This idea has already been done, but its lack of success lies in the math, which is where I hope to pick up the slack.
A car runs faster with more Oxygen in the intake, and this is why superchargers (compressors) add power.
An engine requires a lot of air, however, so when someone adds a leafblower
as a ghetto supercharger, they may or may not be supplying the motor with enough air, and the net pressure change may actually be negative due to the restriction in the intake.
A 2.2L motor running at 6000 rpm uses approximately 450 cubic feet of air per minute. (466.1536 of air/fuel mixture to be exact.) Most gas-powered leaf blowers top out at this rate of flow, which is why a higher-displacement motor (such as a ford 5.0L) would be starved of air. (A 5.0L spinning at 5000 rpm would use almost 900 cubic feet per minute.)
So, while this idea is a little shy of working correctly, I propose the use of MULTIPLE leaf blowers. Even though a leaf blower's fan cavitates as back pressure is developed, it is still somewhat capable of a slight pressure increase, if only to negate any effects of intake restriction to emulate a well-designed naturally aspirated motor. (Keep in mind that people spend hundreds on naturally aspirated intake setups for the same goal.)
Two leaf blowers would be enough to provide air to the aforementioned 2.2L motor (which happens to be the 4cyl GM ECOTEC). Four might be enough for the 4.6 or the 5.0. Two might be good as well for a 1.8L Mazda or a 1.6L Honda. A motorcycle might even work well with one.
The problem is that the motor will run too lean if given too much "boost" in the lower RPM range, which results in a blown motor. Like a turbo upgrade, this upgrade will require stronger injectors or, in the case of small engines or old cars, a richer carburetor setup.