So I'm swapping an EFI 13b from a 1990 FC RX7 into an old carburetted 1980s FB RX7.
If I understand correctly, I will need to make the following changes:
- an EFI capable gas tank that has baffles and supports an in-tank fuel pump
- larger diameter fuel line
- the engine, ecu, wiring etc (duh)
I suppose I could do an external EFI fuel pump with the stock carby gas tank but then I'd need a surge tank of some sort to deal with fuel slosh. Aka more fab and expense. Seems like the easy button would be to buy a tank from a car that was already EFI from the factory. Is my thinking correct here?
If so, next question is what car should I use as a donor? Hoping for a somewhat square-ish, not too tall tank that already has baffles and already supports an in-tank pump design. Then just fab or reuse the straps and get a performance oriented fuel pump (if needed).
Some of the Hyundai tanks look rather square-ish in pics.
Any thoughts or advice from the hive?
My 924 doesn't have a baffle or an internal pump and it was FI from the factory. When I converted it to EFI, I just re-used the CIS external pump.
I think you're over thinking things. Unless you're racing/track days then go straight to a fuel cell, do not pass go.
What you really need to watch is feed and return line diameters. More than a few Carb to EFI conversions have needed to use the old return line as the feed and a new return line added after removing the old feed line as it wasn't large enough to work with the new fueling requirements or the old return line wasn't large enough to return that volume of fuel.
I don't know if they have all the little pieces, but Atkins Rotary lists the GSL-SE gas tank, although it looks like it'll only fit on a 84-85 models, so if yours is older, maybe not.
Not sure what the budget is, but the aftermarket has you totally covered. They sell drop-in pumps with a foam "canister" around them as a baffle. Like this one from Trifekta/Phantom
Do you need to put in a fuel return line ?
I am doing a 13B TII swap into my 82 rx7. I bought something similar to what Curtis posted but it was made specifically for an rx7 fuel tank. I will dig up the name of the guy that sold them.
I am using a stock 5/16" fuel line for the supply and return lines in my setup. When I looked into it, it seemed to be able to support the moderate power levels I am chasing (~250 hp).
It's possible to build one without a return line - technically, it's just a very long leg on the pressurized side and the return is in the pump. You might need a damper on the rail if you do that. Non-return systems are hard for some people, they'll actually convert cars with non-return systems to a return system because they can see the fuel pressure regulator.
5/16" should be just fine for 250 hp. That's basically every turbo Miata on the street.
The 13b gxl se tank will work for you. 650$ new from Atkins plus shipping. Good luck finding a used one!
That being said, a surge tank is NOT hard to install and is the way I would go. You can Mae one with just a large desel spin on filter and mount. Add an efi pump and you are done!
Thanks for the suggestions. Let me add some more info to frame ideas.
Track / race car only. never driven on the street
Budget = as cheap as possible while being safe
-Curtis' suggestion is way too expensive at $1,400!
-Surge Tanks appear to start at $300 before AN fittings. This seems like a decent option but I don't think I have room under the car for one. Which means I need to mount it in the trunk. Which means I need to build a compartment for it to isolate it from the driver for safety rules.
-Fuel cell appears to be a bit less than $1k and more fab work.
-A well cell is a bit cheaper than a full-on fuel cell and might work but I think I need to build a compartment around that too and it is only a little cheaper.
-Buying the GSL-SE tank is also more than I'd like to spend just for a tank. At that price point, I might be better with a fuel cell.
The above points is what got me thinking of a cheap replacement tank from another car. Gas tanks from various Hondas Hyundai etc are under $200 new..
Thoughts?
Thanks
General thoughts:
As mentioned above, I've heard that GSL-SE tanks don't fit the 78-80 SA cars due to the tire well shape.
The rx7 tank is pretty large at 15+ gallons. Tanks from Honda's and Hyundai's are likely to be smaller. May be relevant if you are endurance racing.
It also seems unlikely that a tank made for a different car would fit very well. A squarish tank from another car might sit too low without cutting out the spare well - maybe that's not an issue.
Cheapest, easiest option is probably to get a cheap ebay surge tank for sub $100 (or the diesel filter housing wvu suggested) and go that route. There are surge tanks out there that use hose barbs and thus you can save $ on fittings. Space is tight which is partly why I did an internal surge tank, but I think there is probably room if you are careful with how you mount it.
Or, if you are confident you have all the fumes out, you could cut open the stock tank and weld in a surge cup like the GSLSE tank.
Fill it with water when you cut it, no room for fumes. I'm running the LS MG on a mostly stock tank with an external fuel pump, although I did open it up to play with baffles. Tough to seal up again afterwards.
In reply to sevenracer :
Can you or someone elaborate on the diesel filter thing?
Also, a link to the eBay surge tank please.
Thanks!
You do not need a larger fuel line. The feed line is 8mm, same as what the FC had. The stock return line is 7mm.
I went for years with just replacing the carb's fuel pump with an EFI rated pump. Supposedly the GSL-SE had baffles in the tank, or something. I didn't bother. Never noticed an issue except the car would stumble in left had corners when down close to empty, or under 1/8 tank, depending on how spirited the driving is. Solution: Keep tank over half full when rallycrossing.
After a while, I decided to make a surge tank setup, as I had it in my mind that I was getting vapor lock in the fuel line going to the pump under some conditions.
I nmade it out of 2" threaded pipe, held to the floor with exhaust clamps. The original fuel pump feed line goes to a rattlebox parts-store carb fuel pump mounted forward of the other bits. Its outlet goes to a 5/16" hardline that threads into the middle of the surge tank. The bottom cap has a 5/16" hardline that goes to the EFI fuel pump, that feeds the fuel filter for a '92 Century with nice quick disconnect fittings. They're for plastic line but EFI hose clamps onto them just fine too 
That goes forward to the engine. The return line from the engine bay has a hose going to a 1/4" hardline that attaches about 1/3 of the way up. In the top cap is the return bleed to the tank.
The idea is, the rattlebox pump ("lift pump") keeps the surge tank (about 700ml in size, enough for 20-30 seconds of WOT) full. It feeds halfway up to minimize aeration. IF the lift pump sucks air, the return line from the engine keeps the surge tank full, and its inlet to the tank is mounted low to maximize how long it is before it starts aerating the fuel.
Works a TREAT. And was not all that expensive to build.
Make sure all your soft lines are rated for EFI fuel system pressure.
NOHOME
MegaDork
11/10/21 7:35 a.m.
Snag a tank from any LH model. Intrepid, Concorde, 300M.
Squarish in shape.
Not a metal tank, its some kind of poly something.
Early models (98-00) have a drain plug on them.
Can probably be had cheap from a junkyard.
Fuel pump is in tank.
Specs say 17 gallon.
Just something to consider
NOHOME said:
Would something like this aid the cause and eliminate the need for a surge tank? The filter/regulator itself is only like $80 so if an intank pump is better roll your own.
Holley also makes an overpriced sock for the in tank pump that is advertised as a way to eliminate fuel starvation and suck every last drop of fuel out of the tank.
https://www.ebay.com/itm/283626849961?_trkparms=amclksrc%3DITM%26aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D20200818143230%26meid%3Ddbf24fde7d474b808460bd16ff4e3253%26pid%3D101224%26rk%3D1%26rkt%3D5%26sd%3D192405424338%26itm%3D283626849961%26pmt%3D0%26noa%3D1%26pg%3D2047675%26algv%3DDefaultOrganicWeb%26brand%3DCustom&_trksid=p2047675.c101224.m-1
Pete
The surge tank is to keep that fed so it never sucks air. You use a lift pump to feed the surge tank, doesn't matter if the lift pump sucks air as long as the surge tank has enough volume to cover the momentary loss of feed.
Think of it like a dry sump fuel system
Is the image I linked above working? Not seeing it on mobile.
wspohn
SuperDork
11/10/21 9:04 a.m.
More compact if you use an FI intended tank, but you can also do it the other way.
My rebodied MGA uses a 3.4 GM V6. I use a primer pump - regular old electric low pressure pump - to fill the 'tank' which is nothing more than a head bolted to the car frame that takes a truck sized spin on fuel filter that acts as the surge and return tank. The gas flows from there to the high pressure pump which lives low down on the frame (FI pumps are rubbish at priming themselves) and thence off to the fuel rail on the engine.
My particular set up uses a return line of smaller diameter that sends fuel not used by the engine back into the filler neck of the tank, but you can get returnless set ups.
The return uses a Moroso kit intended for that purpose
I think you're over-thinking things. Unless this is a max-effort race vehicle, I wouldn't worry about it. Drop a high pressure pump in the tank and go. I autocrossed my old 96 Impala SS last year and it has a big, wide, flat belly tank under the trunk with next to zero baffling and it never had a hiccup.
Curtis73 (Forum Supporter) said:
I think you're over-thinking things. Unless this is a max-effort race vehicle, I wouldn't worry about it. Drop a high pressure pump in the tank and go. I autocrossed my old 96 Impala SS last year and it has a big, wide, flat belly tank under the trunk with next to zero baffling and it never had a hiccup.
There is no in tank fuel pump. There isn't even a sending unit that has lines going through it that you can bodge a pump onto, those are part of the tank. The fuel pump sits under the driver's seat after going up and over the rear axle.
Fuel starvation happens in left corners and, perversely, under acceleration, because the fuel pickup is at the driver's front of the fuel tank.
The middle of the tank is very short due to the spare tire well, the only high spot is on the driver's side.
For my Dart project, I took a bread pan, cut it to about the shape of a sump, and welded it onto my fuel tank. I'm using an external pump.
wspohn said:
More compact if you use an FI intended tank, but you can also do it the other way.
My rebodied MGA uses a 3.4 GM V6. I use a primer pump - regular old electric low pressure pump - to fill the 'tank' which is nothing more than a head bolted to the car frame that takes a truck sized spin on fuel filter that acts as the surge and return tank. The gas flows from there to the high pressure pump which lives low down on the frame (FI pumps are rubbish at priming themselves) and thence off to the fuel rail on the engine.
My particular set up uses a return line of smaller diameter that sends fuel not used by the engine back into the filler neck of the tank, but you can get returnless set ups.
The return uses a Moroso kit intended for that purpose
Yep, this.
The only difference is that you need to tap the middle of the filter housing and run a rod down to the bottom of the inside of the filter so that the FI pump picks fluid off the bottom.
wspohn
SuperDork
11/10/21 10:20 a.m.
wvumtnbkr said:
The only difference is that you need to tap the middle of the filter housing and run a rod down to the bottom of the inside of the filter so that the FI pump picks fluid off the bottom.
That is already taken care of - one of the shorter fittings attached to an extended tube that picks up from the bottom of the filter and goes to the HP pump down on the frame.
