jimmyblue

iTrader: (11)

I see in the tune files, a strong MAP-dependence of
VE. I also have encountered what seem to be MAP-
driven fuel surge, which I pin on an incorrect MAP-
axis profile (RPM changing little, WB AFR changing a
lot).

The only thing I can come up with is acoustic type
affects - air density changing resonant characteristics -
and maybe "viscosity" / density affecting flow somehow.
But I have never encountered a sensible explanation
based on science or measurement.

What I'm after is a sense of what's sensible, for sanity
checking VE tables. Of course the VE table is often a
catch-all for all the nonidealities, airflow and other,
that roll up to an AFR skew. Which is a good reason
to try and understand when you're compensating for
stuff that ought to be modeled elsewhere.

3.4camaro

iTrader: (1)

My guess is that:
IF the MAP is very low (a vacuum condition),the piston must do work against the air inside the cylinder in order to complete the rotation cycle. This work takes away from the output of the engine and should therefore lower the VE.

rsz288

LS1 PCM right?

Try the EFILive or HPT forum. Maybe some insight there.

With MAP simply measuring the intake manifold depression from atmospheric, any oscillation that shows up on a WB is usually due to spark or fueling or some other issue.

Idle or cruise or WOT power?

1 FMF

iTrader: (1)

Why VE varies strongly with MAP:
the manifold absolute pressure (map) is the main factor that allows air into the cylinder, without pressure air would not flow into the cylinder. So unless i'm missing something you should see a good direct correlation between map readings and volumetric efficiency... higher pressure means more air into the cylinder equals higher VE. And from the mass air flow (MAF) sensor you know how much air is being injested which in turn allows you to calculate VE.
I get confused in your 2nd paragraph about what your asking, but density at least is already accounted for when you combine the MAF reading with the IAT (intake air temp) reading.
but for sanity checking VE tables i would think they would correlate with the throttle position sensor, you're going to have low VE when the throttle is closed and high VE with throttle open.

jimmyblue

iTrader: (11)

MAP is already a direct term in the speed density air mass
calculation (MAP*RPM*VE(RPM,MAP)*sometempstuff). My
question is about the VE dependence on MAP, particularly
what the normal profile might be.

I've seen some SAE papers saying that the intake/exhaust
pressure ratio is a primary factor of volumetric efficiency,
so maybe it's just using the MAP index as a cheap & easy
"good enough" (there being no exhaust pressure sensor).

rsz288

The VE table is probably misnamed to a degree. It is really the air mass flow based fueling table.

Yes it has the VE numbers, which fueling is calculated from based on predicted air mass flow from the VE table.

RPM and MAP point the table cell which fueling is referenced from and then temperature correction/s are applied to factor for the actual air density based on inlet air temp and engine coolant temp.

Yes MAP and VE are directly related from an air volume flow viewpoint. The temperature factoring turns this into air mass flow.

When the throttle is 100% open, assuming minimal airflow restriction the inside of the intake will see near enough atmospheric pressure which is MAP ~102kpa/14.7 psi. So the cylinder is potentially filling 100% of what it can at that throttle setting.

And if the exhaust clearance is inadequate...if the exhaust cant get out then the incoming air wont fill as well with new air/oxygen. (intake to exh pressure ratio).

When the throttle is at idle, the intake airflow is "throttled" with MAP at ~30kpa so air mass flow is a lot less and fueling as well.

i.e. an engine like an LS2 may achieve cylinder filling equivalent to VE of 105% at WOT, but at idle something around 55%.

MAP tells the PCM where on the the air flow curve the engine is so it can calculate fueling.

Not sure if this helps though........