CJM
05-06-2012, 11:11 PM
So while Im not a real tech nerd on this stuff I wanted to discuss this in laymens terms and explore why a higher comp ratio piston is better than say just slapping on a big bore kit and why its possible to run a higher compression ratio than 11:1 safely on an EX. I hope to get the point across and feel free to correct me.
Go to the end of the post if you want the bottom line
So you have a stock EX. Stock comp ratio is around 9:1, this is further reduced by gasket thickness, the quench area (distance between topend components like the head and cylinder) is actually too much due to the thick stock EX gaskets and lets not forget cam duration. Longer duration means valves open longer, which means less overall compression.
So you swap to a thinner base and or head gasket you will obviously reduce quench area getting a better overall comp ratio closer to that of the advertised 9:1 honda claims.
Furthermore if one were to run a higher comp piston they obviously see a jump in power due to the way the piston creates energy. It creates the energy differently b/c it ignites the fuel at a much better and efficient part of the stroke b/c of the space the higher compression piston occupies vs stock. Thus igniting the fuel more completely and with more available oxygen, perhaps making close to the ideal stoichiometric mixture of 14:7. In other words the ideal air/fuel mixture is 14:7.
So how does this relate to your EX or just any engine? Well for starters if you want power (as mentioned earlier) you need to up the comp ratio. Yea boring it will give you a modest to slight increase, but nowhere near that of using a higher comp piston.
So you ask-which comp ratio should I use to make the most power? Well this is a two fold answer:
1. Obviously the highest possible you can use. this means if they made a 25:1 piston you could use it and see HUGE gains in power. BUT, being that the EX is air-cooled and subject to overheating issues this brings us to a problem.
1a. If you use too high of a comp ratio you subject the bike to more heat obviously due to the fact that the bike is now compressing fuel and air in a smaller space and although more efficient-less space to dissipate heat even inside the cylinder.
So where does that leave us you ask? Well the main thing in an aircooled engine is heat, heat will RUIN these things if you get it to hot. Say goodbye to headgaskets, pistons, rings, cylinders, etc. Proper jetting and proper octane will help to keep the bike cool to some degree
2. So then, you use the highest comp ratio piston possible with the best gas and jetting you can do to avoid detonation. Sidenote: Detonation is when the compression of the engine ignites the fuel-not the spark. This is a BIG problem. Using fuel with a higher octane-it takes more compression to ignite and thus you have less chance of detonation. Dont confuse ignition from the spark plug with ignition from compression-2 very different things.
What helps greatly in most cases revolves around jetting the bike PROPERLY and using the proper octane. The bike must get the proper amount of fuel into the engine from the correct size jet or fuel injector, as well as use the correct octane, a lower octane will ignite that much easier outside the spark on a higher compression engine.
Sidenote: Do not use 87 in even a stock 400ex or any powersports machines, not only does it contain more ethanol which can absorb water in some cases (and is bad!), its safer to run a higher octane within reason than save a few lousy bucks at the pump. This doesnt mean you should run 110 octane in a mostly stock or slightly modded bike. Thats just totally pointless and your wasting your money. in most cases premium pump gas of 91+ octane is good enough. If not mixing 50/50 with 100 octane or better will suffice.
So you ask-whats the highest comp ratio you use do then and still remain safe on a 400ex? Most agree (and I myself as well as a few I built) say 11:1. 11:1 is safe due to a variety of factors we will get into a bit later. Suffice to say that so long as you jet it properly you will be fine.
Now. What if you wanted a bit more power. Everything being equal why not just go to 12:1? Sure can, just gotta use a mix of 50/50 91+ octane and 100+ octane and or race fuel if you want to run that setup in an EX. But wait, theres a hidden catch here (and its a doozie!)-because in most cases the quench area isnt truly ideal your not making the advertised 12:1 or whatever comp ratio. How much lower it is I do not know for sure, but unless you deck the head (shave the head thickness slightly) its still not truly ideal, even decked it might not be ideal.
So that being said, what about cam duration (how long the valves stay open) playing a role in it all? HERE, HERE IS THE KEY! Dynamic vs static compression ratio! Wikipedia explains it best:
The calculated compression ratio, as given above, presumes that the cylinder is sealed at the bottom of the stroke, and that the volume compressed is the actual volume.
However: intake valve closure (sealing the cylinder) always takes place after BDC, which may cause some of the intake charge to be compressed backwards out of the cylinder by the rising piston at very low speeds; only the percentage of the stroke after intake valve closure is compressed. Intake port tuning and scavenging may allow a greater mass of charge (at a higher than atmospheric pressure) to be trapped in the cylinder than the static volume would suggest ( This "corrected" compression ratio is commonly called the "dynamic compression ratio". http://en.wikipedia.org/wiki/Compression_ratio#Dynamic_compression_ratio
So what does this mean in simple terms? The longer the duration to the cam, the less compression your actually making in some cases. Thus your dynamic compression and static compression will never be the same. The longer those valves stay open due to the cam you run, means that although the piston maker says the piston makes 11:1 comp ratio its actually slightly lower.
What does this mean? It means that if your cam has a long duration it is possible you could gain more power by using higher than an 11:1 compression piston and run it on 91+ octane! Why bother??? Well MORE POWER SILLY!
To sum it up:
Everything being equal, if you wanted to run a 12:1 piston or similar (id say 12:1 is about the limit here) on pump gas then its doable. Doable means you have the longer duration cam, the proper quench area, proper jetting and you build the engine right. HOWEVER no two engines-even if they run in the same exact area, will be the same. Different elevation, temperatures, engine setups, exhaust, cams, gasoline octanes, etc all play a role in this.
Go to the end of the post if you want the bottom line
So you have a stock EX. Stock comp ratio is around 9:1, this is further reduced by gasket thickness, the quench area (distance between topend components like the head and cylinder) is actually too much due to the thick stock EX gaskets and lets not forget cam duration. Longer duration means valves open longer, which means less overall compression.
So you swap to a thinner base and or head gasket you will obviously reduce quench area getting a better overall comp ratio closer to that of the advertised 9:1 honda claims.
Furthermore if one were to run a higher comp piston they obviously see a jump in power due to the way the piston creates energy. It creates the energy differently b/c it ignites the fuel at a much better and efficient part of the stroke b/c of the space the higher compression piston occupies vs stock. Thus igniting the fuel more completely and with more available oxygen, perhaps making close to the ideal stoichiometric mixture of 14:7. In other words the ideal air/fuel mixture is 14:7.
So how does this relate to your EX or just any engine? Well for starters if you want power (as mentioned earlier) you need to up the comp ratio. Yea boring it will give you a modest to slight increase, but nowhere near that of using a higher comp piston.
So you ask-which comp ratio should I use to make the most power? Well this is a two fold answer:
1. Obviously the highest possible you can use. this means if they made a 25:1 piston you could use it and see HUGE gains in power. BUT, being that the EX is air-cooled and subject to overheating issues this brings us to a problem.
1a. If you use too high of a comp ratio you subject the bike to more heat obviously due to the fact that the bike is now compressing fuel and air in a smaller space and although more efficient-less space to dissipate heat even inside the cylinder.
So where does that leave us you ask? Well the main thing in an aircooled engine is heat, heat will RUIN these things if you get it to hot. Say goodbye to headgaskets, pistons, rings, cylinders, etc. Proper jetting and proper octane will help to keep the bike cool to some degree
2. So then, you use the highest comp ratio piston possible with the best gas and jetting you can do to avoid detonation. Sidenote: Detonation is when the compression of the engine ignites the fuel-not the spark. This is a BIG problem. Using fuel with a higher octane-it takes more compression to ignite and thus you have less chance of detonation. Dont confuse ignition from the spark plug with ignition from compression-2 very different things.
What helps greatly in most cases revolves around jetting the bike PROPERLY and using the proper octane. The bike must get the proper amount of fuel into the engine from the correct size jet or fuel injector, as well as use the correct octane, a lower octane will ignite that much easier outside the spark on a higher compression engine.
Sidenote: Do not use 87 in even a stock 400ex or any powersports machines, not only does it contain more ethanol which can absorb water in some cases (and is bad!), its safer to run a higher octane within reason than save a few lousy bucks at the pump. This doesnt mean you should run 110 octane in a mostly stock or slightly modded bike. Thats just totally pointless and your wasting your money. in most cases premium pump gas of 91+ octane is good enough. If not mixing 50/50 with 100 octane or better will suffice.
So you ask-whats the highest comp ratio you use do then and still remain safe on a 400ex? Most agree (and I myself as well as a few I built) say 11:1. 11:1 is safe due to a variety of factors we will get into a bit later. Suffice to say that so long as you jet it properly you will be fine.
Now. What if you wanted a bit more power. Everything being equal why not just go to 12:1? Sure can, just gotta use a mix of 50/50 91+ octane and 100+ octane and or race fuel if you want to run that setup in an EX. But wait, theres a hidden catch here (and its a doozie!)-because in most cases the quench area isnt truly ideal your not making the advertised 12:1 or whatever comp ratio. How much lower it is I do not know for sure, but unless you deck the head (shave the head thickness slightly) its still not truly ideal, even decked it might not be ideal.
So that being said, what about cam duration (how long the valves stay open) playing a role in it all? HERE, HERE IS THE KEY! Dynamic vs static compression ratio! Wikipedia explains it best:
The calculated compression ratio, as given above, presumes that the cylinder is sealed at the bottom of the stroke, and that the volume compressed is the actual volume.
However: intake valve closure (sealing the cylinder) always takes place after BDC, which may cause some of the intake charge to be compressed backwards out of the cylinder by the rising piston at very low speeds; only the percentage of the stroke after intake valve closure is compressed. Intake port tuning and scavenging may allow a greater mass of charge (at a higher than atmospheric pressure) to be trapped in the cylinder than the static volume would suggest ( This "corrected" compression ratio is commonly called the "dynamic compression ratio". http://en.wikipedia.org/wiki/Compression_ratio#Dynamic_compression_ratio
So what does this mean in simple terms? The longer the duration to the cam, the less compression your actually making in some cases. Thus your dynamic compression and static compression will never be the same. The longer those valves stay open due to the cam you run, means that although the piston maker says the piston makes 11:1 comp ratio its actually slightly lower.
What does this mean? It means that if your cam has a long duration it is possible you could gain more power by using higher than an 11:1 compression piston and run it on 91+ octane! Why bother??? Well MORE POWER SILLY!
To sum it up:
Everything being equal, if you wanted to run a 12:1 piston or similar (id say 12:1 is about the limit here) on pump gas then its doable. Doable means you have the longer duration cam, the proper quench area, proper jetting and you build the engine right. HOWEVER no two engines-even if they run in the same exact area, will be the same. Different elevation, temperatures, engine setups, exhaust, cams, gasoline octanes, etc all play a role in this.