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Thread: Intake and restrictor

  1. #21
    @Patclarke

    How do we adjust the ignition timing and fuel mixture as you suggested if I use a single caarb?

  2. #22
    At the most basic level, the pipe length calculations are pretty much modifications of the pressure waves you must have learned in physics class. Your restrictor is a converging diverging nozzle, which you should have learned how to analyze in fluids.

    As for the ignition timing and fuel mixture for a carb, look into how you would adjust them on any car.
    Chris Noll
    UB Motorsports Formula SAE: RIP 1987-2010

    "A turbo: exhaust gasses go into the turbocharger and spin it, witchcraft happens and you go faster."- Jeremy Clarkson

  3. #23
    The Kawasaki engine i am using is dual carburetted with a 34 mm bore. since the restrictor comes at 20mm for gasoline, there will be large pressure reduction. Plus, its difficult to get the runners made coming from a dual carb and converging onto a single restrictor. I am thinking of using a 28mm mikuni carb that comes Enfield 350/500 cc.

    My doubt is : Will this new carb be compatible to the bike? What all tuning changes would be needed to be done on the carb?

  4. #24
    I'm guessing you are running a twin cylinder

    In this case you'd probably want to have two intake runners the same diameter as your intake ports. Join the two intake runners in a Y shape at your desired length. Taper the feed pipe down to 20mm then back up to 35mm and connect to your single carb.

    That would make your restrictor have a 35mm intake, 20mm choke, 35mm outlet

    The Y shape join is a guess, a T shape may work better in directing pressure waves down to the closed valves of the opposite cylinder for improved wave coupling...

    A carbed setup will have low power potential as there is no plenum to damp the intake pressure waves. A plenum most likely wouldn't work with a carb as the air fuel mix would slow down and fuel may drop out of suspension causing very poor running. Unless you can inject fuel into the intake runners, remembering no throttles allowed after the restrictor...

    Take a look at Megasquirt ECUs. Low cost if you have the time to make one
    Curtin Motorsport Team 07-08
    http://motorsport.curtin.edu.au/

  5. #25
    Yes ours is twin cyl engine. Dual carb (34 mm bore) in it go virtually no plenum. Its like the A/F mix from one carb feeds into one cyl and similar with the other cylinder. It has individual intake runner system. Now because of the 20mm restrictor in between the carb and intake runners, pressure drop will be high. For pressure recovery, i am planning to use a log type plenum with the simplest of construction. But as you say, using a plenum wont work with carb as the A/F mix would slow down causing poor filling of cyls.

    Now, I am left a bit confused to make this trade off. Please help.

  6. #26
    I wouldn't say you're confused, I'd say it sounds like you've figured out exactly why carbs don't work in FSAE!

    But, in the interest of getting the car running, I'm imagining you're best off to compromise more on the side of making the carb work well over trying to make power. Mostly because the only thing worse than not making much power is not making much power and also having it not start, run, and get around the track well. If it were me I'd try doing the simple log, and make it small-ish, maybe with a volume roughly the same as the displacement of the engine.
    Dr. Adam Witthauer
    Iowa State University 2002-2013 alum

    Mad Scientist, Gonzo Racewerks Unincorporated, Intl.

  7. #27
    I am in a process of designing the restrictor section (I am using a venturi type with maximum pressure recovery design) of the intake. I surfed for stuff and found some calculations from other teams/authors. I fed in my data to obtain the exit diameter of the venturi. But Iam having confusion over fixing the lengths of the converging and diverging sections. I am not able to account for the pressure that has to be generated by my intake. Please suggest some ways to:

    1) Calculate the lengths of each section i.e. conv and diverging.

    2) Calculate optimum pressure to be generated after at the exit. Rough estimations would help as this is my first time. I dont have a data log of testing the car.

  8. #28
    Yeah, the very nature of an engine that is restricted the way ours is makes it difficult to fuel with a carbuerator. In other restricted carbuerator setups (NASCAR comes to mind) you can actually size your venturis to the same diameter as the holes in your restrictor plate.

    By Running a venturi restrictor you're making the lowest pressure part of the system downstream of the carbuerator - where for efficient fuel metering you need the lowest pressure point to BE the venturi.

    In this case, it seems like the best option would be to design a carbuerator with a 20mm venturi. And in your case, that doesn't really seem like an option. So, in backwoods carbuerator talk, you need to maximize the vacuum signal that the venturis see to properly meter fuel flow and jet your carb appropriately. You'll have to jet up and likely modify air bleeds in order to get sufficient flow from a motorcycle carb.

    As far as carbuerator selection goes, you might consider a downdraft carbuerator from some type of V8. If you can get ahold of a single Weber or Stromberg carb it might be better suited for your purposes than a motorcycle carb (whose floats aren't usually well suited for high-g turns anyways)
    Wesley
    OU Sooner Racing Team Alum '09

    connecting-rods.blogspot.com

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