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Old 01-23-2015, 12:16 AM   #1 (permalink)
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subaru Impreza: lean burn and valve lift experiment

I have been self tuning my impreza for power for the past couple years and learned a lot in the process. But the worst gripe I have about my car is mileage. I made some tuning posts before but I never had this much tuning 'power' as I do now. i can change anything that is ECU controlled including but not limited to EGR, lean burn (AFR), timing and my favorite valve lift which subaru calls i-AVLS (exclusive to their NA SOHC powerplants).

this is same as Honda's VTEC but restricted to intake side only. So it opens or nearly closes one set of the intake valves.


The rocker arms of each cylinder’s two intake valves are locked together, and the high-profile camshaft lobe operates them both. The two wide valve openings reduce resistance to intake air, helping to improve horsepower.


Each intake valve is operated via rocker arm by its own camshaft lobe – one lobe with a high profile and the other with a lower profile. The resultant staggered valve openings increase the speed of the air entering the combustion chamber, which increases torque.

Stock valve lift is set to a questionable 1500 RPM!! This seems to only help with idle emission and quality. However I raised this up to around 2900 RPM which would equate to about 75 MPH on the highway (which is the fastest I go on the streets). Now I have to open the throttle more to make the same power or hold the same speed on the highway. Not just a few percent more but about 10% more throttle!! This has got to reduce pumping losses right? The manual indicates with AVLS off (no lift) swirl and tumble is hugely improved. This is why I think the ECU switches to a retarded timing map. In any case I even advanced the retarded/no-lift spark timing map quite a bit without knock to improve efficiency. I have seen good results but non of it is benchmarked and scientific. I did manage to score 40mpg with 65mph avg speed without any 'tricks', just using the throttle to keep speed. This was a good improvement and I think pretty good for a car that makes nearly 200hp.

My next steps are EGR and Lean Burn tuning. I think I'm going to induce lean burn and EGR for cruising conditions so I can run more timing and open the throttle even further. However I wanted to get some opinions here. I'm thinking a lean mixture of 15.6:1 AFR should give me best BSFC? I know some Honda lean burn engines experience crazy 20:1 AFR or Lambda ~1.4 but I'm not too sure about how it will play out with EGR and my current rather retarded spark timing map from low lift. Ie. How much timing should I add when going from stoich to such lean mixtures?

thank you for your attention.

Last edited by ever_green; 01-23-2015 at 12:33 AM..
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