Interesting Thread ....things that make you go hmmmmmmm.............
For me, with very limited experience the gravity feed is my choice. This is because of the simplicity of it. However, there are issues that perhaps aren't immediately obvious with gravity fed systems:
-Aux tank must be higher than primary tank (duh) ..this means you'll be taking either pillion space or rear luggage space. So for two-up riders and/or those who carry the kitchen sink with them this can be an issue. for me it isn't
-IMO gravity fed requires a higher degree of attention when turning the valve on/off. There are many variables ...some answers may seem easy but one can fool themselves. For example
- Turn the valve on too early and you risk pushing gas into the primary tank which *could* cause overfill down the vent tube. But, to the contrary, on my system, if on an incline I loose a fair amount of gas out the aux vent tupe from splashing and gravity. I'm working on ways to help avoid this.
- Turn the valve on too late and you risk out-running the speed of the transfer. While this may not seem likely, you'll be surprised what inclines in the road can do!!! Your testing in the garage or on flat ground will not be the same as real world application.
This is easy enough to overcome (just don't leave the valve open exteneded periods while going up hill). I learned this the hard way and managed to embarrass myself while riding ahead of Jim owens in the Utah 1088. Up hill, out of Zion park and I started surging ...yep gas had started flowing back, to the -point that I was running out of gas in the primary tank!!. LOL, believe me, I learned that lesson in the most embarrassing of times.
For this simple hick from Nevada, gravity is great but you must factor certain things in. Do *I* think some of the fancy tail-dragger cells with electric pumps are better?? Yep, no doubt, especially if you need the pilliion or luggage space. Would I give my left nut for a fancy set-up like Kaitsdad??? No, but I might give the right one
Irregardless, once you experience the advantage of aux fuel you will not go back. there is math that can't easily be explained but in simple terms: While you do not get 100%more range, you stop for gas LESS than 50% as much. why? Good question, but the answer falls into those times when you need three or four gallons to get to the next convenient gas ....the aux cel does away with the vast majority of those situations. While my range is not doubled, I stop for gas about 40% as often ....that is HUGE!!
OHfjr: Interesting concept for fuel transfer. What you said does not seem like a vaccumn type fuel style of tranfer. To me it represents a pressure and volumne difference. If there was a vaccumn it would trandfer no matter what the temperature is. Have you aefer put a vaccumn gauge on your tank to find out id a vaccumn exsist???
Pressure is huge factor, but IMO temperature etc does not come into play as much as the "back pressure" of fuel in the primary tank. IE: with lots of gas in the primary, the transfer will be slow, no big deal, as the primary runs low, there is less "back pressure" and the flow will be faster ....now you must also factor in the pressure of the gas in aux cel ..more in it means more pressure, hence why it is likely possible with many systems to overfill the primary if it too is full. In some ways the fighting pressures are a self enforcing good thing, but just another variable with gravity.
