compression and domes?

[woodsracer144]

does a larger CC dome give you higher compression or does a lower one? on a 2 stroke the only way to raise the comression is to change the domes out or shave the head... you cant run a high compression piston like a 4 stroke right?

what are the compression ratings for a
18CC
19CC
20CC
21 CC
22CC domes?

thanks

[kouki]

Larger cc domes = lower compression Smaller cc domes = higher compression. As far as what size dome = what compression ratio, that will vary slightly as the bore size changes.

[fulltiltrider]

cranking compression can also vary drastically depending on porting. Never assume that a engine with 180 psi will run on pump fuel, my zilla only puts out 190psi but has a uccr of 16.5:1, anything other than methanol would nuke it.

[Honda 250r 001]

Originally posted by fulltiltrider
cranking compression can also vary drastically depending on porting. Never assume that a engine with 180 psi will run on pump fuel, my zilla only puts out 190psi but has a uccr of 16.5:1, anything other than methanol would nuke it.

So does a drag port require higher octane fuel even though the compression is under 200. Or do you have the timing advanced so far that any more compression would detonate too early?

man this stuff is confusing!

and how do i find out my compression ratio?

[kouki]

The compression ratio is the ratio between the volume of the cylinder/dome at bdc and tdc, not to be confused with the displacement.

[woodsracer144]

ok well this just keeps gettin me messed up! i got a LRD 300 resleeve kit cyl on a 89 and my dome is a really deap one thought i would run pump gas with it but i run race gas... im 225psi and i stopped kicking i was goin on to 230... so how is this so?
how do you find the crank crompression? i think i got more a MX style porting

also since porting came up... after you have lets say a XC port can you change i to a MX or something like that?

[Honda 250r 001]

you can get a low end (xc) port job and jump up to a mid range (mx) it may not work as well if it had not been ported before, but you defenatly can change the power band. You can easily go from a xc to a top end port job i think. Hopefully a porter will get in here and confirm/reject.

[fulltiltrider]

Originally posted by woodsracer144
ok well this just keeps gettin me messed up! i got a LRD 300 resleeve kit cyl on a 89 and my dome is a really deap one thought i would run pump gas with it but i run race gas... im 225psi and i stopped kicking i was goin on to 230... so how is this so?
how do you find the crank crompression? i think i got more a MX style porting

also since porting came up... after you have lets say a XC port can you change i to a MX or something like that?

So many parameters when you speak of head design...squish, sar, m/s velocity, dome shape, I would say you probably have a good setup going. Definately no pump gas at 230psi lol.

Lots of ways to skin a cat when it comes to porting, its always safer to cut less and test but that doesnt mean you cant fix a jug that has been taken a "little" too far

[Honda 250r 001]

Originally posted by fulltiltrider
So many parameters when you speak of head design...squish, sar, m/s velocity, dome shape, I would say you probably have a good setup going. Definately no pump gas at 230psi lol.

Lots of ways to skin a cat when it comes to porting, its always safer to cut less and test but that doesnt mean you cant fix a jug that has been taken a "little" too far

true dat. An old porters sayin, "Cut until you see light, weld up and cut again!"

[mxduner]

Originally posted by woodsracer144
ok well this just keeps gettin me messed up! i got a LRD 300 resleeve kit cyl on a 89 and my dome is a really deap one thought i would run pump gas with it but i run race gas... im 225psi and i stopped kicking i was goin on to 230... so how is this so?
how do you find the crank crompression? i think i got more a MX style porting

also since porting came up... after you have lets say a XC port can you change i to a MX or something like that? you need to run race gas with that psi.

you could send it to neil or a good porter. xc porting to 1 guy can be different to another guy as may be so with a mx port. just make sure which ever way ya go, make sure your pipe matches the port job

[woodsracer144]

well right now im runnin a big bore pipe with a 300CC motor.... so i think i need a new pipe anyways...

port wise... its a more top end one... the more she revvs the harder it pulles.. its not a low end hitting motor...