John, (and others)
I know about the normal way of calculating compression ratios, and I
know it takes a fair bit of carefull measurements to get anything like
a corect measurement as I have done it that way in the past
That is exactly why I asked if using a measured compression test could
be used to give a compression ratio.
Think about it this way: piston at bottom of stroke, valves open and
cylinder full of air which connects to outside through open carb
venturi, so it is at 14.5 lbs per sq inch pressure (at sea level)
Piston rises up the bore and produces a pressure (as read on the
compression guage) of 200 lbs per square inch, so Ratio is 200 to 14.5
= 13.8 to 1
Seems high. The question was could this produce the blue smoke
problem that I am experiencing on a rebored engine (+ 30 thou) new
pistons etc and 3000 miles use.
Guy
----- Original Message -----
From "John Holliday" <fprodget at yahoo.com>
To: "Guy Weller" <guy@weller-lakes.freeserve.co.uk>;
<spridgets@autox.team.net>
Sent: 24 July 2001 15:41
Subject: Re: Remember Blue smoke?
> Guy,
>
> I have never heard of that computation for a
> compression ratio. But I do know that 13.8 to 1 would
> require a fuel octane of 104 or so.
>
> Compression ratio is the comparison of total volume of
> the area in your cylinder and combustion chamber of
> the head at bottom dead center verses the volume at
> top dead center. more technically described, here is
> the formula from the Avonbar Racing Site:
>
> Compression ratio
> CR = Swept Volume + Total Combustion Chamber Volume
> Total Combustion Chamber Volume
>
> Swept volume = Bore x Stroke (pr2 H)
> Total Combustion Chamber Volume = Cylinder head
> Chamber Volume + Head gasket volume + Piston crown
> volume at T.D.C. to Cylinder block face.
>
> Example:
> Swept Volume 249.63cc
> Chamber Volume 24.2cc
> Head Gasket Volume 3.4cc
> Piston to block face volume 2.2cc
>
> 249.63 + 30.00cc 30.00cc = 9.32:1
> Total Combustion Chamber Volume required for a given
> CR.
>
> = Swept Volume CR. - 1
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