I have a 30cc gasser with a 18x8 prop getting about 6000 RPMs I need to get to 8000 RPMs what size prop do I need to get 8000

Sort of depends on what 30cc engine and what 18x8 prop you're using? Also is the engine broken in and properly leaned out?

If you're convinced the engine is broken in and leaned out, I would drop down to a 16x8, but that is quite small for most decent 30cc engines.

And why do you need to get to 8000? Agreed 6k is low for a 30cc engine, but the flight performance is what matters.

Only because the Mfg. says 1800 idle and 8000 eidw open

Manufacturers generally are too optimistic about their RPM figures on both ends. The engine will unload a bit in the air, so it will turn more RPM in flight than on the ground. If you are a sure you have a good tune, fly the plane and see what you think. You may also have a look at the tachometer readings forum here on RCU and see what numbers others have gotten with your engine or similar ones.

Manufactures give a working RPM and you want to have the engine working within those specs. Most gas engines use 2K to 8K, as there norm so you don't harm the engine.
6K is a bit low though even on the ground. Of course it can be the tach is off too, I have one that never gives a good reading. It is so bad I broke down and spent the big bucks for a good one.

You can check your tach by pointing it at a light bulb can't you? 60 hertz household power, should read 120

An optical tach will read 3,600 RPM when set to a 2-blade prop reading and pointed at a light bulb. The 60 Hz AC current will cause the light bulb to flicker 120 times per second, which is 7,200 "flickers" per minute. 1/2 of that is 3,600, or the same as the RPM reading you'd get from a 2-bladed prop. Your tach should also read 2,400 RPM when set to 3-blade, and 1,800 RPM when set to 4-blade.

My two crap tachs both read correctly when pointed at a florescent light but don't read a prop. I broke down and bought a TNC when I was working on a lot of different type of engines. At the time it was well worth the money but these days it sits in my flight box a lot. My tower and globee are useless but I have tested my TNC against a couple of the combo tachs sold by Horizon, the one that is a volt meter and a tach and they came up the same as my TNC at less then half the cost.