banshee coolhead dome compression

[EricB]

hey i did a quick search and didn't find anything about actuall compression ratios on coolhead domes. what is a 20cc running at? what octain do you need?

[Chef]

Have a rocky mountain atv catalog? They might have it on their site too, but they have a banshee compression guide in it. At sea level, 20cc domes should have 169 psi. 1000ft has 151, 2000 145, 3000 139, 4000 133, and 5000 127...Hope that helped you some. I have 16cc domes at 500 ft and I am getting about 215 psi, so that chart is pretty dang close.

[mx517]

go to www.thebansheezone.com

[EricB]

i don't have very much 2stroke technical knowledge, about what would that be running? like 10.5:1? or do 2strokes not work like that? i'm asking for a friend too so i really don't know hehe.

[Texan32]

Yes, they do work like that, but its not very often refered to as such. 2 Strokes are much easier to work on (assemble/disassemble). The are much easier to get higher HP #'s from. A banshee only comes from the factoy with like 6.5:1 compression ratio. (don't quote me, but i know its way low) By simply pulling the head and changing the dome size (this requires an aftermarker head), you accomplish 2 things. 1 you increase the COMPRESSION RATIO. 2 you increase COMPRESSION. They both effect one another. Usually, they smaller the dome=higher compression ration=more compression. Alot of 2 stroke racers carry many different size domes. They do this so that for any given race and any different weather condition, they can dial in the motor for what they want. I'm sure Alky472 will tell you the same thing (assuming a AM correct:rolleyes: )

you don't hear about stroker motors on 2 strokes because is just easier to find power else where.

[EricB]

so i figure that he has a theoreticle 169psi so what would that convert to "white person" compression? like 15:1? i was looking at a TRX250r compression deal, but the shee has two pistons obviously. so i'm a little confused, i'm sure that two pistons work out differntly. so is there some easy way to convert the metric psi measurement to what i'm used to?

[Chef]

Originally posted by Texan32
Yes, they do work like that, but its not very often refered to as such. 2 Strokes are much easier to work on (assemble/disassemble). The are much easier to get higher HP #'s from. A banshee only comes from the factoy with like 6.5:1 compression ratio. (don't quote me, but i know its way low) By simply pulling the head and changing the dome size (this requires an aftermarker head), you accomplish 2 things. 1 you increase the COMPRESSION RATIO. 2 you increase COMPRESSION. They both effect one another. Usually, they smaller the dome=higher compression ration=more compression. Alot of 2 stroke racers carry many different size domes. They do this so that for any given race and any different weather condition, they can dial in the motor for what they want. I'm sure Alky472 will tell you the same thing (assuming a AM correct:rolleyes: )

you don't hear about stroker motors on 2 strokes because is just easier to find power else where.

Yep, you are exactly right. When a person does stroke something like a Banshee, they get alot of HP for the cash. 10 mm strokers are usually in the 500cc range and 110+hp range...if done right. But you have to get into things like shaving gears and trenching the cases to make clearance.

Stock compression on a Banshee is 135lbs at sea level. So to get 13:1 compression, you would, in theory, need like 270lbs of compression. 169 PSI is probably 10:1, or 9. But, it may not work like that. 12:1 in my EX is like 160 PSI. I have also heard that at 215 PSI, I am about 12.5:1 compression. I would say you are safe to mix the gas half and half with race fuel, maybe even on 93...but dont take a risk if you dont have to. Two strokes dont take too low of octane too well.

[EricB]

not to sound ungratefull for that input, it is what i needed. but do you know of the formula to go from 2stroke PSI to static compression?

thanks though alky, i'll tell him. i was thinking that 12-13:1 was a little high for the 20cc domes. plus i don't know what else he has done to the head and cylinder, so compression might be a little higher depending on the deck heighth and stuff (right?).

[WyoDuner]

Eric, I assume that you know that 20cc domes on a Banshee at sea level is too much for pump gas. That being said....

I don't know why you want to know static compression ratio but... it was said that stock is around 6.5:1 and that's correct.

I suppose that you could say that 6.5 : 1 gives 130 lbs compression and knowing that 20 cc domes gives 169 compression you could work that backwards and derive 8.45:1 compression with 20cc domes... Just my best guess.

Either way, it doesn't matter what static compression is. 2 strokes are different that 4 strokes in that regard.

Do you have a specific application that you need advice on?

[EricB]

yeah i hate posting stuff like this, it just makes me look like an idiot. but my friend is being so damned persistant about this. he just wanted to know the static compression so he could relate it to what octane needs to be ran in it. so at sea level you wouldn't be able to run pump gas? alright, well thanks man this will get him off my back and let me quit looking like the fool.

[out4sand]

I ran 20cc domes at sea level from 2000 - 01 on my banshee before doing the motor work I currently have and always ran a 50/50 mix of super and either VP C-12 or Unocal 110. I contacted Pro-Design to ask their advice on what gas to run before installing the cool head and the 20cc domes. Here is what they had to say -


Without your porting specs We can only give the following advice:

Stock domes are 24.5cc

With stock port timing 22cc domes at sea level should produce 145 lbs cranking pressure. This is the maximum pressure that should be run with 92 octane fuel.

for every 1cc progression a difference of 8 - 10lbs should be expected (once again, at sea level with stock porting).

Thank you for your inquiry,

Vance, Q &E, Inc. & Pro Design

Race fuel is also a must if your compression is over 145lbs.

:

When I did tear my motor down last winter to do more work everything looked good other then the crank...it was certainly time to true & weld that and also time for a billet clutch basket.

[Predator36]

Be carefull of using cranking pressure as any thing other than as a way to check the wear on your engine. Worn rings will effect the cranking pressure as well as out of round cylinders, and the method of kicking it. Have 2 guys kick the bike to check the compression and the chances are high that you will get 2 differant readings.