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Discussion Starter #22
I have the big three and and happy with it but tried a 13.5-1 last rebuild and didnt notice one difference and neither did the dyno so I kept the same setup on my msd. This rebuild I went back to stock cause the $150 piston was a waste. The forged piston looked great though! I have a buddy thats a builder that told me to get it ported and bigger valves if I want to make use of a higher compression. Ang get a second job to buy the gas to support the higher compression piston. Standard is 12.7-1 is the highest you sould run on pump 93 octane but then i see the 2013 R1s are 13.1-1. Who knows! All i can say is seat if the pants, lap times, and pocket book sayes buy a stock piston!

spot on.... but that is a whole 'nother topic....

too much to write now.... but the big deal is HOW you get the compression... there is soooo much tech to every little detail in engines....in the beginning of the yfz, everyone was sure you couldnt get it to work with above 12:1 compression...and that the yfz would lose power if you went above that....

the answer is that the yfz with its 5 valve head is more sensitive to a wrong combustion chamber shape than 4 valve heads are..... the magic of the 5 valve is to have an open middle section for overlap flow...and efficient burn.... if you add your compression with a big hunk of aluminum dome in the middle of the piston.... you supress the burn and kill the overlap flow.... which is all the "development" any aftermarket company put into their offroad pistons...

so i made the Venom pistons ...and the jump from stock to the 14:1 piston made +3hp across the board... tested to death.... and i added 1.5HP while staying with 12.25;1 pump gas.... there is more to it than that....and sometime i will write it all .... it has to do with getting the compression by design...not just a bigger dome...

right now, cp and cosworth pistons are the ones with the best designs going....
 

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Discussion Starter #23
mixxer what year cam should i get to improve the stock 2010x head?
the cam that yamaha put in there is lower performance to make up for the bad head design .... see the big porting stickied thread for details....

stock intake off the carb models is my personal fav intake cam.... check the stickied cam testing for some details there too
 

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Can u put too much comoression on a stock motor i have a 2010 yfz450r 08 intake cam full hmf and efi tuner and i want to out in 14-1 compression is that too much?
 

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Can u put too much comoression on a stock motor i have a 2010 yfz450r 08 intake cam full hmf and efi tuner and i want to out in 14-1 compression is that too much?
You will be fine with 14:1. It will be a race gas only motor though.
 

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Mixxer,

When installing the GYTR High compression piston in the 450R to increase compression to 13:2 - what should the piston/head clearance be?
 

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The GYTR piston is actually supplied to Yamaha by JE pistons. Is there a minimum piston/head clearance for it to be high compression and from where you have to use 100+ octane fuel?
 

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Discussion Starter #29
the aftermarket has typically added material to the rest of the piston while keeping the squish clearances the same as stock... that is how they raise compression....

anything in the 12's is still good for correct jetting and high octane pump fuel....

above the 12's its good to use better fuels for a better safety margin.... you will only be sorry if you have too little octane... not too much
 

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Discussion Starter #31 (Edited)
good info for people sweating bullets over compression ratios:


there are lots of factors that go into compression and power production...

and one of the biggest is the change in compression also comes with changes in combustion chamber shape.... if you could get more compression without making flat / obstructed chambers that block efficient burning and mixture swirl.... or domes that block the scavenging communication from intake to exhaust during cam overlap at the end of the exhaust stroke....

in the beginning of the yfz world.... all the news was that the yfz ...for some unknown reason... will not make power over 12:1 compression...and beyond that it will lose HP..... i told everyone that the problem was not compression...it was the specific needs of the 5-valve combustion chamber that were being defeated by the lousy designs of aftermarket pistons at that time.... and told everyone to take some clues from the engineers that made their 5-valve in the first place.... not to worry, i was designing pistons that would make more power with the high compression ratings....

Terse was one of the first independant testers to dyno the diff from stock to my own 14:1 piston...and it was a full and solid +3HP over the stocker... debunking the myth that the yfz could not tolerate or use compression above the 12's.... it was all in the design specifics...not the compression ratios in the pistons that didnt add power....

so...anyways...other than the design stuff that adds power...:

effective ring seal...low ring tension/ friction... tight squish / high turbulance & swirl .... unbostructed combustion chamber for fast burn and overlap communiucation....

if you could focus on compression alone ...

if you start at the low end of the compression scale...say around 8:1...

and change nothing else but compression (tough to really do) to 9:1...
you will gain a little more than 4% power....

then from 9:1 to 10:1 its about 3.5%


10:1 to 11:1 its about 3%

11:1 to 12:1 ~ 2.5%

12:1 to 13:1 ~ 2%

13:1 to 14:1 slightly less than 2%

14;1 to 15:1 = 1%


quite a few factors kick in on the "law of diminishing returns"... and those percentage gains are assuming correct piston design to get the comp, not heavy hunks of aluminum domes stuck in the middle of the piston....

so think about the power you will actually gain VS the cost of race fuel that you never quit paying for...... as an option, big bore / pump fuel is a nice gain across the board...

thats the straight dope on the gains to be expected if the variables are minimized.... hope it helps some people who are sweating bullets over what to spend on pistons and race fuel budgets.....

and the stock yamaha piston is a VERY good design... right from the guys who designed the whole 5-valve engine :).... and its VERY inexpensive.... throw in a thinner head gasket arrangement and you can get almost all you can get from a pump gas setup:clap:

and it would have to be a VERY well made race gas piston to make a diff you could even feel......
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Discussion Starter #34
just so you guys know what makes a fast / powerful burn in a combustion chamber....


is a dish design piston....


for piston and head as combustion chamber, a hockey puck shape is ideal.... and what you dont want is a pancake or obstructed chamber...


the more dome....the tighter the combustion chamber....with more obstructions in it....


all of that kills the swirl and turbulance that is so important for a complete and fast burn of the charge.....


that's why yamaha spent so much time on its 5-valve design... the five valves already meant that there was going to be a fight to not have too wide/flat of a combustion chamber...


here's the deal of not having a tight obstructed chamber

tighter....flatter.... more obstructions...all works against swirl + turbulance... swirl and turbulance are what define a "fast burn" combustion chamber.... and fast burn styles do everything right...from being more powerful to being more efficient to being more detonation resistant....


and it works like this.... if you light a static/ non moving charge is fuel/air / it actually burns slowly when you consider engine speeds.... it would burn from the center and propogate slowly outward to the chamber edges....like a whole bunch of dynamite fuses layed out like daisy flower petals with them all touching in the center


now...if you add swirl and turbulance to the chamber you just lit.... you can think of it as those same fuses inside a running blender.... spinning fuses being chopped and flung into other fuses....


which one will burn faster and create more combustion pressure?? that's pretty easy to see....


this will help explain how you can add compression the wrong way...and get no gain, or even a loss of power from combustion chamber shape.... there's lots more to it than that...but that's a big component of power production....especiallly in a 5-valve
 

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i have answered so many posts and PM's about the mythological dangers of high compression... i figured it would be a good idea to just write a thread on the topic and be done with it... so here goes

in no particular syntax... here's the dope...

if there is truly ONE magic
powerband bullet for a 4-stroke... high compression IS it...no question

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 torque 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
How come dirt bikes can have 13.5:1 compression off the show room floor and still run pump has?
 

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Discussion Starter #40
it all has to do with advanced engine controls like air fuel ratio and ignition timing....

when you have full control of all those things ...adjusting everything for gear selection, throttle position, and rpm.... you can run anything you want...

the dirt bikes come set up as competition machines right off the showroom floor.... the factories sell them as ready to compete race machines.... they get a lot of advances that the quads don't get..... so do street racers.... the atv market has always been slow to get hand me downs.... step children of the technical word
 
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