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Discussion Starter #1
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
 

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Great job!!!!!
 

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Outstanding write up, very discriptive & easy to understand.
Now PM me for what I need.....:lol
 

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Discussion Starter #9
thanks for the thanks , gentlemen.....

there are some other factors that come into play with compression.... one of the most important being tight squish clearances....which does triple good by adding compression....upping detonation resistance... and keeping an open combustion chamber that burns fast and scavenges well....

i will add back in when i can write up something good on all that....

you could....and many companies have... added compression and lost power because the whole combustion chamber design is wrong with a big lump of aluminum piston top sticking into it....

but...if it's done right.... going from stock to a good pump gas mid 12 comp piston will add 1.5hp across the board....and going into the 14's with a good design will get you +3hp across the board....

not like adding nitrous....but since pistons are wear out items that need to replaced regularly.... if you can add power from doing maintenance...it's sort of like a freebie
 

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Stickied for you.
 

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Discussion Starter #14
Wanna get into knocking and detonation a little bit and why it occurs?

octane is a rating of how stable a fuel will burn when compressed heavily or excessively heated....

high octane burns in a controlled manner much like heavy rifle bullets need slow burning powders to get the bullet to speed without blowing a barrel apart.....

the intake charge needs to burn fast, but in a controlled/progressive manner.... detonation is when the whole charge ignites at one time from the fuel not being able to handle the conditions the engine is giving it

lean jetting, too high of compression, too much heat, low octane fuel, too much load on engine... all those things contribute to detonation.... which is the 100% death rattle of an engine.... those pressure spikes will turn your fav engine into your fav boat anchor in no time....

there is LOTS more to it than that.... but that is a start
 

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Great info well explained
 

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Bro........ That was the absolute best post on yfzcentral i have ever read. On here we get so many guys tha make a point and can't back it up at all, so it just becomes an opinion. That post was all facts aqnd facts can't be refuted. Facts are what we need to get the most out of our bikes and know everything that comes with it. For every positive there's a negative......or least the possibility of a negative and the key is to knowing how to avoid it and what causes it. You did that perfectly.

Serious "bump" to you bro, that's a true gear head boyz........this is a guy we all can learn from. Thanks guy
 

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Discussion Starter #18
thanks so much again, gentlemen.... much appreciated.... i enjoy sharing and talking / exhcanging tech ..... its nice to know it can be of help to those who are interested....
 

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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!
 
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