Dangers of cylinder head machining
 

I am building a 3.6 track motor. Started life as a 1993 964. I would like to raise the compression as it is less than I had expected but that partly is due to the flycut work on the valve pockets. I would do this by cutting the cylinder heads. I have dealt with issues with the earlier 3.0 and 3.2 engines in dealing with the resulting chain slack due to the decking of the cylinder heads, but not with the 3.6 setup. Wondering what I can get away with as was wanting take out .030''. I appreciate all advice. Bob

Bob, maybe someone here has done that and can give you exact numbers, however, even then, you should measure the minimum clearance between your pistons and the heads, at the edges and wherever the piston dome comes closest to the inside of the head. In 3.0 and 3.2 engines, these are already pretty close, so you are very limited in how much compression you can get by flycutting the heads. I'm not sure about 3.6 964 engines. Measure yours, then you will know. Do not go below .030"/.75mm minimum clearance. Also check the valve to piston clearance when at overlap with the cam you want to run. Do not go below 1mm/.040" at the minimum valve clearance point. More is better here, and the general consensus is that .060"/1.5mm gives a better margin of safety if you over-rev the engine.

Other possible option: If you have any shims between the cylinders and the case, you can remove those to increase the compression slightly.

To get a significant bump in CR you almost always have to use new pistons with different domes.

I also recommend you talk to William Knight, as he specs and builds racing engines. "Knightrace" here on the forum.

the piston to head clearance is .050'' at the step and .070'' on the dome with no shims. These cylinders take an 0 ring, and I guess you can include shims if needed in your build. My thought was to take .010'' off the cylinder to get the piston to head numbers a little better and .020'' out of the head to bring up compression. Problem with shortening the cylinder is you are reducing the CR. Valve to piston has been satisfied even with making the new mods. Chain slack issue is my present concern. Thanks, Bob

I am guessing there is not enough slack to take a link out of the chain?
Alan

I wouldn't recommend cutting that much-geometry is going to get weird.
Been there done that.
If you're intent on more compression just change pistons.

Your piston-to-head clearance of .050" will limit your fly cutting of the head to .020". That will reduce the step clearance to .030". If you do this, do not cut the cylinders even shorter. Calculate how much that will increase your CR, but my educated guess is it will only raise it 0.2 or less. More will require new pistons.

New pistons would be the more logical solution, but these are new and would like to work with what I have and it can run just as is and will probably serve my purposes. The flycut won't effect the step clearance, as the mating face of the head protrudes inside the cylinder wall by a couple of mm. That is the reason for the step. just the dome to crown clearance is altered. To shrink the step dimension the cylinder has to be cut. Was considering the possibility of cutting the head area just where the cylinder mates which leaves a couple of mm of head mating surface that will accomodate the step uncut. That then would close up both step and crown simultaneously.

r lane: Yes you re correct about the step, my mistake. I was confusing that with removing shims (what I did) and cutting the cylinders. :)

Anyway, If you are willing to spend the money on that machine work, including shortening the cylinders, that's about the most you can do, short of changing the pistons. BTW, shortening the cylinders will not "reduce the CR," that will increase it.

You could shorten the cylinder up to .020", which would give a step clearance of .030" which is as tight as I would want to go. That would also reduce the piston-to-head clearance from .070" to .050". Then you could flycut the head mating surface .020" to get that clearance down to .030" which again is as tight as I would go.

These cuts would move the cams .040"/1mm closer to the crank. Can your tensioner take up that additional slack? IME, in earlier engines, they can. I don't know about 3.6 engines. Check your tensioners before you take it apart, if you haven't already. Do they have plenty of room?

Re cutting just the mating surface, so that the remainder of the head actually sticks down past the top of the cylinder--I have not heard of anyone doing this. Maybe Neil or Henry have tried it. It would save machining the cylinders.

Edit: I just did a quick calculation of CR change. Shortening the cylinders .020" will increase the CR by about 0.6. That seems high to me, so check it.

If the valves have been sunk enough to lower the compression, you may have other issues affecting performance. I've understood that taking that much off the heads isn't normally recommended, but I'm just a DIY guy. Hopefully a builder can chime in.
If you need to reduce the piston to head clearance, personally I'd consider cutting the cylinders before the heads.

Do not forget, when lowering the cam centerline that much, you will probably have to machine the cam chain housings as well.

Sorry I didn't read every reply, only enough to know some weren't answering the question (although giving very helpful advice). I hear you can get fatter chain sprockets, which will deal with your chain slack issue. However you will need to shave the chain holders to make sure your cams sit in the middle of their openings.

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P.s. Dempsey's book mentions machining your con rod bushings long in order to increase compression ratio without adjusting the engine dimensions.

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