What compression will this come out to..

AzBoostin

Member
Wanting some more thump. looking at the fatheads with changable inserts. wondering what a 1997 cr125 with a 5cc head will give me....

sixds

Member
more size will not give you more compression, it will burn more fuel to give you more power.

AzBoostin

Member
a smaller size chamber in a cylinder head will give you more compression what are you talking about?... I am not sure what the stock size is i imagine 6-7cc and I am wanting to go down to a 5cc with stock piston, along with some porting to the head. and race gas for hopefully a few more hp... I am just trying to figure out what the static compression with be with a smaller dome.

sixds

Member
compression has nothing to do with the size of bore.

AzBoostin

Member
I am not talking about the bore. I am talking about making the dome size smaller in the head. using a phat head and insert.... that wont give me more compression?

Chili

Yes it will, sixds is either confused or doesn't understand the concept. I can't tell you exactly what the compression will be, but it wil definitely raise the compression.

AzBoostin

Member
right... anyways... does anyone know the stock compression and size of the chamber in the head

rmc_olderthandirt

IT can get pretty complicated. When the volume of the head is provided it probably would be the volume that you would have if you bolted the head to a flat plate and then filled it with water through the spark plug hole. There may be an additional volume that must be factored in if the piston does not come to the very top of the clyinder or if the piston top is not flat.

The displacement is the bore times the stroke, which on a 125 cc might be something like 124.5 cc.

The compression ratio is the volume at bottom dead center divided by volume at top dead center. The volume at the bottom will be the displacement plus the head volume plus some unknown, while the volume at top will be the head volume plus the unknown.

So, the equation will look something like this: compression ratio = (124.5cc + HeadVolume + Unknown) / (HeadVolume + Unknown).

If you knew the compression ratio was 12:1, and the volume of the stock head was 7cc, then the equation becomes 12 = (124.5cc + 7cc + X) / (7cc + X)

Solving for X , X = 4.3 CC

Now, if you swap out the 7cc head for a 5 cc head the equation becomes

CR= (124.5cc + 5cc + 4.3cc) / (5cc + 4.3cc)

CR = 14.4 : 1

High octane race gas, for sure!

Note that all that was based on guesses for the actual stock compression ratio and stock head volume.

Rod

76GMC1500

Uhhh...
But to really complicate things, 2-stroke compression ratios are generally not calulated as static compressino ratios that use the BDC volume divided by the clearance volume. Instead, they are generally calculated with a dynamic compression ratio which takes the volume at the beggining of compression divided by the clearance volume. Compression doesn't begin until the exhaust port is covered by the piston.

AzBoostin

Member
ahh ic ic... well thanks for the info I should be able to figure out somthing from this. Thanks guys.... 2 stroke is nuts haha

AzBoostin

Member
I am hoping to get some good power out of this little 125 to enter the indoor series this fall.. Anyways plan to port,polish the cylinder, as far as I can go without hurting performance, bump up the compression, full pipe, reeds, and race gas ect... should be fun.

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