fuel pressure regulator placement

[EricB]

What are the benefits between the lower two scenarios in the attached schematic. The difference being where the fuel pressure regulator resides and how the fuel is routed to/from the carbs...

-e

[240ZX]

Eric, Based on my limited knowledge on the subject, I think the middle picture is the way to go for a carburated systems (3-way bypass system). This sytem will maintain regulated pressure equally to all carb. Basically, anything downstream of the regulator will have regulated pressure. And this sytem is very similar to the factory carburated Z car system, with the exception of an adjustable regulator. The original system uses a metering block to obtain sufficient pressure to the carbs.

The bottom system appears to have the bypass line (return to tank) at the end of the carburator chain. This is strange!!? The only way I see this system maintaining pressure would be the restrictiveness of the return line. The regulator in the bottom picture would do nothing if the return line had no restriction.....basic fluid dynamics!!!

Some people use a deadheaded system. This would be a system with no return to the tank. Also with this type of system, the fuel pump relief valve (most electric fuel pumps have one) will dump the excess fuel back to the inlet of the pump. It works and has one less line to deal with. However, it is my feeling that this system may tend to operate with the fuel at a slightly higher temperature since the fuel is not constantly circulating back to tank. And as we all know, keepin the fuel cool in a plus!

Is this helping at all?

[240ZX]

Eric, I just took a closer look at the bottom picture....this sytem is not as I explained in my previous post! It will also work just as well as the middle picture. Sorry, my vision is not as it once was.

[240ZX]

Geeez am I absent minded! The only thing missing in both systems is a filter. Generally, you want to run a filter between the tank and pump and also a finer filter just before the regulator. So, I would still recommend the middle pictured system.

[EricB]

I guess what I didn't understand is why on the bottom most one the fuel pressure regulator was moved to after the last carb in the diagram... why couldn't you keep the same fluid low but put the fpr before the first carb???

Thanks for your help 240ZX

[240ZX]

OK, let's see if I can make this a bit easier to digest. I'm not going to mention the filters just to make the explaination shorter. Also, understand that a pump will not create pressure without a resistance down stream and will only deliver flow. A regulator is an adjustable resistance! With a 3-way bypass type regulator you have an inlet port, a return to tank port (bypass), and a regulated output port. Let's not forget that the needles and seats in the carbs float bowl are the key resistance. That how we determine what pressure you need to set your regulator at. For example, on SU's, the needle and seat do not want to see much more than 4 psi. Pressure over 4 psi will force it's way past the needle and seat assembly and cause a rich condition. Also, do not confuse pressure with flow! So, let's now move on to the pictured systems.

MIDDLE SYSTEM - As mentioned previously, the key resistance is the needles and seats in the carbs. If you operate this system with the regulator backed all the way off, you would not get sufficient pressure to the needle and seats and all the fuel would flow unrestricted back to the tank. Now, adjust the regulator to a 4 psi (for SU carbs anyway) reading at the gauge. There will now be 4 psi of pressure from the output of the pump all the way to the needle and seats. All extra fuel will bypass, through the regulator back to the tank. Note: this system bypasses the extra fuel before it reaches the carbs.

BOTTOM SYSTEM - The only difference between this system and the middle system is, the extra fuel is bypassed after the carbs. So, the only advantage with this system is that you have a continuos flow of cooler fuel to the carbs. The middle system will only have a continous flow of cooler fuel to the regulator. The fuel between the regulator and needle and seat (middle system) is flowing slower and can pickup more heat radiated from the engine).

Does this help....I wish I was better at explaining things in writing. Please tell me if this helped.

[EricB]

Hey 240ZX

Absolutely that helps. I hadn't thought as far as a constantly flowing fuel would be cooler... should i mention i didn't do too good in my fluid dynamics class when i was in engineering school?

ok then i think that i might have my fuel delivery to the Z all mixed up then... Didn't drive the Z to work today so I'm going off of memory, but as I recall I've got the fuel tank feeding to one of those low pressure Holley fpr's which I've got mounted to the firewall. From there it feeds the first carb, which feeds, the second, which feeds the third and from there I believe returns back to the tank.

My fpr having one input and two outputs, I used one of the outputs for the fuel pressure gauge. Should I change my system around completely?

It seemed to be working fine, but who knows maybe it could be many times better???

Thanks again for your help,

-e