vera_jr

Im looking to upgrade my engine a little. I have a 455 out of a 74 Trans Am in my 69 GTO. Looking at the codes, that is one of the smog engines that was produced in the early 70s. It states that it has a compression ratio of 7.4:1 which is obviously not great. What is the best way to obtain a 9:1 compression ratio with this engine?
New pistons? heads?

joesweeting

I have a 455 with "96" heads and with that combo I have a 9.5 to 1 comp ratio.

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vera_jr

where did you get the heads? any changes to the pistons?

geeteeohguy

Pontiacs changed compression ration with cylinder head chamber volume, not piston changes. So, a set of smaller chambered heads will get you where you want to be. I'd recommend no higher than 9:1. There is a good compression ratio calculator for Pontiacs onthe Wallace Racing website.

BearGFR

The general "rule of thumb" is that compression ratio needs to be matched to the fuel --- i.e. 9.1:1 for 91 octane, 9.3:1 for 93, etc.
This is for iron heads and a relatively stock cam. Note that these aren't hard and fast limits, just guidelines. It is possible to push them some, sometimes by a lot - but you have to really know what you're doing and also be willing to invest significant time/coin in building the rest of the engine to do it. In the end, unless it's an a dedicated race motor where every last fraction of power counts, the benefit in terms of increased power usually doesn't justify the cost or the risk involved.

Bear

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BearGFR
Garland, TEXAS