So here is my question… What is the benefit to the high compression piston? The parameters being stock bore & stroke. Pipe / Carb / Ignition info probably not important at this point.. I think.

At first I thought that this would REDUCE displacement but because the bore and stroke are unchanged, this WOULD NOT be the case, but in fact the air/fuel charge drawn into the cylinder ( the same amount as if it was totally stock) would be compressed into a smaller volume than stock thus, creating the higher compression at ignition……………… correct?

So then what is the real advantage to this? Does it provide a better burn of the fuel? Does it allow you to run a different kind of fuel that provides more horsepower? What exactly is the bonus here???

I must know …. I’m loosing sleep over this one………….

 

M2C here along w/ running better fuels because lower octang fuel will detonate w/ higher compresion. the tighter you can compress the fuel and air the bigger the boom in the cylinder creating more down pressure in turn more power. some one corect me if im off here but this is the way I understand the concept of higher compresion.

Sam

 

yes higher compression = more power plain and simple, the higher the compression the bigger the boom so to speak.

Most stock motors come with a compression ratio lower than 10:1 because you can safely run low grade pump gas without worrying about pre-ignition. The higher combustion heat associated with higher compression causes pre-ignition/detonation - a scenario in which fire occurs before the spark actually sparks, while the piston is still on its way up during the comression stroke.

The higher the octane rating, the slower the fuel burns, and the more controlled burn combats pre-ignition. Therefore, the higher the compression, the higher the octane needed to counteract the potential for pre-ignition which is detremental to the motor. 89 octane is fine for the stock motor, 92 octane fine for 11.5:1, but step up over 12:1 and now you gotta go to race fuels over 100 octane which can be hard for some people to find. 110 octane is used with 13.5:1 safely.

using an octane higher than necesary is futile and unecesary, and its possible to actually lose power.

Higher compressions are not harmful to the motor as long as the correct octane is used and the carb's fuel/air mixture is tuned properly. motor life is not significantly effected except for the obvious slight increase in wear due to added HP, as any HP enhancing mod will introduce technically speaking.

 

So why does Bomb recommend 91 octane minimum on a stock DS? I have not been able to figure that one out.

 

Thanks guys ... and Extra thanks to you HT! Crystal clear !!

Are we talking about a tremendous amount of extra heat by running a 12:1?
What about plugs? Do you need to run a different heat range now ?

 

Additional concept (Hightower spoke briefly about it.):

Building pressure creates heat. . . seriously think about this for a while. It relates to weather, turbos, compressors, combustion.

The reason it is important is increased compression = more pressure. . . more pressure = more heat. . . heat + pressure in the presence of fuel = combustion (even without spark!)

What is the temperature of a compressor hose? What is the pressure of the air as it is released to atmosphere (lower pressure?)
Weather: leading edges of storms are always warm (higher pressure) and after the front passes it gets cold (lower pressure)
Superchargers and turbos - why do they have air coolers (intercoolers)? Just like the compressor above, the more air is compressed, the more heat builds. Hot air has less tightly packed air molecules than cold air so, if we feed hot air into a cylinder we get less air (compared to cold air.

Why do diesel engines not need spark-plugs? They build enough heat under the compression to self ignite.

As for high compression piston, yes. . . you will be slightly less cc but, a 12.5:1 stock bore will be faster than a 2mm larger bore at stock compression.

The DS responds very well to high compression piston and cams.

 

Probably just to cover their a$$, they figure there is no harm running the higher octane gas.

On the same note, why does Bomb only recommend you use their own brand on synthetic oil and nothing else!

The Suzuki Z400 comes stock with 11.3:1 and they recommend 87 octane.
The Yamaha YFZ450 comes stock with 11.9:1 and they recommend 91 octane.
So go figure!

 

I run an 11.5:1, stock plug heat range, no heat issues at all. There's really no down side to going to the higher compression as long as you use the correct fuel and tune the carb right. I stayed below 12:1 so I never have to worry about fuel availability on ride trips. Nothing worse than being 1000 miles from home and outa gas.

if/when you change the piston, be sure to have the cylinder wall honed for proper piston ring break-in, its the rings that seal all that compression in.

 

piston spead comes into play too, the 400's and 450's have a shorter stroke so much less piston traveling distance is covered at a given RPM as compared to the DS's long stroke. 7000rpm piston on the ds will cover a lot more distance than 7000rpm on the smaller bikes which would produce more heat, and a higher octane can help combat this heat as well.

 

Sweet dreams [img]i/expressions/face-icon-small-wink.gif[/img]