EFI fuel rail plumbing

[woody80z28]

I've read several contradictory points of view on this. Maybe it's splitting hairs...

Some say to run a single deadhead fuel pressure line up to the rails with the regulator back by or in the tank to eliminate problems with heating the fuel on long duration trips.

Others advise to run the feed through both rails and put the regulator after for best accuracy with fuel pressure at the injectors because the deadhead setup can cause fluctuation.

I've also heard a Y in front and back with the reg after is best.

I would rather not have pressure fluctuations or hot fuel problems. What about a compromise? I'm thinking running the feed to the regulator just before the fuel rails would keep the bypassed fuel from heating up in the rails, and should stay fairly consistent pressure-wise by only running the length of the rails deadheaded. Has anyone tried a setup like this? Any thoughts?

For my application the pump is a Walbro 255HP to feed 500-550hp. 3/8 feed and 3/8 return already plumbed.

[cornfedbill]

I have designed production fuel systems for more than 15 years. In that time I have designed both full recirculating and deadheaded fuel systems.

Deadheaded systems took over the market to reduce evaporative emissions and to reduce cost. It was not a performance improvement.

Yes, recirculating fuel systems can increase the temperature of the fuel in the fuel tank. However, this increase is small and primarily affects evaporative emissions. Remember that your tank is a very large reservoir with a lot of surface area. When you are driving, the fuel tank dissipates its heat into the air moving under the car.

For hot restarts, the recirculating systems are much better because they purge the hot fuel and vaporized fuel from the fuel rail. Deadheaded rails are notorious for poor hot restarts when compared to recirculating systems.

The recirculating systems also maintain more constant fuel pressure. When we changed the LS car engines to deadheaded rails, we had to add dampers to the inside of the rails to compensate for the increased fuel pressure pulsations. Converting to aftermarket or homemade undamped fuel rails can cause fuel distribution errors due to high pressure pulsations in the rail tubes.

The other advantage of recirculating systems is that you can use a regulator with a vacuum reference so the pressure increases at high load and decreases at idle. Deadheaded systems are not as well suited to high output engines that idle at low RPM. The OEM's fix this by using variable fuel pressure controlled in tank pumps. This is not easy to do on a retro-fit application.

The early LS1 fuel rails were not really recirculating. The regulator was on the rail, but did not purge the hot fuel on hot restarts. It can be prone to vapor in the rail on hot restart just like the deadheaded rail.

I would use a Y at each end of the rail tubes with a vacuum referenced fuel regulator. This will give you the best performance overall.

[woody80z28]

Thanks for the reply. At what point does the heated fuel become a problem? Is this just with huge fuel pumps running at cruise for hours? I know some people have had trouble on Power Tour. Maybe the Walbro 255 is small enough that it is moot.

[cornfedbill]

I cannot answer that question. We never had problems with constant running on any of the engines I worked on. OEM pumps are not that big. Our issue was always restarting after a 20 minute soak with a hot engine. We had vapor in the rail at crank. Sometimes it was so bad the engine would not start. Once we were running, we never had a problem.

I have heard of coolers on the return line to reduce this problem, but do not have any personal experience. You just need to make sure there is enough air to cool the fuel or a fan.

Sorry I cannot give you a better answer.