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Hi,
Any thoughts on a "safe" amount of material that can be removed from the cyl heads on the 2.5L engine in order to increase the CR ? According to my calculations removing 1.0mm will increase the CR from the standard 9.0:1 to 9.53.
Removing 1.4mm brings the CR to 9.77.
Removing 1.8mm gets the CR to 10.03.
Removing more than 2.0mm will result in almost no aluminium from the cyl head being "below" the valve seat.
Regarding the valve to piston clearance I intend to stay with the standard camshafts. Anyone know if this clearance will be a problem ??

Thanks,
Brian.

Brian16
This is what I have in my collection of Alfa Busso knowledge .Don't remember where it comes from or who wrote it, but it seems to make sense.


"Everything shows that machining .020" off an Alfa head will result in a .4:1 compression ratio increase. I have run heads with .070" machined off.

Split cam timing works well, improves torque. Heavy springs on the intake valves and stock on the exhausts works up to about 7-7200 rpm.

I would try to use a cam that does not have more that 290 degrees duration (seat to seat), or 260 degrees at .050" lift for a hot street engine. That would improve the torque, and it keeps the rpm down, and the engines last longer.

A light aluminum flywheel is worth every penny that you spend.



I would agree with George regarding the increase in compression ratio. With my race engine, I measured a .25 point increase in compression for each .010" cut off the head. This is pretty consistent with his 4/10 for .020". I would also recommend checking clearances, though. For piston to head clearance, an easy way is to put strips of clay on the #1 piston, then fit the head (with an old gasket). Just leave the cams out. Rotate the crank through top dead center. Remove the head and measure the thickness of the clay strips. I've been told .050" is the bare minimum. You can then check the clearance on the valves. With the head fitted and the cams timed, put some heavy rosin core solder through the spark plug hole and into the cam pocket on top of the piston. Rotate the engine carefully through the interference position. If you feel some resistance, that is the solder being compressed by the valve. Pull the solder out and measure the thickness where it was compressed. I'd want .090" or so minimum on a street engine.

With these measurements, you can figure out exactly how much you want to cut the head. On my race engine, I will be running the Spruell 85mm pistons. I was surprised that I only need to cut the head .030" to get to 12.8:1 compression. And with this set up, clearances aren't an issue even with a 12.5mm long duration intake cam.

By the way, the "how to powertune" book has a great write up on how to calculate compression and measure clearances.

Hi Murray,
Great, thanks for the info.

Brian.