Compression Question

[orangedust]

One of the other mechanics, putting on my exhaust said that my 340
in my 70, was of a lower compression being a Calif. car. I am going
to rebuild the rear- end and it was suggested to go to a slightly higher
gear due to the lower compression. I don't know if this is true, its not a
race car so I don't really care if the compression is lower. I'd just like
to know if it is a lower rated engine.

 

[AdamR]

Not true. 70 340s were all rated at 10.25:1 compression. I think 72 was the first year of low compression.

 

[rabius75]

wow! 10.25? oh right.. leaded gas...

 

[orangedust]

I had a feeling it was same as other 340's. Thanks for the information!!

 

[moper]

Orange, the factory rating is marginal at best, and compression is the last thing on a mostly street car you worry about when choosing a ratio. Realistically, the engine has around 9:1 compression in its factory state, due to production variances.

 

[mikesduster]

adam,
i believe they changed compression in 73. rofiler:

 

[AdamR]

Nope it was 71 or 72 pretty sure it was 72.

 

[Coyote Jack]

Adam is right I believe. 72 was the first year of the horsepower decline.

Jack

 

[dgc333]

The horsepower didn't decline in 72, they way they measured it did. Prior to 72 the horsepower was gross HP, that is, it was measured at the crank without accessories (i.e., no water pump, alternator, AC compressor, power steering pump, air filter or exhaust system). In 72 the manufacturers were required to report NET HP which included all those items on the engine just as it would be in the engine compartment.

73 was the first year of emission standards that resulted in compression ratio's being lowered to meet them.

 

[AdamR]

I'm pretty sure my 72, 340 has the lower compression pistons.

 

[3404speed]

72 lowered compression.