Compression ratio

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les
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Compression ratio

Postby les » Tue Jun 24, 2014 8:53 am

How would someone go about lowering an engines compression ratio, other than changing the pistons? Would it be practical to have a plate made to fit between the block and head? ( doubt there is one available) Any thoughts please.


bmcecosse
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Re: Compression ratio

Postby bmcecosse » Tue Jun 24, 2014 9:26 am

You could do that - but would not be my choice. I assume this is for a super/turbo charger application? Do you know what the CR is at the moment? And what level are you aiming at? Best way is to open up the combustion chambers to 'near' the gasket line - I have seen the 'peak' pretty much completely removed, leaving a 'bath tub' chamber shape. You could also skim the tops of the pistons -but I wouldn't take more than say 20 thou off them. However it would be an opportunity to make them all equal distance down from the block face -so do a trial assembly and then decide how much to take off each piston. You would then need to balance them for equal weight of course.....
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alexmcguffie
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Re: Compression ratio

Postby alexmcguffie » Tue Jun 24, 2014 9:30 am

I guess the first question is why? Second is what is your CR now and what do you want it to be?

The easiest way is to fit 2 head gaskets - far from an ideal solution although plenty have done it in the past! I believe at some point in the past thicker head gaskets were available, maybe try Mini specialists. If you are running a standard Minor head you could have the combustion chamber re-ported to that of a 12G295 head, that would gain you approx 5cc.

Burton Power supply 3mm and 3.5mm thick gaskets but these are only for 1275's and are £190 each!
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les
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Re: Compression ratio

Postby les » Tue Jun 24, 2014 12:02 pm

Thanks for the replies, at the moment cr is 8:1, ideal I think but have noticed the top piston ring should be a better fit thickness wise, therefor really needing replacing. It seems, at this stage, only HC pistons are available, so will be putting the cr too high for my needs. Ital 1275 unit. Bores are fine, and std, so it would be a shame to rebore. Crank is also standard and good. It's a disappointment for the sake of the top ring (4 ring piston) Another option maybe getting the top ring groove machined for a wider ring. The opening of the combustion chamber is a thought, wonder if the 'peak' has a purpose?


Tudge
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Re: Compression ratio

Postby Tudge » Tue Jun 24, 2014 12:22 pm

You can definately get compression lowering head gaskets. http://www.vmaxscart.co.uk/supercharger-parts/

Under 'decompression plate'... Pretty pricey though.
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bmcecosse
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Re: Compression ratio

Postby bmcecosse » Tue Jun 24, 2014 12:25 pm

You don't want to go below 8:1 !! Ideally you want 10:1 for an aspirated engine. Don't worry too much about the ring fit - if it's just reasonably standard engine. And if renewing the pistons - not worth doing without a rebore in my view - get the HC pistons! The 'peak is there to 'swirl' the incoming gases but also to reduce the chamber volume while still allowing the valves decent lift.
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les
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Re: Compression ratio

Postby les » Tue Jun 24, 2014 1:31 pm

There may be a misunderstanding here. The cr is 8:1 with the LC pistons I now have, if I can only get HC pistons as replacements I will need to restore the CR to 8:1. To my mind that is the preferred CR if the engine has a supercharger fitted. The slack in the top ring is more than the tolerance allowed unfortunately.


bmcecosse
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Re: Compression ratio

Postby bmcecosse » Tue Jun 24, 2014 2:08 pm

Ah right - so it IS for that supercharger project. Well indeed - just open out the chambers and take 20 thou off the piston crowns. Should do it nicely. And with forced induction - the 'swirl' etc much less important anyway.
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les
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Re: Compression ratio

Postby les » Tue Jun 24, 2014 4:19 pm

I suppose the beauty of fitting HC pistons and working the head is that if, in the future, it was returned to normally aspirated another head would bring it back to a decent CR. Oh well, anyone got a set of standard Marina pistons? :D


philthehill
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Re: Compression ratio

Postby philthehill » Tue Jun 24, 2014 7:11 pm

les
There is a set of Marina NOS Hepolite pistons on 'e' bay item No: 251503873463.


les
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Re: Compression ratio

Postby les » Tue Jun 24, 2014 7:40 pm

Thanks Phil but they're +20. The bores on this unit are remarkably good, so reluctant to get a rebore at this stage.


bmcecosse
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Re: Compression ratio

Postby bmcecosse » Tue Jun 24, 2014 10:39 pm

They are also flat top - so v high compression ! :cry:
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