Choosing Dome Size

Dome CCs determine how much compression your engine will have. A lower CC dome means less volume, leading to higher pressure when the spark ignites, resulting in better performance. Compression increases as CCs decrease. Domes with 16-19cc are for race gas only, 20-21cc is a 50/50 mix of pump and race gas, and 21cc+ should use 100% pump gas, with a minimum of 91 octane recommended. These specifications are for altitudes up to 1500 feet. At higher altitudes, greater compression domes can (and should) be used with lower octane fuel.

(Sea Level Chart - going up in elevations will lower Compression levels because air is thinner)

(Every 1000' above sea level expect 5-10 lbs to fall off from this chart)

Dome size Compression Recommended Fuel

23.3cc 120-140 lbs 91 octane (stock)

22cc 150 lbs 93 octane (pump gas)

21cc 160 lbs 98 octane (pump gas/Race Fuel)

20cc 170 lbs 100 octane (Race Fuel)

19cc 180 lbs 105 octane (Race Fuel)

18cc 190 lbs 110 octane (Race Fuel)

17cc 200 lbs 112 octane (Race Fuel)

16cc 210 lbs 115 octane (Race Fuel)

Approximate Cranking Compression Chart

These pressures are estimates based on fresh STOCK bore, piston, porting, and deck height.

It is recommended to check your compression after installation.

Here is a nice set from Vitos available on Amazon - Comes with 21cc domes for higher compression AND still able to run 93 pump gas at sea level

*There are obviously a couple other factors that come into play when choosing the right dome for you.

Example: Compression Ratio, what type of riding you'll be doing, Squish bands, we will be making more blogs on those topics as time goes on.