I would do it around a heater hose. I think the exhaust gets too hot, and its too dangerous. Anything goes wrong with a fuel line wrapped around the exhaust and you have an engine fire.
Fire! That's what the insulation on the inside of the hood is for so it can melt the plastic push pins and drop down and smother the fire. No, really I have to agree that the exhaust temperatures are a little too dangerous.
I think this idea in itself is just bad... and sorta nonsense. Like mentioned in a hundred other threads (especially the one about the guy in the V6 Stealth getting 60+mpg from this mod), the tubing may reach a good temp of 200*F from the coolant return line, but even if you get your fuel LINE to 200*F at a small spot, the fuel passing through the line at a significant pace isn't going to be warmed enough to make that big a difference.
If the fuel system is a returnless type (one supply, no return) the fuel flow is quite slow really. With enough surface area the fuel might acually get up to a pretty good temperature.
Most of us though have older vehicles that have a fuel return at the injector rail. What little heating that may occur is distributed over the mass of the entire fuel system, including the fuel itsself.
An alternative would be to install a flat plate heat exchanger to transfer heat from the return to the supply and locate it near the injector rail. This would thermally isolate the fuel and fuel system nearest to, and in, the injector rail from the rest of the fuel system, significantly reducing the mass that needs to be heated.
So far I haven't seen any road tests that confirm or deny that fuel heating works or doesn't where the person measures the fuel temp at the injector. Intuitively it would seem that fuel heating would be beneficial. I remember reading a paper from Mitsubishi on thier GDI engine. In this engine the fuel runs through the head casting like an oil galley. They estimated a gain of like 5% in fuel efficiency from the fuel heating alone.
Someone running thier fuel line around a radiator hose a few times on a return type fuel system and expecting 20%-50% is just plain wishful. There is no magic bullet.
I did something like this to my truck. Although I didn't want to modify my fuel lines. So I routed my heater hoses through some copper pipe along the fuel supply and return lines, and also made a coil of copper pipe under the rear fuel tank. I installed one of those outdoor digital thermometers with the sensor on some wire, so that the sensor was on fuel line just before the injector rail. I insulated around the sensor with some header wrap.
On a normal day with a fairly full tank, I could increase the fuel temp about 10 deg above ambient temp in about a half hour in the winter. When the tank gets below 1/8 of a tank its possible to get the temp up to about 110 - 120 deg F on the hottest day in summer.
I would not recommend to do it this way, my results were not very good. It takes way too long to heat up the fuel on systems with a return line. As other people have said it evaporates the lighter ingredients of gasoline in the tank. (which spews out when you open the gas cap)
I think that in order for heated fuel to work properly on fuel injection, only the fuel injector rail should be heated. This way heated fuel only goes to the injectors and not the tank. One problem I see is a lot of injector rails have the return line paralleled with them. One other thing to think about, the faster the fuel is flowing, the more surface area you will need to conduct heat from the coolant to the fuel. If you just heat the injector supply part of the fuel rail, you wont need as much surface area as you would with heating the fuel line. The little "vaporizer" devices talked about in one of the other threads here really have no chance of heating anything unless the fuel is moving really slow.
On my truck I pretty much decided I would have to fabricate an entirely new fuel injector rail to do it right, and went on to other things more promising..
Hmm yeah, I really want to try this but is does seem it'd require a lot of fab work to do it right. I think wrapping 5 times or so a fuel line around a radiator with insulation should allow a good amount of heat soak. But I'd really like to get more like 350-400F since I was told that's about the point when all fuel particles would vaporize.
Even if I didn't have a return setup and I have some heat shield or heat conduit from the exhaust system regulated to 350F and even if I could get it to fuel rail only, I still worry a bit, above 200F things will start to vaporize and I'm not sure how large pockets of gas in a fuel rail would work or what happens when the injectors fire. The trick I think would be a very high pressure fuel rail so we can heat but stay liquid due to pressure but it's just about impossible due to gasoline vaporizing between 200-400F.
I've thought about just injecting onto a hot plate or heating the intake manifold (with precautions so heat doesn't soak to the fuel line), but wouldn't heating the entire intake air mass be bad? Seems like more chance for NOx and I'm sure my old pistons can only stand so much.
Maybe a phased approach with extreme 400F heat to vaporize and then before cylinder we mist water to cool the charge, but when cooled wouldn't the fuel just condense again?