I am going to do some headwork soon, and i want to get the head milled. How much would i need to mill off to get 10.5:1 and 11:1 compression. Im still deciding what compression to run right now, thats why im asking for both.

thanks, Dan

 

That's interesting. I haven't heard of milling the head to raise compression. Most people just use high compresion pistons. Can anyone explain this theory of milling the head?


[Modified by laughin2.2, 8:40 PM 3/7/2002]

 

From what I heard, when you mill, you remove some of the stuff off the bottom of the head. It supposed to be a cheaper alternative to raising compression with pistons. But once you mill the head you can't use high compression pistons. For example my friend has a gsr in his eg (civic hb) he recently put itr cams/valve springs/retainers, port and polished and milled. The shop told him he can't put CTR pistons if he milled it cuz it would raise compression higher than 13.

 

its the same theory. same amount of air is drawn in when the piston goes down, but less room for the mixture to go when the piston returns, so it gets compressed more. type-s pistons have a domed top rather than stock which are flat. milling the head can only be done so many times so you wanna leave some safety gap.

I can't remember how much raises it 1 point. I have about half a point from milling the head and 1 from the pistons. Next time I talk to my mechainc I'll ask how much was taken off...

 

thanks Flavour ask your mechanic for me, i really need to know

 

stock is 10.1:1 CR
if u only change to S or euro R pistons you go up to 11.0:1 CR

i would recommend using spoon's h22a gasket instead of milling the head

that should give a +0.2:1 CR
what would be a good combo is type S pistons + spoon gasket
if u don't wanna invest in a spoon gasket only get the head milled so much that it ads only .3 -.5:1 CR
u do not want to go too muhc on head milling..cause that is irreversible..and can cause gasket leaks if u go too far on certain occasion/heads

be sure of what u are doing..and don't goo too high in compression or u will have to run timing changes or higher octane rating fuel to limit predetonation



but raising the CR is the best way to make god power with H22a
smart move

btw ..spoon gasket should be aournd 150$
if u are paying almos taht for head milling..just get the thin gasket ..

 

sorry for the delay...

my mechanic says that 25-30 thousandths would be equal to about an extra point of compression.

also news to me was he only took off 5 thousandths off mine so my compression isnt as high as i thought it was. around 11.2:1...

 

Well i talked with the guy doing the work...he said to do about 20 thousandths. He said it would be about .5 increase in compression. So i just went with that. I should have the car back this friday and have baseline dynoes by first weekend of april

 

..for every additional point in compression, you should expect a 7% increase in power accross the entire powerband. However, this has a point of diminishing returns, go to high, and you car will reach a compression ratio where there no longer is a power increase.

Milling the head, essentially, reduces the volume of the combustion chamber, so all things being equal, the pressure inside should increase. We can prove this, assuming the gas/air mixture behaves as an ideal gas, by using the relation pV=nRT, where P=pressure, V=volume, n=moles of gas, R= gas constant, t=temp. in Kelvin. Rearranging the equation to solve for pressure while varying volume, you should get p = nRT/V, so you should see that pressure and volume are inversly proportional. One goes up, the other goes down, and vice a versa.

Any questions?


Hope this helps.


[Modified by bb6h22a, 2:06 PM 3/20/2002]

 

ya, where did you pull 7% from?