putting together a drivetrain in hopes of a fall17' build

[chuckster]

Hello all as the title says My son and I hope to build a 289 usrrc roadster over the winter of 17'/18'. We're in the process of assembling the the drivetrain this winter. We've cleaned out (mostly) the garage, and are in the process of insulating and installing heat.
We've got the engine mostly built, and are negotiating for a 2016 Mustang IRS. Given a choice between 3:73s and 3:55s which would you folks choose? I'm leaning towards the 3:55s as a 2200 lb. car with 300-320 rwhp may have trouble hooking up with 3:73s.

[Jeff Kleiner]

Welcome!
To effectively answer your question you need to tell us what transmission you intend to use and the first gear ratio so that the whole package and overall gear reduction comes into play. For example T5s can have either a 2.95 or 3.35 first gear so a 3.35 first coupled with a 3.55 rear =11.89:1 overall while a 2.95 first along with 3.73 in the rear = 11.00:1. In this case the 3.73 would be less aggressive. My opinion having build roadsters in the 300-350 HP range with various gearing combinations is that 2.95 first with 3.55 rear configuration (10.47:1 overall) hits the sweet spot.

Good luck!

Jeff

[edwardb]

I agree with Jeff that 3.55 hits the sweet spot for most of these builds. But do the math with a calculator like this one: http://www.tremec.com/calculadora.php.

Something else you're going to find is that with the 2015/2016 IRS components, the center sections come in various ratios and either aluminum or iron case. Ratios available are 3.15, 3.31, 3.55 and 3.73. I would avoid the 3.15 as the internals are not interchangeable with the other ratios. It will always only be 3.15. The rest are interchangeable if you should want to try something different down the road. The 3.31 and 3.55 come with regular clutch type traction lock. The 3.73 is only available with a Torsen limited-slip setup. That's a whole other subject. But for starters they are double the price and maybe not the optimal ratio. Factory Five initially recommended only the iron case versions used on manual shift Mustangs. Later said the aluminum versions used in automatics was OK. They aren't quite as strong but a bit lighter.

I chose a 3.55 iron case, FWIW.