Craig Bentley wrote:
>
> -----Original Message-----
> From: owner-triumphs@autox.team.net On Behalf Of Trevor Jordan
> Sent: Sunday, October 12, 1997 9:43 PM
> To: Egil Kvaleberg; triumphs@autox.team.net
> Subject: Re: tr6 head questions
>
> At 16:47 +1000 12/10/97, Egil Kvaleberg wrote:
>
> >
> >Problem with your theory is that you seem to get a higher pressure from
> >the compression reading than you get by multiplying the atmospheric
> >pressure by the compression ratio. For instance, my 10:1 compression
> >engine gives a reading of around 180 PSI.
> >
> There are at least two factors which would account for this discrepency.
> The compression of a gas will increase its temperature and therefore its
> pressure in a fixed volume. There will also be some heat transfer from the
> engine block if it is warm (above ambient) temperature.
>
> I do not know which is the more significant factor, but I would suspect the
> gas compression is more important than heat transfer (which would need more
> time). Compression testing a cold engine would eliminate the heat tranfer
> (in fact, there may be some heat transfer from the gas to the block) and
> should produce an result which is closer to that calculated from the
> compression ratio and local atmospheric pressure.
>
> Also keep in mind that a compression gauge reads pressure above atmospheric
> and not absolute pressure. So in the example above, gas (fuel/air mixture)
> at about 15psi (0+15) is drawn into the cylinder and compressed to about
> 195psi (180+15).
>
> If you want to measure a compression ratio, it would be best to use a
> liquid to measure the volume at BDC and TDC. (I am not suggesting that it
> would be easy, as leaks around the rings, etc could cause misleading
> measurements). Is there an easier method?
>
> Trevor Jordan
> 74 TR6 CF29281U
>
> [] Yes while were on the subject inquiring minds want to know, how does one
> go about measuring compession ratios. My engine is still in pieces and I would
> like to be somewhere in the neighborhood of 9.5:1 to 10:1 when finished
> Craig Bentley
Craig, if you engine is apart, then measure the volume of your cylinder
head cavities. Buy or borrow a measuring cylinder marked in cc's.Fill
the each cylinder cavity in turn to the top and take the average value.
You alrady know the displacement from the bore and stroke.
Compression ratio = displacent +volume of cavity / volume of cavity
Displacement = stroke x diameter of bore / 4
Stu
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