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- - Fuel line sizing. (https://www.miataturbo.net/general-miata-chat-9/fuel-line-sizing-49776/)

The dyno will tell the tale no matter what theory is behind it.

I am currently using the stock fuel lines. Eventually I will switch both over to feeds and run a single -6AN return line.

Dumped a new 6-speed into it today, I'll be at the dyno this week. It made 310whp at 18psi last time, on 100 octane through the stock intake manifold. That should be 340-350 on 100 octane, 360-370 on E85. If the head will flow it, I think 400whp should be obtainable at ~24psi. In reality, I'll be happy with ~370 at ~21psi.

Stock return FTW.

I've never thought of using the stock return as another feed to the rail. Great idea.

Quote:

Originally Posted by Rennkafer (Post 601652)

This is something we do on race cars regularly. The reason is that if you run the same size lines in/out then any flow restriction in the return line can act as an uncontrolled fuel pressure regulator. A larger return line gives you some overhead for "stuff" happening (like rocks/curbs/etc hitting the return line and restricting it).

My comment about needing more flow was meant to be as compared to the 350HP engine, which if it's running on straight gas needs less fuel flow than the same 350HP engine on E85.

Bill..I am def going to hit you up when it's time to convert my fuel lines. Just waiting for engine and turbo to be finish...

Quote:

Originally Posted by y8s (Post 602684)

So if changing the size of the return line doesn't affect fuel pressure at all, why is it there? Changing the size to zero or nearly zero (say .0001 inch diameter) is still changing the size, right? How much fuel pressure would you run with a .0001" fuel return?

It's pretty clear that you made up the part where anyone is trying to "control" the fuel pressure through return size. They're trying to PREVENT it from controlling fuel pressure. If the resistance of flow through the fuel return line is higher than the FPR, it's going to affect the pressure in the rail.

You want some proof?

Blow as hard as you can through your mouth with your mouth wide open and measure the pressure in your mouth.

Now blow as hard as you can with your mouth ALMOST entirely closed except for a tiny hole and measure the pressure in your mouth.

which is higher?

large orifice or small orifice?

To check your return line, turn the pump on and back the fuel pressure down. As fuel pressure goes down, flow rate goes up, which will test the return line. If you can drop it 10-15psi below your minimum PSI at idle with vacuum (if you have a vacuum/boost referenced regulator), your return line will suffice. I'm using an AN6 feed line with the stock return line and a GSS340 pump on my car. When testing the min/max with my Ebay regulator and stock OEM return, fuel pressure can be dropped as low as 20psiG, and up to 85+ psiG (done once to check for any possible weak links).

Quote:

Originally Posted by ZX-Tex (Post 601532)

FWIW I am using the stock feed line and the stock return line on my 1994 LS1 Miata. I am flowing enough fuel for a guesstimated 350 BHP with good AFRs, verified with a WB02 sensor. I have an adjustable regulator holding it at whatever the LS1 rail pressure is, I think 53 psi IIRC. It works fine.

The stock LS1 fuel pressure is 58psiG

Sav- Now that you've had the lines installed for awhile, can you smell fuel when you sniff the lines? It seems most fuel lines these days are emitting fuel vapors through the lines.

Thanks in advance for your input :)

Which lines, the PTFE or rubber?

Any return line restriction is going to show up at idle as elevated base pressure. Once the injector duty comes up, the return line has to flow less and less to keep up.