I got into a debate with my local Honda shop here in san diego (Hontech Automotive...they're EXCELLENT). He was convinced that the highest compression that would run on 91 octane gas was 10.5, and even that was pushin it. But I've seen people run 11:1, and even 11.5:1 in street hondas, and they all claim to run pump gas. Anyone here who can beat down this argument once and for all?

 

12:1, 93 octane.

Ask Dubster, his roommate has 11.5:1 I think, maybe higher than that... on 91 octane.

 

The highest compression ratio you can run is going to depend on more than just the octane grade you run. Tuning, engine construction materials, combustion chamber shapes, piston dome shapes, how efficient the engine's cooling system is to prevent hot spots, and how many sharp edges there are in the cylinder all affect what the highest compression ratio you will get away with will be.

Now, in the ideal engine, I would imagine that you could run about 14:1 compression. Of course, my ideal engine would be a kevlar/carbon/teflon composite direct gas injection engine with a constant-volume electric pump cooling system and a matching semi-spherical combustion chamber and piston dome

 

I've seen just over 12:1 compression on 91 octane, and my roomate is at around 11.6:1.

 

i know of a kid running 12:1 pistons in a 400 small block chevy on pump gas if that helps. But then again, we have a gas station where i live that has Trick 101 avail from a pump does that still count as pump gass..<evil grin> but you can run 12:1 safely on pump gas.

 

Like 1stgen said, you can't just say "12:1 will be fine on pump gas" for every motor.

That said, I would stick to about 11.5:1 if all you can get your hands on is 91. That's what the Toyota 2ZZ-GE (Celica GT-S/Matrix XRS/Vibe GT) runs, and it's a production motor intended to be sold anywhere.

 

that's a very good point, I understand that engine build affects max compression ratio. Let me rephrase.

I'm thinking specifically of a D16A6 combustion chamber, using 89 Integra pistons (big domes), to achieve approximately 11:1 compression. I understand that sharp edges are a no-no, and my funds a limited, so I'm thinking basically of a drop-in piston swap to raise compression to 11:1. Would 91 octane fuel be sufficient?

 

You will be fine

 

Yeah, you'll be fine.

 

If not you can always back down on the timing. But you should be good.