Big Cube VE tables

[joecar]

Is VE affected by RPM? If so, why do I get the same value ~2.468 when backing into "max" air by using/comparing VE table in both numerical units and percentage. For example

@ 400 RPM and 15kPa I see 1.029883 g*K/kPa or 41.72228% which indicates a "max" of 2.46825

@ 8000 RPM and 15kPa I see 1.268945 g*K/kPa or 51.4071% which indicates a "max" of 2.46824

Wouldn't the "max" increase as RPM increased? I took chemistry, but it was a long time ago. So, I probably just don't get it.

Isn't "max" your theoretical 100% volume (which shouldn't change with RPM)....? :?:

[superls1]

Is VE affected by RPM? If so, why do I get the same value ~2.468 when backing into "max" air by using/comparing VE table in both numerical units and percentage. For example

@ 400 RPM and 15kPa I see 1.029883 g*K/kPa or 41.72228% which indicates a "max" of 2.46825

@ 8000 RPM and 15kPa I see 1.268945 g*K/kPa or 51.4071% which indicates a "max" of 2.46824

Wouldn't the "max" increase as RPM increased? I took chemistry, but it was a long time ago. So, I probably just don't get it.

Isn't "max" your theoretical 100% volume (which shouldn't change with RPM)....? :?:

I guess I should say per unit of time. The engine definitely moves more air at 8000 RPM's than it does at 400 RPM's. No? So, my assumption was that if the VE table was used in a fueling equation that the values in the table might increase as RPM increased.

It is probably just a lack of understanding of the units on my part. I guess I was thinking that the table represented actual air moving through the motor. The units do not indicate that, but the algorithm could by accounting for RPM.

[Ira]

I guess I was thinking that the table represented actual air moving through the motor. The units do not indicate that, but the algorithm could by accounting for RPM.

The table refers to the amount of air going into each cylinder on each intake stroke. It usually has the biggest numbers at the torque peak.

Ira

[Blacky]

It is probably just a lack of understanding of the units on my part. I guess I was thinking that the table represented actual air moving through the motor. The units do not indicate that, but the algorithm could by accounting for RPM.

The VE table does not show actual air mass (or even air flow), but percentage of theoretical maximum air mass. The maximum won't change*, that's a fixed number: 8.5 grams per cylinder in a 5.7 liter LS1.
But as MAP and RPM change so does the mass of air that enters the engine. The VE table is just a prediction of what percentage of that 8.5 grams of air is expected to enter each cylider at various rpm/map combinations.

* except for temperature and barometric changes, but the PCM compensates for those (sometimes not very well).

If the VE table reads 50% then 4.25 grams of air is expected to be in that cylinder. That is how the PCM "knows" how much air is in the cylinder and therefor how much fuel to add to achieve the current "desired" AFR.

The whole point of a MAF sensor is to calculate the mass of air entering the engine - that is why when you have a MAF sensor you don't need a VE table. Except for transient throttle conditions and MAF failure conditions.

Paul

[superls1]

It is probably just a lack of understanding of the units on my part. I guess I was thinking that the table represented actual air moving through the motor. The units do not indicate that, but the algorithm could by accounting for RPM.

The VE table does not show actual air mass (or even air flow), but percentage of theoretical maximum air mass. The maximum won't change*, that's a fixed number: 8.5 grams per cylinder in a 5.7 liter LS1.
But as MAP and RPM change so does the mass of air that enters the engine. The VE table is just a prediction of what percentage of that 8.5 grams of air is expected to enter each cylider at various rpm/map combinations.

* except for temperature and barometric changes, but the PCM compensates for those (sometimes not very well).

If the VE table reads 50% then 4.25 grams of air is expected to be in that cylinder. That is how the PCM "knows" how much air is in the cylinder and therefor how much fuel to add to achieve the current "desired" AFR.

The whole point of a MAF sensor is to calculate the mass of air entering the engine - that is why when you have a MAF sensor you don't need a VE table. Except for transient throttle conditions and MAF failure conditions.

Paul

Thanks. Great info.

I was just thinking about it in terms of expected air flow through the engine for each RPM and MAP intersection as oppsed to maximum air mass.

[superls1]

I would still like to see some tuned VE tables then. Forget the big cube request. There just don't seem to be very many posted. Why? I haven't tried doing mine yet. I guess I just want to know everything before jumping in including "what" it might look like.

[superls1]

I do have one more question: How does SD account for humidity? Humid air is less dense. So, if I tune my car when humidity avg's 50%, but in the summer it averages 80% what will happen? Just run pig rich for a while until LTFT's go really negative?

[GMPX]

I would still like to see some tuned VE tables then. Forget the big cube request. There just don't seem to be very many posted. Why? I haven't tried doing mine yet. I guess I just want to know everything before jumping in including "what" it might look like.

Try one of the web sites to download bin / tun files, there is sure to be an 8.1L file on one of them.

Cheers,
Ross

[GMPX]

I do have one more question: How does SD account for humidity? Humid air is less dense. So, if I tune my car when humidity avg's 50%, but in the summer it averages 80% what will happen? Just run pig rich for a while until LTFT's go really negative?

SD compensates for humidity just like a MAF does...it err, doesn't!!!.
No humidity sensor in a MAF.
We have similar weather here in Aus up in north Queensland, PLENTY of MAFless cars up there.

Cheers,
Ross

[Ira]

I do have one more question: How does SD account for humidity? Humid air is less dense. So, if I tune my car when humidity avg's 50%, but in the summer it averages 80% what will happen? Just run pig rich for a while until LTFT's go really negative?

The change in humiditY from 50% to 80% doesn't affect the % of oxygen in the air to a significant amount. It runs a tiny bit leaner and the extra water makes it less likely to detonate so I think in the end it's basically a toss. You've never heard anyone with a carb complain about humidity messing up the mixture.

Ira