I would research on Nitrous and how it works before taking on that risk.

You have a 98 Eclipse GS which would need a WET nitrous system. A DRY system will not work, they can only be used on a 95 safely since they have a different style of fuel system. The safest shot for a 420a would be 50 shot. 75 has been done and you could have some good results but some have not so its a 50/50. Anything higher then that on stock internals your 100% risking having engine damage.

Before going and buying a Nitrous System I suggest you have a compression test done on the motor thats in your GS.

For more information you can go to:
2GNT.com
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