DECOMPRESSION PLATES
The difference between a decompression plate and a solid copper head gasket – is very little.  These two items are essentially the same (i.e. a metal version of the original head gasket), although the solid copper head gasket has extra locating holes machined for Ferriday Sealing Rings.

A solid copper head gasket is needed when a conventional head gasket is not strong enough for the amount of power a modified engine can produce.

A decompression plate is used -

• As well as a conventional head gasket to lower the compression ratio of a turbocharged engine or turbo/supercharged conversion (a 1.5mm thick decompression plate will lower the C.R. of an RS Turbo CVH to 7.4:1 without compromising piston strength).  Decomp plates for Zetec & ZVH turbo conversions are available, and for any other turbo conversion.  Available in a variety of thicknesses, we can help calculate your compression ratio and supply exhaust manifold adaptor plates (e.g. RS Turbo manifold onto Zetec head).

• To restore the piston to valve geometry or compression ratio of a heavily skimmed head (especially useful for rare multi-valve heads, Sierra Cosworths etc).

• A decomp plate simply becomes an extension of the block or head, and allows the head gasket to do its job in the normal manner. Ferriday Sealing Rings are not needed with decomp plates.

When using a Decompression Plate - only one head gasket must be used. All that is needed to seal the plate is a thin smear of high temperature non-setting sealant across the two metal surfaces only.  Let the conventional head gasket do its job in exactly the same way, as if the spacer plate wasn’t there.  Don’t forget that with OHC engines it may be necessary to use a vernier pulley to allow correct cam timing settings.  It is not necessary to anneal a decomp plate.

Aluminium-alloy or Copper? – The majority of engines now have aluminium cylinder heads, so the decompression plate can also be made from aluminium alloy.  The plate should be placed next to the aluminium head, so effectively becoming an extension of the alloy combustion chamber.  Customers often express concern as to whether an aluminium decompression plate is strong enough - both your pistons & cylinder head are made of aluminium alloy and perform their separate tasks without problems during normal engine running.  An alloy decomp plate will also perform well under normal conditions.  If an overheat occurs (localized or general) then all these components are in danger of destruction, but it is quite conceivable that the plate may fail before the pistons or head.  This should be regarded as a good thing as it is cheaper & easier to replace than a set of pistons or a cylinder head!  Another advantage with aluminium is that the end product is some 20% cheaper than copper.  As far as the decomp plate is concerned there is no limit to the amount of boost you can run, but pinking/pre-ignition, engine management limitations, piston strength, and head gasket strength will still be the limiting factors.

Some of these items can be ordered online - here.

Click here for a list of some decomp plates currently held on file.

Please e-mail or phone if you'd like any help in calculating compression ratios.  I've written a program to get an immediate list of new compression ratios for different plate thicknesses.  There are plenty of engines already on file, but for anything new all I need is the bore, stroke, current compression ratio, and the compression ratio that you'd like to achieve.  A complete table of options can then be e-mailed as an MSWord document.

If you're undertaking a turbo engine build and are struggling to get the compression ratio down to an acceptable level, or are unhappy with your combustion chamber design, you may consider having the combustion chambers CNC machined.  More details on the CNC Combustion Chambers page.

ZVH Engines.  The "ZVH" engine is a Ford Zetec bottom end (1.8 or 2.0 litre) with a Ford CVH head fitted.  To fit the head to the block there are 3 problems to overcome - 1) Compression ratio. 2) The redundant oil return passages in the Zetec block that need to be blanked off. 3) There is a waterway in the Zetec block that sits very close to an oil gallery in the CVH head.
A copper decompression plate will tackle all of these 3 problems in one go (assuming this is a turbo engine), and will allow easy fitment of block to head. If you have already sorted out issues 2 & 3, then a (cheaper) alloy plate can be used.