Hello all, I need suggestions.

I run a B21F with a K cam. After polishing the head's combustion chambers, I was planning to raise the compression ratio from my present 9.3:1 to 10.5:1. But shaving the head while still keeping the K would make the B21F very much "interference".

Would I generally be happier by continuing to use the K at 9.3:1 comp. ratio...or by raising the comp.ratio to 10.5:1 and installing an A cam?

I am all ears... Thanks!

Higher compression tends to reduce the effect of "wilder" camshafts by reducing the amount of residual exhuast gas. Neither of these cams is really wild though. Choose the compression you want for its own sake, and the cam you want for its own sake, and don't confuse the two.
Personally, with a B21F I would use lots of compression, the A cam, and the best fuel and most ignition advance I could get out of it.
The interference you speak of is only a concern if the belt breaks, and as most engines out there are interference types, why worry? Keep good cam timing components in the motor and drive it.

Thanks for your reply, Rhys. If you have the energy, maybe you can answer two more questions:

- What is the "A" cam's powerband?

- I calculate that, with pistons at TDC, my piston-to-valve clearance is about .472". The K cam has .471" lift int/exh. (I have been running it, advanced 4 degrees, for 20 k./kms without problems). I am assuming, from those calculations, that I cannot mill the head .050, while retaining the K.

Am I assuming right or am I missing something?

In any case, thanks again for your input. It is always very much appreciated!

What you are missing Pierre, is that at TDC the valves are not fully open, but merely at the overlap point, so they are barely off the seat. The valve only attains max lift for a short period, and when the piston is at midstroke. There is plenty of clearance to mill the head. If the belt breaks you will be just into interference, so don't break the belt, which is not too much to expect as so many cars - including all modern Volvos, are interference.
Iadr has some great info on the cams - he posts here and elsewhere. The A will give more midrange, the K more top end in general.

Rhys, as you may realise, I am trying to avoid having to do any more calculations...

I can live with having to keep an eye on the belt. Apart from that, may I take for granted that I can mill the head .050" or do I have more homework to do?

If need be, is the "figuring out" something a good machinist will do, if provided with the cam specs and others?

Thanks, thanks, thanks...

Just mill the head without concern for valve contact. They won't hit it. The turbo guys know how much you can mill off - Volvo says .5mm or .020 max, but many go further. Biggest problem is not valve contact, but that milling the head will retard the cam timing, which is rarely, if ever, a good idea.

I'll go ahead with .060'' off, for 10.8:1 CR. After a few espressos, I might even consider going for 11:1 by milling about .070'' off...

Thanks for your precious advice, Rhys.