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Thread: Question about Borg Warner S257 sx-e

  1. #1
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    Question about Borg Warner S257 sx-e

    I am beginning assembly of my m50b25 project. It has been bored over to 84.5mm, 8.8:1 weisco pistons, eagle rods, and cut ring head gasket. I am leaning towards the Borg Warner s257 sx-e (purchasing from agpturbo), but am curious about turbine housing options. What size a/r housing is going to get me into boost quickest (I'm thinking .63) for my power goal of 450-500 whp? Also, I'm assuming I want the 'open 4 bolt outlet' option to mate it to my RSI t3 top mount manifold?

    Thanks for any input.

  2. #2
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    The .63 will definitely spool faster at the expense of back pressure. Since you have good pistons, low compression, and only 2.5 liters, I think the .63 is going to be a better choice than the .82. If you had a 2.8 or 3.0, or if you had stock pistons I probably would have recommended the .82.

    i would get the 3” v band outlet rather than a 4 bolt outlet.

    Also what part number of piston did you get? Was going to check for you quick to make sure you got the ones for 135mm rods rather than 140mm


    86 325es, 2.8L m50, S476sxe, ProEFI 128 ecu, e85, solid rear axle, TH400 trans, 28x10.5w slicks, zip ties, popsicle sticks, tape
    best time 9.06 @ 151.8 mph, best 60 foot 1.30

  3. #3
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    I did get the 135 mm rods. Also, the block has been bored over to 2.8 liters (84.5 mm), should I consider the .82? Ultimately I want as much usable power as possible, in the least amount of time.

  4. #4
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    Unless you put an M52 crank in it as well its not 2.8 liters. I was asking what pistons you got, I know the rods you have are 135mm. Wiseco has a part number for pistons to match 135mm rods, and for 140mm rods.


    86 325es, 2.8L m50, S476sxe, ProEFI 128 ecu, e85, solid rear axle, TH400 trans, 28x10.5w slicks, zip ties, popsicle sticks, tape
    best time 9.06 @ 151.8 mph, best 60 foot 1.30

  5. #5
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    Aw dang it, I have a 2.8L and a 0.63 into a 3” exhaust.

    Am I screwed? Or is this going to be a jumpy little turbo?



    Sent from my iPhone using Tapatalk

  6. #6
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    That totally makes sense about the displacement. Also, my pistons are model number ke115m845.

    I see that there is a .68 t4 size option available. I would be willing to purchase a t4 manifold if that would be the ideal choice.

    Thanks for any input

  7. #7
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    Quote Originally Posted by E30round2 View Post
    That totally makes sense about the displacement. Also, my pistons are model number ke115m845.

    I see that there is a .68 t4 size option available. I would be willing to purchase a t4 manifold if that would be the ideal choice.

    Thanks for any input
    Those are the correct pistons, just checking. No need for a t4 for what your doing. Open T3 will be perfect.

    Quote Originally Posted by SilburE36 View Post
    Aw dang it, I have a 2.8L and a 0.63 into a 3” exhaust.

    Am I screwed? Or is this going to be a jumpy little turbo?



    Sent from my iPhone using Tapatalk
    Don’t fret about it, it’s not that big a deal.


    86 325es, 2.8L m50, S476sxe, ProEFI 128 ecu, e85, solid rear axle, TH400 trans, 28x10.5w slicks, zip ties, popsicle sticks, tape
    best time 9.06 @ 151.8 mph, best 60 foot 1.30

  8. #8
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    Some guy, do you have a recommendation on piston rings gaps?

  9. #9
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    .018” top, .021 2nd


    86 325es, 2.8L m50, S476sxe, ProEFI 128 ecu, e85, solid rear axle, TH400 trans, 28x10.5w slicks, zip ties, popsicle sticks, tape
    best time 9.06 @ 151.8 mph, best 60 foot 1.30

  10. #10
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    Thanks a lot for your help. I have noticed how involved you are in the forum, and would just like to let you know that it means a lot that you take your time to help a new guy like me. I won't ramble on and on, but I did want to take a second and let you know that this internet stranger really appreciates your contributions.

  11. #11
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    No problem, just paying it forward


    86 325es, 2.8L m50, S476sxe, ProEFI 128 ecu, e85, solid rear axle, TH400 trans, 28x10.5w slicks, zip ties, popsicle sticks, tape
    best time 9.06 @ 151.8 mph, best 60 foot 1.30

  12. #12
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    So, six plus months later my engine is finally assembled and installed. What a journey. It turns out that the RSI manifold I purchased will fit either a t3 or a t4 turbo housing. My options are t3 .63 A/R or t4 .68 A/R. I'm sure the difference will be minimal, but which would you recommend? I'm sure I'm overthinking it, but I've never done anything like this before. Thanks in advance.

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