Index Home About Blog
From: Dave Baker
Subject: Re: Compression, Displacement and Volumetric Efficiency
Date: 09 Dec 1999 21:12:48 GMT

>Subject: Re: Compression, Displacement and Volumetric Efficiency
>From: Kevin Mouton

>May I address the carbon deposit question in a real world manner. This
>is the one thing that actually does have the most profound affect on
>compression readings on most older vehicles. I have taken compression
>readings on engines that were mildly carboned up which produced readings
>of as high as 210 psi. Then after a chemical decarbonization rechecked
>the compression and obtained readings as low as 110 psi.

I am sorry but I find this hard to credit. I have stripped literally hundreds
of engines over the years and I routinely measure chamber volumes as part of
the process of blueprinting race and road engines. Measuring the chambers
straight off the block complete with carbon and then again after chemical
cleaning shows that even high mileage engines accumulate a relatively small
volume of carbon. Much more than 1cc is rare.

For "decarbonising" to halve the cranking compression reading implies that the
chamber volume doubled - in other words half the volume was being taken up by
deposits. An average engine with 500cc swept volume per cylinder and say 9.5:1
cr has a chamber volume of 59cc. To halve this would mean nearly 30cc of
deposits which would entail a layer of carbon 1cm thick over everything in
there. This doesn't happen - deposits above a certain thickness tend to break
or burn off and disappear - only on exhaust valve heads do deposits tend to
reach any appreciable thickness as the high temps really coat the stuff on with
a vengeance. 20 thou though is still about as thick as it tends to get.

I don't always know the mileage of the engines sent to me for rebuild but I
frequently do. I worked on a known 140,000 mile Golf Gti head recently and
engines in the 80,000 to 100,000 mile region are common. Even high mileage ones
don't have appreciably thicker deposits than low mileage engines.

I can envisage (but only just) the occasional situation where a really sick
engine, burning lots of oil, is run under very light throttle for a long period
and builds up abnormally thick deposits. I can guarantee though that one good
thrash at high rpm would blow most of it out again. You should see what comes
out the exhaust pipe the first time a high mileage car gets a good caning on
the rolling road (chassis dyno for you linguistically challenged colonials
<g>). All I can say is, don't stand behind one wearing white !!

> Regardless of the math involved you will never approach the
>formulated results for an engine during an actual field compression
>test. Naturally, all things being equal, the higher the designed
>compression ratio of an engine, the higher the compression test readings
>will be, but in the real world all things are never equal.

Yes you will - engines don't manage to ignore the rules of physics any more
than any other device. If the CR and cam duration/timing are known, the
compression reading can be calculated with considerable acuracy. Some of the
recent posts have come close but none have shown the full story. I'll post the
complete maths on the web site and a copy on here when time allows.

Dave Baker at Puma Race Engines (London - England)  - specialist cylinder head
work, flow development and engine blueprinting. Web page at

Index Home About Blog