This is topic Compression question for pump gas... in forum Tech Talk at Northern California Ford Owners  .


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Posted by Fox89GT (Member # 1539) on :
 
Whats the highest compression ratio you should go while using 91-93 octane pump gas? Would the higher compression also hurt emissions? Also, whats the highest octane gas around here? I think I've only seen about 92 whereas the east coast might have about 94. Bummer huh...
 
Posted by 2000BlackGT (Member # 283) on :
 
quote:
Originally posted by Fox89GT:
Whats the highest compression ratio you should go while using 91-93 octane pump gas? Would the higher compression also hurt emissions? Also, whats the highest octane gas around here? I think I've only seen about 92 whereas the east coast might have about 94. Bummer huh...
you can get 100 octane at the pump on Almaden @ the 76 station
 
Posted by gR40coupe (Member # 698) on :
 
I believe you can run 11"1`
 
Posted by Mean2001GT (Member # 1750) on :
 
also depends on if you have alunimum heads. if you do you hould be able to bump it up .5 more compression.
 
Posted by Fox89GT (Member # 1539) on :
 
I'm thinking specificly thinking about the GT40P heads and wanted to mill it a bit. So this is an iron head with about 60cc chambers. I think if I shave off about .030 my compression should be close to but not quite 10:1 and would be okay running 91RON and still pass smog. Are my assumtion reasonable?
 
Posted by shade-tree (Member # 298) on :
 
If you back off on the timing a bit, an iron-headed 10:1 or 10.5:1 should survive on 91. I would suspect you would need a little octane booster to run optimal timing.
 
Posted by 93PONY (Member # 60) on :
 
A little known secret....

Run straight water w/2 bottles of water wetter. It'll keep cylinder head temps down & help eliminate hot spots in the heads....thereby reducing the octane requirement of the motor.

You should be fine on 91 w/ 10.5 to one & Iron heads. Like Shadetree said, if it's pingin', drop the timing a few degrees. Or get a custom chip that pulls timing at peak torque (most likely were you'll ping) & throws it back in at higher rpms.

Minor changes in compression doesn't effect emissions too much....as long as you run on pump gas that is. Cam specs are much more important.

BTW, cam profile will also play a BIG part in cylinder pressure. Cams can bleed off cylinder pressure, or help increase it. With a good cam *cough* custom *cough* you can maximize cylinder pressure without having to shave the heads.
 




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