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Rollin' on 15's, baby!
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When they say do a 50 tho skim for the Mazda racing series what actual compression does that give?
Who is "they"?
 

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Rollin' on 15's, baby!
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no one seems to mention the actual compression there aiming for
Probably because no-one is aiming for a specific compression ratio given the relatively small increase over stock ratios afforded by skimming the head.

You can get a rough idea with some simple maths (volume of a cylinder is pi x radius x radius x height).

Bore is 78mm and stroke is 83.6mm, giving a swept volume of 399.47cc.

On manual transmission engine with 9.4:1 compression ratio the combustion chamber volume must therefore be 47.56cc

Decking the block by 50 thou (1.27mm) will reduce the combusion chamber volume by 6.07cc.

Compression ratio of a block decked by 50 thou will therefore be 10.63:1 on manual transmission engine.

If you skim 50 thou off the head then you will probably not take as much out of the combustion chamber volume due to the shape of the bowl in the head, therefore expect a compression ratio slightly less than 10.63:1. Sure fire way would be to CC the combustion chamber bowl in the head with a pipette, after skimming.

Do bear in mind that a 50 thou skim on a manual transmission engine (9.4:1) will make it an interference engine and that service life on an OE spec head gasket may be significantly reduced. Without vernier pulleys your cam timing will also be retarded but this can be beneficial up to a certain point.
 

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first its 30thou....then its talk of 40thou....now its a 50thou skim....any one running with this skim.... though a c/r of 10.6 to 1 sounds about as high as I`d want to go and still run on 95ron fuel....just making the engine more efficient and gain a few bhp and lb/ft.
 

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thats probably true...i would think upping the c/r any higher will need the ignition retarding to avoid pinking as the engine comes on cam..
but who is running a skimmed head and by how much, what were the results (dyno or butt dyno)
 

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Discussion Starter · #8 ·
Thanks niggle that's exactly what I'm after I'm roughly after 11 to 1 so il take 60 and with high octane and some cam re profiles should make a improvement, could u explain why the cam retard is a benefit Iv never thought much into cam timing and am interested to learn more
 

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Rollin' on 15's, baby!
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could u explain why the cam retard is a benefit Iv never thought much into cam timing and am interested to learn more
It is beneficial because it can increase the peak torque or move it to a more "convenient" point in the engine speed range. It's all about gas speed, inertia and volumetric efficiency. We are not talking about huge changes here because the changes in cam timing are very small.
 
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