Robin-Subaru... better airflow

Discussion in '4-Stroke Engines' started by moped-dan, Apr 17, 2010.

  1. moped-dan

    moped-dan Guest

    Well today I took off the air filter cover on my robin subaru.. I cant believe the engine gets enough air through that cover it just seems so restrictive with only those small holes. I then rode the bike without the cover, and left only the thin black sponge in place to prevent dirt and stuff from getting the engine. I secured the sponge with a little tape around the edges so it wouldnt fall off.

    Anyways the engine seems to have more power now. Not a lot.... but enough to be noticeable. It seems to keep more speed against a headwind or a hill. It is a little louder now as well.

    Does anyone know if this could damage the engine somehow? I think ill go for a long ride tomorrow and see what happens :driving:
     

  2. G-Superior

    G-Superior Member

    you can seize your engine up!:eek:
    because there is more flow, air can get in easier so less vaccum will be avaliable to suck the fuel up the carb. jets
    its not so typical in a 4 stroke but it happens sometimes and belive me its not nice to see a hole in the top of a piston when you are a fair few miles from home:eek:
    Engines normally run much better and give much more power when they run leaner(until you make a hole on the piston)
    good luck with you ride and last thing, Its noisier because you can hear the intake valve open and the piston sucking the fuel/air in, its a tipical mod. that boy racers do to their cars and mopeds to make it intake sound louder hahahahhaha :D
     
  3. moped-dan

    moped-dan Guest

    Well I went for a ~40 mile ride yesterday and the engine didnt blow up :eek: Everything went good and today I went for a few miles after dinner.

    It seems to be doing fine but I really dont want to sieze the engine up or anything, is there any way to tell if im running too lean? Thanks G-superior!
     
  4. give me vtec

    give me vtec Active Member

    check the plug...
     
  5. Happy Valley

    Happy Valley Active Member

    Just drill a few holes in the outer air filter cover if you think it needs more air....it'll save you from loosing the filter element. Personally I don't find the EHO35s need it though.
     
  6. give me vtec

    give me vtec Active Member

    I agree... not enough to risk sacrificing reliability anyways.
     
  7. G-Superior

    G-Superior Member

    thats true :rolleyes7:
    you dont want to ruin a perfect very reliable japanese made engine(if it was a cheap chinese one you could:whistling:)
    a mini moto velocity stack will probably fit it! and you can even use a K&N filter whick is better looking and works better for what you are doing:D
    I would not drill holes in an expensive air filter cover just for high flow, just get a cheap one of ebay or something like that(a K&N is only a few $) :D
    G-Superior
     
  8. Happy Valley

    Happy Valley Active Member

    We're talking about the stock, plastic lid on the air cleaner here.
     
  9. G-Superior

    G-Superior Member

    I will take a picuture of one i did a few months ago for a racing suzuki lt50
    it was really good looking and worked perfect(much much better than stock)
     
  10. On one of my Robins I discarded the white filter and run only the black foam element with the stock cover. It seems to work fine. The stock cover allows plenty of air, it sucks air in through the bottom holes, which are much larger than the intake valve opening. Clean the filter element often and it will be fine.
     
  11. graucho

    graucho Active Member

    In my opinion/experience I found that the small intake on the stock RS35 can suck all of the air it needs the way it is. It's the "rear end" LOL, thats restricting of flow. Currently I have 2 RS35's and both are running a lot sweeter by "slightly" grinding the exhaust port. I didn't have time so I had it done for me by a local shop. I think you can mess with the intake all you want but it's not going to do much good if your restricting the burnt air/fuel out of the exhaust port. Its like running your furnace with 6" ductwork when you should have 8". Removing the furnace filter isn't going to make the furnace run less sluggish. Just my 2cents. :whistling:
     
  12. give me vtec

    give me vtec Active Member

    not harsh at all... what you said was absolutely correct.
     
  13. moped-dan

    moped-dan Guest

    graucho, I think you bring up a good point.

    I ended up drilling 4 holes in the front of the air filter box, I really do believe it makes a difference. I went for a nice ride yesterday and it felt more willing to go up hills without slowing down and didnt slow down as much against a headwind.

    [​IMG]

    [​IMG]

    I didnt really think about the exhaust side of things, I wonder if I could increase flow in that direction. I was thinking of going for a ride with the exhaust how it is now, then remove the muffler and try it again and compare the runs. Is it the muffler that would be restricting flow or the exhaust port itself?
     
  14. Old Bob

    Old Bob Member

    You guys need to bone up on head design and porting. I've never seen so much misinformation in my life.

    Look up David Vizard and read his book on head porting, before you mislead people into ruining their engines with half assed theory and conjecture.
    Read this book or keep your mouth shut.

    The Theory and Practice of Cylinder Head Modification
    Author: David Vizard
     
    Last edited: May 19, 2010
  15. Happy Valley

    Happy Valley Active Member

    Well Bob, I've noticed a couple things about your contributions here, while you have displayed you have working knowledge and experience with small utility engines and have offered what I consider to be good information, you seem to awfully defensive and particularly disdainful of anyone who doesn't.

    What is the purpose of a site like this?

    Maybe you could offer a bit of synopsis of Vizard's work to those less informed?
     
    Last edited: May 19, 2010
  16. GearNut

    GearNut Active Member

  17. Old Bob

    Old Bob Member

    Yes it is not cheap. Books on this subject are not cheap.

    I can attempt to share the relevent information.It gets tedious at times and really needs some study in other areas of intake and exhaust tuning and camshafts to get the full picture.
    There is only so much that can be done with a given displacement and rpm range.You won't make a monster out of one of these engines, but can improve it.
    One of the first things to grasp is torque and horsepower relationship.
    HP is a function of torque, without torque there is no hp.The primary objective of engine modifying is to improve torque in a given rpm range.

    I'll dig up some info from my book.

    Just for kicks, anyone know why intake valves are bigger than exhaust valves?
     
  18. GearNut

    GearNut Active Member

    Yeah, there is more mass to the intake charge, less mass to the exhaust charge since some of the mass got burned up during the combustion process.
    I have been involved in building Harley engines for 17 years. The ones I built were not rip snorting asphalt burners mind you, just good reliable daily drivers. I am not a fan of short life HiPo engines, My mentor was all over that scene though.
     
  19. graucho

    graucho Active Member

    First off....
    Who in the heck are you to tell anyone to shut up on a public forum? No one gave advise to do any porting themselves. No one explained what tools to use or how to do it. The bicycle movement will be slow growing by those who shut down a thread by making others afraid to post because you belittle them. This is a teaching place with all levels of experience. Anyone can have input here. I've seen thousands of threads with misinformation, with people finally chiming in with good bits of advise without telling others to keep their mouth shut.

    Secondly to answer your exhaust valves question,

    The flow over the valves (or any port):
    m_dot = CD*A*rho_0*sqrt(R*T_0)*(P_r)^(1/y)*(2y/(y-1)*(1-P_r^(y/(y-1)))

    where
    CD = discharge coefficient
    A = area of the port
    rho_0 = gas density before the port
    T_0 = temperature before the port
    P_r = pressure ratio over the port
    y = cp/cv

    It's the pressure diference over the valves that drives the flow, and the pressure difference is greater on the exhaust side. Hence... allowing the use of a smaller valve.
    Unless your burning nitrous. I know with nitrous you want a slightly bigger exh valve than just a regular NA motor. Different pressure ratios.
     
    Last edited: May 20, 2010
  20. Old Bob

    Old Bob Member

    The reason the intake valve is bigger is simply because there is only 14.7 psi of pressure to push a/f mixture into cylinder while the cylinder pressure is typically 50-75 psi when the exhaust valve opens.
     
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