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I'm going to build myself a few sets of the opaque polyamide fuel lines with the cohline cone 12 x 1.5 fittings. I have a choice of the 1.85mm ID lines or the 2.75mm ID lines. The original 45 year old steel lines were 1.85 ID. Anybody want to weigh in on the advantages or disadvantages of the smaller or larger ID lines?
Speedo
I have used poly fuel lines for many years on MFI engines and my MFI flow bench and found no difference with the fuel flow volume's between 3mm I.D. ( 2.5mm I.D. ) and 4mm I.D. ( 3.5mm I.D. ) as long as the lengths are 36" or less. I haven't used the 1.85mm.
What I have found is the polyamide has a 500 psi bursting pressure @ 70 degree's F. Normally this is no problem with a street engine but with a track event engine the heat in the engine bay lower's the bursting pressure a little, and if a fuel injector problem occurs it may cause the line to burst. I use 3mm I.D. ( 2.5mm I.D. ) x 5.5mm O.D. ( 6mm O.D. ) Nylon fuel lines with a 800 psi bursting pressure @ 70 degrees F. This is a little safer but still not as safe as the original steel lines.