Announcement

Collapse
No announcement yet.

m20b29 cam question

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    #16
    Originally posted by mrlucretius View Post
    Can you explain this some more? What do you mean by "matching"?

    To explain this fully, it would be a long post and in a place not many people would see in the bottom of a thread. Do some reading up on dynamic compression to get a good grasp on it. Just be aware some of the info out there is pretty dated, but it really helps visualizing what I meant by matching.

    In short the cam overlap (both valves open simultaneously) can help or hurt depending the the intentions of the engine. A lot of duration/overlap (can be lower duration and tighter center lines too) hangs the pair valves open longer, allowing intake gasses to pass into exhaust without combustion, and also exhaust pulses to revert to the intake.

    Now if you have a really high CR with a low overlap cam, you are trapping more mass (increasing dynamic CR) and it gets to a point where the fuel mix will pre-ignite from pressure no matter how little timing advance. In a lower compression you have the opposite. Too much mix is being expelled and will get to a point, the engine won't even start - the flame front will just go out the exhaust instead of pushing the piston down - path of least resistance.

    So, in a running engine things change as RPM increases (dynamic CR) due to all kinds of variables. Such as back pressure in the exhaust (high RPM, running condition), a long duration cam won't bleed as much as if cranking to start, there's already pressure in the exhaust helping it stay in the chamber.

    With that in mind, you can see how cam specs will kind of indicate the temperament of your engine. Low compression takes more RPM for a big cam to reach the point where it's most efficient. Many times people will over cam the engine and all the real power it "could" make is outside of usable range. When we design custom cams for the domestic makes, we try and walk that fine line of too much trapped mass (dynamic compression) so that as much of the combustion/flame is pushing on the piston as possible (maximizing VE). Over cam-ing them sure does make a cool idle though lol - just don't expect to have any brake assist or crankcase ventilation since vacuum won't be there. We actually sell a "thumper" cam people like because has a crazy sounding idle, but doesn't make as much power as the others since the peak power is above the redline.

    It wasn't an m20 and has a fair bit better combustion design, but we had a client with 13.8:1 compression running pump fuel (93 octane) with a matching cam (stock engine was 9.6:1). He is in FL, so we don't have any emission standards and never tested his, but based on remote tuning for other clients with a little less CR, his could be made to pass (specially if EGR was retained).
    john@m20guru.com
    Links:
    Transaction feedback: Here, here and here. Thanks :D

    Comment


      #17
      you CAN NOT make very good VE without a fair bit of overlap, this includes at low rpm like 2500rpm. but it is a double edge sword. while it is working for you it is great but when it works against you it is a nightmare because of the effects of reversion.

      while an exhaust is scavenging you can almost not have too much overlap, however on a street engine you cant design an exhaust that scavenges across all rpm from idle to redline at all load points so you need to compromise.
      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


        #18
        Originally posted by ForcedFirebird View Post
        I lost a bunch of dyno runs/winpep when the dyno PC HDD gave up the ghost, but IIRC removing the spacers on the ITB's (shortening the runners) made the peak greater than the full length, but it was very sharp and much lower RPM than the long runners. Peak power was greater, but area under the curve suffered severely. Maybe these particular files made it to the SD card, will have to look if I ever get a free moment.
        people think if you use shorter runners you make more topend power, it isnt always the case and to some extent it depends what your starting point was.

        the example is the dbilas system its got short fat runners, people think this will make more high rpm power. in reality the engine will make its peak hp at a higher rpm but the absolute hp peak will actually be lower and at all other rpm it will be down on torque.

        similar on mine shortening by 2" the RHD result in about 1hp less peak and a chunk of midrange was less by ~12 ft-lb.

        if the rest of the engine isnt designed for the rpm of the engine then changing one part of the system wont produce the results you expect. the inlet, cam head and exhaust and bottom end shouldnt be considered as separate systems
        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


          #19
          Originally posted by digger View Post
          you CAN NOT make very good VE without a fair bit of overlap, this includes at low rpm like 2500rpm. but it is a double edge sword. while it is working for you it is great but when it works against you it is a nightmare because of the effects of reversion.

          while an exhaust is scavenging you can almost not have too much overlap, however on a street engine you cant design an exhaust that scavenges across all rpm from idle to redline at all load points so you need to compromise.
          Ironic you mention overlap. It is believed that little/zero overlap is optimal for turbo, which I never believed. Had an order for a 3.8l Camaro a few years ago and ran many simulations through Dynomation to come up with a great cam to match the highly modified head/intake we worked over. The engine builder/tuner (assembler?) argued tooth and nail the cam was "too hot" with ~58° lap. Stepped it down just a bit per the client request, engine builder still was hesitant. Long story short, they couldn't believe they gained so much power. Instead of being 400whp@22psig, they were well over 500 at only 14psi. Old heads were "ported" by another company (no numbers or bench proof), and was running their shelf "turbo cam".


          Originally posted by digger View Post
          if the rest of the engine isnt designed for the rpm of the engine then changing one part of the system wont produce the results you expect. the inlet, cam head and exhaust and bottom end shouldnt be considered as separate systems
          This. All. Day. My above example wouldn't have made near the power if only the heads or cam were used. The new cam would be too large for the head flow, and vice versa for the heads.

          I have found, though, power comes from the top end. As long as your cam is designed around the CR/RPM, the rest is just how efficiently you can move the air in and exhaust out.
          john@m20guru.com
          Links:
          Transaction feedback: Here, here and here. Thanks :D

          Comment

          Working...
          X