The Tech-F1, Vampire and Swiss Auto were all built to the same regs when the CIK were concidering switching to 4-strokes when they got rid of 100cc 2-strokes. All are around the same weight and produce basically the same power.
Formula 6 is 4-stroke only and all the above engines race on the same grid (seperate to the tkm 4-strokes). The EcoMoto's and Yamaha have been overbored to produce again similar levels of performance to the others (being older designs they were down on power).
As for recomendations, i will always be biased towards the Tech due to my past involment with the company and it has always served me well since i have been racing it. However the Swiss Auto does appear to be a good unit for the price, and all three have good second hand motors floating around at the moment.
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