Volumetric efficiency and fuel mileage

Iv'e been wondering this for a while. If you increase the volumetric efficiency of your engine by 20% without increasing the boost, will you gain fuel mileage at all?

In theory, yes. Increases in VE that do not involve boost are beneficial to breathing at all RPM (including low RPM where you use the least fuel). But boost-derived VE improves efficiency only when you're in boost, which is when you're drinking the most gas = not so much MPG improvement there.

That being said, our SC 3800 engines have decent VE when not in boost, which is why MPG numbers can be very good if you stay out of the throttle.

Uhhh duhhh what's VE mean?

Volumetric Efficiency

Its how the air going into the motor is metered without a maf sensor if i recall correctly. Been a few years since ive done any tunning.

In Thermodynamics, we learned that volumetric efficiency is just how it sounds. It is the percentage of the max possible output that you actually obtain. In a car, making the most of the combustion chamber. I would imagine that a motor with a perfect flowing exhaust and intake would be considered at 100% volumetric efficiency. A supercharger or turbo allows you to reach over 100% VE. Modifications, aside from boring and stroking an engine, when you gain power at least, increase volumetric efficiency.  So basically, by dropping to a 3.4" pulley, you're showing your knowledge of fluid dynamics without realizing it! Haha

One way to think about VE (the term, not the tuning table) is a ratio of the volume of air entering the cylinder compared to the swept volume of that cylinder, between top and bottom positions of the piston face.

Example for L67:

3.8" dia piston and 3.4" stroke: swept volume = 3.1416 x 1.9^2 x 3.4 = 14.74 cu in.

If on the intake stroke the engine can suck in 14.74 cu in of air, it's operating at 100% VE.

An engine operating at 80% VE will suck in only 11.79 cu in of air for the same stroke.

A boosted engine doesn't suck.  It will take in more than 14.74 cu in of air, operating at +100% VE.

Of course, when talking cu in of air I mean uncompressed cu in.

AA if it's sucking in 14.74 cu in of air, it's operating at 100% volume. But that doesn't mean you have 100% efficient burn. You will be able to tell by an average of the O2 sensor crosscounts (if you have a wideband) how efficient your burn is, and therefore what your volumetric efficiency is.

Forgive my crude math... Assuming a 14.7:1 A/F ratio desired, you would theoretically want 1 cu in (?) of fuel for 14.7 cu in of air. But in reality, you don't see 100% volumetric efficiency, so based on the rich or lean exhaust detected you adjust the amount of fuel higher or lower.

I think of VE simply as a measure of air flow in relative to displacement. The burn certainly impacts VE, but it is a different thing (thermal efficiency). If combustion efficiency is low, it will inhibit the engine's ability to pump air, which results in lower VE.

I would argue that to have 100% VE (if an engine claims it), thermal efficiency would need to be within engineered spec. If it drops below, then VE would also drop.

So you are a believer in VE tuning huh Devin? That's probably the most controversial tuning area for 3800's. Different credible tuners swear one way or the other and personally, I still can't really decide if it does anything. It doesn't help either than the engineers have been known to change stuff like that throughout the evolution of the PCM software/hardware. I want to positively know if the VE map is used at all in normal operation. I know it can be referenced in the event of a maf fail but otherwise, when is it used?

I've never said that it's the only way to do things. But what do you do if you exceed the flow capability of the MAF? (which you are probably near)

After I was done tuning my VE the way I wanted it I could drive with no MAF and the car ran exactly the same. What's that worth? I don't know. I just know I like to have that table correct.

If maf tuning works for you then go for it! I always do MAF tuning after VE tuning and never have knock and barely any STFT or LTFT. (some timing table tuning as well, of course)

VE is how GM used to operate their cars. Way back when, they didn't have MAF sensors. They used a Speed Density system which really only looked at the MAP, TP, and O2 sensors for fueling. The MAF sensor is more for fine tuning the fueling. In all reality, one could tune the Riv to basically ignore the MAF sensor if they wanted to...like the deek has said.

Initially, I started out tuning VE first using tablemodifier during which I unplugged the maf and disabled power enrichment. Even though real VE was a great deal different than stock, about as different as it can get without changing to a different power adder or something, the car still ran ok on speed density (stock VE map). I started trying to tune the VE map and I ended up producing bigger and bigger spikes throughout the table if you look at it graphically. It just looked wrong. I do want to get a more true VE tune in there for testing sake but not just for the event of a maf failure. I already know it will still run.

Sometimes if you "raw" Ve tune you get a lot of spikes and valleys, and then you also get a bunch of KR spikes...  I find that by manually 'smoothing' out extreme high and low spots after tuning you can get it running damn near perfect. Some peaks and valleys are supposed to be there for whatever reason, I'm sure, but I find it works better when you don't have extremes.

I take my VE table after tuning, export, then open it in Excel. I graph it as a 'surface' and look for crazy peaks and holes and look to see where, and possibly why. Then I smooth out what I want by hand, save out the numbers and re-import. I'll try to attach an Excel file from my last VE table.
(something seems to be missing in my VE table, not sure why. But this isn't from a recent read)
Also, I forgot to include the axis labels. but just looking at it you'll get the idea.

turtleman wrote:

So you are a believer in VE tuning huh Devin?

Wait a second... who's Devin?

deekster_caddy wrote:

turtleman wrote:

So you are a believer in VE tuning huh Devin?

Wait a second... who's Devin?

This got merged to the VE thread. Devin was the OP in the thread this discussion was in previously!

Sorry, guys. Missed that. Should have merged it earlier.

So I was scanning VE today out of curiousity, and I was averaging ~60% VE.

What does this mean? Is my tune way off or what?

It probably means you were measuring at idle/cruise. VE increases as the throttle is opens. The engine pulls in the most air at WOT at high RPM. That's when you should see numbers above 100%.

If you don't see higher numbers at WOT, I think your app is miscalculating.

I should see above 100% @ WOT? I will test tomorrow....

Deek, yeah I pretty much did what you said about smoothing the spikes out after doing probably 4-5 data-log/generate-table cycles. I know it needs a lot more work because the changes in the numbers didn't start getting smaller. I do want to revisit this to see if it helps driveability at all but I haven't found myself with the time to drive around all day lately. HP Tuners does all that graphing for you in the editing software so that's what I smoothed the map with. I feel so dirty about manually typing numbers into the cells of big tables like that. haha