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afr change in relation to compression change.

Understanding AFR

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hi could you explain what effect to the transition of afr will be if i lower the compression ratio of a engine with boost. will my afr targets stay the same to aim for within vac range of at the moment being about 0.86 at 100kpa atopsheric so will it be right to correctly match back to these targets. im aware ill have to increase timing in that range to correct for the loose of lower compression to maintain power but with this i realise my timing reduction will be less over the boost range as a result as well to give better power.

but my question comes to what affect the lower compression has on afr? do i get a slower afr transition everywhere or just on boost? being u can then run more boost with lower compression so do u get a longer range on boost of fueling afr target transition? as i aim across my mbt fueling targets between mbt lean,mbt max and final max flame front speed to my current target but being i can now run more boost does these afr targets just move up the boost range as u use lower compression?

Could you please edit that with punctuation and capitals, where required? It is very difficult to decipher exactly what you're asking and doing so will be more likely to get a good response.

have reworded it bit better hope this helps.

The mixture will not change when CR is altered.

yeah but will the transition of mixture change?? e.g. at the mo im transitioning from 12.6 100kpa to 11.8 at 1 bar so if i lower compression to allow for a higher boost level to be achieved i was looking to stick with the same start and end targets within the boost range of afr but have a wider slower transition range of afr accross the boost range to end at the same afr max but at the new max boost target? if that makes sense?

This is an overly hypothetical thesis IMO. I highly doubt you will see any measurable difference.

Simply revisit your fuel and especially timing maps after a CR change and adjust on a sensitive load bearing dynamometer accordingly.

ideal thank you for your insight, just gives me lot better understanding before doing the change and trying to understand its affect fully prior to getting it there on the dyno to make the most of the change and be safe.

Higher compression ratio = more thermal efficiency and lower exhaust temps, allowing you to run less rich for some areas (in theory), but only if you can prevent knock or run at lower loads. In actual day to day use, it doesn't change much, because AFR tuning is about half rule of thumb and half actual checking of knock, exhaust temperatures, dyno numbers, etc.

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