Home Forums Shifter Karts reed valve vs. piston port

This topic contains 5 replies, has 3 voices, and was last updated by  Rob Kozakowski 4 years, 11 months ago.

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  • #13449

    Bill Schmidt
    Participant

    At work today we went through a 6 hour school on Stihl brand engines. I was aghast to hear the instructor say that piston port engines make significantly more HP than reed valve engines!!??!! I asked him why the Japanese have been using reed valve engines in their MX bikes for the last 20 years or more. He said he didnt know, but maybe it was a cost issue. Anyone care to shed light on this dilemma?

  • #13457

    James McMahon
    Participant

    There’s no dilemma. Reed will nearly always make more power, over a wider band with more overrev because the inlet timing isn’t as compromised. Whatever the instructor’s experiences were I’m guessing there were other factors at play aside from induction type.

    If PP made more power, guys wouldn’t be adding reeds to “stock appearing” PP motors ;)

  • #13458

    Bill Schmidt
    Participant

    James,

    The instructor briefly mentioned adding reeds to pp engines, but gave no details. Can you give me some examples of which pp engines guys are adding the reeds to?

  • #13460

    James McMahon
    Participant

    KT100’s, you’d see them talked about on 4cycle.com, but it seems their historical posts are gone.
    What was his reasoning behind PP’s making more power? I guess you could maybe make more peak power than some reeds, but overall, at least in my experience with karting the PP’s were never as fast as reeds or rotaries. At least a second off on a sprint track with a 20lb break.

  • #13466

    Bill Schmidt
    Participant

    He didnt have any reason for the alleged power advantage.

  • #13517

    Rob Kozakowski
    Participant

    My (admittedly limited) understanding is that piston ports can actully make more power – albeit in a relatively narrow band – it’s all a matter of what the purpose of the engine is designed to do.  A piston port designed for a lot of top end power, with a pipe (remember 2-pipes play a huge role in 2-strokes) designed for that could potentially produce higher peak hp than a reed or rotary?

    One problem with a piston port is that it is relatively inefficient in the lower to mid rpm range, but is apparently quite efficient at high rpm’s.  When a piston port is designed to be used for karting (and most other) purposes, my guess would be they sacrifice the ultimate power available at the top end for increased bottom end.  What good is top end power if you’re a slug out of every corner and never get to use the top end power?

    Taking that a step further, I’d also guess part of the reason most PP classes tend to run clutches is to run the engines where they’re designed to work best – basically bypassing the low-mid range where they suck.  I’d also guess that reeds and rotaries are generally run as direct drives because of their wider useable power bands.

    Stihl is mostly involved in chainsaws, where I would guess that purpose, combined with design and packaging limitations (i.e. where do stick a tuned exhaust?) could possibly allow for a piston port to produce more power than a reed?

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