Coolant Pipe Diameter

[AndyW]

Is there any optimum size or requirements for the diameter of coolant pipes and hoses?

 

I'm looking at replacing some of the top and bottom hoses on my Superspec Rover engine, but they seem to be a bit of a mish-mash of diameters. The thermostat main outlet, the water pump inlet, and both the radiator stubs are all 32mm OD.

 

But the rubber top hose from the thermostat increases to 38mm ID to fit a 38mm OD alloy joiner pipe, then another hose tapers back down to 32mm to fit on the radiator top inlet. Same for the bottom hose, this starts as 32mm, increases in diameter to fit a 38mm alloy pipe, then back to 32mm to fit the water pump.

 

I don't know if this increase and then decrease in pipe diameter is necessary and "by design"? Or just because they were the hoses and 38mm joiner pipes supplied.

 

Can I simplify everything and use 32mm diameter thoughout, or will this cause flow restrictions? Is there really any need to increase the diameter of pipe runs? Or am I over-thinking all this :-)

 

Andy

 

[Snapperpaul]

Over thinking

If its 32 out and 32 in just use 32

[brumster]

What he said

 

Probably was done for convenience. You are right, diameter is a factor, and the fluid flow will slow when it enters a larger diameter pipe, but since it's going back into a 32mm it won't make any difference in your case (if the pipe is all one section from enlarging to narrowing, with no take-offs inbetween).

[MarkBzero]

I was thinking about this the other day, not just pipe diameters but also the number of joiners as I cannot buy pre-shaped coolant pipes.

[AndyW]

Probably was done for convenience. You are right, diameter is a factor, and the fluid flow will slow when it enters a larger diameter pipe, but since it's going back into a 32mm it won't make any difference in your case (if the pipe is all one section from enlarging to narrowing, with no take-offs inbetween).

 

Ok, so 32 > 38 > 32 is unnecessary normally. Keeping it all at 32mm makes it simpler.

 

But you mention takeoffs. How does that affect the flow? At the moment the output from closed thermostat that normally goes to the heater, returns into the top radiator, which is stupid for fast engine warmup. I intend to tee the 15mm return pipe into the bottom hose instead before it goes into the water pump.

 

Can I still tee that into the 32mm hose entering the pump, or should I increase the diameter of the inlet bottom hose? As far as I can understand it, the 32mm of the pump inlet is the restricting aperture so what happens before it doesn't really matter.

[fry61]

Just to go against the flow !!! A lot of Florin's pipes are in 22 mm copper with copper adaptors to suit radiator hose connections. I thought that the smooth bore of the copper would balance the reduction in size --- do a calculation of water pump inlet/outlet cross-sections -- on our 1800 CVH it was less than the csa of 22 mm pipe. Plus also the copper will liberate far more heat than the rubber hose.

[brumster]

 

But you mention takeoffs. How does that affect the flow? At the moment the output from closed thermostat that normally goes to the heater, returns into the top radiator, which is stupid for fast engine warmup. I intend to tee the 15mm return pipe into the bottom hose instead before it goes into the water pump.

 

Can I still tee that into the 32mm hose entering the pump, or should I increase the diameter of the inlet bottom hose? As far as I can understand it, the 32mm of the pump inlet is the restricting aperture so what happens before it doesn't really matter.

 

Well, I won't attest to being a fluid dynamics expert, I suspect there are a lot of variables - but what I meant to point out was that if there was a take-off somewhere in your run, then obvious a proportion of coolant will pass down that versus staying in the "main" hose run... assuming there is nothing blocking it or otherwise adding resistance in to the circuit, of course, like a heater matrix for example.

 

So your 15mm return pipe should provide enough resistance that general flow when the thermostat is open, will be round the complete circuit. The water will find it "too much bother" to go down it versus follow the flow through the engine, BUT when the thermostat is closed there will be enough pressure that it decided that narrow 15mm tube is the best way to go... given enough pressure behind the water, it will pass through it (quickly) and.... well, ok, into the rad at the moment but I get what you're saying, better to direct it to a heater or else bypass the rad.

 

In this case, you're not fighting the resistance of the pipe diameter, it's the thermostat - which will be far narrower than any hose equivalent - so I wouldn't worry about it. Like you say, be it 32mm or 38mm, it won't be the significant restriction in the system during warm-up.

[Longboarder]

I'm sure the diameters should be by design but more the design of the engine designer (and they don't always get it right) and subsequent testing (as demonstrated by the odd head gasket holes for flow pattern in the engine). I would be directed by the stubs on the engine not the radiator. If the stub on the water pump is 32mm then 32mm hose to its destination. No narrower but I can't see a section somewhere of larger pipe along its length would make any difference. You do see sections on tintops that start at 32 at the rad and end 38 at the pump on the zetec for instance but that one has a 15mm joining part way along the hose length so the 38 section has a feed of a 32 and a 15. My hoses are a mishmash of what I had in the box of spares, a bit from the sierra, bits from zetecs, a bit I bought and some copper.

[AndyW]

Great. Thanks everyone. Just wanted to make sure I wasn't going to cause problems by changing everything to 32mm.

[agent_zed]

I'm just doing that same as my bottom hose blew the other day, thankfully as i pulled into the drive after returning from work.

 

I've found that most of my connections are 32mm but the water pump seems to be about 38mm. It's always been like that and nothing i can do about it as the radiator inlet and outlet is 32mm. I am going to replace all of my pipes as they are all pretty old but annoyingly i am going to have to buy various bends and straights and a reducer for the bottom hose to be able to replace what i have. I'd rather have less joins but i hey ho.

 

If everything is 32mm i'd use 32mm.