Fueling for 400whp
 

I want to start a build for 400whp and I need to get my fueling straight. I'm somewhat ignorant about cars despite reading about car tech all day so hopefully I can get a little help here. I need to upgrade my fuel system of course but there's a few details I have questions about. My car is an 03 with a BEGI S5 and 3071 750cc injectors w/ Hydra. I'm upgrading to a Walboro 255HP.

First of all, I'm guessing that I can leave my existing hard lines in place. I see that there's a FPR or anti-surge device where the hard lines terminate in the engine bay. Do I want to use that leave that FPR (if it even is one) as it is or should I upgrade this to a different FPR even though i'm running a Hydra? Next, I have a FFS dual feed fuel rail (FTW). Would it be best if I hooked this up just by "T"ing into the line that comes off the FPR?

Thanks...

This is something I'll be dealing with soon. I'm considering a few different options, some of which would not apply to your (newer) car. However almost everything I'm looking to do involves more fuel line between the tank and fuel rail.

Part of what I get to do at work is I'm exposed to some cutting edge stuff on race motors. Lately we've been playing with different injector style/spray patterns, different injector positioning (rotary application), fuel pressures, etc. Some of this stuff is amazing. But the more I see, the more it's reinforced that injector type is very important. Atomization is everything. RC's design is pretty damn old at this point. I'm looking for some high flow injectors (500+ cc/min) that have patterns like some of these ~300 cc/min OE mazda injectors.

Super high fuel pressures and direct injection is absolutely the future. I've also been looking at running a Speed3 fuel pump and having my block tapped for injectors. And no, I'm not kidding.

direct injection is amazing. Its like the next generation of the ICE...and we don't get to play.

Previous Miata fuel system (made 475whp) on 2002:

Walbro 255
-8 Line to Fuel Rail
-8 Line out of Fuel Rail to FPR (Aeromotive A1000)
-6 Line from FPR back to Tank
1000cc/min injectors
Fuel Rail was aftermarket Vishnu Dual Feed.

Worked great.

As for what a typical miata owner would do, I don't know. But that's what I did and I had great experience with it.

A Walboro 255, a good AFPR (like Aeromotive), some braided feed and return lines and 660+ injectors is about all you need. So you're on the right track.

Did you ever test the flow limits of the factory hard lines?

No, I did it while I was putting the built motor in. Fueling is not one of the things I take lightly. But, I was told that past 400whp you should replace lines. But, I don't know where the magic spot is.

I've always wondered that myself. 400HP seems like a reasonable number to swap them out.

So you had a return based fuel system? Anyone got any experience with returnless? I'd prefer to not replace my hard lines unless I have to do so. Paging eliminator77....

There are a few guys in 323s making 400+ with just a 255 and hog injectors...But I don't know anything about the miata fueling, you'd think it would be similar.

You have aroused my curiosity. Where exactly on the block (or the head, I assume you meant) is there space for a set of direct-injectors?

This is how I did it when I ran 498hp.

- Bosch Inline Fuel Pump
- -8 Line to Rail (Duel Feed)
- Big Bore Fuel Rail (One off at the time, but similar to the M-Tuned Rail we sell now)
- Aeromotive A1000 FPR
- -6 to stock hard return line
- 930cc Injectors @ base pressure of 60psi

I had fueling issues with a 255 and stock lines @ 350ish hp with my Big Bore Rail and 930cc Injectors.

Did you use a modified pickup at the tank or did you still have a main fuel pump?

Good lord, that's 10.6 cc/hp. Whew!

4 PC PRO's /thread.

Here's my very thought out plan for my 99'.

I already added a 3/8 tube to the sending unit for a pickup or return. I'm thinking put a 255HP and remove the in-tank FPR (plug it or something) so that all the fuel the pump spits out goes into the factory fuel line. Let that go to the engine bay like normal. Let it go through all the factory damper things. Remove line that goes from damper to fuel rail. Hook up a fuel injection hose there and run it into my Mallory 30-100 PSI adjustable regulator. Since I added a return already, hook that up. Now, the regulator has 3 outputs. Hook one up to the fuel rail.

The real question to me is, how to properly tap into the factory hard line and run that into the fuel injection line I bought. I guess slide a piece of 5/16 fuel injection line over the factory hard line with some JB weld to help seal it and and 4 hose clamps and hope it never leaks. :eek5: Then run it to the regulator.

Needless to say, if somebody made a factory fuel fitting thingy to 5/16 barb fitting I'd buy one.

So you get all the disadvantages of a return system, while maintaining a returnless rail? Brilliant. 1,000 points of failure, I think the candidate called it.

I did it 2 ways and found no one better than the other.

1. Removed stock fuel pump and ran -8 line instead to fitting on stock the the steal Fuel Pump mount. -8 from there to front of car.

2. Sumped the bottom of the and ran -8 to the front. The 2nd option worked great for Drag Racing but was pretty terrible on the street and road course due to the fuel sloshing around and running the pump dry. Probably needed foam in the tank if I were to that style again.

You don't understand. I'll explain later. But this will be much better than the factory returnless or factory return setup.

Alright Abe, consider the following.

Returnless fuel system. Pump put out exactly 62 PSI of fuel pressure. We have 1000 miles of fuel line between the pump and the fuel rail. If we turn the pump on and are not using any fuel at the rail, it will pressurize the entire system to 62 PSI and that pressure will be uniform throughout the fuel system. 62 PSI everywhere. Agreed?

Now we start the engine. Fuel has to travel through the 1000 miles of fuel line. The faster it has to go, the more losses there are and pressure will drop. Pressure at the rail is now a function of resistance in the fuel line. So the more fuel we try to flow, the more the pressure drops. Fighting a loosing battle we can't do anything about.

In desperation we install a dual feed fuel rail. Makes no difference cause fuel pressure delivered to it is STILL a function of the losses between the pump and the 1000 miles of fuel line. Say pump puts out 62 PSI and then it looses 12 PSI between the pump and the rail so 62-12=50 PSI. No matter what, pressure will drop as fuel flow increases.

To minimize pressure drops, we need to reduce the frictions between the regulated supply of fuel and the rail. So, using larger fuel lines would help. So would reducing the length the fuel has to travel between the regulator and the fuel rail. Or better, both.

So I'm gonna put a regulator next to the fuel rail and plumb it with say 1 foot of fuel line. Now the losses in the fuel line between the pump and the rail are much much smaller, almost insignificant. If the pump puts out 124 PSI and looses 14 PSI due to losses in the fuel lines between the pump and the regulator, then my regulator sees 124-14= 110 PSI. But then it just drops it to 62 PSI. But wait, what if the pump puts out 124, but then it goes through 5 bends, one of which is pinched and causes a 9 PSI loss in pressure. Then it goes through a dirty fuel filter cause the gas I just bought had some trash in it, so minus another 12 PSI. Finally it snakes its way to the engine bay, loosing another 2 PSI. So 124-9-12-2 = 101 PSI. Plumb that to the regulator and it still puts out exactly 62 PSI. But wait, what if the alternator goes out and we loose some voltage that causes the head pressure of the pump to drop to 100. Still, 100-9-12-2= 77 PSI which becomes exactly 62 PSI after going through the regulator. So the losses before the pump no longer matter. (as long as it never drops below 62 PSI going into the regulator)

And now let's say I install a dual feed rail. Now, I have 3 outputs on my regulator, so I can feed regulated fuel pressure to the rail. The fuel I'm feeding it with is exactly what I want, not what I want - losses between the fuel pump, fuel filter, fuel lines between filter and engine bay, and a fuel pressure damper. I can now put say 62 PSI of fuel pressure on both sides of the rail. Now a dual feed rail actually DOES reduce the pressure difference between the regulator and rail because the lines going to each side of the rail are connected separately to the source. With 2 lines I reduce the pressure losses between the regulator and the rail. Where as putting a dual feed rail on a stock 99's system that looses X PSI at Y fuel load only helps to make that loss more uniform.

Brilliant?