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Could the low compression ratio be a problem?

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Hello everyone,

I've tried looking for a similar question on the forum but couldn't find it.

I am building a 5 cylinder Fiat turbo engine with wiseco pistons, which give a compression ratio of 7:1

Can this low compression cause me any problems more than benefits?

I have to mount a Garret GT2871R turbo with the intention of being close to 400cv

greetings and thanks

It shouldn't be a 'problem' as such, but it will mean you're missing out on potential performance because the overall efficiency will be reduced.

The main areas it will show will be off boost, when there will be less torque than may be expected.

On the positive side, you will be able to use a lower octane fuel and/or run more boost.

It is a very low CR, you may be able to remedy it in part by using longer rods, if it's a low deck height on th4e pistons, skimming of the head and/or block, or possibly thinner head gaskets - but those will depend on the engine design?

The other downside of extra low CR is very poor turbo spool... It becomes more noticeable with bigger size of turbos installed...

Hi, thanks for the answers.

I will put a 1.8mm Spesso brand head gasket, I do not want to make it thinner since the piston protrudes 0.85mm from the block, and the combustion chamber of the cylinder head is not hemispherical, so the piston cannot "enter" into it and needs a minimum tolerance, for my taste 1mm (surface of the cylinder head with respect to the piston head).

That's why I can't remove material to the block surface either.

The only option I have is to shave material from the flat of the cylinder head.

I would really like an 8:1 compression...

I'll see what I can do.

thank you

I have seen an instance where the piston top was machined to relieve the head clearance so the block could be machined with the pistons protruding further - but that was by an experienced machinist using a CNC machine programmed for the chamber shape, and may be a bit over-board for you.

Several piston manufacturers will do custom machined piston domes/reliefs from moulds taken of the head chambers - not much help if you already have them, but may be worth keeping in mind for next time, if required.

If you're skimming the head, don't forget that the valve to piston clearances will also close up and need re-checking, as will valve timing, if you aren't able to adjust it.


Yes, next time I'll be more sure of the material they bring me.

The problem was that we asked for some forged pistons and connecting rods and they brought us these, without paying attention to the compression that was left... and it's been a while since I can't return them...

What compression ratio is used to leave in prepared turbo engines?

greetings and thanks


Returning to the subject, after lowering the stock a little more (0.6mm) I have a theoretical Compression Ratio of 7.2:1.

Original specs are 8'5:1

What is your opinion?

maybe I can compensate with the adjustment of the cams?

greetings and thanks

I think you'll need about 18% more boost with 7.2:1 compression pistons to make the same power as 8.5:1 pistons.

¿Pero también es cierto que en algunas preparaciones se prefiere bajar la compresión para tener más presión de turbo?

No tengo otras opciones para aumentar la compresión, y no tengo experiencia de que esto pueda hacer que salga mal.

I mean, I don't know if it's really going to cause problems.


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