The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: spyke on 22.09. 2010 20:35
-
Hi chaps,
Im trying to calculate what my comp ratio will end up once Ive got my A7 ally head onto my A10.
Ive just checked the volume of the combustion chamber and make it 40 cc.
I sort of know that ive got to compare swept vol. with combustion chamber vol. but am not sure about other variables like head gasket
and piston head shape , valve pockets etc.
Im using standard flat top hepolite pistons 69.989dia.
Any body give me the total formula or want to work it out for me!
I thought the pistons should be 7.25 or 7.5 :1 but the A7 combustion chamber must be smaller vol than the A10 one so would raise it up abit more ,wouldnt it?
Cheers Spyke
-
G'day Spyke,
yes the comb chamber of the A7 is smaller.
The best way is to bring it up to TDC, bolt on head with gasket, tilt motor till spark plug hole is vertical. Use a syringe to measure the amount of fluid it takes to come to the bottom of the spark plug hole. Use same syringe with some vac tube to suck it back out.
325cc divided by the volume = comp ratio.
To work out the different cc a thicker head gasket will give. Pye X Rsq X h, so a 1mm gasket on a 70mm bore is 3.14 X (3.5 X 3.5) X 0.1 = 3.84cc. So if you use a 1.5mm gasket the increase will be 1.92cc.
Hope this helps.
Cheers
-
Spyke,
The fluid Muskrat talks about must be XXXX or Fosters.
What ever you do don't use Coopers.
That stuff will rot your socks.
cheers
-
If you check it with a trial assembly you can put grease around the edge of the piston to stop any fluid going down past the rings.
-
HI All,
Just a slight hiccup in Muskrats formula ???? ???? ???? ????
To the best of my knowledge the compressoin ratio formula is
swept volume + combustion chamber volume, divided by combustion chamber volume.
for example,
325 + 35
_______ = 10.2 :1
35
HTH
John O R
-
So, if the head were already off, why not just use a graduated syringe to fill it up with the plug installed. Just avoid a surface tension dome of liquid. I suppose mineral spirits does not do surface tension like water.
-
Hi manosound,
You have to consider the shape of the piston top, and as Muskrat says also the headgasket
Regards
John O R
-
Yes, Muskrat calculated head gasket, good and well. Flat-topped pistons, should be no problem. Shaped pistons, like my 9:1 domes, I drew the piston profile in AutoCad then calculated the volume of the dome. 10:1s could be real tricky. How about sticking them in clay and measuring the mould per the idea for the combustion chamber. There is just something about tipping the bike 45 deg. to get the plug hole verticle that doesn't sit well with me. Then, syringing out the liquid seems iffy.
I suppose the next issue that will come up will concern the measured PSI. So you might think 7:1 yields 7 x 14.7 (std atmos) = 102.9 PSI. I think you'll find your pressure gauge reads higher, which, I think, is due to adiabatic heating of the compressed air resulting in higher pressure. Come on you thermodynamologists, set me straight if I am wrong.
Richard L.
-
Doh, your right (as usual) John OR. I forgot to add the comb chamber.
Yes Richard L the psi is slightly higher, my 10.5:1 reads 178psi. Can't see any harm in tilting the bike over. Any fluid (i use engine oil) left after syringing out will be burnt off in no time.
Cheers
-
No one done a Redex treatment ?
No plugs 3 or 4 swift kicks and there ain't going to be no nothing left in the pot..
-
That didnt go unnoticed Olev, there's no acounting for some peoples taste.... *beer*