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Re: Compression Ratios

To: Chris Kantarjiev <cak@dimebank.com>
Subject: Re: Compression Ratios
From: Trevor Boicey <tboicey@brit.ca>
Date: Fri, 27 Mar 1998 01:57:04 -0500
Chris Kantarjiev wrote:
> Someone is almost certain to say "oh, 1 atmospher is 14.7 psi,
> so just divide by 14.7 and you get CR". That's soundly wrong;
> the air gets heated as it gets compressed, so it's not a simple
> calculation at all.

  Not to mention that this relies on a cam that is perfectly
tuned for "starter cranking speed" which of course never happens.

  Very hot cams can give really low compression readings because
of overlap and latency. The intake valve is still open when the
piston starts rising, which works great at 7000rpm but at low
speeds like compression test rpms, it pushes a lot of the
air back out the intake before the valve closes. This makes any
attempt at relating it to CR impossible.

-- 
Trevor Boicey, Ottawa, Canada.
tboicey@brit.ca, http://www.brit.ca/~tboicey/
[ Seeking some miscellaneous MG parts, see the list on the web page... ]

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