Water pump flow rate?

[kenmorton]

Anyone know what the flow rate of an average 3ltr petrol engine water pump is when the engine is near maximum revs?

[GreenV8S]

kenmorton said:

Anyone know what the flow rate of an average 3ltr petrol engine water pump is when the engine is near maximum revs?

Depends on the layout, a V engine will typically need a much higher flow rate than an inline one, and of course engine designs will vary anyway. But assuming it's a V then rough order of magnitude 2-3 liters per second with a pressure differential probably around 2 bar.

[knighty]

hmmmm, the coolant flow is really a factor of the amount of power/heat that the engine generates........all I can do is say that for every bhp, you should have 1/3rd that figure in L/min as a coolant flow rate.......so lets say its a 300bhp engine, you should ahve 100L/min coolant flow rate......I have used this on many professional race engines and kit cars and it works a treat.

[ridds]

I would agree with that.

[GavinPearson]

You'd want a pump with an output of about 80 litres / minute.

[kenmorton]

So the Davies Craig 110 litre/min pump should just about cope with my 360 bhp.

My problem is a kit car with the engine in the back, radiator at the front. The belt driven pump works fine at medium to high revs but at idle and up to 1500 rpm with all the extra pipework it just won't push the water round. Now I'm sure the electric pump will work well at low speed but just a bit concerned weather it will flow enough at max revs.

BMW have started fitting an electric water pump to the twin turbo straight 6's (300 bhp) anyone know what these flow like?

[GreenV8S]

kenmorton said:

So the Davies Craig 110 litre/min pump should just about cope with my 360 bhp.

The flow rates they quote are IMO extremely misleading. As soon as you offer the slightest back pressure the flow collapses. The combination of a low power motor that can only produce torque at low speed, and a small centrifugal pump that only produces pressure at high speed, result in a terrible performance curve. There's a good reason that mechanical water pumps take several thousand watts of mechanical power off the crank, and the little electric pumps take about a hundred watts. Look inside them, although they take a decent size hose the inside of the pump is so small your little finger will get stuck; it's seriously restrictive even if you were planning to keep the original mechanical pump too.

If you have an engine with extremely low water flow requirements you might get away with it for a while, because it doesn't take much water flow to control the average engine temperature. But without decent flow you'll be vulnerable to hot spots in the head and durability is likely to be severely compromised. If you're pushing out 360 bhp that implies a hefty water flow requirement and the little EWP just can't provide it.

[kenmorton]

So the other solution of using the EWP as a booster pump for low speed flow would then become a restriction to the high speed flow of the mechanical pump. I can't put a smaller wheel on the pump to over drive it either because it uses the back of the timing belt to drive it so it would cock the timing up (unless I can work out exact belt pitch difference and find a new length belt)

How have BMW overcome the weakness in the electric water pump flow for the 300bhp 335i motor, or are they not expecting people to be thrashing it flat out untill it's out of warranty?

[GavinPearson]

kenmorton said:

So the other solution of using the EWP as a booster pump for low speed flow would then become a restriction to the high speed flow of the mechanical pump. I can't put a smaller wheel on the pump to over drive it either because it uses the back of the timing belt to drive it so it would cock the timing up (unless I can work out exact belt pitch difference and find a new length belt)

How have BMW overcome the weakness in the electric water pump flow for the 300bhp 335i motor, or are they not expecting people to be thrashing it flat out untill it's out of warranty?

You need to remember that you have a cooling system, i.e. hoses + block passages + head design + the pump. If you match the parts as a system well, then you may be able to downsize certain components.

In BMW's case they would have made sure that the system was pretty damn reliable before releasing it, I don't think their track record would suggest anything different to that, but remember that they may also have a helper pump somewhere in the system that you're not aware of. They also may be using a coolant that can take higher heat levels than most.

In your case you need to optimise what you have to the best of your ability.