Knife edging cranks - pros / cons. Discuss

Discussion in 'Engines' started by A.N. Other, Oct 20, 2008.

  1. A.N. Other Banned after significant club disruption Dec 5th 2

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    Spotted a thread by Brooky this morning, and it set me off thinking.

    Knife edging's the thing to do and all that, but what does it really deliver?

    Pros

    * Lighter bottom end
    * Cuts through the oil, less drag etc

    Cons

    * What spec of engine do you need to get the benefits?
    * Low down torque effects?
    * What are the likely risks of interfering with factory crank balancing and ending up with a less balanced result, and rougher engine?


    Experience of this on my side is that my track car engine has an unlightened crank, and I know of a 1.8T crank that was balanced at Vibration Free and the result was not a great job and a less apparently balanced engine.

    Discuss away....
     
  2. GVK

    GVK Paid Member Paid Member

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    Mine didn't last long enough to get any benefits from, but hey that's old news.
     
  3. RobT

    RobT Forum Junkie

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    ask yourself the question - what are the balance weights for?

    spin a kg round on a piece of rope and feel the forces involved. Then 2x kg weights either end of a piece of wood holding the center with a nail or suchlike. smoother yes?

    as far as I know, they are to counteract the mass of the piston and connecting rod assembly. They need to be matched together - engines are a complete unit, not a collection of individual pieces which is where a lot of people go wrong.

    if you go chopping lumps off the counterweights without considering this, rough running may result.

    having less drag through oil sounds like a good idea to me but not at the expense of imbalance. Dry sunp it and its not an issue.
     
    Last edited: Oct 20, 2008
  4. Matt82

    Matt82 Forum Addict

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    are you being serious rob? they knife edge the crank weights and dont reduce the mass of the rod etc that theyre supopsed to be balancing? that seems a bit nuts. i thought thats what the idea of balancing was
     
  5. Hotgolf

    Hotgolf Paid Member Paid Member

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    It actually doesn't cut through the oil as such, unless you have an extra 2 litres in there;)
    The crank is far higher than the oil level in the sump, but it does cuts through the oil mist.
    Although again, if you mill them down/plane them flat you're not gaining anything apart from weight saving.
    So, yes it's lighter, quite a bit lighter, depending on how it's done and how much is taken off.

    What sepc would be be best on, well something you want to take past the nromal rpm limit of a stock engine. The higher you go, the less weight you want, same as the valve gear, which means if you are going that high, your spec would have to change through the whole engine to guarantee(or help guarantee) that it won't fall apart.

    Low down torque effects. So many other factors it would be hard to determine. You add a set of lumpy cams, and you're in the sameboat.

    Risks of interfereing with factory balancing. For a start you'd loose all the factory balancing so it would need to be re-balanced properly. And, if done properly it should be ok.
    But as mentioned they are there to counter balance the rods/pistons so you will have overall imbalance due the the weight removed. but, it would be the same(but to a lesser extent) if you added lightweight pistons. 1 end is going to be lighter, either big end or small end.

    As for rough running. The engine in my mk1 was exceptionally smooth.

    GVK's engine died due to thrust bearing failure, not improper balancing. The John Jones/Reeves engine I have here was suffering the same fate, but luckily I noticed it before I rebuilt the engine and had the faces re-ground.

    And Matt, you're right, they don't reduce the piston/rod combo, but a number of people sell the cranks like this as do Autotech:
    [​IMG]
    and LR engineering
    [​IMG]

    The LR engineering ones seems to have the big ends drilled to loose weight so as least they have had some thoughts about this, but I haven't seen one to verify it yet.

    At the same time, with the amount of weight removed from the counterweight, you're never ever going to be able to remove the same amount from the opposite side. You'd have sod all left lol
     
    Last edited: Oct 20, 2008
  6. chopperoli Forum Member

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    Just to clarify, the crankshaft counterweights are there to counter the forces of the reciprocating parts (piston + rod assy).

    You would only remove mass from the counterweights when the pistons / rods are being lightened too. Or if you found you needed to when blueprinting the engine.

    EDIT oops too slow
     
    Last edited: Oct 20, 2008
  7. Hotgolf

    Hotgolf Paid Member Paid Member

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    Having had a little think(and it hurts this time of day still) and just speaking out loud, but even though there's an imbalance you still have counter-weight to counter weight and rod/piston-rod/piston throwing in opposites.

    If things are done right, the opposite throwing weights will be the same 1 rod up, 1 rod down, and also 1 couterweight up, and 1 counter weight down, so, you're still throwing equally and opposingly, and as it's a 4 cylinder it's not un-even, but your imbalance doesn't come from this it comes from the amount of weight, rather than an imbalanced/out of balanced weight.
    So, you won't shake the engine apart, you'll just tend to have premature wear on the big ends as there's more forced being excerted on these areas.
    And as a note, I was told by my engineer that Knife-edging this was may cause premature wear on the big ends.

    As for how much. My old 2042 has 30k on it now, and has been abused from day 1. I'm having it back for a rebore/rebuild soon so I'll take some pics and sizes of the bigend bearings and journals.
     
    Last edited: Oct 20, 2008
  8. RobT

    RobT Forum Junkie

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    another one to add in:

    why are multi-cylinder engines smoother?

    less imbalances in the rotating crank assembly.

    The cranks hotgolf posted up have 180 degrees between the masses. A 8cyl could have 90 degrees between the masses and a 16 cyl 45 degrees. You may get to the point that you dont need the counter balance weights at all as the postons/rods balance themselves acceptably.

    The pistons and rods do try to balance each other which is why you can take mass off the counterweights and get away with it. But as revs rise, those imbalances can come into play.
     
  9. Hotgolf

    Hotgolf Paid Member Paid Member

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    Too true, look at an old beetle engine. 6k and it's all over..........all over the road in a thousand bits usually!
     
  10. Brookster

    Brookster Paid Member Paid Member

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    Ok all very interesting.

    Balancing is a hot topic, in Motogp Kawasaki developed the Screamer engine which had the crank on each cylinder set at 4 rods @ 90 deg each 90, 180, 270 & 360 so it ran like a 2 stroke, and was better balanced than the standard 2 @180 deg, 2 @ 360 deg.

    this gave a very higher revving equally balanced engine.

    any thoughts on that one ?

    my mate Paul Bargate has a knife edge Bildon Motorsport Crank in his 2.1L 16v and it revvs to 9500rpm no problems it is a 9A though,

    would a 9A have totally difference characturistic to a ABF Like mine which has a different Rod length ! i.e. Torque
     
    Last edited: Oct 20, 2008
  11. Hotgolf

    Hotgolf Paid Member Paid Member

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    ABF has a better rod ratio to add a stroker crank.
    Also your mate probably has a steel billet crank, not a forged crank.
    His rod ratio isn't the best for a stroker crank, and the stresses being put on it at 9 1/2k arereally high, but it's proven it works.
     
    Last edited: Oct 20, 2008
  12. Brookster

    Brookster Paid Member Paid Member

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    arrrr your alive!! :lol: any chance of doing mine ?

    Yes its Billet Steel.

    so wouold i get better torque out of my 2.0L 16V ABF than a 9A 2.1L ?

    Cosmo Crank
    [​IMG]
     
    Last edited: Oct 20, 2008
  13. infinity

    infinity Forum Member

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    ran one in my old 9A for around 15k and had no issues at all. engine rev's were limited to 7500 rpm with the odd 8k down shift. I think about 1 kg was removed from the crank. I think its best to look at the system as a complete unit. ie clutch / flywheel / crank / and harmonic damper.

    On another note the old BTCC engines used to run one counter weight per rod and the rev's were limited to 8.5k (90 era) all they cared about was making the engines as light as possible so they could spin quicker into there rather narrow power band.
     
  14. mark25 Forum Junkie

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    Unless you drill holes in the counterweights and fill them with heavy metal pellets to help the balancing.
     
  15. mark25 Forum Junkie

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    Straight six and V12 engines are inherently balanced. An in-line 4 isn't, because the inbalances occour at a frequency that can't be balanced by the moving masses, which is why Porsche used balance shafts rotating at 2x engine speed to balance their big 4 pot.

    But the six's and 12's don't automatically rev any higher than an in-line 4, quiet often they max out lower than a small in-line 4.
     
  16. RobT

    RobT Forum Junkie

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    maybe its because of the high overall weight of the rotatiing assembly - maybe they would if you could make the whole lot as light as a 4-pot - a 2L V8 would spin a lot higher and easier than a 2L 4 cyl I would have thought
     
  17. Jaundice

    Jaundice Forum Member

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    Sorry to throw a swear word in but what do Honda do to their cranks to get the Vtechs revving so high. When I test drove the new Civic type R the salesman claimed the engine was built to rev and tested at 15k RPM. Thats a lot for a 4 banger.
     
  18. RobT

    RobT Forum Junkie

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    what a load of tosh.
     
  19. A.N. Other Banned after significant club disruption Dec 5th 2

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    Never trust a salesman. If the trailer trash pikeys in America hadn't bought into **** mortgages from salesmen, we wouldn't have a credit crunch.

    The salesmen are clean gone (albeit wiv their commission ;) )

    /OT
     
  20. Hotgolf

    Hotgolf Paid Member Paid Member

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    No counterweights on the old beetles ;)
     

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