Vance,
Yes, you're absolutely correct. I did not include the head gasket thickness.
And .015" off of the cylinder volume will increase the CR more than .015" off
of the combution chamber volume.
However, my impression is that some listers believe raising compression to a
like degree can be accomplished by either decking the block or milling the
head. When an owner wants a TR engine bumped up to 9.5 CR there is no way that
this can be done just by decking the block. On this point we both agree.
I believe Kas's Competition Manual added 0.5 to the CR when the block is
properly decked.
Increased CR may be the result of decking the block, but is it not more to
insure a flat sealing surface, more similar CR's in all cylinders, as well as a
perpendicular surface for other machine work such as cylinder boring?
Otherwise it seems to be a whole lot more trouble than simply cutting the head
to raise compression if complete engine disassembly was not part of the plan.
Ideally, yes, both block and head should be cut to insure a good sealing
surface, especially with a higher compression. Again, though, there seemed to
be a basic misunderstanding about decking the block versus milling the head
with regard to raising compression.
Thanks for the clarification to my oversight.
Greg
>
> From: "Navarrette, Vance" <vance.navarrette@intel.com>
> Date: 2003/12/22 Mon PM 05:38:49 EST
> To: "Greg Dito" <dito9561@bellsouth.net>,
> "Timothy Holbrook"
> <tjh173@yahoo.com>, <6pack@autox.team.net>
> Subject: RE: Does 0.120" really get 9.5:1 compression?
>
> Greg:
>
> You are forgetting the head gasket, which is 0.035" or so thick.
> Common practice when modifying engines for power is to "0 deck" the
> block, i.e. decking the block so that the piston crowns are flush with the
> block deck. You then rely on the gasket thickness to provide clearance
> between the piston crown and the head.
> Obviously, if you are using some exotic gasket (steel, for example)
> then you need to make the appropriate changes. The hypothetical example you
> cite (assuming 0 head gasket thickness) would not leave enough clearance
> for thermal expansion, etc. The piston would contact the head.
> Because of squish, decking the block 0.015" will make more power
> than shaving the head 0.015", assuming equal compression ratios. Obviously
> the limit to raising compression via decking the block is piston to head
> clearance, and you can mill the head a great deal more than the block.
> But decking the block makes more power than milling the head by the same
> amount (assuming you have a squish area on the head).
> Obviously, milling the block deck insures a flat mating surface,
> but likewise milling the head provides a similar function for the head.
> It is equally valid to use these means to raise compression and power, and
> so one can kill two birds with one stone via decking/milling.
>
>
> Vance
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