What method you select to reduce the compression ratio depends on how much
you want to reduce it and how hard you intend to drive the car.
The traditional cure is a thicker gasket of the normal sandwich type, or
two standard gaskets, or two gaskets with a solid steel or copper spacer
in between. Sometimes these work reliably, but the probability of leakage
and/or blowing a head gasket increases as the number and/or thickness of
the layers of gasketing increases. I wouldn't do this unless the car is
always driven gently.
A solid copper gasket such as you described is more reliable IF conditions
are right. It is not as compressable as a standard sandwich-construction
gasket, so is not as forgiving of imperfect block and head surfaces. If
you go this route, have a machinist verify that the block and head
surfaces are flat enough. Otherwise you'll have sealing problems. Good
solution for a hard-driven car, but only if done very carefully.
Grinding out the combustion chamber to increase volume avoids gasket
issues, and gives you an opportunity to improve the chamber shape as well.
If you're doing it yourself, or if the machinist doing it isn't familiar
with the flow characteristics of the XPAG head, concentrate on unshrouding
the valves (creating more space around them) BUT be careful not to cut
past the inner edge of the gasket. A side/face cutter piloted in the
valve guide will give you better control and uniformity than a hand-held
grinder. If you need to gain more volume than that will give you, go to
someone who knows the XPAG head intimately, not just any old "speed shop".
Good solution, but expensive if your goal is just reduced compression
rather than increased high-rpm performance.
Reducing the compression height of the pistons will also reduce combustion
chamber volume. This is relatively inexpensive (compared to combustion
chamber reshaping), but nowhere near as inexpensive as a thick gasket.
It has the advantage of not being permanent, in that you can always refit
standard pistons, but you can't return reshaped combustion chambers to
original if the job is botched. It is also more reliable than a thick
gasket, if not carried to extremes. The only limitation is in how much
you can safely remove from the piston crown before it becomes too thin.
That will vary from piston to piston, and there is no way to know until
you try it. However the amount you want in this case (.085 minus the
compressed thickness of a standard gasket) shouldn't be a problem.
On Fri, 23 Jun 2000, lbprince wrote:
> The XPEG head on my TF 1500 has been shaved too many times. With the
> static timing set at TDC, it is on the edge of pinging even when premium
> fuel is used. By interpolating the stage tuning data, the compression ratio
> is around 9.6. Readings on a compression gauge are between 175 and 180 psi
> on all cylinders. The engine was recently overhauled. Octane boosters seem
> to help, but they are expensive and a hassle to fiddle with.
>
> To reduce the compression ratio to 9.3, my calculations indicate that a
> .085" gasket " (compressed) should be used. The Gasket Works in California
> makes copper gaskets that are this desired thickness. I am told, however,
> that copper gaskets tend to leak unless special precautions are taken. Is
> this true? What are the special precautions?
>
> There are other options like shaved pistons and increasing combustion
> chamber volume. Are these other choices advisable?
--
Chip Old 1948 M.G. TC TC6710 XPAG7430 NEMGTR #2271
Cub Hill, Maryland 1962 Triumph TR4 CT3154LO CT3479E
fold@bcpl.net
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