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 head 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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