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Supercharged S2000 prefers rich mix, why?

Understanding AFR

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My tuner mentioned years ago, that it wasn’t until 10-10.5AFR that my car would start making power. I questioned accuracy of my wideband but his external wb confirmed accuracy.

Fast forward 4yrs later, I’ve adjusted the piggyback back to 11.2AFR and felt noticeable power dip. I added fuel back.

These supercharged S2Ks are known to run well at 12AFR but my S2000 prefers a pig-rich 10. What gives?

I'd suggest you have a dud injector or a cylinder that isn't sealing. Do you have individual egt sensors? Have you had injectors flowed? If you do a ramp run and shut it off do the plugs all look the same?

Unless your cam timing is such that you aren't getting anywhere near the same overlap that others are and are getting a more representative figure of what is actually going on in the combustion chamber rather than a mix of burned exhaust and overlap bypassed charge air. It is possible your intake cam was advanced to get a bit miller cycle style.

I've personally never seen an engine running on pump gas perform well at 10.5:1 and in every case I've tested I'd expect to lose power at that sort of AFR. I can't obviously speak for every engine out there but I have tuned a couple of turbo S2000's in my time and I don't recall anything unusual about them.

The only real advantage from running a very rich mixture may come from the cooling affect of the fuel allowing a little more timing to be added so it's very unusual that you state you've noticeably lost power after only leaning out the AFR. Ultimately you need to give the engine what it wants in order to achieve optimum performance, however this would make me very suspicious that something else isn't right.

Thank you both for the response and I agree that it doesn't seem right. All pre-FI health checks of the motor came back positive, leak & compression. No individual EGTs nor have I done New Plugs > Ramp Run > Pull Plugs. Also a deep-dive of overlap and miller cycle would be awesome so I'll have to research.

I will say that I'm on my 3rd set of DW injectors. I've tracked intermittent misfires on Cyl1 since 2011 (when car was modified) and have lived through 2 dead ones... which btw DW kindly replace (good customer service). These were older, early 2011 style, so their current line might be better yet I'd rather try another brand just in case.

Once I get new injectors, re-tune and track down the issue, I'll report back. I'm hoping that'll solve it since I already have re-gapped new plugs, coils, checked grounds, etc etc. Something just doesn't seem right.