Understanding VE | AEM
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The Hellrazor
Last seen: 6 months 2 weeks ago
Joined: 03/31/2016 - 01:24
Understanding VE

So after dabbling with many different ECU's in the passed few years the Infinity has because by far my favorite. Though I do feel more like workspaces take 95% of the time to dial in then the actual tune. Now that I have them where I like them I must ask this question.

So my car runs wonderful I love it. But, I do not understand how VE is calculated clearly. I have seen many videos and they all look the same.
At IDLE the VE is sitting around 60% and some at even 80%.
Considering the throttle is essentially suffocating the motor shouldn't VE be close to say 10% VE and at WOT and 98KPA close to 100%?.

Fully warmed up and 800rpm Idle my 350z is currently at 48% VE. I'v seen pro tuners sitting on a dyno fully warmed up engine idling at 70% VE.

What am I missing? Is VE Calculated based on TPS as well? Are you all ways trying to achieve close to 100% regardles of Throttle? My injector rates look correct and my MAP is calibrated correctly. So should my VE not be close to 10% when I am essentially demanding 10% power output?

Last seen: 2 hours 5 min ago
Joined: 06/06/2014 - 09:31
You got it backwards.  VE isn

You got it backwards.  VE isn't caculated.  The ECU is calculating the engine's airflow and VE is an input to the calc and it's actually up to you to provide its value - which is why there's a VE map for you to tune.  What's really going on is a VE based Speed Density fueling control that works on two major parts: engine VE data and injector data.  The ECU is calculating the engine airflow as mass and since it knows the injector's flow and battery offset data, it essentially controls fuel mass delivery to match air mass delivery to achieve a certain target AFR.  That in a nutshell is how it all works.

I like to think of the relation between engine air mass flow and injector fuel mass flow as a balance.  If you have good (accurate) injector data then you'll have fairly good (more accurate) VE values - this is a good balance.  When you don't have good injector data and the injectors actually flow more or less fuel than you've told the ECU, then the VE values will have to be skewed in order to make AFRs on target - this is when things are out of balance.  For instance, the most common "imbalance" is injectors delivering less fuel than the ECU thinks they deliver.  For the same engine air flow, if injector size goes up, the ECU compensates by reducing injector pulse time since a higher flowing injector will need less on time to deliver the same amount of fuel as a smaller injector.  Well if your injector isn't actually delivering the full amount of fuel, your AFRs will be leaner than target and you'll have to artificially increase your VE value in order to get the injector pulse width to increase in order to get AFRs on target.  So that's one reason why you might see higher than normal "VE" values in some calibrations.

IMO at the end of the day, it really doesn't matter what your VE values wind up being.  They can be artificially high or low or totally dead nuts on.  It doesn't really matter.  What matters is that whatever VE values you have get your AFRs on target without feedback.  If you can accomplish that then everything is kosher.  FWIW it's worth I've seen accurate idle VE values be anywhere from 30-60 depending on cam profile.  More aggresssive cams will typically have higher idle VE values.

Last seen: 2 weeks 2 days ago
Joined: 09/14/2014 - 10:21
VE is simply a feed-forward

VE is simply a feed-forward input into the ECU, but it does have a physical representation that is the percentage of air that could fill the swept volume of the cylinder at that pressure. 50-70% VE at idle makes sense, as you have a significantly higher exhaust port pressure (above ambient) versus intake port (~30-40% of ambient). A four stroke engine will never be able to get 100% cylinder fill when it's pumping against an external pressure differential, even with extremely good valve timing. 


Conversely, with pressure pulses creating additional cylinder filling/evacuation at higher engine speeds, and where the cam timing starts getting into its sweet spot, you can easily see 95-130% VE values at higher RPM/high throttle angles.


Understand that you are inputting a value that is representing a physical outcome of an engine cycle, but the control system of the EMS has other feedback mechanisms to adjust to that being slightly off from reality (based on relatively recent inputs to the EMS). If all the EMS inputs are reinforcing your feed-forward inputs as being correct - that's a good indication that's actually what's happening in the engine.