Maxie,
If you should be fortunate enough to have the resources, find a fast combo and be able to set a record, or be a top qualifier, you should also be ready to go to "the barn"!
It's mandatory when setting a record and it's possible to be "chosen" at any other time, but you need to be prepared to tear the engine down to verify that everything is legal and within the rules!

It's also usually cheaper to buy a proven combo than spend a lot of time & money figuring out exactly what it likes and needs to go fast. That also applies to the obscure, under-factored combos that you might find. You need to have a good combo to start out with because the classes all have a respective "index" which is a minimum performance standard that needs to be met. You can run faster than the index, and many fast cars can run and dial-in more than one second under. But if you can't run the index, you are dialed in at the index anyway and you start out at a disadvantage.