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Old 10-26-2011, 08:06 AM   #1
Wattner OP
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CycleWizardry: Thumper Myths Cleared, Tips and Tricks

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!
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Old 10-26-2011, 08:24 AM   #2
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very nice!

Im in........................
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Old 10-26-2011, 08:34 AM   #3
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Quote:
Originally Posted by Wattner View Post
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!
Good idea!
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Old 10-26-2011, 09:36 AM   #4
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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.

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




http://www.thumpertalk.com/forum/sho...light=velocity

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cyclewizard screwed with this post 10-26-2011 at 10:32 AM
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Old 10-26-2011, 10:37 AM   #5
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Squish/quench and artificial octane/ piston heights/ rod ratios next on the list if anyone wants to discuss it?
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Old 10-26-2011, 10:46 AM   #6
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that was a great read.

thanks for sharing your knowledge.
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Old 10-26-2011, 10:51 AM   #7
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Originally Posted by cyclewizard View Post
Squish/quench and artificial octane/ piston heights/ rod ratios next on the list if anyone wants to discuss it?

Bring it on.
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Old 10-26-2011, 10:55 AM   #8
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Bring it on.
When I get back I'll do just that, I have to go make some money...
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Old 10-26-2011, 10:56 AM   #9
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that was a great read.

Thanks for sharing your knowledge.

10/4.........
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Old 10-26-2011, 11:24 AM   #10
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Squish/quench and artificial octane/ piston heights/ rod ratios next on the list if anyone wants to discuss it?
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
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Old 10-26-2011, 11:26 AM   #11
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Originally Posted by cyclewizard View Post
Squish/quench and artificial octane/ piston heights/ rod ratios next on the list if anyone wants to discuss it?
That is what I never understood......
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Old 10-26-2011, 01:32 PM   #12
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Old 10-26-2011, 02:00 PM   #13
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Signing in for this class , Hp 1301.
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Old 10-26-2011, 02:02 PM   #14
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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 ?
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Old 10-26-2011, 02:21 PM   #15
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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!...
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