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2.7 stroker, want info on 2.5 cutt or custom pistons.

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    2.7 stroker, want info on 2.5 cutt or custom pistons.

    I have ran a couple ideas across some members here and i appreciate your replies.
    I want to do the 2.7 conversion but with the 2.5 longer rod. From what i have researched the 2.5 rod on a 2.7 crank spins faster than the 2.7 crank/rod.
    I know the other possible alternatives - 2.7 (eta)crank/rod/piston/ i head. or - 2.7 (eta)crank/rod, seta/custom pistons. I have read all there is to read on these two (twice). I can't find the seta pistons and customs with this setup are difficult to get pricing for. Had difficulties trying to get correct specs to order pistons.
    Regardless, I like the more tourqe to come from the 2.7 crank and the faster rotation from the 2.5 rods. A few have brought this configuration up, but i have not noticed much detail on the subject from those that have actually gotten this done.
    1) I have only seen the reference to .080 shave off and relief recut once.
    a) Is there a certain amount of shave verses CR amount?
    b) will any machine shop know how to do this?
    2) Details/specs of custom pistons with CR.
    Anymore information would be appreciated.

    Oh, this will stay a NA motor.

    Thanks, rvaughnp
    Last edited by rvaughnp; 09-21-2009, 06:21 PM.

    #2
    Ok, i have been continuing to research this and i have ruled out the shaving bit.
    please help me !! I have shave my piston ( picture ) This is a good and seemingly logical post for not doing the shave.
    I have contacted Ross and will get info from them on a set of customs. I plan on getting info from other piston companies as well.
    I will keep this thread updated on this matter, being there haven't been to many with correct info on this particular combo.
    Thanks
    rvaughnp

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      #3
      Ok, here is where i am at so up to now.
      86ETA27 from another site states that he has done the shaving; actually he was the one I was referring to about the statement of “shaving .080” in my previous post. We are emailing back and forth and will try to get any info on the shaving, who what where, how much, when.
      Found 2.9 piston at MM. Talked with Teddy, great help, they have the 2.9 or 2900 series piston.
      86 mm bore 9:6.1 with the 2.7 crank and 2.5 rods. Was told these are direct bolt, $1170.00 a set (with wrist pins and c clips). Rings $180.00. They use this combo in one of their build motors.
      Here is a good and informative post 2.7 Rebuild info - Page 3 - R3VLimited Forums
      I will have information from JE, brcperformance and Wiseco in the next couple of days. I have to get a little bit more info and email a data sheet to Ross to get their information.
      rvaughnp

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        #4
        ,,,
        Last edited by DSP74; 09-25-2009, 02:03 AM.
        sigpic


        88 325is

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          #5
          Anyone know where i can get 11.1 EURO Pistons or there specs so i can have them custom made?
          sigpic
          1986 "C2 2.7 Alpina" Sedan
          1987 325ic Black Vert
          1986 327i Red Track Car RIP 10/10/10
          1989 325is Henna S50 track car SOLD


          Transaction feedback here please!!
          http://www.r3vlimited.com/board/showthread.php?t=170548

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            #6
            search 11:1 on here youll see i had some. there arent many sets. search mahle m20 pistons in google to get an image with all m20 pistons and dimensions.

            Comment


              #7
              if you go for custom pistons you would want to use the correct shape dome to suit your head
              89 E30 325is Lachs Silber - currently M20B31, M20B33 in the works, stroked to the hilt...

              new build thread http://www.r3vlimited.com/board/showthread.php?t=317505

              Comment


                #8
                Originally posted by digger View Post
                if you go for custom pistons you would want to use the correct shape dome to suit your head

                What's the correct shape dome?

                Even those fancy Metric Mechanic pistons have a different dome profile than stock.

                I would think as long as the dome profile doesn't create hot spots, or interfere with flame front travel it should be fine.
                sigpic


                88 325is

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                  #9
                  yes but OEM 11:1 pistons and any others that went with the older style heads are flat top. not well suited for the later chamber.

                  Comment


                    #10
                    It's a 4.5cc difference in cumbustion chamber volume.

                    FWIW the majority of the "dome" on a Metric Mechanic piston is also a flat top piston.

                    My point is that until one dome profile is proven more effective than another, I don't see any reason why the flat top would be less effective than the dome/dish of a I piston face.

                    To raise compression to high levels there isn't a way to accommodate ANY kind of dish like in the stock pistons.
                    sigpic


                    88 325is

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                      #11
                      11.1 euro pistons will yield a 10.2.1 C/R with an 885 head. Ship that piston to a custom piston manufacturer and tell them you want a copy of this with a pin 5mm higher.
                      The 5mm higher pin compensates for the 5mm longer rod.
                      With all due respect...all that did was add another piston in the mix of pistons that can't be found or very hard to find.

                      When this process (by me) in completed, I will have manufacture names and specs from said manufactures of piston that will work with this build. No more guess work for the 2.7 crank/ 2.5 rod.

                      I was able to get a 2nd choice... "m 20 fever" from e30tech had some made by bcrperformance. Scott of bcr said he would email the specs on those, as I have the order number from "fever" but I have to get his ok to post that number first. (that number ties to those pistons that he had made.) "fever" has had those pistons in his motor for going on a year and states they have no issues. He had them made to an 8:8.1 cr but Scott said that they could match them to any cr; all for $774.00 including wristpins.

                      Wiesco was supposed to call me back today but didn't. I expect to hear from them tomorrow. I doubt I will go through sending the info to Ross. We'll see.

                      Any who, like I said I will post a full report on the manufactures and the specs from said manufactures when this is done.

                      rvaughnp

                      Comment


                        #12
                        Originally posted by DSP74 View Post
                        It's a 4.5cc difference in cumbustion chamber volume.

                        FWIW the majority of the "dome" on a Metric Mechanic piston is also a flat top piston.

                        My point is that until one dome profile is proven more effective than another, I don't see any reason why the flat top would be less effective than the dome/dish of a I piston face.

                        To raise compression to high levels there isn't a way to accommodate ANY kind of dish like in the stock pistons.
                        i was just stating my opinion on the matter. look at an eta piston and an "i" piston. forget about compression and displacement. bmw didnt change the combustion chamber for nothing, and they didnt change the piston shape for nothing. sadly the 11:1 pistons or other rare ones are not the secret recipe for an easy badass m20. ive been down all those roads. anyone doing something like this should really just get custom pistons or pistons being produced for their application (plenty would not have to be one-off customs). when you bore the 20yr old block the 20yr old pistons will be loose anyways.

                        Comment


                          #13
                          I have looked at the combustion chamber of the 325i and it’s not built any different than any other American made head (in shape).
                          I am going out on a limb here, but if the Mahle pistons that are posted here...
                          http://www.bmwccn.no/rogaland/Mahle_m20.html
                          are identical to stock, then it’s odd that only the m20 motors have this unusual shape. My guess is they wanted the gases to flow a certain way. I can’t think of any American made heads that ever where made where one side was offset from the other.

                          I am no engineer, but I can't think of why this shape would make an engine run any different especially when this design is so rare compared to all the other designs throughout the market.
                          JMO.

                          And I don't want to highjack this thread with this, but someone could think this through and make this arguement part of their decision so...

                          Why would BMW make a head of a piston in this fashion... who knows. I think it's a gas flow thing. We do know that virtually any other motor that we exchange pistons in, as far as "hot rodding" works. Why would this motor be any different from the hundreds of motors that allow us to do this.

                          I will finish with this, when I am done with this build and I wind up using a custom domed piston and it works, the shape of that piston moving up and down isn't going to cause any lack of sleep. as i am feeling right now.

                          rvaughnp

                          Comment


                            #14
                            the piston shape most certainly is for the swirl and flow of combustion chamber gasses. the chamber is different than the old style. i guess bmw couldve made a flat top piston that fit, but they didnt. just cause you can "hot-rod" it together doesnt make it optimal. i myself have assembled and ran an m20 with 11:1 eta pistons and 885 "i" head. theres not much room for any valve lift without changing the top of the piston anyway. do what you want. i bet you a dollar that in the end when those custom pistons arrive at your house, they wont have a flat top. :)

                            Comment


                              #15
                              Comparing US V8's-
                              I wouldn't. A 2.5L engine makes right about 170 HP stock. That's 68hp per litre.

                              A 5.7L american V8 would have to make 387hp. That is in stock form with a smooth idle etc. Not to mention we are talking about 25+year old technology in the BMW making that power.

                              HP/L of a 2.5L BMW is damn good.

                              lennon I get the point your trying to make. And think you are right. I just want to explore more options, and honestly those 11.1 pistons will get more than a pin height change, including a dome profile change...
                              sigpic


                              88 325is

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