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Re: engine compression

To: mgs@autox.team.net, ldove@macomb.com
Subject: Re: engine compression
From: jurrasm@genesis.torrco.com (Mark Jurras)
Date: Tue, 1 Oct 1996 15:16:22 -0400
> Reply-To: Lewis Dove <ldove@macomb.com>
> 
>  Here is what 
> I found, before and after squirting oil into the cylinders:
> cylinder #  compression (psi)before oil  compression after oil
>    1                  124                        133
>    2                  132                        135
>    3                  127                        131
>    4                  133                        137
> the plugs in cylinders 2 and 3 are a nice clean tan; the plugs in the 
> cylinders 1 and 4 are sooty and black. My conclusions are:
> All the pressures are low, but perhaps I have a bad gauge
> The rings are not sealing properly and are probably worn
> Nigel is a '65 MGB with a US spec. engine
>   What do you think?  

I do not know what the pressures should be but being that they are all
even is a good sign that you don't have a major failure. #1 may be a
little tired though. I just did a compression test Sunday with a
friend. He had 5 at 165 psi and one at 125 psi. Three squirts of oil in
the offending cylinder and it read 165. Anyone know what the pressures
are for the low and High compression engines? If not Boyles Gas law may
help P1V1/T1=P2V2/T2 Pressure and Temperature are absolute.

I am not sure you did this so here goes. When you perform the
compression check you need to watch the gage. The needle should rise on
the first compression stroke to near the maximum pressure and should
only bump up a little on the subsquent strokes. If the gage reading
pumps up with each stroke then the rings are suspect. Oiling the
cylinders should make the gage act normal. An engine that turns slowly
will not build the pressures. Also a higher performance cam usually has
greater valve timing overlap. This will also lower readings.

- -Mark

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