@R Thorpe posted:As far as fuel air mixture is concerned does an engine know the difference between 3000 RPM sitting your garage and 3000 RPM out on the road?
Yes, the difference to the engine in terms of what A/F mixture is needed has to do with how hard the engine is working.
When the car is sitting in the garage, the engine is only using the amount of fuel needed to overcome the friction in the engine (not much). On the road, to get to 3000 rpm, more fuel is needed to overcome drive train losses, air resistance, tire rolling resistance, uphill/downhill resistance or assistance, etc.
The amount of fuel drawn in by the carburetor's jets is controlled by the manifold vacuum and RPM (traveling air volume) which is a good measure for how much load is on the engine. Fuel injection uses a combination of manifold pressure and RPM to approximate the engine load/need for fuel. It's basically the same thing.
Many of us have a whole stack of carburetor jets to adjust the amount of fuel drawn into the carbs at different times, speeds and loads.
Great question, by the way.