'Power-to-weight’ is a term that gets thrown around a lot and we all know how much difference it makes to how a car performs and handles. Or do we?I recently sold my Birkin S3, a replica of the old Lotus 7, a car specifically designed to prove that smaller engines and less weight is a better recipe for a race car than an aircraft engine bolted to a fire-truck chassis.Image gallery My S3 weighed in at a very slim 540kg and under the bonnet lurked an imported Toyota 20-valve blacktop (capable of 120-150kW) engine with modified throttle bodies and exhaust, the finely tuned Gotech management system promised 121kW at the wheels. 'DOESN'T CORNER LIKE ON RAILS'This doesn’t sound all that impressive but consider that ratio and you’ll notice it’s pretty much on par with my current car a 2006 Merc SLK 55 AMG.Off the line there are very few cars and even bikes that can keep up with the Birkin, the open diff and lack of weight at the driving axle makes the perfect launch a fairly difficult achievement, but once mastered, the lack of inertia on a car this light makes it fire off the line with such violence it feels as if a truck has driven into the back of it. Once you get over about 140km/h however the shove starts running out as the 1950’s aerodynamics take over and pull the car back much like it was attached to a bungee cord.Contrary to popular belief, the Birkin does not corner like it’s on rails… it’s better! Drive it like a fool and it will tend towards slight understeer and push the nose wide as it scrubs off the speed. Turn-in is razor sharp, but once it starts changing direction you have to use the throttle to shift the weight around and make it turn. It’s incredibly involving to drive and just puts a massive smile on your face.The ride is quite firm, but it remains very composed over bumpy roads, only under hard acceleration have I ever noticed it ‘jumping’ over bumps in the surface. WHAT'S THE CATCH?Stopping a car this light is highly entertaining too, even with a very basic braking system it stops instantly, because the lack of weight not only means little inertia off the line but also little momentum when you want to stop or change direction.So what’s the catch? There’s no heater, no aircon. No power windows, mirrors or power-anything. The boot is barely big enough for my laptop bag and there’s no useful cubby-holes inside. It doesn’t have air bags, satnav, parking sensors or cruise control.There’s no radio and even if there was, you wouldn’t be able to hear it, the engine and the wind is deafening. Despite this I often used mine for the daily commute to the office, a 45 minute drive each way. I enjoyed every single second.540kg might not be much for a car but this thing is 540kg of pure fun and adrenalin in equal quantities. I’m not a doctor but that sounds like a lot to me!"Have you purchased a new ride? How about that old clunker you'll always remember fondly? Send us your Reader Road Test and we'll publish it on Wheels24.