The CW will accelerate the availability of its primetime programming on its website, shortening what had been a three-day delay to just eight hours starting Thursday.

The shift is aimed at curbing illegal streams and downloads that siphon viewers of heavily pirated series including “The Vampire Diaries” and “Gossip Girl” away from both TV and CWTV.com.

The decision comes on the heels of a test of the eight-hour gap that CW quietly conducted with some affiliates in four markets between October and January. There was no change in broadcast ratings in those markets and a slight increase on CWTV.com.

“It tells us that when you present next-day streaming, those who want to stream will be more predisposed to watching on CWTV.com or Hulu Plus than a pirate site,” said Rick Haskins, exec VP of marketing and digital programs at CW. “It also shows it didn’t really change the broadcast numbers.”

But shrinking the time between telecast and website nationwide could still cut into CW’s C3 ratings, a measurement that includes viewing on DVRs in the first three days after the premiere airdate.

The 75-hour window was imposed more than five years ago to help bolster the net’s C3 numbers. CW has consistently seen significant lift from DVR viewing in its otherwise flagging primetime ratings.

Another risk: The quick online turnaround could lure more of CW’s young-skewing audience into watching shows online instead of on TV, which draws more ad revenue and may be particularly problematic for the CW’s affiliates.

All in all, CW execs are crossing their fingers that broadcast ratings won’t shrink, and if they do, that those viewers will be watching on legal digital platforms instead of via Bittorrent. Worst-case scenario: Piracy levels are unaffected, and both TV and online auds suffer.

CW’s TV ratings are already foundering, down 20% season to date versus the same period a year ago in primetime among viewers 18-49 and 27% in the net’s target demo, 18-34. On Monday, a repeat of the CW series “Hart of Dixie” actually scored a 0.0 rating in 18-34.

CW often registers big gains for its shows when viewing is measured across ensuing days. “Vampire Diaries” and “90210” saw their auds grow by nearly 50% from the live telecast to seven days later.

Nevertheless, CW was compelled to change policy after internal research revealed that about 20% of all streams online of CW programming were of the illegal variety, with levels reaching 28%-29% for “Vampire Diaries” and “Gossip Girl.” Even worse: 50% of that piracy occurred within three days of the premiere broadcast. Those numbers aren’t growing at a particularly alarming rate but aren’t decreasing either.

The eight-hour window will bring CWTV.com in line with the window already in place for Apple’s iTunes and Hulu Plus, the latter thanks to one of several deals CW parent companies Warner Bros. and CBS Corp. made last year to put the network’s programming on digital outlets, infusing enough new revenue to stabilize the money-losing netlet.

The shift may not be good news for new Hulu Plus subscribers who may have decided to begin paying $8 per month to get an earlier start on online versions of CW programming. However, those subs still get to watch those episodes with lighter ad loads than those that run on CWTV.com, which has the same volume of ads as the telecast (but is not a simulcast).

CW could see some relief online in the event that it adopts Nielsen’s Extended Screen measurement tool, which will be capable of rolling online viewership numbers into C3 provided CW runs a simulcast of the TV episode online with identical ad pods. But that tool has seen little uptake among TV networks to date, with CW opting to do its own measurement via a combination of data from Doubleclick and Nielsen VideoCensus.

The affiliate-level test was conducted in San Francisco, Dallas, Baltimore and Minneapolis with affiliates belonging to the CW’s three biggest station groups: Tribune, Sinclair and CBS. Those groups are onboard with the changes, and other station groups with CW affiliation are being notified.

To promote the CWTV.com shift, the network will begin airing the same 10- and 15-second spots promoting the tagline “See it tonight, stream it tomorrow” that it’s used in local tests.

The eight-hour gap will get its first test on a big night for the net: one that sees the first original episode of “Vampire Diaries” in several weeks.

Shift has the blessing of the network’s ad sales department, which has been slowly bringing the ad load online in proportion to TV over the past few years via its “convergence of screens” strategy. The Big Four haven’t been as aggressive on that front.

While the possibility that accelerated online availability will distract TV auds may not please them, moving to a next-day stream puts CW in line with other broadcasters’ online windows. However, the older skew of those nets’ auds made piracy less of a problem. Median age for the CW is 37.2, up from 34.6 a year ago, but still well younger than at the Big Four nets, whose median ages are in the 45-55 range.

The new online window represents perhaps the biggest policy change put in place under Mark Pedowitz, who is midway through his first season as president of the network. The netlet cut a pair of lucrative deals late last year that put CW content on the ad-supported part of Hulu eight days after broadcast and even more broadly on Netflix at the close of the season.