Date: Sun, 22 Dec 1996 22:19:48 -0800 From: Mike West Subject: Carb CFM and Jet sizes (numbers) I went to the net and searched for drill sizes for the carb jets. And there they were! At least a few. The drills I speak of are wire gauge drills, in numbers from 1 to 80. In drill sizes the larger the number the smaller the drill. It turns out that the carby jet size is in hundredths of a millimeter. Example: a 132 jet is 1.32mm diameter and is a #55 drill. This holds true for air jets too. A #78 drill is a 41 air jet. Any drill chart will give you the others if it has the metric equivalence. Looking at the sizes available, each drill size is approx. 10% larger than the one before it and 12% greater cross-section. A plan used by my machine supply guy and avid dune crawler, he buys two of every jet he is going to open up. He says you can't find the max till you go over it. Then he whips out the fresh one and drills it back to the last size. If he has dual carbs he has four on hand. On a single carb, I expect you could solder the end closed and then re-drill, but on duals, I doubt you could ever balance the flow with a random sized solder blob. Whichever method you use, be sure and chamfer the jet end after drill- ing. Just a slight one to break any burr there. The Cubic Feet per Minute of our Carbys: I worked up a formula based on the Liters and rpm so you guys can figure what you have with bigger engines and so forth. The formula is (Liters X rpm)/ 56.64 Using a 1600, it's (1.6L X 4500rpm)divided by the 56.64 equals 127 CFM. If you're running two carbs then each carb will draw half that. Now you can steal parts off Chevys or whatever. Keep in mind that there has to be some restriction at the carb or you can't boil gas. Vacuum, remember? :-) That formula is just based on stroke and bore so if you have too small of valves or restricted exhaust you won't achieve the full potential. So go forth Monster Makers and create new legends! Proving there is more to a VW than too small a backseat. :-) west