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Compression Ratio again


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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

Thank you John that is a good article.

 

So, will TR Gold to be used for Technical articles too?

 

Peter W

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The other serious problem with the using the head height is that is does not account for valve height in the chamber - which can vary and has a big effect.

 

My opinion is that 10.5:1 is a bit high for a CP cam if you want to able to run 95 octane fuel. With EFI and 3D ignition it is possible, but leaves no margin for hot weather and stale fuel - both of which occur in the real world.

 

I ran my PI saloon with 10.25:1 and the "132" cam (admittedly needing lower CR than the CP cam) and that was on the edge with 95 fuel and would pink in hot weather or with some brands of fuel.

 

My Vitesse is nearer 11:1 but has a longer duration cam and the 2L head has more squish area which helps too.

 

Nick

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I had already picked up on your article in Totally Triumph John, and very fine it is too.

Thanks for that info Nick. I was coming to the conclusion that I should probably be aiming for something less than 10.5 as both Peter and John's suggestions I think assume 97 RON (unless I misread that). I usually fill the TR about 1 in 3 tankfuls with a premium fuel, otherwise she gets 95 RON supermarket grade.

Jerry

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Hi Jerry

 

10.5 is as I said as far as you want to go. If you deck the block, blueprint the crank and rods and balance everything, it will run on supermarket junk fuel definately with EFI both of mine non-EFI have run like that for years with no pinking.

 

Cheers

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it all depends on yer rings too.

if its a new engine, an no bedded in correctly,

then it,ll use oil,

 

if its an olde engine, rings worn,bore worn, it,ll use oil

 

Ol will give seveere nok in yer engine

even at lowish comp ratios

as oil is lowering the octane rating of the fuel

 

M

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