Why is compression ratio so low?

[Viper X]

I searched the archives and couldn't find an answer to this question. Maybe I'm just old, but someone has to know the answer to this question. We used to run 11.0 to 1 in street engines. I know that gasoline isn't the same, but why is the compression ratio in these engines only 9.6 to 1? My 02 vette had 10.5 or so. Wouldn't increasing the compression ratio to about 10.5 increase the hp level and performance of this engine, relatively inexpensively, without sacrificing driveability? The valve sizes seem to be about the same as a small block chevy's performance heads, so what's with the combustion chambers on these engines?

I'm looking into getting my heads upgraded and the packages I see all refer to keeping the compression ratio about the same as stock? Is this a "smog" / emissions issue? Won't the engine components stand the additional compression? (I doubt that).

Any one have any experience with "heads" upgrades here in southern California? Any recommendations? Will the engine need to be "re-tuned" after the head work? Thanks for the info / help.

Love that Viper.

 

[Skip White]

91 octane fuel is just to low, 93 would be ok for a point more compression, cars run hotter than they used to, because of emisions, etc. Engines last longer at lower comp. and they have been able to achieve great HP numbers with out it. If you are going to do some head work they can easily get you another half point or more, but if you ever want to supercharge the engine or Nos it, then that would not be to good an idea. Just because the Vette had higher comp. doe's not mean to much, I think the valve timing interplays with this, not to mention the computer is pulling a good bit of timing out.

But if your going to do head work and no forced induction, then I think it would be a good idea to raise it a bit.

One of the hybrid cars on the market is running at around 14 to 1 comp. ratio, but valve timing changes that. Would love to know whats up with that.

If all you can get is 91 octane, you will need a boost additive, and many of the boost additives on the market are no good, they all tend to lie.