Decompression plates

A decompression plate is a solid metal copy of a head gasket, and can be used as well as one conventional head gasket to lower the compression ratio of a turbocharged or supercharged engine without compromising piston strength, and/or to restore the piston to valve geometry or compression ratio of a heavily skimmed head (especially useful for rare multi-valve heads & very old castings).

Decomp plates are available for any engine in a variety of thicknesses, we can also help calculate your compression ratio, and can make exhaust manifold adaptor plates to help with the fitment of a non-standard inlet or exhaust manifold (see bottom of page).

A decompression plate is not to be confused with a head gasket, it is an entirely different component that is incapable of sealing combustion pressure by itself.  Therefore it can only operate when used with one OE type gasket.
Often made from aluminium alloy, the plate simply becomes an extension of the block or head, and allows the head gasket to do its job in the normal manner.

As a very rough guide, an OE type gasket is often capable of handling more than twice the original power output.  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 whole area of the plate between the 2 metal surfaces only, let the conventional head gasket do its job in exactly the same way, as if the plate wasn’t there.  In my opinion, the use two head gaskets is a bad move – each gasket is designed to crush by a certain amount as the head bolts are tightened, and as you’ll still only have one set of bolts, the crush is likely to be insufficient.  An OE type head gasket is different thicknesses at different points to create different clamping loads, the greatest of which is around the bores to contain combustion pressure.  These clamping loads are simply transmitted through the decomp plate to seal on the other side, with the help of a tiny amount of sealant.

Please note that the thicker the plate, the less directly those forces are transmitted through to the other side – so thinner is nearly always better.  As a guide, a 2.0mm aluminium alloy plate is the maximum that should be considered.

Surface flatness & finish of both the head & block is very important. Always resurface the cylinder head, perhaps the block as well, always use an aluminium plate next to the aluminium cylinder head, always use a copper plate against the flattest surface.    Metal to metal joints are very unlikely to seal completely by themselves so a soft non-setting, high temperature sealant must always be used.

Don’t forget that with OHC engines it may be necessary to use a vernier pulley to allow correct cam timing settings as the original crank to cam geometry will have changed.

A list of drawings on file is here.

It is not necessary to anneal a decomp plate.

Aluminium-alloy or Copper? – The majority of engines now have aluminium cylinder heads (some have alloy blocks as well), so the decompression plate can also be made from aluminium alloy.  The alloy plate (& sealant) should be placed next to the aluminium head, so that the plate effectively becoming an extension of the alloy combustion chamber.  Customers sometimes ask whether an aluminium decompression plate is strong enough – both the 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, and is probably one of the strongest components inside the engine.  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 considerably cheaper than the copper version.  When block & head are both made from cast iron, the most suitable plate material is copper.

You can see more info 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.
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 thin 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.

Manifold adaptor plates – If you want to use a different inlet or exhaust manifold on your engine, then an adaptor plate will allow easy fitment.  We have many drawings on file and most combinations can be catered for, and produced within a few days.  Just find an easily available turbo exhaust manifold from an engine that has similar port shapes & spacing, send both manifold gaskets, and I’ll make a plate that will get you closer to a turbo conversion without the need for a custom manifold.  Consider turning the exhaust manifold upside down – this can sometimes make things much more accessible.  More info here.

Mike Tanski. © – Copyright Mike Tanski, Ferriday Engineering – not to be reproduced without permission.