PI fuel pipe lenghts

[Jason]

PI is pretty high presure, injectors trigger at 60 psi I think.

[shenderson]

The pressure is much higher in diesels, which is why they use steel pipes.  The Lucas system injects at about 80 psi, but the pipes are neoprene so much more likely to expand and elongate under pressure.

Best practice is to use equal length pipes; the reason they aren't is probably because Triumph were penny pinching.

Steve

[JohnD]

Please explain further!
Equal length pipes to minimise friction losses?
I don't understand why that is important.

John

[JohnD]

Flow through a tube is directly proportional to the pressure difference across it.
And inversely proportional to the fourth power of the diameter.
And directly to the length.
(ignoring viscosity etc.)

So the loss of pressure at the far end of a longer tube will be proportionally MORE with a lower pressure system, than with a high pressure one.  So length should be much more important for low pressure than high pressure engines, something that Lucas clearly were not bothered by, or the hoses would bethe same length

I suspect that the answer is in timing.
Injection timing is unimportant in a petrol injection engine.  There are continuous flow systems out there, like Tecalamit, and while the Lucas Pi is pulsed, the timing is only approximate, nothing like the accuracy that is needed for ignition, as anyone who has installed a metering unit will know.
  However, it is vital for a diesel engine, where the fuel enters the combustion chamber as droplets, rather than vapour.  Mistimed injection would lead to rapid 'drop-out' and collection of liquid fuel, that will not evaporate easily, let alone burn.

Rather than suffer significant pressure loss down a longer hose, it is the time taken for the pressure pulse to travel to the injector that becomes significant.  Only if the hoses are the same length will the pulse take the same time to travel and so arrive at the right moment.

Same length hoses are good practice on diesels, but not required for petrol injection.

John