Why are aftermarket surge tanks so big?

[oldcorollas]

i'm bored/procrastinating at work, so here is more hectic diagramness

so.. tanks are usually flatter and wider than the one above...

A is height of tank, B is width of tank.

if there is 1G cornering (or accel, whatever, just think of looking at tank from different side)
then fuel will be at 45deg..

the fuel in tank to start with = AxBx(tank fullness)

the fuel when cornering(etc) will be:
0.5xAxA + CxA = A(C+A/2)

then you balance the equations, cos the fuel has to be same amount

AxBx(tank fullness)=A(C+A/2)
Bx(tank fullness)=C+ A/2

and you want C+A to be AT LEAST half of B, or else it's gonna surge (assuming pickup is in middle)
ie C+A= B/2
or C = B/2 - A

so
Bx(tank fullness)=B/2 - A + A/2
Bx(tank fullness)=B/2 - A/2
2Bx(tank fullness)=B - A
which gives us...
tank fullness = (B-A)/2B

in other words..
if you have tank where B=2A (ie twice as wide as high), you only need 1/4 full tank
if you have tank where B=4A (ie 4 times as wide as high), you need 3/8 full tank
6:1 ratio tank = then 5/12 full etc..

that doesn't work for square tank, cos C is 0.. but most tanks are not square...
for square tank (B=A), then 1/8th tank is ok...
or more basically, you just need fuel height 1/8th of the width (if height is taller than width) to keep pickup covered.

that make sense?

anyway, then you need to consider inertia adn the initial fuel slopping around, as that will be more likely to uncover pickup momentarily than steady state.... (which is what the baffles are there for), and surge tank is more for the momentary direction transitions, rather than steady state...

if you need surge tank for steady state, then you need a fuel cell

[oldcorollas]

and instead of using letters and shit, you can put in measurements, and then work out the tank fullness directly.. even with offset fuel pickup (measure from the long side)

ie tank is 65cm by 13cm, and fuel pickup is 40cm from one side (and 25 from the other)
A=13, B=65, and C+A = 40

C+A = 40, and A is 13, so C=27cm

Bx(tank fullness)=C+ A/2
(tank fullness)=(C+ A/2)/B
(tank fullness)=(27+ 13/2)/65
= 33.5/65 = 51.5% full
gotta keep tank just above half full to stop surge


edit, plus you can do the same type of calculation for the surge tank itself, bu tthey are usually tall enough and thin enough that they have to be almost empty to run out of fuel...esp if they taper at the bottom

[IH8TEC]

mines 2L, and after a drag race 12sec at full noise, you can hear the std intank pump catching up filling the surge tank again, i run a bosch 044 external, thats with 550cc injectors which flow alot more (874cc at 77psi 100% - flow tested today), -6 returns into the surge tank, and excess from tank goes out top of surge tank into the main tank. run a -8 feed as well.

[TurboRA28]

Thats some good information OC!

[Cuzzo]

OC you can either bullshit really well or you know too much info.

[RAd28]

Lol... that's a pretty good lesson in taking a simple easy to understand argument, then turning it in to something that makes you go cross-eyed.

and what is 'C' OC?

After all that most enlightening discussion, does anyone have a valid reason why a 1L (or smaller) surge tank would NOT be good enough for a sub 2-300hp street driven car with fuel return to surge tank?

[JustenGT8]

Lol... that's a pretty good lesson in taking a simple easy to understand argument, then turning it in to something that makes you go cross-eyed.

and what is 'C' OC?

After all that most enlightening discussion, does anyone have a valid reason why a 1L (or smaller) surge tank would NOT be good enough for a sub 2-300hp street driven car with fuel return to surge tank?

It'll be fine

[oldcorollas]

Lol... that's a pretty good lesson in taking a simple easy to understand argument, then turning it in to something that makes you go cross-eyed.

and what is 'C' OC?

After all that most enlightening discussion, does anyone have a valid reason why a 1L (or smaller) surge tank would NOT be good enough for a sub 2-300hp street driven car with fuel return to surge tank?

the poin tis you don7t need to guess when you will surge

C...
i just assumed 1G side loading, so the fuel sits at 45deg. when it is at 45deg, the slope of the fuel will be A high adn A wide. (cos that is height of tank). C is just the distance from side of tank to where fuel touches top of tank.

1L is fine but so would half a tank

[Sciflyer]

engine isnt under full throttle under braking/cornering so the tank would be more likely to fill upto being full during this time. so by time you open the noise up exiting the corner its full of fuel.

if the tank is completely dry you still have the surge tank full plus the fuel thats in the main/return lines, then as soon as the lift pump comes back into fuel it starts filling the surge tank again. add to that a lot of you guys are talking about EFI tanks which are baffled from factory hence less likely to have fuel surge in the first place.. 1L like the ones i sell should be heaps for most 4cyl appt

Doesnt always work like that in practice, some cars will get fuel surge quite easily if you corner hard with a higher-flowing pump in the tank - both my soarers will do this with Walbro pumps in, much less than 1/3 a tank of fuel and its surge time (although i dont have a problem with the stock pumps)

[Duk]

and instead of using letters and shit, you can put in measurements, and then work out the tank fullness directly.. even with offset fuel pickup (measure from the long side)

ie tank is 65cm by 13cm, and fuel pickup is 40cm from one side (and 25 from the other)
A=13, B=65, and C+A = 40

C+A = 40, and A is 13, so C=27cm

Bx(tank fullness)=C+ A/2
(tank fullness)=(C+ A/2)/B
(tank fullness)=(27+ 13/2)/65
= 33.5/65 = 51.5% full
gotta keep tank just above half full to stop surge

edit, plus you can do the same type of calculation for the surge tank itself, bu tthey are usually tall enough and thin enough that they have to be almost empty to run out of fuel...esp if they taper at the bottom

Where's the body roll correction factor?

[oldcorollas]

you really want? it is not hard if i get bored i will do some general calcs
you will have to guess the "slosh" factor tho.

[LeeRoy]

Maybe i can ask here and someone can answer then. I have one of Lindens surge tanks, not sure how big, maybe 1L?

I wanna run two 044s from this into 1/2" fuel line, with a 3/8" fuel return. Probably just 1 big lift pump, but would be okay? Will the surge stay full for most driving styles???

Might also be tuned on e85 with 1000cc injectors...???

Cheers,
Jase

Only if you return to the main tank would you empty the surge tank. If you return to the surge tank you will only deplete it by what the engine is using if the lift pump is not adding to its fullness.

if the tank is completely dry you still have the surge tank full plus the fuel thats in the main/return lines, then as soon as the lift pump comes back into fuel it starts filling the surge tank again. add to that a lot of you guys are talking about EFI tanks which are baffled from factory hence less likely to have fuel surge in the first place.. 1L like the ones i sell should be heaps for most 4cyl appt

If you run out of fuel to feed the main pump it will drop pressure and the engine will start to sputter.

Chances are the tanks we are talking about have as much baffles as george bush has brains. In a friends old VN that used to surge with anything less than half a tank and a good corner.

1L would be fine in the majority of cases

[RAd28]

ae71 - As LeeRoy mentioned, the initial question was asked with retrofitting surge tanks and EFI to older cars with no baffles etc.

answred my question, This time around, i'm going for a baby surge tank...