Listers,
I follow this thread with interest. The reason we shave cyl heads and port
match combustion chambers is to equalize pressures and then raise them.
Seat of pants will tell you if you have a problem, testing will explain it.
A CONSISTENT reading is most desirable, whatever the number. For eg, lo
comp TR6 motors run about 130 psi, fresh. So do spitfire 1500's in US trim.
A generaq rule is take your CR and multiply it by 15 (roughly atmospheric
pressure). Thus a 10:1 motor "should" produce about 150 psi. Of course,
there is always carbon buildups, etc that actually RAISE compression, they
may do so unevenly - as where one plug has been oiling due to a bad guide or
ring. This cyl in question will then read high.
An italian tuneup may fix such problems. FAiling that, the rule I use is
no more than 10 percent of the median reading as a total variation before I
look further (ie: on the 120 readings, I'd go +/- 6 psi for a range of 126
to 114. Obviously, there is a problem in the motor more serious than a bit
of carbon. What, I do not know, but my next trick would be a leakdown test:
Take you air line and feed the cylinders while said cyl is at TDC. (use the
end of your compression tester if it has a quick release hose- mine does -
o/w brasie an airline fitting to a hollowed out sparking plug.
Some leakage is normal. Air sounds from the rad (cap off) is head gasket,
tailpipe is ex. v, carbs is intake v,. rocker cover is rings (general) or
valve guides (specific, remove v cover and listen ata point).
Good luck all
Dave
-----Original Message-----
From: Martin Hargreaves <martin@datamodl.demon.co.uk>
To: photoray@flash.net <photoray@flash.net>; spitfires@Autox.Team.Net
<spitfires@Autox.Team.Net>; carbuff@nac.net <carbuff@nac.net>
Date: October 24, 1998 12:59 PM
Subject: Re: Spitfire compression
>
>> At 09:06 AM 10/23/98 -0500, Ray Bryant wrote:
>> >I just did a compression test at the urging of a friend. I didn't want
to
>> >do it for fear of what I might find.
>> >Here are the results:
>> >#1 - 118 pounds
>> >#2 - 90 pounds
>> >#3 - 78 pounds
>> >#4 - 117 pounds
>> >This was done with the distributor center wire disconected so that it
would
>> >not start.
>> >Was this the correct way to test?
>>
>> Did you have the throttle open fully when you took those readings? This
is
>> more accurate.
>
>I recently did one on my UK MkIII (1296 cc) and got:
>
>#1 - 163
>#2 - 161
>#3 - 173
>#4 - 163
>
>Throttle open, dist. disconnected at coil. Any ideas why #3 should be
higher? #2 oils its plug
>pretty efficiently but looks OK from this, yes? Are these the right sort of
number, they seem
>quite a lot higher than Ray's figures?
>
>Cheers,
>
>Martin.
>
>'70 Spitfire MkIII
>'86 Jaguar XJ-S V12
>
>
>###################################################################
># Martin Hargreaves (Director/Consultant) #
># Datamodel Ltd - Open Systems Management and Security #
># martin@datamodl.demon.co.uk http://www.datamodl.demon.co.uk #
>###################################################################
>
|