Jon_Valjean

Hey all,

I've searched the forums about this, and although I've found a good post in the archives I've got a few questions I was hoping someone could clear up for me:

Car = 81 GS. I can't get a GSL-SE fuel tank here, before anyone suggests that.

1) I'll be using a Walbro 255 EFI pump feeding a 13B-T engine developing over 400Hp. I know I need to replace the hard fuel lines from the tank to the firewall - I'm assuming I can use the existing delivery line as a return, then get a new delivery line made - question is, does anyone know the best size for the new delivery line, preferrably in metric terms?

2) Does anyone.. ANYONE have a picture of a swirl tank (also called 'surge tank' or 'swirl pot') that has been installed on an FB designed to be on the road? I simply can't figure out where in the hell to put it.

3) Do I really need to increase the fuel tank outlet feed in diameter, if it's going directly to a swirl tank? I'd rather avoid welding anything to the tank.

If anyone could help it'd be much appreciated

Thanks!

coldy13

I'm not sure, but hopefully the '81 tank has a drain plug on the bottom. Take that out and put a fitting in. If I remember right the thread pitch is M10x1.25. That'll work much better than the stock pickup and you might not even need a surge tank.

You can just make a new metal feed line and use the stock metal feed as the return, should work fine. For the new feed, I'd probably go with 1/2", I don't think it's really possible to go too big with the feed/return lines.

Jon_Valjean

Hey thanks for that - unfortunately my tank doesn't have a drain I suspect they were used on the 84+ models?

So if I was to have a M10x1.25 (or larger) pickup tapped into the bottom of the fuel tank, I wouldn't need a surge? I understood a surge was pretty crucial in most turbo EFI applications?

steve84GS TII

For my TII conversion I kept it simple and just copied the factory setup on the TII.

Keep the large feed line and small vapor/smog line.Replace the stock small return line with another stock FB feed line.Its the right length/shape and its the same size as both the stock TII lines.

The fuelpump holder from a 2nd gen will fit perfectly inside a 1st gen fuel tank.Its just the right height.Just get a hold of any complete FC fuel tank and remove the mounting rings for the pump and vent, and remove the internal slosh tank.
Then cut the appropriate holes in the top of the FB tank and weld the 3 items in place.Obviously,be extemely carful welding to a fuel tank.Take all precautions neccesary or just have a pro do it for you.Its really not a lot of welding,you could even solder or braze it.
It all fits perfectly and exactly duplicates the stock FC tank.Then you can upgrade to a SupraTT or Cosmo or whatever internal pump you want.

This setup has powered my car for years with a SupraTT pump rated for over 400hp,is totally silent and never loses prime or sucks air.Basically,its factory reliable......

coldy13

It might be needed in an EFI setup. I don't know much at all about EFI turbo setups, just about blowthrough carb setups. For the tank pickup, the larger the better. But if you're gonna go through the trouble of changing the pickup and adding a surge tank, why not just get a fuel cell?

nevarmore

I apologize in advance. I understand the basic theory, but do not have enough experience to make reccomendations for a specific application.

It should hold true for return lines, those are like the big pipes for drains. They are under lower pressure and do not have to move the fuel as effeciently as the feed line. Your feed lines should be properly sized because they are under pressure and do have to meet demands.

It requires more work to maintain pressure in a larger line, simply a greater volume to pressurize and pump along. There are also effeciency issues. You can run a garden hose up to the injectors and rely on the return line to handle the excess, but that is wasteful. The initial cost of the lines is more, the cost of the pump to handle it is more, and the power (electrical) to work that pump is more.

To determine how big of a line you should have you need to know the engines maxium fuel consumption and the required pressure (likely 55-65psi, ~6 BAR). I'll try and look for the exact forumla or table tonight. The idea is to have lines and a pump capable of supplying a little more than the engine will be ever able to consume. At redline it'll be getting enough with a saftey margin and at lower speeds the return line will handle the excess.

Jon_Valjean

Hey thanks for all your input guys - I'm still a little confused. I understand the fuel lines now, so that's all cool, but I don't know if I should be using an in-tank pump, or an external.

I suspect any major modifications to the existing fuel tank would be pointless given that I could get a fuel cell made up for probably just a little extra.

So, in summary, with an in-tank pump you don't need any type of surge tank?

So a good way of doing this would be to get a FC tank, cut out the necessary pickups, weld them into a fuel cell and buy a high-pressure aftermarket pump?

steve84GS TII

Im sure there are fuel cells out there that are made to accept intank pumps.Intank pumps have many advantages.They are quiet,never lose their prime,are well protected and keep cool since they are immersed in fuel.Thats why I chose to go that route with my car.Its how almost every car on the road is setup.
Whether you modify a stock tank or use a fuel cell,the intank is the way to go.With an intank pump,you just need a slosh box to keep the pump immersed in fuel.When the tank gets low on fuel,the slosh box keeps the pump covered in fuel.

A surge tank is method used in Aus a lot.It involves a low pressure pump drawing fuel from the tank,then feeding a small external fuel tank,inside which is an intank,high pressure fuel pump.The high pressure pump stays immersed in fuel inside the surge tank,and provides pressure to fire the injectors.
It works, and it doesnt require any mods to the main tank,but its also complicated,requiring 2 pumps and associated wiring,plus mounting the surge tank somewhere.

Jon_Valjean

^^ thanks for that - v. interesting.

I just received an email from AJC13B in response to some questions regarding his setup - he used two Bosch pumps feeding a 1/2" pipe to the engine. He had the stock tank modified by welding two 1/2" pickups at the base, and originally gravity-fed this to the two high pressure pumps. He said he didn't need to use a swirl tank until he started running slicks.

I think the best thing is to investigate the cost of a custom fuel cell & in-tank pump, vs. the cost of a redundant twin-pump setup such as AJC13B - I must admit, I'm liking the latter option.

Problem is, I want it to be very quiet as well...