pistons and compression / U dont Know!

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

Very good, mixxer. I like higher than stock compression in CRF motors sometimes but not always. The PEMEX variable and cracked/broken expensive after market high compression pistons taught me to go back to stock pistons in the 450X and also 450R.

Edited by cubera

Cool write up! Very interesting!

glad you guys liked it :smirk:

Little bit of..... AWESOME!

:smirk:

thanks gentelmen.... i was being heckled for not covering detonation and compression in another thread...

that is a big big topic on its own.... brief answer from the other thread;

detonation is similar to picking the right burning rate for doing your own bullet loading....

a fast burn rate compared to the mass it has to accelerate will result in higher pressure and that in turn creates a faster burn rate.... pressure spikes come from all of it literally igniting at once....

same deal with octane as detonation supressant..... higher octane is like slower burning powders... can handle more compression/ heat/ leaner mixtures and still burn in a controlled manner...instead of auto igniting the whole mixture....

i will write way more than 500 words to fully cover the topic of detonation and fuels...

latent heat refers to the thermal mass and the caloric value a compond can absorb ... you don't want big droplets of fuel for many reasons...unburned fuel ...besides being unburned / not adding to power / and dampening the burn of the fuel that will ignite...also washes the lubricating film from your cylinder wall...not a good way to slow and control a combustion event

Interesting write up and I'm hoping to revive it a bit with opinions solicited on which piston I should replace my original stocker with this winter/fall?

Everyone has an argument to each side I know this.

I'm looking for a little extra oomph to go with possibly stage-1 porting for a bike I use for everything from an occasional desert race to single track and enduro type exploration riding.

Whatever I get it will be a forged piston.

I want to be able to still use pump gas 91. I ride at elevations from 4500-7500 ft.

What are your thoughts and or recommendations (esp on compression ratio I should go with to achieve pump gas running?)

The 12.5:1 CR pistons will work with pump gas just fine. Adds a nice little boost to the power.

Most aftermarket brands are good. We prefer JE right now, but Wiseco and CP both produce a nice piston.

Rumor has it that Cosworth will be back in the OffRoad game with a new design once testing is complete.

is there anything to take note of when purchasing a piston to worn on my stock 450x head: domed/vs non vs compression and height of piston and/or skirt?

is there anything to take note of when purchasing a piston to worn on my stock 450x head: domed/vs non vs compression and height of piston and/or skirt?

Most all increased compression ratio pistons are domed. Moving from stock 12:1 to 12.5:1, you won't see a huge difference in dome height, but it's there.

Clearances won't be an issue with 12.5:1...they are all clearanced to work with virtually any valvetrain set up. As you move up in CR, things get more dicey.

If you are planning on pump gas, 12.5:1 will be the choice.

Many come with their proprietary slick coatings on the skirts, they all have fancy names for them.

Wiseco calls theirs "Armorglide", Cosworth "Xylan coating".

I think the JE and CP may be the only ones that don't have them.

good info. What's the highest comp ratio I can do with stock valve train, 12.5:1?

good info. What's the highest comp ratio I can do with stock valve train, 12.5:1?

You should be able to run a 13.5:1 without any troubles with a stock valve train. It's when you start pairing a high comp piston with a high lift/long duration cam that issues arise.

Remember that the 13.5:1 won't run well on pump gas in the 450...in the 250 it will, but not the 450.

Ahh that's my key. ON PUMP GAS though?

I think u can gain compression while sticking with the stock piston by using a "R" base gskt. Im not sure how much though probably not enough for the rev heads out there.

I think u can gain compression while sticking with the stock piston by using a "R" base gskt. Im not sure how much though probably not enough for the rev heads out there.

Nope...that's a 250 trick. The 450R and X gaskets are not interchangeable.

The 450R has the trans breather coming out of the case. The 450X has the breather coming off the cylinder.

Use a 450R gasket on an X and you'll block the trans breather...

ok some basic questions: say I take my piston out, throw my rings back into the cylinder and measure clearance. How do I know if I can get away with simply a new piston/rings or if I need to rebore and get a new piston/cylinder?

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