So if you build an engine with the high compression pistons and the high
compresson head what is the resulting compression ratio?
Larry Hoy
>-----Original Message-----
>From: owner-mgs@autox.team.net [mailto:owner-mgs@autox.team.net]On
>Behalf Of John DiFede
>Sent: Monday, January 29, 2001 8:47 AM
>To: mgs@autox.team.net
>Subject: Ideal Advance Curve
>
>
>Rick wrote:
> All of this brings me to a few conclusions regarding my
>1970 MGB-GT.
>
>1. I need to know if my car is high or low compression. The
> piston crowns in my '70 B-GT, the car in question, have the
> same crown geometry as those in my '79 MGB. The heads
> seem to be roughly universal, for the most part. Therefore,
> how is the compression ratio changed? Piston crown? Head
> gasket thickness? Something else?
>
>I wrote;
>
>Rick, I can telll you how the compression works, as I just figured it out
>this past weekend. First there are two
>different piston types. Shallow dish (high compression 6.2cc dish) and deep
>dish (low compression ~16 cc dish). Also,
>although the cylinder heads look alike, they are different. The early heads
>had a deep combustion chamber (~43cc) and
>were used with the shallow dish (high compression) pistons to arrive at a comp
>ratio of about 8.8:1. The later heads had
>a shallow combustion chamber (~39cc) and were used with the deep dish piston
>to arrive at a comp ratio of about 8.0 :1.
>The changeover occured (in the US) I believe for the 1972 models which used
>the later shallow head combusion chamber and
>the deep dished pistons. At this time the compression ratio on US cars
>dropped to 8.0:1. These shallow heads also had
>larger intake valves for about 2-3 years, when the head design reverted back
>to the smaller intake valve. Blocks which
>use the large intake valve he!
>ad must have the eyebrow cut into the top of the bore to allow for the exhaust
>valve.
>
>This web site can provide some additional information.
>http://www.mgbmga.com/index.html
>
>If you have any questions e- mail me off the list and I will ty to answer
>them. I currently have each type of engine and
>cylinder head disassembled in my garage.
>
>PS. Could it be that your 79 MGB was rebuilt using High Comp pistons?
>
>John
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