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pistons and compression / U dont Know!

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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

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Not bad :smirk:

If ya get bored, feel free to drop about 500 words on detonation and why you want the A/F mixture to burn instead of "explode".

If you want to teach yourself about Gasoline etc...

http://www.faqs.org/faqs/autos/gasoline-faq/part1/index.html

Its a huge read. Real info. No BS here.

If you read this and then think about you'll understand why running the highest octane rated gasoline you can get is not a good idea. Run as little as you can get away with to make best power.

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''If ya get bored, feel free to drop about 500 words on detonation''

ALL of that ,and I don't think there was one word about detonation, as Shawn said YOU BETTER RUN ENOUGH OCTAINE WITH THAT COMPRESSION , If your too cheap to run race gas or at least mix , watch out for melted pistons

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''If ya get bored, feel free to drop about 500 words on detonation''

ALL of that ,and I don't think there was one word about detonation, as Shawn said YOU BETTER RUN ENOUGH OCTAINE WITH THAT COMPRESSION , If your too cheap to run race gas or at least mix , watch out for melted pistons

His description is dead on.

Mixing your race gas down is a bad idea. You can easily end up with less octane of the regular 91-93 pump gas by itself.

With a decent combustion chamber and not too much lead time you can run a 13.5:1 piston with no problems. You can run it leaner on good gas and make more power safely too.

Think about his rich mixture descriptions and what happens with a rich (bigger droplets)mixture...Ill give you a hint....Latent heat removal...what does it do for you?

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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

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His description is dead on.

Mixing your race gas down is a bad idea. You can easily end up with less octane of the regular 91-93 pump gas by itself.

With a decent combustion chamber and not too much lead time you can run a 13.5:1 piston with no problems. You can run it leaner on good gas and make more power safely too.

Think about his rich mixture descriptions and what happens with a rich (bigger droplets)mixture...Ill give you a hint....Latent heat removal...what does it do for you?

Then why doesnt Honda build thier 450s,(bikes and quads), with 13.5:1 compression to start with?? Doesnt the 250 have 13.5:1? And why not the 150?? It seems the 150 would benifit most by higher compression.

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Because they design the engine to deliver the power they feel it should have. The first CRF450s only had 11.0:1 (11.5?) compression. That doesn't mean that 11.0:1 pistons are the best compression for 2002 engines. In fact, the 2002 Camshaft benefits the most from the high compression pistons.

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Then why doesnt Honda build thier 450s,(bikes and quads), with 13.5:1 compression to start with?? Doesnt the 250 have 13.5:1? And why not the 150?? It seems the 150 would benifit most by higher compression.

nobody builds factory engines at 100 % of the power capacity

I ran my WR444 at 13:1 on pump gas

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nobody builds factory engines at 100 % of the power capacity

I ran my WR444 at 13:1 on pump gas

so 13.5 is 100%? what cmp do all the factory race guys use?

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so 13.5 is 100%? what cmp do all the factory race guys use?

it was near it for 93 pump gas, any higher or longer cam intake duration and it woulda been pinging

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Then why doesnt Honda build thier 450s,(bikes and quads), with 13.5:1 compression to start with?? Doesnt the 250 have 13.5:1? And why not the 150?? It seems the 150 would benifit most by higher compression.

factories also take into account fuel quality and availability (not great) and possible customer modifications (sometime REALLY not great)

and make a compromise ...sometimes for safety...sometimes for rideability...

from the first year of the liquid cooled '86 CRF500.... honda detuned it for many successive years to make it faster on the track...the 86 made too much power for most mortals...

smaller combustion chambers are naturally more detonation resistant...and can handle higher compression with pump fuel....

in the 250class power is more important than the 450 class... 450's typically need to concentrate on suspension and rideability / tractability.... 250's need all the power they can make to be competitive...and it shows in the showroom differences

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Because they design the engine to deliver the power they feel it should have. The first CRF450s only had 11.0:1 (11.5?) compression. That doesn't mean that 11.0:1 pistons are the best compression for 2002 engines. In fact, the 2002 Camshaft benefits the most from the high compression pistons.

right on the money.... more camshaft tends to pizzz away compression at lower rpms via reversion.... compression helps combat low rpm power loss..

on the other hand...long duration camshafts also make an engine more detonation resistant at lower rpms for that same reason... less efficient cylinder fill due to reversion... thats why engines with radical cams can run lower octane gas and get away with it

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it was near it for 93 pump gas, any higher or longer cam intake duration and it woulda been pinging

less intake cam duration.... or different lobe centerline that would close the intake earlier in the cycle...would make it more prone to the death ping

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so 13.5 is 100%? what cmp do all the factory race guys use?

That really depends on the rider, the track, the type of racing etc.

And you can run out of space to create compression too. Once you get into the 14's:1 you end up fighting with chamber volumes and figuring out what your going to do with valve reliefs etc because stuff starts running into each other.

On an IR intake engine like the CRF (single cylinder) you generally dont have an issue with intake shapes and volumes so you can still fill the cylinder well and you dont need to push the compression ratio through the roof. The short stroke and high piston speeds take advantage of the shitty gas to an extent also.

Longer stroke engines and engines with slight rod angles that allow lower piston speeds and greater dwell time at TDC can have a greater tendency to ping if everything isnt just right as cylinder pressures skyrocket.

Shorter rods create greater piston speeds at lower RPM. This usually means low RPM torque when everything else is right. Longer rod engines can run very very high RPM while maintaining reasonable piston speeds and can continue to make torque at higher RPM as the result.

I love this shit... :smirk:

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nice to meet you shawn! i can tell you love this shit because you are pretty damn good at it !

on the tight/inefficient combustion chamber thing.... deck height of a piston at tdc is the best way to add compression in a piston instead of just adding more dome...

the stock squish clearance in many machines is excessive

when i design venom pistons for a 450class machine i change the pin position to tighten squish clearance up considerably (and tight squish has both power enhancing and detonation resisting qualities)

so now i have higher squish velocities...and havent added a big dome that inhibits combustion AND blocks flow and cylinder scavengine during cam overlap at TDC... :smirk:

everything has its limits.... and the 14's are where the law of diminishing returns kicks in pretty hard in our thumpers...even when everything is done right...

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As this thread progresses I just wanted to add ,that at first I thought this guy must be a piston manufacture wanting to sell pistons, because to the reg rider to just throw a tech statement out there without mentioning effects of high comp ,without the mention of proper fuel seemed careless , but ive read some of your other posts where you have addressed it , your post here was a great one IE most over my head , but from an initial standpoint the post turned me off because of the missing detonation problems that come along with high compression .

obvoiusly your intentions were to just let everyone know the benifits of higher comp,

shawn tell me how you could end up with lower octain fuel by mixing 50/50 premium fuel and say some sunoco purple ?

Ive done this in alot of high dollar motors running semi high comp that really didnt need 108 oct ,but needed to be say 95 with succsess.

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nice to meet you shawn! i can tell you love this shit because you are pretty damn good at it !

on the tight/inefficient combustion chamber thing.... deck height of a piston at tdc is the best way to add compression in a piston instead of just adding more dome...

the stock squish clearance in many machines is excessive

when i design venom pistons for a 450class machine i change the pin position to tighten squish clearance up considerably (and tight squish has both power enhancing and detonation resisting qualities)

overlap at TDC... :)so now i have higher squish velocities...and havent added a big dome that inhibits combustion AND blocks flow and cylinder scavengine during cam

everything has its limits.... and the 14's are where the law of diminishing returns kicks in pretty hard in our thumpers...even when everything is done right...

Was looking at your website and saw the 5 valve piston-dont know what CR it is-but it does appear that there is alot less dome on top. Was wondering how far up in the cylinder would one of your typical high compression pistons be or what deck clearance do you shoot for? Do you use non oem head gaskets?

Also would like to see one of your high compression CRF pistons.

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As this thread progresses I just wanted to add ,that at first I thought this guy must be a piston manufacture wanting to sell pistons, because to the reg rider to just throw a tech statement out there without mentioning effects of high comp ,without the mention of proper fuel seemed careless , but ive read some of your other posts where you have addressed it , your post here was a great one IE most over my head , but from an initial standpoint the post turned me off because of the missing detonation problems that come along with high compression .

obvoiusly your intentions were to just let everyone know the benifits of higher comp,

shawn tell me how you could end up with lower octain fuel by mixing 50/50 premium fuel and say some sunoco purple ?

Ive done this in alot of high dollar motors running semi high comp that really didnt need 108 oct ,but needed to be say 95 with succsess.

Unintended chemical reactions is how you end up with less octane possibly. About 25 years ago back when Circle Track Magazine was still edited by Smokey Yunick (RIP) they did a study on fuels and did chemical analysis on several fuels ( pretty sure it was like 10-15 different fuels) leaded and unleaded trying to figure out what does what, how they performed and how to get the best out of them. They did this with a test mule 350 Chevy and adjusted the compression ratios and mixed the fuels up and down and ultimately came to some results.

Now with all that said, they then sent the fuels out to a lab to find out exactly what was in them. Amazingly enough many racing gasoline's have very little actual gasoline in them. Trying to remember all this off the top of my head, one had as little as 5% gasoline and then a mixture of all kinds of other petro-chemicals in there. One of the most common petro's you can find to raise octane with out hammering out the BTU content of the fuel is Toluene. Its got an octane rating of 118 MO. But it doesnt like to vaporize. If youve got a heated intake tract it can work well to raise the levels of shit pump gas. But using it to raise the octane level of a motorcycle with an intake tract thats only an inch long that isnt very hot to begin with it dropplets out bad and you end up suppressing detonation but for the wrong reasons. Which takes us back to the latent heat question. If you can vaporize the toluene it burns well and controlled and allows you to lean the shit out of it and make real power. But its a long road to haul for the avoidance of properly formulated fuel.

If you need a 95 octane fuel and all youve got or can get is a 108 vs pump premium, Id run the 108 lean. Right to the edge really. Thats where the power is anyway.

These are fourstrokes. A perfect plug looks like it just came out of the box on gasoline in a fourstroke. But you'll never get there with pump gas because of all the other shit in it. Water, milk, detergents...who knows what...its in there when in comes out of the corner pump.

All this goes out the window when you start playing with methanol. Or nitro-propane. Loves me some Nitro-Propane baby :smirk:

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As this thread progresses I just wanted to add ,that at first I thought this guy must be a piston manufacture wanting to sell pistons, because to the reg rider to just throw a tech statement out there without mentioning effects of high comp ,without the mention of proper fuel seemed careless , but ive read some of your other posts where you have addressed it , your post here was a great one IE most over my head , but from an initial standpoint the post turned me off because of the missing detonation problems that come along with high compression .

obvoiusly your intentions were to just let everyone know the benifits of higher comp,

shawn tell me how you could end up with lower octain fuel by mixing 50/50 premium fuel and say some sunoco purple ?

Ive done this in alot of high dollar motors running semi high comp that really didnt need 108 oct ,but needed to be say 95 with succsess.

i'm not an expert on fuel chemistry... so i have to take information from the experts.... there are many different formulations used to create higher octane in race fuels...and mixing brands may or may not have components that work well together....

adding tetraethyl lead to non leaded fuels always seems to work well.... while you could still get real lead additive from kemco oil, i have built and dyno tuned many machines that used pump premium mixed with their lead additive at compression ratios pushing 15:1 with absolute reliability...

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Was looking at your website and saw the 5 valve piston-dont know what CR it is-but it does appear that there is alot less dome on top. Was wondering how far up in the cylinder would one of your typical high compression pistons be or what deck clearance do you shoot for? Do you use non oem head gaskets?

Also would like to see one of your high compression CRF pistons.

deck height is pretty close to zero on all my piston setups.... used with stock gaskets, a 450 class machine is between .030"~.035" total squish clearance

which is tight...much tighter than oem ....but more than safe for endurance racing use

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