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Relationship between VE & load

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Kind of a technical question. I'm curios the relationship between VE & load as the title states. I find that there are sites on the fuel map which you can reach with the engine loaded more than 1 way. For example the same site but in different gears.

The most extreme example is the cells which can be reached free-revving which can also be reached driving around at very low load. Under actual load driving around my AFR comes in right around stoich however free-revving will be very lean, up to 1.15 lambda. I've learned to not be too concerned with this but I find it interesting.

A more common example is I find just going through the gears the higher the gear in the same cell the richer the AFR will come back. I know some ECUs provide an ability to do compensations by gear.

So science wise, is it that the less loaded or maybe put another way the less resistance the engine is experiencing at a specific manifold pressure and RPM the more volumetrically efficient the engine is in that moment?

What are you using for load or VE look up? I have never observed what you describe with a common SD type tune, MAP is usually a pretty good indicator of load and MAP x RPM usually has a strong relationship to airflow. If using Alpha-N or you have very poor/unstable vacuum then you will definitely see some differences with load. Are you sure the difference you are seeing is not due to some other variable you are not compensating for correctly such as charge temp?

Hi Adam, first I should start be noting I'm very novice, so I may not always be on point... / here to learn! Regarding your question I'm on Hondata s300 which isn't really a VE based tuning system however as I understand it, often when you make a change (say an intake) and your AFRs go leaner as a result that's because of improved volumetric efficiency, or likewise if you're tuning adjustable cams on the dyno a change that results in leaner AFRs as I understand it means you've made a VE improvement and need to update the maps the go along with it.

That said, I find a ramp run in 2nd gear vs 3rd gear the 3rd gear version will be a little richer. Same with 3rd vs 4th. The most black and white example is free-revving; for me this is the 30 kpa column (Hondata has load on the x-axis and rpm on the y-axis). If I free-rev I can move through the 30 kpa column and the higher the rpm I go the leaner I go, up to as lean as 17:1, however I run the same sites with actual load on the engine I can get to the same sites but they are more or less stoich as tuned under those circumstances.

I figured the reason for these examples is the engine must be more volumetrically efficient the less load that is placed on it. Being in the novice category every idea I come up with is a hypothesis until verified by someone who really knows their stuff or I find it written somewhere authoritative. I find tuning very interesting and like exploring these little discoveries so that I can understand them.

Sounds like you have something else going on that is not being compensated for correctly. As mentioned earlier MAP x RPM should be a pretty good indicator of air mass flow (assuming charge temp is constant), so if you have the same MAP and RPM and the same Inj PW, you would expect a similar AFR result coming out the other side. I don't know Hondata well so I don't know what "load" means in that system, it may be using something else besides just MAP in the load calculation - or they may switch to some completely different type of fuel control under certain conditions.

Have you got any logs that show a case where you have the same inj PW, same RPM and same MAP under two different loads and AFR is different?

On a little family vacation, will try make some logs when I get back. Thanks.

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