Ok idk much about using juice. Can you use a good amount on a stock 98 Gt motor? I've heard you gotta change head gaskets or something? Any info on this would be appreciated

 

The amount depends on the wear and tear on the motor. How many miles are on the motor and how well has it been taken care of? Normally they say your motor can handle half of its original horsepower in juice. So I would say you would be safe with a 100 shot or so assuming that your motor doesn't have any existing issues. I also reccomend getting your car tuned after to correct any A/F issues.

 

I have a 96GT that saw spray, they are very similar. I would recommend a 100 shot w/ a good tune. Any more and you are getting into a grey area. You will want the tune for sure. That will get the most out of the juice and keep it the most safe. Make sure you clear up ANY issues w/ your car before you do it. The juice will compound any issues with it. You will not have to replace headgaskets(or at least shouldnt) as long as you have good compression. You will prob. want to look into a better clutch if you are an M5. Dont forget any safety things like a WOT switch(wide open throttle). You will also want a bottle heater to get the most out of the juice and stay as consistent as possible. Good luck.
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Agree with the others.

If you have any issues now(skipping, surging,etc.) DO NOT install. get these problems fixed first.
Go with a 100 shot & good tunes with retarded timing.
Premium fuel is a must.
Get colder plugs
Get accessories--bottle heater, purge, guages, etc.


Like anything else, if you are gonna do it, do it right.