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Uncontrollable Boost - M20 w/ 6262

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    Uncontrollable Boost - M20 w/ 6262

    Alright guys I am pulling my hair out over this damn car and am not sure where to go next. Please help me lol. I’m running a 6262 .82A/r turbo on a RSI manifold with a 46mm Precision wastegate and no matter what I do I can not control the boost over 5,000 rpm. It will stay somewhat near spring pressure (tried 4.5 and 7.5) until around 5,000 then spike to wherever I set the fuel cut, in this case 14.5psi.

    I confirmed the wastegate is opening as it should and had a feeling the manifold design was the issue, so I removed the wastegate entirely this morning and found I can hold 0psi until high rpm then it still spools up to 7-8psi! How do I fix this? Will recirculating the wastegate make a difference? I’m just super confused because I know other people have run this combo before and not had this bad of issues. If recirculating the WG doesn’t help I’m not sure what else to try besides a different manifold.

    Any help is appreciated.

    #2
    Exhaust gasses have inertia, and the wastegate branch of this manifold or most like it makes spinning the turbo up unavoidable because of that. For any significant portion of the flow to take that turn and go down to the open wastegate there will have to be a more significant restriction upstream. I'm kind of surprised that you can't stay below 15psi because even with my little internal wastegate I see only 13-14psi at the limit with it open, but my 57mm turbine is more of a restriction than your 62mm turbine even if we're both using a .63ar housing. The best you can do with the current setup is make sure your wastegate is opening fully and run higher boost or run a bypass valve to bring the boost down. But really, with such a big turbo, good for ~700hp or so were you planning on running less than 1bar of boost in the first place? I'd ditch the screamer pipe in any case so you have a better chance at hearing detonation, I hate tuning cars with screamer pipes because of that.
    @turbovarg
    '91 318is, M20B25, T3/T04E 60 trim (18psi), megasquirt, coilovers, Z3 rack, cold AC
    [b u i l d]
    [Car of the month: April 2018]

    0c8b7c9527af628a346878feb14bf757

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      #3
      Thanks for the Input! No, I was not planning on running that low of boost long term. I just wanted to get everything dialed down low before turning it up. I’ll throw a 15lb spring in there and see if I can keep it under 20psi. If that works, I’m happy with that. But if it then wants to run 20+ I’m back to the manifold drawing board.

      Comment


        #4
        Update: I put a 15 psi spring in and set fuel cut at 20psi. It hit 20 faster than it was hitting 14.5 earlier. I don’t see any way to make this work other than changing the manifold. Sure, I could likely run a light spring and probably keep the creep to 17-18, where I want to be, but the spool up sucks because the wastegate opens so early.

        The one thing I don’t understand is how other people are running this same manifold/turbo combo and able to hold anywhere from 13-17 psi without issue.

        Comment


          #5
          hey this is probably a stupid suggestion but what does your timing look like?
          going too low on the timing will cause this on top of a crappy wastegate exit.

          Comment


            #6
            Click image for larger version

Name:	830F9653-D4B7-462A-B994-3E329D6FD087.png
Views:	156
Size:	282.8 KB
ID:	9924418 Ha- I found your earlier post mentioning advancing timing helped with your creep issues and already tried it. I’ve attached a screenshot of what I’m running at the moment. I tried a couple more degrees of advance too but it didn’t help so I went back to the attached table. I have a dyno appt coming up but am in a bit of a catch 22- I want the tuner to find the appropriate timing but can’t bring it to the dyno if I can’t control the boost. Thanks for the idea though.

            Comment


              #7
              Try a boost controller in conjunction with that lighter spring for better control, but it might just be a runaway situation since more boost leads to more exhaust gas volume. Are you absolutely sure the wastegate is opening fully?

              I haven't seen anyone else running the same combo as you like you mentioned, and don't know what size turbine housing you have so I can't tell you more about it being a quirk of your setup. You're only running a couple of degrees less advance than me at 200kPa and more than I am above 5,000rpm, so I don't think it's your issue. The design of this header does promote boost creep, that much is known, but I haven't seen a case as bad as yours and I haven't yet seen a header for M20 turbo applications that has the optimal routing.
              @turbovarg
              '91 318is, M20B25, T3/T04E 60 trim (18psi), megasquirt, coilovers, Z3 rack, cold AC
              [b u i l d]
              [Car of the month: April 2018]

              0c8b7c9527af628a346878feb14bf757

              Comment


                #8
                I have a .82 A/R hot side. Maybe it’s not a super common combo, but I know the .84 6262 with RSI manifold was at least coming in the KAMotors kits for a while. I’ve seen at least one person running this manifold with a .63 A/R 6262 as well, which is partly why I’m so damn confused at this point.

                As far as the wastegate- I can hear it opening, and it worked perfectly on the bench (opening fully, etc.) but I suppose it is possible that on the car it is not opening fully... I am not sure how I would test that. I have a 1/4” line going straight to the compressor housing. I suppose I could try to drill out the barb fitting on the compressor housing and see if that makes a difference.

                Edit: and thanks again for the suggestions!! I appreciate any help I can get.

                Comment


                  #9
                  I can hold 0psi until high rpm then it still spools up to 7-8psi! How do I fix this?
                  You don't, there's no way to prevent making some boost at high RPM WOT. How much boost you make is a function of the geometry of your parts, the size of your turbine housing, WG passage and size of the wastgate. Most OEMs are shooting for near natural aspiration with WG fully open, but it is rarely achieved, they are usually making 3-5psi (even with large electronically controlled internal WGs).

                  Can you post a data log? Is it very suddenly spiking or does it slowly ramp up?

                  A smaller turbine housing A/R might help because it would increase your turbine inlet pressure and drive more flow through the wastegate. This come with the risk of increasing the pumping losses of the engine which will result in less power at higher RPM, so it will need to balanced against your peak power target.
                  Last edited by citizen_insane; 05-11-2020, 06:39 AM.

                  Comment


                    #10
                    The issues is 100% the RSI Manifold
                    I have the same setup (6266 on an RSI manifold). With a 10psi spring it sees 13psi by the end of the rpm band.

                    What I have done is used my boost controller to level the boost. So for 20psi I have the duty cycle set to decline. So for instance at 4000 its set to 40, at 4500 39, at 5000 38, and 5500 37, at 6000 36, and so on.
                    You can play with these values to get it to level off at the boost pressure you want. But if youre trying to just run 10psi, you may as well be ready for it to creep to 13-14psi
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                      #11
                      There's a pretty decent difference between a 6262 and a 6266. Turbine housing A/R has an significant effect too. Agreed on boost control, but my guess is the turbine and housing combination are the primary factor at play here. Very 'free flowing' and resulting in a good boost to exhaust pressure ratio, to the point that the wastegate is not effective due to it being a sub-optimal flow path compared to the turbine. It should be noted that a wastegate manufacturer will recommend vastly different placement than we have on these headers, as seen below in Precision's manual for the wastegate OP has.

                      Click image for larger version  Name:	Screenshot_2020-05-11 - PTE-PW46-Wastegate-Instructions pdf.png Views:	0 Size:	215.1 KB ID:	9924793


                      Click image for larger version  Name:	image_151177.jpg Views:	6 Size:	67.3 KB ID:	9924791

                      ^This is my boost control duty cycle table. Keep in mind my turbo is essentially an internally wastegated 5857 in precision's more sensible terms (The T3/T04E 60 trim designation is essentially gibberish if you want an intuitive sense of how big the turbo is) with a .63A/R turbine housing, and I still have boost control issues, but can run anything above 15psi with decent control by tapering my boost duty as seen above.
                      @turbovarg
                      '91 318is, M20B25, T3/T04E 60 trim (18psi), megasquirt, coilovers, Z3 rack, cold AC
                      [b u i l d]
                      [Car of the month: April 2018]

                      0c8b7c9527af628a346878feb14bf757

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                        #12
                        I'm thinking I am going to take the easy way out and weld a wastegate flange onto the turbine housing. Yes, there are drawbacks if I decide to switch housings or turbos altogether, but I know I will be able to control boost accurately and can have it done quickly without buying another manifold.

                        Appreciate the boost control table! Once I get things manageable on a spring I am definitely going to wire up my EBC.

                        Comment


                          #13
                          Will that even fit under the hood? I know the design of the RSI header is sub-optimal but I don't rag on it because packaging is really a significant issue in these cars.
                          @turbovarg
                          '91 318is, M20B25, T3/T04E 60 trim (18psi), megasquirt, coilovers, Z3 rack, cold AC
                          [b u i l d]
                          [Car of the month: April 2018]

                          0c8b7c9527af628a346878feb14bf757

                          Comment


                            #14
                            I think I can make it fit under the hood. Obviously that pic is just one I pulled off google for an example, but heres another one of a similar setup on an M20. I've got an hour or so left at work before I can go mock everything up. If it looks like it'll work I'll drop off the turbo tomorrow and hope for some good results!

                            Comment


                              #15
                              My setup looks very very similar to that last picture. My wastegate works so well it's getting blown open as it spools up with a ~9psi spring (VGT is a big factor there too). The same spring would be worth about 12psi at sea level, fwiw.
                              Main issue is the cast -> stainless weld, can be tricky to prevent cracking. Be sure to preheat and cool down very slowly.

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