Hi Randy,
I have Aussie Cleveland heads on Isabel, "302C" they are affectionately called (even though they were cast in Geelong, AU). I understand the Aussie 351C heads are the same in configuration as USA C heads.
Okay, so here is the scoop on the 302C heads. They are just like US 2V heads in every way except the (quench) combustion chambers are the smallest of any C heads, mine CC'ed at 58cc. Stock 351C 2V are in the 74-76cc range.
So what you get by using a pair is compression, too much compression in fact for pump premium. Mine were a true 10.5:1 (which would be like 11.5:1 advertised, read on). I used custom Cometics MLS gaskets made in .066" thickness in order to reduce the compression to a true 10:1 measured using a Katech Whistler (
http://www.katechengines.com/motor-sports/services/whistler/http://www.katechengines.com if you would like to know more about that). True 10:1 is on the bleeding edge of what today's pump premium with ethanol can power.
While I was at it I measured a pair of '70 4V closed chamber heads with the Whistler, they came up at 10:1. The advertised specification is 11:1 so that tells you something about advertised CR values.
A 2V 351C in '70 was advertised as 9.5:1, in reality it was probably more like 8.5:1.
The best way to do a true performance engine using the 302C heads is to use dished pistons of the right size to get a manageable CR (unless you are building a race engine in which case they'll work well with flat tops or a slight pop-up). What I did using thicker gaskets is not the best way as it increases the "squish" area (the area/distance between the top of the flat part of the piston and the flat part of the head deck surface on a quench chambered head). Too much squish hurts the flame propagation characteristics that closed chamber heads are desired for. Too much squish can cause detonation problems. I have managed to get away with it since I only increased squish .025" over stock and I have tailored my ignition timing very carefully (see my recent installment in the ECI thread). In a true performance engine you would want to zero deck the block (my pistons are ~.040" down the hole in an undecked block), use a stock gasket and make the desired CR with a piston of the right volume (dish or dome) to get the desired CR. Building an engine this way yields the best flame propagation characteristics the quench style chamber can deliver.
How's that Randy? Probably more than you wanted to know! If you have any questions either post up or I'll give you a call to discuss.
Hope this helps.
Regards,
Bob