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High-Compression Pistons for your Motorcycle: Everything you Need to Know


Billy@JEPistons

Whether you're racing or looking for increased performance out on the trail, there are a plethora of performance upgrades to consider to increase the power of your machine. Piston manufacturers like JE Pistons offer high compression piston options for many applications, but there are important merits and drawbacks you should consider when deciding if a high compression piston is right for your application. To better understand, we’ll take a look at what increasing compression ratio does, what effects this has on the engine, detail how high compression pistons are made, and provide a high-level overview of which applications may benefit from utilizing a high compression piston.

JE High-Compression Pistons

Bumping up the compression in your motor should be an informed decision. It's important to first understand what effects high-compression has, the anatomy of a high-comp piston, and what applications typically benefit most.

Let’s start with a quick review of what the compression ratio is, then we’ll get into how it affects performance. The compression ratio compares the volume above the piston at bottom dead center (BDC) to the volume above the piston at top dead center (TDC). Shown below is the mathematical equation that defines compression ratio:

Compression Ratio Formula

The swept volume is the volume that the piston displaces as it moves through its stroke. The clearance volume is the volume of the combustion chamber when the piston is at top dead center (TDC). There are multiple different dimensions to take into account when calculating clearance volume, but for the sake of keeping this introductory, this is the formula as an overview. When alterations to the compression ratio are made, the clearance volume is reduced, resulting in a higher ratio. Reductions in clearance volume are typically achieved by modifying the geometry of the piston crown so that it occupies more combustion chamber space.

Piston at BDC Piston at TDC

Swept volume is the volume displaced as the piston moves through the stroke, and clearance volume is the volume of the combustion chamber with the piston at top dead center.

How does an increased compression ratio affect engine performance? To understand how increasing the compression ratio affects performance, we have to start with understanding what happens to the fuel/air mixture on the compression stroke. During the compression stroke, the fuel/air mixture is compressed, and due to thermodynamic laws, the compressed mixture increases in temperature and pressure. Comparatively, increasing the compression ratio over that of a stock ratio, the fuel/air mixture is compressed more, resulting in increased temperature and pressure before the combustion event.

The resulting power that can be extracted from the combustion event is heavily dependent on the temperature and pressure of the fuel/air mixture prior to combustion. The temperature and pressure of the mixture before combustion influences the peak cylinder pressure during combustion, as well as the peak in-cylinder temperature. For thermodynamic reasons, increases in peak cylinder pressure and temperature during combustion will result in increased mechanical efficiency, the extraction of more work, and increased power during the power stroke. In summary, the more the fuel/air mixture can be compressed before combustion, the more energy can be extracted from it.

JE High-Compression Piston

Higher compression allows for a larger amount of fuel/air mixture to be successfully combusted, ultimately resulting in more power produced during the power stroke.

However, there are limits to how much the mixture can be compressed prior to combustion. If the temperature of the mixture increases too much before the firing of the spark plug, the mixture can auto ignite, which is often referred to as pre-ignition. Another detrimental combustion condition that can also occur is called detonation. Detonation occurs when end gases spontaneously ignite after the spark plug fires. Both conditions put severe mechanical stress on the engine because cylinder pressures far exceed what the engine was designed for, which can damage top end components and negatively affect performance.

Detonation/Pre-Ignition Piston Example Detonation/Pre-Ignition Piston Example

Detonation and pre-ignition can spike cylinder pressure and temperature, causing damage. Common signs of these conditions include pitting on the piston crown.

Now that there is an understanding of what changes occur during the combustion event to deliver increased power, we can look at what other effects these changes have on the engine. Since cylinder pressure is increased, more stress is put on the engine. The amount of additional stress that is introduced is largely dependent on the overall engine setup. Since combustion temperatures increase with increased compression ratio, the engine must also dissipate more heat. If not adequately managed, increased temperatures can reduce the lifespan of top-end components.

JE Pistons EN Plated Crown JE Pistons EN Plated Ring Grooves

JE's EN plating is a surface treatment that can protect the piston crown and ring grooves from potential damage caused by high cylinder pressure and temperature. EN can be an asset for longevity in a high-compression race build.

Often, additional modifications can be made to help mitigate the side effects of increasing the compression ratio. To help reduce the risk of pre-ignition and detonation, using a fuel with a higher octane rating can be advantageous. Altering the combustion event by increasing the amount of fuel (richening the mixture) and changing the ignition timing can also help. Cooling system improvement can be an effective way to combat the additional heat generated by the combustion event. Selecting larger or more efficient radiators, oil coolers, and water pumps are all options that can be explored. Equipping the engine with a high-performance clutch can help reduce clutch slip and wear which can occur due to the increased power.

Pro Circuit Motor

High-level race team machines are great examples of additional modifications made to compensate for increased stress race engines encounter. Mods include things like larger radiators, race fuel, custom mapping, and performance clutch components.

Let’s take a quick look at what considerations are made when designing a high compression piston. Typically, high compression pistons are made by adding dome volume to the piston crown, which reduces the clearance volume at TDC. In some cases, this is difficult to do depending on the combustion chamber shape, size of the valves, or the amount of valve lift. When designing the dome, it is essential to opt for smooth dome designs. Smooth domes as opposed to more aggressively ridged designs are preferred because the latter can result in hot spots on the piston crown, which can lead to pre-ignition. Another common design option is to increase the compression distance, which is the distance from the center of the wrist pin bore to the crown of the piston. In this approach, the squish clearance, which is the clearance between the piston and head, is reduced.

JE Piston Crown Details JE Piston Crown Side View

Piston Compression Height 

Higher compression is commonly achieved by increasing dome volume while retaining smooth characteristics, as pictured here with raised features and deep valve pockets. Compression height can also be increased, which increases the distance between the center of the pin bore and the crown of the piston.

A high-level overview of which applications can benefit from increased compression ratio can be helpful when assessing whether a high-compression upgrade is a good choice for your machine. Since increasing the compression ratio increases power and heat output, applications that benefit from the additional power and can cope with additional heat realize the most significant performance gains. Contrarily, applications where the bike is ridden at low speed, in tight conditions, or with lots of clutch use can be negatively impacted by incorporating a high compression piston. Keep in mind these statements are generalizations, and every engine responds differently to increased compression ratios. Below are lists of applications that may benefit from increasing the compression ratio as well as applications where increased compression may negatively influence performance. 

Applications that may benefit from utilizing a high compression piston:

  •  Motocross
  •  Supermoto
  •  Drag racing
  •  Road racing
  •  Ice racing
  •  Flat track
  •  Desert racing

Weston PeickSutherlin WORCS

Motocross and less technical off-road racing are two of multiple forms of racing in which high-compression pistons can benefit performance due to higher speeds and better air flow to keep the engine cool. Peick photo by Brown Dog Wilson.

Applications that may be negatively affected by utilizing a high compression piston:

  •  Technical off-road/woods riding
  •  Trials
  •  Other low speed/cooling applications

Gary Sutherlin

Lower speed racing and riding may not benefit as much from a high-compression piston, as heat in the engine will build up quicker due to lessened cooling ability.

Fortunately, if you’re considering increasing your engine’s compression ratio by utilizing a high compression piston, many aftermarket designs have been tested and optimized for specific engines and fuel octane ratings. For example, JE Pistons offers pistons at incrementally increased compression ratios so that you can incorporate a setup that works best for you.

For example, high-compression pistons from JE for off-road bikes and ATVs are commonly available in 0.5 compression ratio increases. Assume an engines stock compression ratio is 13.0:1, there will most likely be options of 13.5:1 and 14.0:1, so that you can make an informed decision on how much compression will benefit you based on your machine and type of riding.

Various Compression Ratios

From left to right are 13.0:1, 13.5:1, and 14.0:1 compression ratio pistons, all for a YZ250F. Notice the differences in piston dome volume and design.

If performance is sufficient at an engine’s stock compression ratio, there are still improvements in efficiency and durability that can be made with a forged piston. Forged pistons have a better aligned alloy grain flow than cast pistons, creating a stronger part more resistant to the stresses of engine operation. In addition to forged material, improvements can be made on piston skirt style design to increase strength over stock designs, such as with JE’s FSR designs. JE also commonly addresses dome design on stock compression pistons, employing smoothness across valve reliefs edges and other crown features to improve flame travel, decrease hot spots, and ultimately increase the engine’s efficiency.

JE Standard Compression Forged Piston JE FSR Style Skirt

Even if stock compression is better for your application; forged construction, stronger skirt designs, and more efficient crown designs can still provide improved performance and durability.

If it’s time for a new piston but you’re still not sure what compression ratio to go with, give the folks at JE a call for professional advice on your specific application.

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I have answered so many post 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 endpower 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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The one time I tried a high comp piston I could never get it to start as easy as it did with the stock piston no matter what jetting changes I made. I loved the way it felt though on my 250f, revved better and built power so much faster.

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Great article! Well done. However you forgot to mention one of the advantages being high elevation applications. I live in Colorado and only ride at higher elevations in CO and NM. So my upgrade to a higher compression piston has been a great choice for my bike.

Thanks again for the great article.

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