Lowering the compression ratio

"Feeling under pressure?"

The compression ratio in short is the amount of air that an engine can squish ready for the bang phase of combustion. A 9:1 compression ratio simply means that 9 units of air will be compressed into the space of just 1 unit.

The compression ratio has a dramatic effect on an engines performance and the problem of knock, (where the air fuel mix prematurely ignites) is controlled to a large degree by the compression ratio.

Using higher octane fuel will also minimise the issues here as will adding water injection, but the real solution is simply to reduce the compression ratio.

To calculate your compression ratio you simply divide the swept volume (which will not change unless the engine is bored out and/or the crankshaft is replaced with one of longer throw) into the combustion chamber volume.

Compression ratio is worked out by dividing the volume above the piston when it is at TDC into the volume above the piston when it is at BDC.

If you are looking to supercharge (adding a turbo, supercharger or air compressor) an engine then you will find you are limited in the amount of boost you can add by the compression ratio. (* see note below)

The lower your compression ratio the bigger margin of error you have to play with which makes tuning much easier.

If you have a high compression ratio there is not much margin for error and detonation and knock are real problems.

We look at the best ways of lowering an engines compression ratio.

While you are reducing the compression ratio it would make sense to strengthen the internal engine parts.

This makes even more sense if you using forced induction to increase the power of your engine.

A handy formula to keep in mind is:-
CR=(swept volume+combustion chamber volume at TDC)/combustion chamber volume at TDC

*Don't make the mistake of thinking that compression ratios determine the maximum boost you can safely run. This is only a small part of the equation.

The critical thing is your fuelling, the fuel air mixture, and ignition timing are the key ingredients here.

A lower compression ratio will give you more of a margin for error and in the main allows you to run higher boost than you could otherwise.

We have heard of some engines running 15psi or more boost on a 10:1 compression ratio, and these usually have a fairly sophisticated engine management and fuel delivery system. (Recent advances in direct injection allows petrol engines to run at much higher compression ratios. This innovation came from the world of diesel engines which run extremely high compression ratios.)

A few notes when working out your final compression ratio. When you replace the head on your engine it will typically have needed to be skimmed and this increases the compression ratio, so will need to be taken into account with your calculations.

The thickness of a new gasket will also be slightly more than it will be when the head is torqued down onto it, so take a gasket thickness measurement from the old gasket.

Methods of reducing the compression ratio

Low compression pistons. This seems to be the way to go. The pistons are much shorter than conventional ones. A small plus is that they are also often lighter so the engine will rev a little more freely.

We would recommend combining low compression pistons with a shorter stroke to get the most benefit.

The shape of the piston crown will also have a bearing on the amount of compression that takes place in the engine.

This will require a strip down of the engine and whilst the engine is apart you may just as well perform some of the other mods listed below.

Shorter rods & reducing the stroke. A shorter stroke will have a dramatic effect on the compression ratio.

By combining this method with low compression pistons one can start to think about running very high boost pressures when adding a turbo.

The crank will also have some impact on the throw of the engine and the crank, piston crowns and rods should ideally all be matched up.

Head work, again increases the volume of the cylinder but the effectiveness depends a lot on how the intake and exhaust valves are sited, and how much space there is for you to work with.

Removing the head is relatively simple and does not require as much effort as other compression lowering mods. However, it requires great skill to do a proper job on the head and achieve the lower compression ratio you are seeking.

Thicker head gaskets. This option is a bit of a bodge, but we should mention it as a lot of people do run thicker gaskets to achieve a lower compression ratio.

We have also seen people using 2 gaskets (or more) to achieve a lower compression ratio! Using multiple gaskets is certainly not recommended and introduces a major weak spot in an engine.

A thicker gasket will reduce the compression ratio by a small fraction, probably only by .1 or .2.

This is by far the easiest method of reducing compression but the risk is that you are more prone to head gasket failure and the gains in lower compression are minimal.

Decompression plates are essentially an extension to the head and can be very effective at reducing the compression ratio.

The block side needs a conventional gasket seal but the head side generally only requires a non setting high temperature sealant (in the case of aluminium decompression plates).

Plates can be made of a variety of metals and we suggest you talk to a specialist about your options here.

The decompression plates may fail prematurely in high boost applications where high temperatures are involved.

Many view this as a good thing as replacing a decompression plate is a lot easier to do than replacing pistons and heads.

To discuss all aspects of engine tuning and car modification, or to get more information on reducing your engines compression ratio please join our friendly international car forums.

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Your Constructive comments on this article

6 Responses to “How to lower the engines compression ratio”

  1. TCJBOLDIE says:

    To get a 100% accurate CR in a turbo motor you need to add to the cylinder head volume the dish of the piston volume,HG gasket compressed thickness and the distance. the piston is below the top of the block at TDC IF the head has been resurfaced,chambers moded/polished then you will have to have the chambers cc’d to arrive at the real/actual CR.

  2. TCJBOLDIE says:

    IMO just install the correct pistons that will give the C/Ratio you need.
    There are so many makers of forged pistons out there so it is not too hard to get the CR you are wanting.
    Decompression plates and thick head gaskets are band aid fixes.

  3. Rodney says:

    I have a 302 with 9.5.1 compresson my head gasket it’s 0.40 thick I need the compresson to be in the 8.5.1 area what size gasket do I need to achieve this

  4. MIchel says:

    have some question , I have set of piston on order , they told me the piston make 9.14 compression with 92cc heads . if I wanted 10.5 compression in my engine , do I need lower cc cylinders heads or higher cc cylinder heads . because I have 73.5 cc heads on my engine , and I not wanted to have low compression because is build for street machine , wanted to have 400 to 450 hp .

  5. Eugene says:

    changing the compression can be made by a lot of factors or other way to lower if .as we all should know that cams are used to bleed off compression . as per a note must be taken when concidering this fact . example = a larger cam very much will change the compression . a small cam with same parts will read at a higher rate . but install a larger cam the compression will be less . in this regard one can and is often done in racing …by off seting the cam ( retardng or advanceing ). also one must remember in pistons a + on the piston is not what a person thinks. a + on a piston is lets call down in the hole . a – is really a taller piston at top. its n reverse as one thinks ( + = more – = less ) . in pistons think as this back wards . hope this helps some one in some little way . Hansen’s Racing

  6. Stuart Lang says:

    I have a 638 BBC has big chief heads and 15 to 1 compression. Could you change pistons to drop it to say 13 to 1 with out having to change any other major parts?

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