HP gains with underdrive pulleys are rpm-dependent and they also depend on what other mods you have. On an otherwise stock 5.0, the gain is something like 3-4rwhp. On a highly-modified 5.0 you might see a 10-12rwhp gain.
well to be exact as rpm increases hp from pulleys will increase. So you will only see more gains on a modified motor (vs. stock) if you spin it up to like 5 or 6 grand or more. But if you have a mild modified setup that makes power in the stock powerband than you wont see any gains over a stock motor w/ pulleys.
For years now, peeps have time slips showing 1 to 1.5 tenths reduction
in 1/4 mile et times and in rare cases, some have found 2 tenths
The old 1/4 mile rule of thumb is 10 hp is good for a tenth
so
If you found 1.5 tenths that would be around 15 at the crank
and
Somewhere around 12.75 at the wheels using 15x.85
I been running them for quite some time with no probs what so ever
I do idle at 850 to 900 and that increased rpm helps with
charging issues at idle.