Hey guys,

We can only get 91 octane gasoline around where I live. I was wondering what is the safest/highest compression ratio I could use with that. I plan on using non-VTEC cams such as the crower 62404, 62405A or Crane Stage 5.

I was thinking 11.0-11.5:1 would be good, but could I go higher since these cams have a lot of overlap?

Thanks for your help.

 

keep it under 12 and good tuning.

 

yeah i would say anything under 12:1 with good tuning will be fine on 91, i think even 12:1 would be ok with some good injectors

 

I had 11.5:1 with 93 octane and still detonated! And yes, that was after tuning it.
Then again, alot of the gas station owners are making me wonder if its really the correct gas!

 

My car runs like a million bucks at 11.5:1 on 91 octane.

 

i currently got 12.5:1 running on 91 octane and pings like a ****. going to dyno tune it later today hopefully i can tune it out..i would really stick with somthing under 12:1....i bought this motor at a dirt cheap price so i couldnt resisit

 

You can crank the CR up quite a bit, but combustion chamber design and ignition timing are going to have a lot to do with how well it runs. The good folks at http://www.theoldone.com have some great pistons for this. The best thing to do would be to get a wideband O2 sensor so you can target a 12.5:1 AF ratio and then maximize your ignition for that setup.

If you wanna get really sexy, use a water injection system hooked to the throttle position sensor that injects a little H2O when you're getting into it. That will not only keep your pistons 'steam cleaned' but it will allow more aggressive spark tables to be used.