How much?
 

How much H.P. do you think it takes to knock a half a second off your e.t.

That's a loaded question. I went from 13.712 to 12.89 without any additional horsepower. Drag radials, drag struts, upper and lower control arms, 4.30 gears, 3500 stall speed converter. If you car is a stock 5 speed you can drop a 3/4's of second with drag radials, 4.10 gears and practice. To get that extra 3/4 off you'll need exhaust, tune and intake mod's and a built rear end. Once you hit the high 12's to low 13's you'll need to really launch hard or have a power adder. Normally each 10 rwhp equates to 1 tenth of a second. So 150 rwhp = 1.5 seconds. This explains why so may blown GT's run so slowly at the track. They just can't put all that power to good use.Traction is king! Since I bought my GT and started racing at the track I have met many guys with more power than my car has who are much slower because they cannot get off the line. You can run low 13's very easily in a manual 99-04 GT with just a few power mods such as pullies, X pipe and catback and setting up your rear end properly to handle hard launching. Set up properly these cars can run low 12's with only 300 rwhp.

I second Blue's post, roughly 10hp = 1/10
Or, taking away 100lbs is worth 1/10 gain

Also depends on what times your talking about. On a 14 second GT, I would say that something like 50 - 70 hp should knock-off a half second w/o any other mods. But, on a 8 second modded GT, you need a lot more than that.

They used to say that until you got to 11 seconds that it took 1,000 dollars to take off a second. After that it starts to get really expensive like 1,000 per tenth. With all the power adders available now that may not be accurate anymore. I do know that once you hit the low 12's or high 11's you start to break everything (transmissions, rear ends etc.) I takes alot of money to go fast and remain reliable. The drive line just is not designed for much over 350 rwhp.