2 Stroke compressions versus 4 stroke

[turbowrenchhead]

First of all I would like to thank everyone for there input into my questions. I'm a first time blaster owner and I can see why they are so popular. I'm an auto mechanic of 17 years and have a great understanding of 4 strokes, compression ratio's and the octane level needed for them.
Well 2 strokes is a whole new ball game I'm finding out. I see that the stock CR is 6.6:1 and it requires 93 octane. I think raising your compression is one the cheaper and easier ways to do it. So when I found someone that has a raised dome piston on ebay and stated that you can still run 93 octane I thought the only way to do this is by running a bigger jet. Or you can also raise your octane too. I can get 110 octane here in town no problem. It would just be a hassle taking it along with you on the trails. I would probably have to take a gallon with me to fill up in the next town. But the power increases might be worth it.
So my question is why does a lower compression 2 stroke require higher octane versus a 4 stroke? Just curious in trying to understand the physics behind it.

 

[Blaaster]

I am led to understand that adding oil to fuel degrades the octane rating, but I am open to correction.

 

[Racer51]

You can use lower octane gas in a 4stroke cause you can get away with it without major reprocushions. The clyinder and piston from predetonation excess heat charge from race gas requires a forged piston otherwise youd melt right through in no time. Since race gas burns cooler and has a more potent flash point causeing a hotter exploseive reaction. The reason for this is because it explodes hot and burns out cool into exhaust fumes thats why you get cooler engine temps.(Blaaster) nope its simply used to lubricate thats all it's there for. However the brands matter couse some seal better than others so yes it does play a part to compression. but if your expecting to get gib HP boosts useing race gas thats a thing for the built engines with porting or pig pistons that require cooler burning fuels to hep stress on internal parts to a minimum......

 

[Awk08]

from my understanding
you can go as high as 165lbs. compression on these and still run 93 octane
i was running close to that with the first modded head i got from ken oconnor racing.
no race gas needed

 

[mmajay]

Sounds to me the original poster is talking about compression ratio and not cylinder compression.

 

[Blaner]

im not sure if you know this or not but there are two methods of comprssion calculation on a 2 stroke.there is a distint difference between the two. they are: Corrected Comprssion Ratio and Uncorrected Comprssion Ratio.

Uncorrected compression is calculated in the same manner as a 4stroke, the full volume of the cylinder + the head volume / by the head volume. this will give you a figure around 9.5:1 or so

Corrected compression is calculated as the volume of the cylinder only being measured in terms opf stroke from where the exhaust port closes. before your exhaust closes, you actually have NO cylinder compression as its got a free path out the exhaust. so if you use the this method, your compression turns out around 6.6:1.

i wanted to post up the real blaster figures but seem to have accidentally deleted by two stroke folder! EISH!

in all honesty, i believe you better off having your cylinder head redesigned, the squish bands are far to wide and at the wrong angle and the combustion area is come shaped...have you ever seem anything explode in a cone shape? no- it needs to be hemispherical! compression does not add as much performance as you may think, its more about the dynamics of atomization of the fuel at the squish bands.

high compression without a redesigned head is going to cause detonation. you can compentsate with higher grade fuel but if a band aid to a shark bite...

 

[Larry's Shee]

Another reason for the lower compression ratios is the "stuffing" factor of the exhaust. Fuel/air charge is drawn down into the pipe right behind the burnt charge. Then just before the ex port closes the reflected sonic wave pushes the charge back into the cylinder, thus the need for lower comp ratio. Just like a four stroke uses a lower CR if using forced induction.

 

[hudack]

First of all I would like to thank everyone for there input into my questions. I'm a first time blaster owner and I can see why they are so popular. I'm an auto mechanic of 17 years and have a great understanding of 4 strokes, compression ratio's and the octane level needed for them.
Well 2 strokes is a whole new ball game I'm finding out. I see that the stock CR is 6.6:1 and it requires 93 octane. I think raising your compression is one the cheaper and easier ways to do it. So when I found someone that has a raised dome piston on ebay and stated that you can still run 93 octane I thought the only way to do this is by running a bigger jet. Or you can also raise your octane too. I can get 110 octane here in town no problem. It would just be a hassle taking it along with you on the trails. I would probably have to take a gallon with me to fill up in the next town. But the power increases might be worth it.
So my question is why does a lower compression 2 stroke require higher octane versus a 4 stroke? Just curious in trying to understand the physics behind it.

the part in red are you asking if you can just run a higher octane instead of rejeting?
if so the answer is no you must rejet when you add more air in or out of the motor

 

[Blaner]

true larry! built in boost system!

 

[turbowrenchhead]

So why does the manufacture want you run 93 octane fuel?
There has to be a point where re jetting won't cut it anymore and you must go with a bigger carb. I would think you can probably re jet if you add a redesigned head and replace the exhaust. If you add more cc's you should go with a larger carb, right?

 

[Blaner]

yes, rejet whenever you add/remove any 'go faster' bits. the stock 26mm mikuni is restrictive when porting and head mods are done, they work fine but a larger one unleashes the potential.

if you put in a hi comp piston (the likes of which i am not aware of) you will most likely have to rejet because, with the added compression, on 93 octane, the chance of pre-ignition is great. this is NOT because of the higher compression alone but mainly because of the poor squish band on the stock head. as the mixture is compressed, an ideal head will allow the air/fuel mix the be squished towards the center of the combustion area (why center spark motors are best!) and in the process increasing squish velocity. in the case that the mixture is squished too violently, pre ignition of the fuel/air mix will occur due to the heat build up caused by friction of the overly active molecules.

this pre ignition will also be promoted by the presence of casting flaws and rough edges in the combustion area, of which the blaster head has MANY!

All these factors point towards a head that needs attention. BUT as i was saying, if you did manage to find a hi comp piston, these problems will persist.

you are right however in that rejetting to a richer setting with your hi comp piston will help the probelm as fuel has a cooling effect and pre ignition will be reduced. BUT you will be making your motor very inefficient. you are counter acting a problem by smothering it with fuel.

The idea of the redesigned head is to make the motor burn its fuel MORE EFFICIENTLY. this means more power and less fuel usage while still getting better compression!