Why are aftermarket surge tanks so big?

[RAd28]

Is there a particular reason why EFI surge tanks are always so big? I think the smallest i've see has been a 1L tank, and the biggest a whopping 7L... what is the point? My understanding is that they're there to provide fuel whilst under hard acceleration (cornering or straight line) where the fuel tank pickup might be starved... now... i'm figuring that some pretty hard driving would see your fuel economy drop to 30L/100k's... which means a 1L surge tank would be good for 3.3k's of driving... so basically 2 laps of a decent race track... even at 100L/100k's your getting 1K of driving on a tiny 1L surge tank... which is 2 quarter mile passes...or most the way around a good long track...

so the question is... is there any reason why i should avoid a 500ml surge tank?

[TurboRA28]

I would like to know also.. Can't fit much of a surge tank under the RA28 so i've got a 4ltr one in the boot. But if this is overkill and I can get away with a smaller one under the car it'd be good.

So interested to know the smallest size I can safely get away with for track days.

[RAd28]

Have you monitored fuel comsumption at the track at all? I'd be using that as a fairly good gauge of surge tank size...

Anyone know whether there's any harm in letting the EFI pump run dry? or getting air into the EFI side of the system?

[pope]

EFI fuel pumps are fuel lubricated. Opened up a Bosch 070 the other day and it wouldn't last too long at its normal operating speed without lubrication.

[WDE_BDY]

You also need to factor in the difference between what your pump flows and what gets used by the engine. Quite a bit of fuel goes down your return line and back to the tank. If your return line goes to the main tank rather than the surge tank ( many do to keep from recycling heated fuel) then your useage is much more than just what the engine needs. I would think 1L is the minimum level for a high horsepower engine.

Callum

[Youngy]

I think a 1 litre is pretty small really - are you guys really thinking of going smaller than a litre?

[LeeRoy]

When an 040 flows 300LPH...

thats 5L per min,
or 83ml per second.

so if you return fuel to main tank you have 12 seconds of surge from the lift pump before you run dry if using a 1L surge tank. Dismiss the bottom 166ml as it would probably sputter at that level.

....so 10 seconds. Whats the duration of a decent corner?

[KARNAGE]

when i helped my mate do his xu1gtr torana first we put in a 1L surge tank and its mainly a drag strip car and even after the warm up of the tires the tank ran out just after the quarter mile so we through in a 5L tank in the boot. this is a very high horse power engine though so most wouldnt need 5L tanks but as leeroy said a 1L tank in most will run dry under heavy driving

[Hen]

I have a 700ml surge tank shoehorned under the rear of my Sprinter. Admittedly not a high horsepower engine (110rwkw) but I have NEVER had fuel surge in 5 years of hard drifting and a plenty of normal and fun driving.... well the only times it's surged was when I totally ran out of petrol.

Doing the maths for your fuel consumption (injector cc and max duty cycle help there) give a good idea how fast the surge tank empties. Then decide how long you need. This gives the size of tank you want.

Hen

[IN 05 NT]

When an 040 flows 300LPH...

thats 5L per min,
or 83ml per second.

so if you return fuel to main tank you have 12 seconds of surge from the lift pump before you run dry if using a 1L surge tank. Dismiss the bottom 166ml as it would probably sputter at that level.

....so 10 seconds. Whats the duration of a decent corner?

yes, BUT, you would need a massive motor to use 1ltr of fuel in 12 seconds!....plus you still have a constant supply of fuel from the return and the lift pump filling it.

my surge tank is a 2ltr in the ra23, done many laps in a row and never had a surge problem,

im looking at using a small surge tank in teh corona....600ml or so....but am still interested to know other opinions aswell.

[RAd28]

IN05NT - The point LeeRoy is making there is that you'll have 12 seconds of your main pickup starving when the return is going to tank, so this'll be lessened by the amount your engine is using... which is a valid enough point, and one i hadn't considered.

HEN - Did you return to main tank or surge tank?

I hadn't considered the return to main tank because i've never done it... the idea that fuel heat soak is an issue is one i also don't understand, as LeeRoy mentioned 5L/min means the fuel is moving fairly fast, and considering that the fuel pump will be at almost ambient temperature (in a mild street car) since it's cooled by the amibent temperature fuel as is the case with the engine bay fuel lines and rail. I really can't see that the fuel temp would increase by any more then a degree or two... therefore if you return to surge tank you should be able to get away with a tiny surge tank...

In my previous surge tank design i used a single feed from the bottom of the surge tank, and lift pump feed from the main tank, return from engine bay and overflow to main tank all entered the top of the tank at a tangent to the internal wall to assist swirling and minimise airation... haveing grown wiser since then i decided that's a waste of time... next time all ports will be at the base (except for overflow obviously) because the tangent idea is a bastard to manufacture...

[oldcorollas]

if you don't plumb return to surge, then you need go by the EFI pumps capacity at pressure.
if you do, you go by maximum flow of injectors.

work out the longest corner or acceleration you will do, say... 10-15 seconds, work out how much fuel flows during that time.. add 50% or 100% for fudge, and then it should be big enough?

also depends on the shape of the surge tank. the bigger the tank, mor elikely to get surge within the tank..

then again, if you are just doing drag with not such big power, or drifting, then you won't get G-forces throwing the fuel around so much, and lift pump running out of fuel is not as much of an issue.
but doing circuit work with heavy braking and high G cornering, then it is an issue.

[Hen]

I run return to the surge tank.

And I think the silly large surge tanks may often be due to the "bigger must be better" mentality. A scant few may need them, but most don't.

Hen

[MWP]

EFI fuel pumps are fuel lubricated. Opened up a Bosch 070 the other day and it wouldn't last too long at its normal operating speed without lubrication.

They use the fuel passing through as a coolant as well.
They overheat if run dry for too long.

so if you return fuel to main tank ...

Why would anyone do that though?
Its obvious that returning to the surge tank means you can get away with a smaller surge tank and possibly smaller low pressure pump.

[TurboRA28]

Yeah I run the return line back to the surge tank..
So if I run 4x 500cc injectors, that is 2 ltrs per minute ?