Piston deck clearance & ignition advance

[carbon]

Does anyone have experience or reliable info about effects of tightening up piston to deck clearance on large bore A-series engines, and specifically how this alters ignition advance curve?

Two related questions :
- what is closest piston-head clearance that can safely be used?
- is there a reference list of compressed cylinder head gasket heights?

Many thanks in advance.

[ianh1968]

- is there a reference list of compressed cylinder head gasket heights?

Not that I've seen, but they generally quote the compressed volume...

Try this:

Code: Select all

Thickness in cm = Stated Compressed Volume (in cc)
                  ------------------------
                  ((7.4/2) x (7.4/2)) x Pi (area of the circle)

(Assuming gasket hole diameter to be 74mm/7.4cm)

The "BK450" is stated as having 4cc volume.
So we have:

Thickness = 4 / (((7.4/2) x (7.4/2)) x Pi )
Thickness = 0.093cm = 0.93mm = 0.037"

Ian

[Spider]

As Ian has said, the compressed thicknesses of most gaskets is around the same, I've measured them from 0.036" to 0.042".

I've built engine having the Pistons protrude by up to 0.012" above deck height, but also kept the Compression Ratio in check, as as is appropriate for the combination of the build.

By getting the piston closer to the Beak in the Head at TDC the 'Squish is increased and so it's one final 'opportunity' (and the last!) to keep the fuel mixture in an atomised state prior to ignition (which starts as we know prior to TDC).

As far as Advance requirements go, not an easy one to answer, but none the less, CR for CR (ie Piston high or lower), it won't change the requirements radically.

[carbon]

Ian & Spider,

Thanks for the comments, both are helpful