tomorrow im taking the head off, replacing all gaskets and changing the timing chain and sproket as well as refitting valve seals polishing the valves i was wondering if its a good idea to change the compression ratio of the car by skimming the head a little, this is why;

it seems the only diference between the 135 and the 136 engine is the compression ratio but the 136 guives more power and better FE

head gasket is going to be worked on (cleanded valve re-seatted and seals changed) so why not "tune it" a little and get better performance

but experience trumps ignorance, so please, any concernes while doing this things or problems i may encounter, and if its ok to skim the head, how much should i take of to set the CR @ 9.5:1 (witch produces more torque and power but requiers 95 RON instead of the 91 RON i use now) do i need to do work on the pistons? (seals are going to be checked and i may fit new seals)

i realy will apreciate your imput on do / dont and specific measures

i wouldn't bother skimming down a head to increase the cr, it's not worth the hassle it will cause you in the future when the head gasket pops every 5 miniuites...

if you are that desperate for an extra 5bhp then just fit a the whole 136 engine

update...

the head is dead, there was a bold that was welded to the head with some tipe of glue that heat and lots of torque did not break, after driling the bold and removing the head we saw water damage, it was basicaly corroded throu. Here in the dominican republic we machine the head when is warped by heat, the head was aparently machine twice so it was not recomended to be fix a third time

the intake and exhaust manifold did not have a gasked fitted on, only silicone paste, so it had an air leak and a exhaust leak

the engine did not have a crank cover or side gasket eyther, just silicon paste.

the chain on the engine was very, very, very, loose and the theeth where hooked throwing of cam and ignition timing

so all engine seals and gasked are going to be new, new head with polished ports an valve, the hole engine is going to be cleaned to remove water and oil gunk mayo an silicon residue and who knows what else

,

to be honest, so long as you use the right type of silicon sealant/ instant gasket there's no reason why it be as good as a pre-formed gasket

I can tell you how to measure a CR from first principles.

Measure the bore and stroke of a cylinder, then (Bore * Bore * Pi *Stroke) / 4 gives you the Swept Volume of a cylinder. Now get that cylinder to TDC, and measure how far the piston sits off the block deck. If it's below, use the same formula to calculate the volume. I'd be surprised if the piston protrudes above the deck, but if it does, make the same calculation, and take note it needs using later.

Next, get a burette, and use it to measure the volume of a combustion chamber in the cylinder head. Remember to leave the valves in! If the piston is below the deck, add the free volume to the combustion chamber to arrive at a combustion space volume. If the piston protrudes, take the head volume and subtract the protrusion volume.

Now things get complicated, with the possibility of domed or dished pistons. A dished piston isn't too bad, since we can measure the dish volume with the burette, and add it to the combustion chamber volume, but a domed piston will require some trigonometry, which I'm not going to do unless forced! Add the dish volume or subtract the dome volume from the combustion chamber size so far, and we are finally ready to actually calculate the CR.

CR is swept volume / total combustion volume.

Rearranging this, combustion volume = swept volume / CR, so in the case of a Skoda 1.3 pushrod motor (using nominal capacity; replace with actual to get a correct chamber size for a 136 enine:-

CV = 325/9.5 = 34.21cc.

Oh and Tom, why should the skimmed head prove less reliable, as long as you use a 136 head gasket?