine tuning a lean tune on a different engine. Along with timing and VVT adjustments, I was able to see perhaps 1-2mpg in a car that was otherwise getting around 45mpg.

I'm not saying a 20% improvement is impossible from lean tuning alone (though the maths suggest it nearly is). Just, often people make a change, and then they also unconsciously change their habits, and together these add up to a larger improvement.

EDIT: As I understand it, most of the gains from lean tuning come from having a larger volume of "inert" gas to heat and expand in the cylinder, and also to create less power at a given throttle opening, so you open the throttle more, which reduces vacuum and related losses. These are not insignificant, but still relatively marginal gains.

Last edited by Ecky; 10-11-2023 at 04:58 PM..
  Reply With Quote