LS6427

iTrader: (2)

I've read a bit lately on nitrous use. Some of things I read mentioned that when you spray a motor you decrease VE (Volumetric Efficiency).

Why?????


And what can be done considering cubic inches and the size of a shot.......to keep the VE from decreasing....?

.

Blown06

iTrader: (32)

Your replacing the air that the engine can naturally take in with air from another source. It's a physical property if you will. Filling a space with air from another source takes the place of air naturally drawn in on its own. Make sense?

I've heard promod guys joke about the carbs just being along for the ride once all the nos is flowing. This of course is exaggeration, but you get the point.

LS6427

iTrader: (2)

^^^
So, VE will always drop way down with nitrous? No way to set it up where the VE stays the same, or close, while the nitrous is being sprayed....?

Sounds like the loss of HP when a SC'er is used.....but it's makes so much more power from the compressed air charge, the net gain is a lot more in the end.

.

smokeshow

iTrader: (17)

The VE of the engine doesn't necessarily change. The dynamics of the engine will remain the same. However, since the oxidizer content of the intake charge is increasing and is accompanied by a proportional amount of fuel to compensate, it will displace air mass which your engine thinks is being drawn into the engine. Now you could reduce the amount of fuel delivered in the VE calibration when nitrous is bring sprayed, but it would not be a reflection of an actual decrease in VE and would only be accurate while X amount of nitrous is being sprayed.

joecar

The actual VE remains the same...

but the effective VE will be raised, and now you need a means for the PCM to add extra fuel and then some

( you want to avoid going lean with the extra oxidizer/oxygen to avoid melting the pistons )

for example, some of the EFILive and HPTuner custom operating systems (COS) provide nitrous VE multiplier tables (triggered by wiring the nitrous activating relay signal to a specific unused PCM pin)...

musthaveLSx

That statement in and of itself, is false.

http://www.epi-eng.com/piston_engine...efficiency.htm

It's all about trapped mass. Increasing the charge density, as when nitrous is used, gives you greater trapped mass at any given RPM (assuming W-O-T). The molecules are closer together, meaning more mass per cu.in. of cylinder volume. Nitrous does 2 things. It increases charge density because it cools the charge as it expands from its trapped pressure to manifold pressure. Secondly, it's chemical make up is 34% oxygen, whereas the air you breath is roughly 21%. The result is more charge mass and greater oxygen, enabling more fuel to be burned per cycle.

musthaveLSx

In considering why that statement would be made, I came up with this possibility:

Let's say your engine is on the dyno at W-O-T and peak torque RPM.
That would be roughly 102% VE, or maximum.

Now, you turn on the Nitrous.
The air meter on the dyno will indicate LOWER AIR FLOW. The air flow rate through the carburetor goes DOWN when the nitrous is on.
The dyno software VE calculation will be false, because you are not measuring the mass coming from the bottle. The carburetor is still delivering fuel proportional to the air flow going through the boosters. If you at the same time measured manifold absolute pressure, you would see that go UP.

Turbotr

Not trying to be argumentative here (really). But it's not about trapped mass- because we're talking about VOLUMEtric efficiency. Air VOLUME has no knowledge of air density (though engine VE does vary slightly with density and other factors like MAP).

Volume is a geometric (only) measure. For example the volume of a box is LxWxH, period. There are no other inputs to that equation like temperature, etc.

Volume flow efficiency is basically the ratio of air volume trapped by the engine per 2 revs vs its displacement. If say a 5.7L engine inhales 5.7L VOLUME of air every 2 revs then the VE is 100%. Notice that did not depend on air density or anything else.

The trapped mass of air of course depends on volume AND air density.

Speed/Density control fundamentals- it knows the air volume flow via the engine speed (intake strokes/sec) and its displacement and its VE. It knows the air density via the MAP and the temperature. Knowing the volume flow and the density it can calculate the air mass flow and thus also brew in the right amount of fuel mass flow to match the a/f ratio target.

The confusion comes from all the general misuse of VE, including whatever the OP saw. Its almost never stated or used correctly IME.

MIKE1444

These guys are right. If you pull information from your mass air flow sensor, and spray a wet shot past it, the input mass/volume mixture will offset some of what you see on your mass air sensor.

Air is 20% oxygen. 70% nitrogen, by mass. Nitrogen and air have similar atomic mass. Volume works on a particle basis. N20 is more dense, by about 50%. This assumes your engine runs 100% VE at that range.

Everybody in my machine design class argues about this. One school of thought is that if you spray a 50% more dense particle with 30% more usable goods, you get 1.95X the output. So, you can spray half your N/A power, at half the speed you used to get it. That's a 200 shot on an LS3 at about 3000 rpm. Should make 750 ftlb. Double the density will burn at probably four times the speed because we're using a squared function for area, so you need to chop your timing by 1/4, so like 9 degrees of advance if 26 was the na limit. Keep in mind, you're doubling the stress on your motor, so it digs into the steel fatigue cycle designed into it.

So, a 572 at 1.14 ftlb/in3, at 3000 should eat up a 373 shot, but that's quite a hit. What a hit. wowee. 650 foot pounds at the push of a button. Oooooh baby.