CycleWizardry: Thumper Myths Cleared, Tips and Tricks

Discussion in 'Thumpers' started by Wattner, Oct 26, 2011.

  1. Wattner

    Wattner Long timer

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    Hey CycleWizard,

    How about clearing up some Thumper Myths, and offering Tips and Tricks that you have learned and keep in your arsenal? I know your first love is the RFVC Honda motor, but many things that we have discussed can work on many thumpers.

    I know you are making parts and offer motor and machine work, so pricing and offering those services may be best discussed in Vendors.

    Here, would you be so kind as to address high compression, cams, rods, valves, gearing, oils, oil pumps, cooling, big fins, head work and options, etc. etc?

    I see you are posting in several threads, but thought it may be GREAT to have a one stop for the masses to refer to.

    Your knowledge is impressive to me and I learn something with every conversation we have and post you make...

    Thanks in advance! :D:clap:D
    #1
  2. elsalvadorklr

    elsalvadorklr southern xr rider

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    very nice!

    Im in........................:D
    #2
  3. sc-razor

    sc-razor Long timer

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    Good idea!
    #3
  4. cyclewizard

    cyclewizard Long timer

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    Great ideas lets clear up the old myth about compression / velocity first..

    4-strokes love... Love...LOVE HIGH COMPRESSION.....!!!

    high compression does not cause a top end power loss in 4-strokes the way it can add a pumping load to a 2-stroke engine.... that is a very persistent hold over myth to beat down.....

    high compression makes your engine perform likeit has a bigger displacement at lower RPM's....and it makes it perform like it has more camshaft at high RPM's.... more bottom...more top...and better throttle response across the board.... a beautiful thing, and very hard trick to beat.. short of forced induction

    so as to not perpetuate any sort of mythological fecal fog here... it needs to be explained exactly how high compression does all of that...

    high compression is NOT just a high dome that squeezes the A\F mix so tight is goes off like an atomic bomb... the tighter pressure squeeze does indeed help the power output...but it isn't all the magic

    it's tough to paint an analogy in layman's terms with words alone.... . as always, i will use exaggerated illustrations for the purpose of clarity...

    your piston and cylinder arrangement has now become a GIANT syringe.... the piston is the rubber plunger...and the clear tube is your cylinder.... and while we are at it..... lets give it 2 needle outlets on top too...one for intake and one for exhaust....

    in our LOW compression model...we will exaggerate and say that the piston\ plunger only goes as high as half way up the tube at the top of its stroke

    and the HIGH compression model goes very close to the end of the tube at the top of its stroke

    that exaggeration will help with understanding all the other dynamics besides how tight the mixture gets squeezed alone....

    so ...besides being used to squeeze the A\F charge before ignition.... you piston\ plunger is also important to how much vacuum is seen during the intake \ suction stroke......

    let's say you could put your finger over the intake side of the LOW compression syringe ...and then feel the amount of vacuum generated as you pull the plunger\piston to the bottom of the stroke...... you will notice that the vacuum builds slowly...and doesn't become very strong until the bottom of the stroke...

    doing the same test with the HIGH compression plunger \piston.... where the piston has a much smaller volume of air trapped above it to begin with.... you will see a very fast...very sharp rise in the vacumm it generates...since it has less trapped volume to dampen the vacuum in the first place....

    so what does that do for a running engine?? a few things...all good!

    the higher compression version provides a STRONGER and EARLIER vacuum pulse into the intake tract... which makes for better\ sharper throttle response by delivering a stronger signal to the carb's metering circuits...

    and also the sharper vacuum drop makes the incoming fuel droplets break up \atomize into a better\ finer air + fuel fog.... the smaller the fuel droplets...the better the combustion...the only part that can burn is the part that comes in contact with oxygen... big droplets only have the "skin" of the drop burn away durung combstion...the reaminder of the drop not only doesn't burn...and adds unburned hydrocarbon emissions to the atmosphere....it also serves to dampen the combustion process by absorbing latent heat/energy from the part that does combust...

    the other thing that the stronger vacuum signal from the higher compression piston does is also wonderful....

    it CREATES a HIGHER VELOCITY incoming INTAKE CHARGE....

    what does that do you ask? one thing that higher velocity does is keeps atomized fuel droplets in suspensioin better than a lower velocity charge does...and we know that is a good thing....

    and we sort of know that higher compression gives back a lot of the tq. that a BIG duration cam loses... but most people think that the tighter squeeze of the A\F mix prior to ignition is what does this (and of course, that's part of it)...

    first we need to know why a big cam actually loses bottom end power and response in the first place

    a modern performance cam opens the intake some 20 to 30 degrees before the piston is all the way to the top of the EXHAUST stroke.... just prior to the beginning of the downward intake stroke....and it doesn't close the intake valve until somewhere from 50 to 70 degrees AFTER the piston has reached the bottom of the intake stroke and has started back up on the compression stroke...

    at high speeds you need to have the intake valve open those long periods of time to simply have enough time @ high rpm to get any sort of decent cylinder fill...and at high piston speeds @ high rpm you will get a stronger vacuum pull into the intake port.... and the velocity generated in the port can sort of "ram charge" the incoming mix into the cylinder even though the intake valve is still open as the piston is traveling upwards for as much as 70 degrees of rotation

    BUT at lower speeds.... you not only don't get as much piston speed generated vacuum signal ...with a BIG cam you are still leaving the intake open long enough after bottom ... that the piston is able to push charge that has already entered the cylinder back up through the open intake valve... i've said many times that you can't compress a charge in a cylinder that isn't sealed...

    SO...

    as we have already discussed....the high compression piston imparts more vaccum...and more signal...and more velocity into the intake tract...in a BIG cammed engine...that added intake velocity helps to give enough inertia to the incoming charge that it helps to counter act tha low speed reversion of the intake flow....

    high comprression one-two punch to help with low end loss on big cams.... tighter squeeze is always bigger boom...PLUS higher velocity \ earlier acceleration of the intake charge making for more cylinder fill AND less reversion loss of that charger by virtue of that greater velocity...

    so could high compression possibly do anything else ...beyond the wonderful stuff outlined already??

    you bet it does!

    on the exhaust stroke it is more effective at getting more of the burned charge out of the cylinder....think of the 2 different piston\ plunger\ syringe's again.... the one that leaves the least space at the top of the cylinder is the one that pushed the most spent charge out the exhaust.....

    and it did it with higher velocity too..... and since higher exhaust velocity has more inertia heading in the OUT direction...it creates a stronger vacuum in its wake....

    which brings us to another good thing....

    at top dead center \ piston at its highest point...at he end of the exhaust stroke...and beginning of the intake stroke...it is during the period known as "cam overlap".... for a brief segment of time ...just before and just after the top...the intake AND exhaust valves are open just a little bit...and for very good reason....

    the exiting high velocity exhaust...and subsequent vacuum tail it leaves in its wake....will pull the last bit of spent charge out of the cylinder... AND use its energy to begin pulling the intyake charge into the cylinder...even BEFORE the piston begins its downward intake stroke... it couldn't vacuum the rest of the combustion chamber out completely...OR begin the movenent of the fresh charge inward from the intake tract unless both intake and exhaust valves were open simultaneously @ TDC...which is exactly why there is overlap timing in high performance cams in the first place.....

    NOW....

    which would take better advantage of a strong exhaust vacuum signal....and both clean out the combustion chamber AND transfer some of that vacuum energy effectively to the intake port??? the large combustion chamber volume of low compression OR the small\ efficient combustion chamber volume of the high compression piston??

    once again..... ADVANTAGE HIGH COMPRESSION.....

    i hope i was effective at illustrating the MANY unseen...and largely unknown...advantages of how a high compression setup works...well beyond the simple "tighter squeeze of the charge" ( which is wonderful in and of itself BTW)

    now...to debunk the RELIABILITY VS HIGH COMPRESSION myth...hopefully for the last time....

    horsepower and torque are a direct reflection of the combustion pressures seen inside an engine......

    ANYTHING that makes your engine have a higher output is a result of it creating more combustion pressure within your engine...... whether the power came from a jet kit...pipe...cam...special fuel...etc...etc...

    as far as the stress on your engine components....they have not the slightest idea wher the pressure comes from...and they wouldn't really care either...more pressure = more power = more stress on everything...

    a 50 hp pump gas setup ..... is putting out more stress on the engine components ....than a high compression engine delivering 47hp.... the compression isn't what is the stress...the actual pressure from combustion is.... and combustion pressure is MANY times greater than cranking compression in any event....

    increased power = stress and accelerated wear.... that is the bottom line....it doesn't have anything to do with what compression you have..aside from the actual power it adds to the engine..

    and BTW....on the piston reliability thing...compression notwithstanding... there are design and material components that will make one piston\ ring setup better in the reliability and longevity arena's


    Smaller ports on a single cylinder 650cc motor is a waste of time and resources, I would up the compression and run better fuel if all out power is what you're looking for.
    If not than leave it the way it is...and be happy with 29hp.:lol3

    Me and Mike and Pop's over at TT are power hungry junkies, we want to shit ourselves when we wack the throttle.:D




    http://www.thumpertalk.com/forum/showthread.php?t=924710&page=8&highlight=velocity

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    #4
  5. cyclewizard

    cyclewizard Long timer

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    Squish/quench and artificial octane/ piston heights/ rod ratios next on the list if anyone wants to discuss it?:ear
    #5
  6. LittleRedToyota

    LittleRedToyota Yinzer

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    that was a great read.

    thanks for sharing your knowledge.
    #6
  7. mcma111

    mcma111 Long timer

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    Bring it on.
    #7
  8. cyclewizard

    cyclewizard Long timer

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    When I get back I'll do just that, I have to go make some money...:lol3
    #8
  9. cyclewizard

    cyclewizard Long timer

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    10/4.........:d :lol3
    #9
  10. cam14

    cam14 Been here awhile

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    Real interested in the finer points of rod ratios. In the context of building a reliable motor, is it really worth the effort to add a long rod and move the wrist pin up to generate a long rod motor. At what point is the rod ratio too short? Since a long rod motor will have more dwell time at TDC, how dose that affect cam timing and demands on lift and duration?

    TIA
    #10
  11. bobbed06

    bobbed06 Cavalera Conspirator

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    That is what I never understood......:ear
    #11
  12. Sourjon

    Sourjon TAT'erd

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    Subscribed

    John
    #12
  13. thebigman

    thebigman bout a dollar 3.98

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    Signing in for this class , Hp 1301. :deal
    #13
  14. thebigman

    thebigman bout a dollar 3.98

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    ? , Is it easier to get more compression my decking the top of the cylinder for example .010 compared to the Head .010 ?
    #14
  15. willys

    willys Long timer

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    I'm in....I'm at my limit now for information on what next to do to my KLR685 StageII head,ported and polished from airbox to exhaust, bigger valves, Mega Camed, Flatslid, modded exhaust powerplant!!!
    I need to know now about how to add compression to a KLR engine with a non adjustable cam timing system! augh!

    Subscribed....please advise.....sitting on the edge of my seat!...:ear:freaky
    #15
  16. techforlife

    techforlife CDI REPAIR

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    This is getting interesting,,,,,,i actually can stay awake and away from beer reading this...:lol3


    :lurk


    B
    #16
  17. cyclewizard

    cyclewizard Long timer

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    I had it 3/4 written out an hit the back button and it all disappeared,,,,,,,,,gawd that pisses me off when I do that.

    I'm going to start over..after I get back from Tim Hortons:lol3:lol3
    #17
  18. cyclewizard

    cyclewizard Long timer

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    Cam timing and lift is a good question. I have never heard anyone give this advice,
    and I dreamed it up myself. Actually, I figured it out using simple
    geometry equations. Any valve lift past 0.25 of the valve head diameter
    is basically a waste of time (or lift, more accurately). So, a 2" valve
    needs no more than 0.500" of lift. A 1.6" valve head (like an exhaust
    valve) needs no more than 0.400" of lift. Flowbench results clearly
    demonstrate this when you pay attention to it. Yeah, maybe flow goes up
    a bit past this 0.25/1 lift/valve head dia. ratio, but not much compared
    with what it does at lesser lifts. I have done flowbench testing where the
    flow actually drops off much past the 0.25 ratio point, although that's
    a bit unusual. Yeah, most cams you buy have the same exhaust lift as
    intake lift and same duration #'s despite the fact on the exhaust side that it doesn't help;
    it's just convenient for the machinist to do it this way. Cam duration
    (all other things equal) will be affected by the rod length/stroke
    ratio. More stroke (for a given deck height) results in a smaller
    rod/stoke ratio and that needs a bit more cam duration ... to get the
    same timing for piston position (which is what you are trying to get the
    valves to open and close for). I've writen articles where a long rod
    engine and a short rod engine (otherwise as identical as possible) were
    both run on the dyno and the long rod engine gets more horespower with
    the same cam. The assumption here is that cam timing is cam timing is
    cam timing. But this ignores where the piston is and where the piston is
    with a short rod isn't where it is with a long rod at the same crank
    angle (like halfway between TDC and BDC for example). So the piston
    position with the short rod is NOT the same as it is for a long rod and
    THEREFORE with an identical cam the VALVE opening does not occur at the
    same piston displacement with different rod ratios. From a piston
    displacement viewpoint, a long rod engine gets more cam duration (with
    the identical cam setup) than the short rod engine and therefore gets
    more horespower ... in addition to lower piston drag which also helps
    the longer rod ratio engines. If you get the same valve opening for
    piston displacement, the short rod engine needs a bit more cam duration
    to accomplish the same actual piston displacement timing. OK, I need a
    blackboard to really explain this. The easier thing to do once you get
    your bore, stroke determined, and head flowed, is give these numbers to
    me C-dub when you want to do your cam, and I will crunch the numbers for you and give you the grind that fits your engines needs.:lol3:lol3 plug plug.......:roflthe mods here will probably put me on probation again..


    On a big 780XR motor with a ported head I've found it breathes the best at or near 480 lift but not enough to warrant the wear and tear on the valve train, 405 lift seems to be the best all around #'s on big stroker motored XR's for longtivity and power, but if you guys want a cam that large no problem I will do it for you.
    The intake valves will come in contact at those lift numbers because of the canted valve design so there are things that have to be done to the head to accommodate these issues.

    For smaller displacement 644's to 675 XR's 370/380 lift is plenty..you don't want to over cam your motor. The larger bore and stroke motors can take much more cam than a smaller bore and stroke, the larger bore will mellow the cam compared to a smaller displacement engine with equal cam events.

    Now if you want a "large" cam on a smaller 644cc 675 XR than you need to up the comp in the 12 to 13 C.R. range to get it's full effect.
    #18
  19. badcooky

    badcooky I see dumb people!

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    Now how do i get all the people i've argued with about 4 strokes to read this Wiz ,that's the kicker.
    The long/short rod thing is called dwell angle and yes the fugs can read that too.
    #19
  20. cyclewizard

    cyclewizard Long timer

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    Another myth I would like to put to rest is the oiling problem people discuss here and over at TT or so they think there's an oiling problem..there's not..it's all in the hands of the opperator.:deal

    The only reason" well one of them" people fry there valve train on these motors is they set there idle to damn low, the pump on an XR does not pump enough fluid at low RPM's sitting in traffic........:lol3 set your idle to 1500 rpm or more..

    If your sitting in traffic on a hot day idling bring your rpm's up to 2000 so the oil can flow and help cool the bike.
    When I build a motor or bike for a customer there's a reason why I have the idle set high so don't fricken play with it please...:lol3 it gives me bad feed back and won't cool properly and you could ruin your new motor...puff it starts to smoke...:D


    Clean your down tube and engine screens and when you pull your side cover off on the clutch side make sure the O-ring and dowel is in place when you put it back on.

    One more very important factor here is you have to use a ZDDP additive on these motors or an oil like redline that has a high percentage of ZDDP mixed in the additive package, one other thing is STP has a high content of ZDDP in it also, mix some in there if Redline is not available in your area and Joe Gibbs oil for breakin......
    #20