Quote:
Originally Posted by Christ
The PPM counts don't tell you much about volume or mass unless you have something to compare it to... namely the number of millions of parts coming out of the exhaust.
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It's obvious that you're not thinking about what "parts per million" really mean. It's the same as percent, except per million rather than per hundred. So you've got burned air:fuel mixture (14.7:1, roughly) coming out the exhaust, and so many parts per million of that are unburned fuel. So if your engine is moderately well tuned and emitting 20 ppm HC, then for every liter of fuel burned, you have (20 * 15.7 / 1000000) liters of unburned fuel, or about 0.3 milliliters. So burning that completely would increase your mpg by 0.03%. (Plus a bit for burning the CO to CO2, but still not enough to even be measurable outside the lab.)