I finally ordered my new heads aluminum 64cc 2.08/1.6. With the increased air flow, shouldn't my mpg go up a little, or will producing more HP decrease my mpg?

I will be adding a Holley DP soon, so I know that isn't going to help .

Also, I will have around 9.3:1 compression, I can still use 87 octane with that right?

 

Generally speaking (very generally), operating in an increased vacuum range will result in greater fuel effciency. By increasing the flow volume, however, you are likely decreasing the flow velocity, which tends to lower the intake vacuum. In this way, increasing the flow tends to decrease the vacuum at a similar throttle opening, so you'll have to train yourself to be a little easier on the throttle to get the same efficiency. The only thing you will likely change is to increase the peak flow, and thus, peak power.

 

One point increase in compression is worth a 10% increase in gas mileage, all else held constant. Obviously, that's not an exact figure, but it's pretty close. Fuel economy increases much more vs. compression than power does (which is only about 4%).

Obviously, you didn't just change the compression when you upgraded so this figure will be offset by the effect of other combination changes. Generally cylinder head flow increases don't have much effect on fuel economy unless they significantly reduce intake velocity (i.e. going from a 160cc intake port with small valves to a 200cc intake port with big ones). From personal expereince I'll tell you that your mileage should net/net stay about the same. UNTIL you realize you're going to be using that newfound power a lot more often which will murder average fuel mileage.

9.3:1 on 87 octane is marginal at best. It will run much better on 89.

 

Cool, thanks for the info.