How much horsepower equals a tenth in the quarter mile. Or how much of a difference does 10 hp make in the 1/4?

i'll guess 10hp is near a tenth depending on the time your running. faster you go, the more hp each tenth becomes

really? I'm only running 15s so would that make it like only 5 hp for me? Cause i took my air filter out and went from 15.69 to 15.49? Which means my purolator would be holding back 20 hp???

ET gain from HP is proportionate to speed. The faster you go, the more HP it takes to get that tenth. A 20 HP gain on a 13 second car isn't the same as a 20 HP gain on a 9 second car.

Lets say you use a 100 NOS system on a car.

A 15.0 second car that weighs 3500 pounds needs 205 HP. A 100 HP shot of NOS will make the same car run 13.14. That's a gain of 1.86 seconds with only 100 more HP. That's why NOS is so popular in a street car.

A 10.0 second car that weighs 3500 pounds needs 691 HP. A 100 HP shot of NOS will make it run 9.56. The same HP increase only gained 0.44 seconds. Seeing 7 or 8 second cars with 300+ dual stage NOS systems doesn't mean much when you realize it takes a lot more HP to go just a little bit faster so getting that tenth in a slow street car isn't as hard as getting that tenth in a much faster car.

^nice i never knew or thought about that

ok gotcha. So how bout a 15 sec car with 200 horsepower weighing about 3300 pounds?

It will be the same thing, at 15 seconds the tenths seem to just fall away with very little work. Then a little bit more and 14 seconds....and so on. Like Stephen said right now at 15 seconds tenths only cost a few bucks, or sometimes even free. As you start to get into the 13's and 12's it will start to cost you some $$, and if you start to try and pick up tenths at the #'s that Stephen is putting down, you better have a good game plan and a deep wallet.

Were the 60' times similar on both runs? How much did the MPH increase?

Stephen spelled it out to a T. The faster you go, the harder it is to punch a hole through the air and accelerate.

I had a car similar to yours years ago. 86' IROC w/ a remaned LG4 and a T5. 3.73's, TPI car exhaust manifolds, Y-pipe, !cat, TPI car catback w/ a blowmaster muffler. Same intake and carb as you, ZZ4 dist., 180* stat. I ran consistent 15.20's on street radials. I yanked the PS belt on a cold night and managed a string of 15.00's and two 14.9x passes. I was right around 91 MPH when I ran 15.20's, and 93+ MPH w/ the PS belt off and good air.

I still had my AC, it gets hot in FL. But, it was a low option car: crank windows, manual locks & mirrors, no tilt or cruise, etc. It was a fun car, I miss it.