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Ideal AFR range for a high boost 4g63 on 92-93 RON

EFI Tuning Fundamentals

Discussion and questions related to the course EFI Tuning Fundamentals


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Andre mentions in this segment that the ideal AFR for maximum power on a turbo charged engine is Lambda 0.82-0.85 (12-12.5 AFR). However, I am currently running 13.2-13.0 AFR (or Lambda 0.88) at spool up and gradually tapering down to 11-11.2 AFR (or Lambda 0.75) at peak boost pressures of 30-32psi on a 3rd gear WOT pull (Lower gears register slightly leaner while higher gears are a bit richer 10.6-10.8). The octane rating here in Bangladesh ranges from 91-93 RON (Not very consistent from station to station). Hence, due to the poor octane rating, I am using water/methanol injection at the intercooler pipe before the throttle body with a ratio of 25/75% water/methanol, to cool the combustion charge and raise octane rating to prevent knock. Hence, I'm wondering, how much power am I leaving on the table with the richer mixture that my evo 8 is using. Can I expect to gain more power running 11.5 AFR (or Lambda 0.78) over 11.0 Afr (Lambda 0.75) in my application?

Hi. Do you have any dyno data to prove that 11 AFR produces more power than say 11.5 AFR on a high boost 4g63 motor? Thanks.

Both the screenshots show ramp runs with AFR's around 11. It doesn't show a comparative Virtual Dyno, between a richer (11) and a leaner (+11.5) mixture. I am still interested to see some data that indicates power gain from running rich.

You made me going back to my 5 years old records. In the future - if you do not trust people sharing their experience with you just check it out yourself and that would be it.

4g63, 1.3Bar boost, race gas. And it is always the same - Lambda 0.74 gives you the best power.

Attached Files

Thank you for sharing your experiences. I definitely value people's experiences when it comes to EFI tuning but truth be told, in many instances, these personal experiences are not very reliable nor accurate. Hence, I trust in data and testing. I have tested both 11.5 vs 11 AFRs on my evo. There seems to be no difference in power according to the virtual dyno. I'm not 100% sure how accurate the software is provided the right conditions are met (flat road etc).

Hence, I have chosen 11 AFR, because a richer AFR would suppress knock, while lose no power or torque. I have a friend who tunes for 11.6-12 afr at peak boost and holds it to redline. We endlessly discuss on what AFR would yield maximum safe power on a stock block 4g63 running 93 RON (89 US octane) and water/meth injection.

I have attached my test result. I wonder if Virtual Dyno can accurately track subtle changes in power and torque.

Attached Files

I don't personally believe that VD is accurate or repeatable enough for what you're trying to test. I've tuned hundreds of 4G63's over my career, including 3 that have held world records on the drag strip. in terms of gasoline based fuels alone, you would expect to see a small but measurable improvement in power at 11.5:1 compared to 11.0:1. As you move into the 10's you'll start to see the power drop off a little more sharply. That being said, if you're running a methanol injection system this changes things. If I was running purely on methanol I'd normally be in the range of 0.60-0.65 lambda (the varying stoich values is why I always use lambda for simplicity), where as I'd be around 0.78-0.80 on gasoline alone. What this means is that as you inject more methanol, you'd typically want top target a little richer. How rich will depend on the methanol volume in comparison to your total fuel volume. The dyno will show you what the engine wants ultimately.

Andre, you might be right as all the best power results with lambda 0.74-0.75 (including methanol injection) on regular gasolin were done using WD. That is not what i found just myself but something 3 other guys in my region came up with independently... So in our area it is common conclusion based on WD pulls since real dynos aren't that available...

Thank you for your response, Andre. I have a 6 speed transmission on my evo 8. I do stock ecu reflash road tuning as we don’t have any dyno. If I run 11.5 afr at 3rd gear, 1st and 2nd gears would run a bit leaner than 11.5. It would probably be at 12 AFR. 4th to 6th would be a bit richer, around 11 AFR. Would a leaner mixture in the 1st and 2nd gears cause any detrimental effects?

Andre, my water/meth injection is sprayed at the intercooler pipe before the throttle body. I use a ratio of 75:25 meth/water. How does this affect the ideal air fuel ratio? Should I target 11.5 in this instance?