195 is stock, 180 is what I always had the best results with, 160 will not let your car get up to the correct operating temperature as will not having one in at all like you are doing now. This can cause damage to the engine and I would HIGHLY suggest you put one in ASAP. Installing one takes all of like 15 minutes and they are $3. No idea why you would not run one.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.