I just purchased a new string for my Great Northern Fireball.  Old string I measured 59 1/4 the BH was 6 3/4 with a 5/8 nock point.  Other than the arrow thwacking the target silence.  I bought it used and the string was begining to fray so I ordered a new string at 59 1/4 put it on the bow and the BH is 8 1/4 so I think they were made wrong take it off and measure 59 1/4!  Lay it next the the old string same length!  I untwisted until I was concerned about untwisting the string and got it to 7 1/4 but some twang versus silent like before.  
How can the same length of string produce such a difference in brace height?
Thanks,
Andrew

B-50 vs FF maybe?

Same material?  Sounds like stretch is the differrence.

Both are fast flight.

try measuring both strings under tension. one way would be to string the bow and measure tip to tip along the string. this will give a rough idea.

I use 2 bench dogs with nails inserted and vice on my workbench. gently crank the vice out and measure.
This is also the way I take stretch out of new strings.

I think you will find the old string "stretches" more then the new thus giving you lower Brace height.

Fast flight strings stretch.

At 4pm one afternoon, I put a 62" D97 flemish style bowstring on my 66",  37# recurve. I set the brace height at 8 1/2". Twenty four hours later at 4pm the next day, The brace height was sitting at 8 1/4". I waited another twenty four hours and at 4pm on the second day, the brace height was at 8 1/8".

I then decided to shoot the bow with the new string. I shot about 150 arrows over the course of 2 hours time. I then checked the brace height and it was 8".

The next morning, I checked the brace height again and it was down to 7 15/16". Since that time, it hasn't stretched at all. With no more stretching, I fine tuned the 2212 arrow for the brace height it likes and it likes a 7 3/4" brace height. This brace height hasn't moved in probably over 750 shots so my D97 fast flight string is "shot in".

QuoteOriginally posted by Night Wing:
Fast flight strings stretch.

At 4pm one afternoon, I put a 62" D97 flemish style bowstring on my 66",  37# recurve. I set the brace height at 8 1/2". Twenty four hours later at 4pm the next day, The brace height was sitting at 8 1/4". I waited another twenty four hours and at 4pm on the second day, the brace height was at 8 1/8".

I then decided to shoot the bow with the new string. I shot about 150 arrows over the course of 2 hours time. I then checked the brace height and it was 8".

The next morning, I checked the brace height again and it was down to 7 15/16". Since that time, it hasn't stretched at all. With no more stretching, I fine tuned the 2212 arrow for the brace height it likes and it likes a 7 3/4" brace height. This brace height hasn't moved in probably over 750 shots so my D97 fast flight string is "shot in".

Takes a little time for all strings to settle in.  If you want ot speed the process, cut the serving off and let set 48 hrs then reserve.

Count strands
Wax it rub in to generate heat to season it