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must have one o' them 2-stroke 1,7's...
I said per cycle, not per revolution...two revolutions and all four cylinders will have done one intake stroke. At 5000 rpm, there are 2500 intake strokes per minute, or 4000 liters of air per minute for a 1.7.
While we're correcting each other, 3-5hp is 300-500 amps at 12 volts, not 30-50. You'd need to run 120v power to only use 30-50 amps. Running at 12v would be pretty impractical, so I'd use 24 or even 48v motors, and run 2-4 normal batteries, just to keep the cable size reasonable. Normal starter motor cabling should suffice then.
The best use I've seen of an onboard "extra" small engine is on an AX car that used it to drive a fan, not to boost power for the engine, but to draw the air out of a rubber, nylon, and steel skirt in the center of the car. Yes, a sucker car. Was only allowed for one season before someone updated the AX rules to outlaw "movable aerodynamic devices". It was scary fast that one season, though.