According to my formula you will be able to get 2600 psi with a 13hp engine and a 5.5 gpm pump. I am using the following formula: PSI x GPM = ??? take ??? and divide that by 1,100 and there you have how much horsepower you need. If you already have the horespower and gpm you could use this formula to figure out what your PSI will be. HP x 1,100 = ??? then take ??? and divide it by your gpm. So with your numbers it would be 13(hp) x 1,100 = 14,300. Then 14,300 divided by 5.5(gpm) = 2600(psi). Others might have other formulas they use. I hope I helped.