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I was reading a post about having your engine professionally tuned for racing. What exactly do they do to your engine when this is done?

 
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Most tuning involves setting the air/fuel ratio to optimun using a computer fuel controler like vafc or e-manage. With a turbo, you need to add more fuel or you'll be running lean (too much air). Other things that can be tuned are cam timing via an adjustable gear, and other engine running conditions. The e-manage can do alot more than a/f, like ignition timing, injector cycle, etc. Basically it's making the car run as best it can with the added mods.

Oh yeah--all is done on a dyno for the most part, so the gains/losses can be measured.

 
Rep Power: 608

^^^ what he said

 
Rep Power: 288

how much does this usually cost?

 
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Usually a few hundred dollars.