I'm glad Vince replied before I saw this thread. I would've said a similar thing but in not such a nice way.
Look at it this way lolcats. If you can't tune a car decently, WHY EVEN BOTHER OPENING A GARAGE??
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It becomes a case of the blind leading the blind, and if people follow someone who can't tune a car properly then they are completely stupid.
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I'm not trying to discourage you though, I'm just advising you that if you do open a garage up, you need to be up to a certain standard that most people expect when they drive a tuner car from one of the tuner garages.
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This is what I suggest:
Like Vince said, read Scaff's Tuning Guide first. 👍 That will give you the fundamentals to actually understand what's happening to the car if you change a suspension/LSD/brake setting. Buy a cheap, decently handling car from the used car lot ('88 Silvia K's [S13] for example) and do a little bit of engine work to get around 250-300hp and get every other part fitted and learn to adjust each item one by one.
Secondly, go and test cars already out on offer to the public to see what the standard of a good tuning company is. MFT, RRV, RedZone, GME, Shogun, ETZ......all of us won't let a car go out to the public unless we're satisfied with it ourselves. Personally, if I'm not happy then I know the customers/drivers won't be happy either so I won't release it until I am happy. The results show with all of us in the top half of any tuner cup field on any occasion we enter and always scoring good marks.
Thirdly, don't be scared to ask if you think you're stuck, we'll always help out, and you may find yourself becoming a tuner at one of the big cheeses (like little 9yr old SVX who's joined shogun!!) because we are noticing the efforts being made by guys like yourselves and some of us like different perspectives of the same issue.