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Thread: Fuel Cell Placement/Debate

  1. #1
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    Fuel Cell Placement/Debate

    The E30 fuel pump feeding my turbo M42 happened to seize winter 2017 so my car has been sitting in the barn for 2 years.

    In the meantime I’ve hatched a plan to do an E85 conversion+MS2 on my turbo setup to get just a little more power out and have more control over the tune (currently a custom motronic tune). I’ll post up a new thread for that project when the time comes, but one of the things I’m considering is replacing the stock gas tank saddlebags with a plastic fuel cell to avoid tanks rusting out in the future. I’ve seen folks put tanks in the spare tire well, but I’m not sure I want to move weight behind the rear axle since I am happy with the weight balance and suspension setup on the car so far.

    Someone please convince me as to where the best placement is and what fuel cell to go with. I am considering putting a 5 gallon cell cell on passenger side in the spot where the current stock tank is and still running an in tank pump unless it really makes sense to put one in the spare tire well or elsewhere.

  2. #2
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    Keep us updated. I'd love to turbo my m42 in 2-3 years.

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  3. #3
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    Quote Originally Posted by sonomaGTLN2 View Post
    The E30 fuel pump feeding my turbo M42 happened to seize winter 2017 so my car has been sitting in the barn for 2 years.

    In the meantime I’ve hatched a plan to do an E85 conversion+MS2 on my turbo setup to get just a little more power out and have more control over the tune (currently a custom motronic tune). I’ll post up a new thread for that project when the time comes, but one of the things I’m considering is replacing the stock gas tank saddlebags with a plastic fuel cell to avoid tanks rusting out in the future. I’ve seen folks put tanks in the spare tire well, but I’m not sure I want to move weight behind the rear axle since I am happy with the weight balance and suspension setup on the car so far.

    Someone please convince me as to where the best placement is and what fuel cell to go with. I am considering putting a 5 gallon cell cell on passenger side in the spot where the current stock tank is and still running an in tank pump unless it really makes sense to put one in the spare tire well or elsewhere.
    In our build I kept the passenger side part of the stock tank. Allows the factory pickup tube assembly. Cut off the crossover fitting and welded patch over the hole I made. 2gal surge tank in the trunk for extra capacity. The weight balance is best in the factory location. If possible I say keep it there and work a solution. Tank is also most protected in front of the subrame safety wise. Factory got the tanks in the right place IMO, when on scales it's apparent how well it is.

    My fuel pump setup is carb so dual carters but if fuel injected I'd just use one carter to feed a surge tank. From the surge use your inline high pressure pump ie bosch 040 or 984 to feed the rail.
    Last edited by autox320; 06-16-2019 at 06:02 PM.
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  4. #4
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    Quote Originally Posted by autox320 View Post
    In our build I kept the passenger side part of the stock tank. Allows the factory pickup tube assembly. Cut off the crossover fitting and welded patch over the hole I made. 2gal surge tank in the trunk for extra capacity. The weight balance is best in the factory location. If possible I say keep it there and work a solution. Tank is also most protected in front of the subrame safety wise. Factory got the tanks in the right place IMO, when on scales it's apparent how well it is.

    My fuel pump setup is carb so dual carters but if fuel injected I'd just use one carter to feed a surge tank. From the surge use your inline high pressure pump ie bosch 040 or 984 to feed the rail.
    I agree with Jody when running a 4 cylinder. With our M20B25 we put an 8 gal in the floor of the trunk, top of tank level with floor of trunk. We also have a full size battery in the trunk on PS outside like the E30s. With me in the drivers seat and 8 gals in the tank our weight balance is 51% front and 49% rear. Our car comes in at 2330 pounds with me in it and 8 gals.
    Last edited by OLD MAN; 06-16-2019 at 10:03 PM.

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  5. #5
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    Quote Originally Posted by OLD MAN View Post
    I agree with Jody when running a 4 cylinder. With our M20B25 we put an 8 gal in the floor of the trunk, top of tank level with floor of trunk. We also have a full size battery in the trunk on PS outside like the E30s. With me in the drivers seat and 8 gals in the tank our weight balance is 51% front and 49% rear. Our car comes in at 2330 pounds with me in it and 8 gals.

    Thought process the 6cyl weighs more so it offsets it better? or uses more fuel so need more capacity? With half the factory tank just using the passenger side and a 2gal surge(passenger side center of trunk) we hold approx 7gal. If thinking fuel usage, It seems plenty even for all out track day filling up between sessions. Drops anywhere between 1/4-1/2 tank each 20-30min session. Even the M when on track with higher output 280-300hp used about 0.25gal/min or about 1/4 tank per session ie approx 3gal. I plan to use similar setup on it as the 320 only having a tank on the passenger side saving the other well for exhaust.

    Weight wise it moves the corner weight around during use if stick it all in the trunk. Some say it's noticeable and some say it isn't bad. I can say this it's much easier to engineer just using the trunk space. If keep low it could help weight balance front to rear. All front engine cars get too light in the rear when cut and start modifying.
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  6. #6
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    Thanks for the responses guys. I have never heard of a surge tank before. How does it attach to the main tank and what keeps fuel moving between the tank chambers?

  7. #7
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    Quote Originally Posted by sonomaGTLN2 View Post
    Thanks for the responses guys. I have never heard of a surge tank before. How does it attach to the main tank and what keeps fuel moving between the tank chambers?
    A surge tank is very basic and simple design but very effective. Idea is never allow fuel starvation to the main pump keeping a "head" on it. Starvation causes surges in power delivery especially in turns. The factory pickup tube and intank pump can eventually see air which will interrupt fuel delivery. BMW's this is mostly hard right handers especially sweeper turns as fuel moves towards the aux tank. They've tried to alleviate it with dual pickups(e46s) but it's only another bandaid.

    With a surge tank your using dual pumps. The first transfer pump is used to pickup from the main tanks into the surge tank. Surge tanks minimize sloshing and keep a head on the main pump used to feed the engine. A tall vertical cylinder is most optimal but compromises can be made on design. Basically a IN (from first pump) and OUT for the main pump at the lowest point. Need an overflow at the top of the tank to feed back to the main tank. That's really it, and some take to extreme by making internal to the main tank to save from running lines. In a nut shell your keeping a head on the main pump and that's it.

    Here's my homemade square surge tank for 2gal capacity and to add a bit of rear weight. It has a baffle at the inlet outlet inside. Google fuel surge tank and you'll see basics.

    88 M3
    91 318is
    91 318i
    83 320is
    08 X3 3.0si

    "If it flys, floats, or f*cks, rent it!"

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