Would a car using air richer in oxygen require high-test gas to avoid knocking?

Question

I have a basic understanding of how a turbo or supercharger works--increasing the amount of air that can enter the engine at any given time, thereby allowing more fuel to be burned. Since there's more air forced into the cylinder, higher-test gas must be used to avoid knocking due to the increased pressure within the cylinder.

But what if you didn't increase the volume of air entering the chamber, but increased the amount of oxygen in the air charge (by introducing pure O2 into the intake)? The atmospheric pressure would essentially be the same, but the added oxygen would allow more fuel to be burned at once than with non-enriched air.

Would you still need to run high-test gas, or could you run good ol' 87 regular? The cylinder pressure shouldn't change, so the temp during compression should be similar. But would the added oxygen make the fuel mixture more prone to early detonation, therefore requiring high-test?

Answer 1

Knock is pre-ignition of the air/fuel mix caused my over pressure during the compression stroke. High octane gas helps prevent knock. By increasing the oxygen content of the air but leaving the volume of air entering the cylinder the same, the pressure during compression will not change (Ideal Gas Law). Rather, the amount of fuel would need to be increased to avoid lean burn. The added oxygen would not cause pre-ignition since you would still have to run at the same air/fuel ratio (stoichiometry) and the result would be more horsepower and torque. So in short, higher octane gas would not be required.

Answer 2

In tuning our older model cars, it was always a balancing contest between carburation mixture settings, fuel jet sizes and engine spark advance.
When I moved from the coast to Denver I had to rejet my carburators to deliver less gas because the air density here is less and the engine would run too rich with std sea level fuel jets, and foul the plugs, however by reason of the lower air density here the ambient pressure is also less so we were able to advance the spark farther before the engine would knock which gave us a little added rpm boost and compensated somewhat for the loss of power due to the lighter fuel-air load.
Some of the braver souls even installed manual spark advance systems on their engines so that they could advance the spark even farther as they drove thru the mountains and their altitude kept increasing.
At that time we found that if our engines were jetted and tuned optimally to give just a slight knock with a full throttle opening, that changing fuels to get the added octane from high test gas didn't make much difference in reducing the knock. (so running high test gas wouldn't let us increase the spark advance any more than if we were running regular).
Does this make sense to anyone out there??

Answer 3

Hop in the car, turn off the stereo, roll down the windows, turn off the A/C, and then turn on the car. Now, turn on the A/C, with the fan all the way down, as low as it can go, and pay attention to the sounds the car makes. You will probably hear the engine make a change in noise. This change in noise is the engine working harder to run the A/C compressor, which is what provides the cool that the fan blows air by. You might also notice the engine noise changing when you click on your rear-defroster. Some cars require that little extra power to heat the back window. When you run your heat in the car, the heat isn't provided by an extra part, such as the A/C compressor. Instead, the heat is provided directly from the engine, typically by blowing the air past a heater-coil that is heated by, often, the anti-freeze in your radiator. On a hot day, stuck in traffic, if you notice your temp-gauge on the dashboard rising into the red, a quick hint is to turn off everything non-essential, roll down the windows, and turn ON the heater. This will help pull heat from the engine. Downside of that is, it blows it on you.

Answer 4

Trojankn's answer sounds good on a theoretical basis.
Now look at practical. If your trying to save the $0.10 per gallon difference between regular and premium, forget it. Look at the cfm of air/oxygen involved and see what the oxygen is going to cost you. Also, how big a tank you need for it, how you are going to regulate delivery, where can you get it refilled on a trip or at 2:00 in the morning, etc.

Nice try though.