Discussion and questions related to the course EFI Tuning Fundamentals
So is it possible to get greater than 100% VE on NA engines? If yes, then how so?
Yes it's possible by utilizing pulses in the intake and exhaust system with appropriate cams.
However, it's also OK to have greater than 100% VE relative measured air to compensate for something not fully accounted for in the air measurement (say the exact location of the manifold pressure pickup is located at a localized low-pressure node in the manifold, or the air temperature measured is not what it is in the combustion chamber)
So don't worry if when tuning the VE table you get more than 100%, and if you're looking to increase the performance of a normally aspirated engine -- consider the intake / exhaust and cam profiles. Engine Simulation software is one way to explore these areas without an unlimited budget.
Thanks for the answer David.
So its not possible with stock cams I assume?
"However, it's also OK to have greater than 100% VE relative measured air to compensate for something not fully accounted for in the air measurement (say the exact location of the manifold pressure pickup is located at a localized low-pressure node in the manifold, or the air temperature measured is not what it is in the combustion chamber) "
So youre saying its possible because of the position of the MAP sensor? Is VE currently measured by the MAP sensor only in the manifold? Is there a way of measuring the exact VE in the cylinder?
Splitting the original question in to two parts -
"is it possible to get greater than 100% VE on NA engines?" - Yes, by utilising a large amount of cam overlap, having the inlet and exhaust optimised for pulse tuning and having the intake pickup taking advantage air pressure caused by the car moving.
"Is it possible to have to enter a figure of greater than 100% VE in an ECU map to get correct the correct air to fuel ratio?" - Yes. There could be a number of aspects of the installation, particularly on a modified engine, that now combine to fool the ECU into thinking the amount of air being ingested is more that it actually is. Such as a incorrectly calibrated sensor and David's examples of MAP sensor location or an air temperature discrepancy. Remember that the ECU is making assumptions based on all its sensors, a lot of things could be incorrectly assumed.
In answer to the question "not possible on stock cams?" - Highly unlikely. Standard engine are so emission/accountant strangled that they would be very unlikely to be aggressive enough to get to 100% VE. Unless it is a motorbike or a pretty serious sportscar engine that is pretty highly tuned to start with.
Thank you for the answer my man! Okay so basically,
not possible with stock cams.
using greater degree-ed cams, it is possible. coupling with tricking the sensor feedback into thinking the intake charge is more than it actually is (Hence the >100%VE) you can 'boost tune' the NA car.
does this make sense?
Depends whether you are referring to "actual" VE or "ECU" VE.
Either way, if the VE in the map is producing the correct A/F ratio the engine will be producing whatever power it produces. Whether that VE figure is 50% or 150% doesn't actually make any difference, it is just a nominal representation of VE within the ECU that eventually leads to a fuel injector pulse width.
Unless the mapping is way off, in order to get more power you need to get more air (and fuel) in and then get the burnt gases out as efficiently as possible. Re degreeing the cams might help a bit but changing the sensor feedback will have no impact at all...it won't be getting any more air in even the VE figure in the ECU might be larger.
I race with Mazda 2L MZR engines with stock internals that I believe have realistic VE values of around 120%. I am using stock cams, but very good throttles (AT Power Direct to Head Throttles), and a custom designed 4-2-1 exhaust system designed from engine simulations that really works.
Tuning the Variable Valve Timing on that engine is very reveling about how cam timing, engine RPM, and the intake/exhaust pulses can make a huge difference in the VE of the engine.
Hi Denis, wouldn't the sensor feedback matter? if the ECU finds a higher VE than the actual amount, wouldn't it alter the duty cycle of the injector to be longer?
Hi David, that is very interesting. How do you calculate your VE to be 120%; or are you getting that based on sensor feedback? You mean individual throttle bodies are a substitute for changing to larger duration cams? Do you mind showing some pictures of your project car?
The ECU calculates the VE from the sensor data. If the sensor data is wrong, for instance because they are misconfigured, then the VE won't match what the engine is actually experiencing. It doesn't matter WHAT the VE value is, as long as the A/F ratio is correct.
If you are completely reliant on the calculated VE figure matching reality and aren't checking A/F ratio and/or power on a dyno then the engine will NOT be running the correct A/F ratio and will not be making the best power (or be about to wear out/blow up).
I do not calculate the VE, I find the necessary VE by tuning the VE table on the dyno. I am very sure that my sensor inputs (air temp, manifold pressure) are correct, and with the wideband lambda sensor feedback, you tune the VE until the commanded lambda equals the measured lambda. That requires a VE of > 120% on my engine at wide-open-throttle (MAP is very close (1kpA) to measured Barometric Pressure).
Here is a link to my project with many pictures of the engine, intake and exhaust systems:
Just to confirm, is VE the percentage of how much the combustion chamber is filled with the air fuel mixture or is it the efficiency at which the air fuel mixture is drawn into the cylinder from the intake manifold, i.e., the amount of volume a mass of fuel-air-mixture occupies in the intake VS the amount of volume the same mass of fuel-air-mixture occupies in the cylinder?
I understand that explanation Denis, thank you. Basically, youre saying that AFR is the defining variable right?
David, by that you mean, you tune the VE based on the wideband exhaust lambda sensor output figures right? Can you clarify what you mean by commanded and measured lambda? As I understand it, measured lambda is what the actual lambda sensor is reading and the commanded lambda is what you want to set it as. So you wouldn't you be tuning the measured to become the commanded? Also, thank you for sharing that link. I will have a proper look at that.
Yes, when tuning you adjust the VE table until the "Commanded Lambda", "Fuel Mixture Aim", "Target Lambda" or whatever it's called in your ECU, produces the same value when measured by your wideband lambda sensor. If your VE table, air-mass calculation and fuel mass calculation are correct, then you can change the Commanded/Aim/Target values, and the measured value will be exactly that. If you've tuned the VE table, and change he commanded lambda and the wideband doesn't measure the same, you could fix it by changing the VE table, but it's really indicating that something in either the air-mass or fuel mass calculation isn't correct (wrong injector calibration, improper sensor calibration, etc). You could change the VE table to compensate -- but then you KNOW that that the VE table is not representative of the true Volumetric Efficiency of the engine.
Whilst I understand the benefits of VE based mapping...I really do think that injector pulse width duration based mapping was a lot less confusing! Took me a little while to get my head round it as well, coming from the old school way of doing things!