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jerrytr5

Compression Ratio again

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OK, there probably isn't a definitive answer as trawling through the Forum seems to indicate but I'll ask the question anyway.

When I get around to putting my engine back together, what CR should I be aiming at?

Six pot, 2.5 with a CP camshaft. +20 overbore running EFI. Standard head & exhaust. Standard pump fuel. Road and the occasional trackday use.

The answer will be between 9.5:1 and 11:1 but that's quite a range which I'd like to narrow down

Jerry

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Jerry, It depends upon the octane of the 'standard' pump fuel you plan to use. For 97 RON I work on a basis of 10.5:1 as the limit (learned from posts on here). There is no simple relationship between onset of knock and octane. If you tried to run 10.5:1 on 92 RON it might be possible to cure knock with a richer mixture at full throttle and maybe some spark retard. But there are limits to that trick and it might not work without killing power significantly, and you'd have to use 97RON.

Peter

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10.5:1 from me too.

 

It's just below the limit, I think, when 11 would be too close or over. My present engine was built to that, and I first ran it on some old fuel left over in the tank - and it knocked! Disaster!

But a tankful of new fuel stopped it, for good.

 

I don't know how much the octane rating of fuel can drop with storage, but the difference was obvious.

John

Edited by john.r.davies

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If the only departure from standard is a 20 thou overbore I'm struggling to see how you can aim for any thing other than whatever that mod gives you, not much over standard I should think. Unless I misread the question.

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Maybe Jerry is planning to shave the head to bring it to the desired c/ratio.

Table allowing for rebores and so forth here:

http://www.goodparts.com/tech_docs/TR6_Compression_Ratio.html

20thou takes a standard thickness head from 9.5 to 9.6

Peter

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What difference would the crank and rods make to the target CR?

 

A CP cam and an essentially standard head (bar skimming to set CR) implies that the planned engine isn't intended to be a high revving, high output screamer so an all steel bottom end is probably not going to be on the cards.

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Pete, I was planning on a dummy build and measure to determine if I need to machine anything to achieve the desired CR. 'Standard head' was meant to imply unworked or flowed apart from a little port matching. Usually when I build an engine I just put it back the way it came apart, but as I have a different block thought I might attempt a little more science on this occasion.

Thanks all.

Jerry

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10.5:1 from me too.

 

It's just below the limit, I think, when 11 would be too close or over. My present engine was built to that, and I first ran it on some old fuel left over in the tank - and it knocked! Disaster!

But a tankful of new fuel stopped it, for good.

 

I don't know how much the octane rating of fuel can drop with storage, but the difference was obvious.

John

John, It will I think be complex, especially as we dont know what is in the fuel originally.

The possibilities I can come up with are:

- If 5 % ethanol was all oxidised or partitioned into a water layer then 97RON would only fall to 96 ( http://newenergyandfuel.com/wp-content/uploads/2012/04/Ethanols-Impact-on-RON-Octane-Ratings-in-Gasoline.jpg )

- The aromatics that are the main anti-knock components, but these are relatively resistant to auto-oxidation so that may not be important (more here:

http://cdn.intechopen.com/pdfs-wm/47914.pdf )

- I think the BS allows octane to be boosted using gases such as iso-butane. If that evaporates during storage the RON might fall, but we dont know by how much without buying the BS we have no idea how much is allowed. Martin ( Clarkey) might have that information.

 

Peter

Edited by Peter Cobbold

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Pete, I was planning on a dummy build and measure to determine if I need to machine anything to achieve the desired CR. 'Standard head' was meant to imply unworked or flowed apart from a little port matching. Usually when I build an engine I just put it back the way it came apart, but as I have a different block thought I might attempt a little more science on this occasion.

Thanks all.

Jerry

Burette method is best method of measuring the combustion chamber volume and hence CRatio.

I expect you know, but there are two different block designs with different head gaskets.

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Apart from buretting, what would you measure in a dummy build to inform on CR?

John

 

Calibrated Hypodermic syringe from the chemist shop?

link https://www.medisupplies.co.uk/Syringes-Needles/Syringes

 

Peter W

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Motored engine, in-cylinder pressure. But needs special kit.

Peter

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Apart from buretting, what would you measure in a dummy build to inform on CR?

John

Deck height is the other factor that I need as well as combustion chamber volume.

Jerry

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Calibrated Hypodermic syringe from the chemist shop?

link https://www.medisupplies.co.uk/Syringes-Needles/Syringes

 

Peter W

Did you read this?

http://www.mintylamb.co.uk/?page=measurecc.htm

 

Peter W

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When were you planning on reading post 5 from the Prof which references the website from Richard Goode which gives you all the information you need and is available from us to turn out the head and "deck" height for whatever compression ratio you want ?

 

Mick Richards

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Hadn't seen that particular one Pete. I've borrowed a burette that's more like a yard of ale glass. Wondering what to fill it with in the absence of liquid paraffin. White Spirit?

Jerry

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Hi Mick, I've printed all that off for reference. Surely it is just that, reference, and in order to get an accurate figure for my circumstances I actually need to take measurements. Who knows how high up the bores a new set of pistons will move?

Jerry

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Hadn't seen that particular one Pete. I've borrowed a burette that's more like a yard of ale glass. Wondering what to fill it with in the absence of liquid paraffin. White Spirit?

Jerry

Water is fine. Seal the valves with smear of grease, spark plug inserted. Flat glass or perspex plate covering most of the chamber, again lightly greased to seal.

Peter

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Hi Mick, I've printed all that off for reference. Surely it is just that, reference, and in order to get an accurate figure for my circumstances I actually need to take measurements. Who knows how high up the bores a new set of pistons will move?

Jerry

 

Sorry Jerry, I'd missed your reply to Peter, as you say you need to measure to confirm what you need to do for the actual deck height and what needs to be done to change it.

 

Mick Richards

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Tables, or formulae to calculate CR from the height of a head face shave, are intrinsically unreliable and misleading, as they ignore any changes made to the chamber to improve flow, or previous work done on the head.

Buretting the head is the Gold and only standard (!) for this work, especially as it allows small differences in chamber volume to across the head to be detected and corrected.

 

May I humbly offer my own description of the process: http://www.totallytriumph.net/spitfire/skimming_your_head.shtml

Ignore the "Script Error" message and scroll down to read the whole thing.

 

OR, see TRaction, May 2015.

 

John

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