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Turbo Selection Advice - BorgWarner S252 vs. S257

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    Turbo Selection Advice - BorgWarner S252 vs. S257

    Hey R3v,

    I've been working toward turbocharging the car (M20B25) and was hoping for some input on a turbo selection. Goal is drivability and early boost with decent top end. Power goal of around 300-350hp would be sweet, but really anything over my current 170hp would be terrifying. My engine is stock with a re-freshed head (40k miles ago) and ARP head studs, so plan is to run a maximum boost pressure of 15 psi with a stock redline.

    I've been looking at BorgWarner's MatchBot to help pick a turbo. I think the AirWerks S200SX-E series are the best suited for my application, besides something from the EFR series which are pretty damn pricey. I'm currently between two turbos that are nearly identical apart from the compressor housing. Here are the selection details from MatchBot:

    Borg Warner Turbo S252 S200 SX-E
    P/N: 12709095019
    T3
    70mm turbine wheel, .83 a/r
    Compressor: 52.2mm inducer, 70mm exducer

    MatchBot reference: https://www.borgwarner.com/go/0J14XG

    Click image for larger version  Name:	5019_Compressor_Map.png Views:	0 Size:	179.6 KB ID:	9861666

    Borg Warner Turbo S257 S200 SX-E
    P/N: 12769095003
    T3
    70mm turbine wheel, .83 a/r
    Compressor: 57.2mm inducer, 76.2mm exducer

    MatchBot reference: https://www.borgwarner.com/go/R4IF87

    Click image for larger version  Name:	5003_Compressor_Map.png Views:	0 Size:	182.1 KB ID:	9861665

    Going off the compressor maps, I'm leaning towards the S252 for better response and mid range. Note that I intentionally set the boost pressure to 17 psi from 5k rpm to 6.5k rpms to make sure I wasn't dangerously close to the choke line for either turbo.

    I managed to find both turbo candidates through BorgWarner via Amazon with a very close a/r ratio - .82, that I'm considering going with but wanted a second opinion before pulling the trigger. This is my first attempt at trying to size a turbo and boost in general so I definitely might be missing something. Which turbo would you choose?

    Turbo considerations:

    https://www.amazon.com/Borg-Warner-T.../dp/B07G9L36J4

    https://www.amazon.com/Borg-Warner-T.../dp/B07G9Q1GVN
    Last edited by zwill23; 08-15-2019, 10:07 PM.


    #2
    I'd go with the S257!

    Comment


      #3
      Another vote for the S257. I'd worry that you're getting pretty close to choke so your compressor efficiency is tanking pretty bad on the smaller wheel (8% points worse!). The larger wheel should result in lower pre-turbine pressure since you don't need as much power to drive the compressor and should generate more torque for a given boost level.

      The rotor group inertia is largely driven by the turbine wheel, so going up to a larger compressor with higher efficiency should more than compensate.

      Comment


        #4
        Originally posted by kid8 View Post
        I'd go with the S257!
        Originally posted by citizen_insane View Post
        Another vote for the S257. I'd worry that you're getting pretty close to choke so your compressor efficiency is tanking pretty bad on the smaller wheel (8% points worse!). The larger wheel should result in lower pre-turbine pressure since you don't need as much power to drive the compressor and should generate more torque for a given boost level.

        The rotor group inertia is largely driven by the turbine wheel, so going up to a larger compressor with higher efficiency should more than compensate.
        The lower pre-turbine pressure is a good point, bringing the boost level back down to 15psi in matchbot it's pretty obvious that the engine delta pressure, or pre-turbine pressure is worse off with the S252. In the higher rpm range I would essentially be working the engine harder to achieve the same boost level vs. the S257. Considering I could theoretically build 7.5lbs of boost pressure at 2500rpms it should still be pretty damn responsive and the larger compressor would be better for future expandability if the engine is ever stroked or has the rpm limit raised.

        Thanks for the help!! I'm going to pull the trigger on the S257 :D

        Comment


          #5
          Originally posted by zwill23 View Post



          The lower pre-turbine pressure is a good point, bringing the boost level back down to 15psi in matchbot it's pretty obvious that the engine delta pressure, or pre-turbine pressure is worse off with the S252. In the higher rpm range I would essentially be working the engine harder to achieve the same boost level vs. the S257. Considering I could theoretically build 7.5lbs of boost pressure at 2500rpms it should still be pretty damn responsive and the larger compressor would be better for future expandability if the engine is ever stroked or has the rpm limit raised.

          Thanks for the help!! I'm going to pull the trigger on the S257 :D
          exciting. I really like these borg warner turbos, cool choice

          Comment


            #6
            Originally posted by hkv View Post

            exciting. I really like these borg warner turbos, cool choice
            Thanks! Yeah I've heard nothing but good things about them, hoping it works out as well as I think it will.

            Comment


              #7
              woooo, biggie smalls, here we go!

              Comment


                #8
                What A/R do you plan on going with?

                also there are better prices to be had if you shop around- https://www.ebay.com/itm/Borg-Warner...W/262576091330

                Comment


                  #9
                  Originally posted by mars_colt View Post
                  What A/R do you plan on going with?

                  also there are better prices to be had if you shop around- https://www.ebay.com/itm/Borg-Warner...W/262576091330
                  The ideal A/R I was looking at was .83, but the S257 Iím ordering is .82. I figure thatís close enough.

                  Good find! Thatís only the supercore though, itís missing the turbine housing. I havenít been able to find a better price, but there might be something out there. Iím ordering the turbo soon, so Iíll peruse around one more time before pulling the trigger.

                  Comment


                    #10
                    Really looking forward to your results with the S257. Your goals of power / spool / drive-ability are exactly what I'm looking for too. Haven't been in an e30 with more than a stock m20, but can only imagine how much fun this little car can be 300-350hp. Keep us posted on this.

                    Comment


                      #11
                      Originally posted by marfcus View Post
                      Really looking forward to your results with the S257. Your goals of power / spool / drive-ability are exactly what I'm looking for too. Haven't been in an e30 with more than a stock m20, but can only imagine how much fun this little car can be 300-350hp. Keep us posted on this.
                      Absolutely, will do! Still in the parts collecting phase and trying to build a healthy system, but hoping to start building the setup soon.

                      Comment


                        #12
                        i dont' have any help on turbo selection but i want to say it's a fine line with boost
                        where its super fun, becomes super slow, then you turn it up , then it becomes too fast to even floor it on the street so now it's like you
                        can only blast it on the freeway, but when nobody is around. plus getting a hardcore clutch, tires, ripping out your diff.

                        limiting yourself to only 300hp with an expensive turbo is a smart choice.

                        Comment


                          #13
                          Originally posted by LowR3V'in View Post
                          i dont' have any help on turbo selection but i want to say it's a fine line with boost
                          where its super fun, becomes super slow, then you turn it up , then it becomes too fast to even floor it on the street so now it's like you
                          can only blast it on the freeway, but when nobody is around. plus getting a hardcore clutch, tires, ripping out your diff.

                          limiting yourself to only 300hp with an expensive turbo is a smart choice.
                          I feel pretty fortunate the fastest cars I’ve ridden in have been an STi back in high school and my buddies S52 swapped E30. I’m not trying to build an internet monster, my goal is to retain my E30 as a reliable turbo daily - a unicorn. The borg is definitely a reflection of that. Turbo selection aside I appreciate the comment, I’m excited to start tearing into the car again. It’s gotten a tad too reliable :)

                          Comment


                          • kid8
                            kid8 commented
                            Editing a comment
                            Hope you're not talking about greyE, because she ain't fast. You should ride with Ryan though...he has your HP goals.

                          • zwill23
                            zwill23 commented
                            Editing a comment
                            I sure am! You’re just jaded from all the hot rods you’ve owned :P GreyE rips. I’ve driven Ryan’s car! But was admittedly too chicken shit to give it the beans...
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