SpudFiles

I have seen mutiple guns with mixing fans, I have seen multiple guns with out them and both guns work?

Do you need them or just make it easier for igntion?

My next cannon is going to be combustion, using propane and a fuel, that why im asking

It makes it easier to ignite, and drastically improves performance. Fans are not necessary, but are strongly recommended, especially for "advanced" designs.

I have one gun that im going to modify to do this with, But how much to put in psi with and ggdt thing didnt work once i down loaded it
I will rig up something to my gun to get the propane in.

If you are building a combustion gun you should not be using ggdt, you should try hgdt.

I know I've mentioned the SpudWiki to you several times before, but you really should look at it.

SpudWiki > Search "chamber fan" (or just fan):
http://www.spudfiles.com/spud_wiki/inde ... hamber_fan

I have seen multiple guns with out them and both guns work?

Do you need them [...] ?

You answered your own question there. No, you don't. As you've noticed, cannons will work without them.

or just make it easier for igntion?

From the Wiki: The fan will improve muzzle velocity, power and usability

I will rig up something to my gun to get the propane in.

Before you make a topic on that too:
http://www.spudfiles.com/forums/how-to- ... t7234.html
http://www.spudfiles.com/spud_wiki/inde ... Fuel_meter

My first combustion has everything just about right, except it didn't have a fan. I made another one just like it but added a fan and I absolutely could not believe the difference. The power increased ridiculously.