So a couple of months ago I purchased a blow through 12AT (with modded Nikki) and proceeded to install it in my 85 GS. Everything is done except for the fuel system. Now people have told me conflicting things. Everything I've read here on the forum says that I need a rising rate fuel pressure regulator, and a higher PSI fuel pump. So I went ahead and bought a Mallory 4309 and a higher PSI fuel pump.

However I did some more research before I started to install the fuel system. I went to a few local rotary shops in the greater Seattle area (mazdas n more, atkins rotary, and Jeffs mazda) and they told me something totally different. They all said that I would not need a RRFPR or a higher PSI pump. They said that if I were to run out of fuel revving it high, that all I wound need is a higher VOLUME pump that stays at low psi. They said that PSI does not matter with a carb, but volume does. So what is it volume, psi, or both????

So what I'm asking is do I really need this pressure regulator and pump? BTW I'm only planning on running 5 PSI boost.

 

Did you tell them it was for a blow-through turbo?

Anyway, don't listen to them. You definately need a 1:1 pressure regulator. Otherwise the carb won't function properly. You also need a pump capable of supplying the base fuel pressure plus whatever boost pressure you're running. If you're only running 5psi then a pump that puts out about 14 or 15psi should be sufficient.

 

1:1 reg means what exactly? anyone know any good readings on fuel systems? im confused, ive read that im gonna need a rrfpr but id like to read more

 

it means that your pressure will rise 1 pound for every pound of boost the turbo makes.

what do you mean by "readings on fuel systems"? do you mean books? there are several out there, but in terms of turbocharging and fuel, i keep hearing about the book by Corky Bell (the guy that owns BEGI) - i don't know the title, but it shouldn't be too hard to find.

 

yep i borrowed that book from a buddy, reading it right now. not a very detailed book, good basics though. by readings i mean websites maybe, or even books. how to make your car handle just came in the mail for me today, so far its pretty good....

 

yes you need a 1:1 regulator preferably or a rising rate one. most people ive talked to and the regulator i "used to own" was a aeromotive 1:1 regulator. once you tune it to what you want thats it. well in a sense those shops are right, you dont really need a high psi fuel pump. you need volume, flow and then psi. if your going to run only 5psi which is not that much then a cheap bosch or walbro would do just fine.

 

I do have a mallory 4309....

So what would happen if I used the stock fuel system now?
What would happen if I just used a high volume fuel pump with no regulator?

 

uh it would probialy ping like hell or just fade out at the top end.
you would kill your carberator floats, with no regulator

 

PSI - Pounds per square inch... Unit of pressure... (Bar or Kilograms per Square Centimeter in metric.) It's how HARD the pump moves the fuel... (Also helpful if you're using it in a flamethrower. ) For a carb, you only need a few PSI... This is mainly to get it up through the fuel filter and into the floats. (Intake air will do the rest. For an example, stick a straw in your drink, Blow hard over the tip of the straw. The drink will move up the straw if you do it right and create a mist in the wind... Instant carb demo... High pressure will **** up little valves in the carb.) Fuel injection, because it sprays a fine mist through a nozzle, requires 40 or more PSI. (For an example, pick up a windex bottle, or your garden hose...)

Volume - Measured in Gallons per hour... (Liters per hour in metric.) This is how MUCH fuel the pump can move. The higher, the better. Obviously, you'll need a lot of fuel for this turbo setup... If your engine begins to consume the same ammount as your pump can move, you'll lean out, creating ping and heat... Ping is bad, especially in a rotary. Ditto on heat.

Low Volume, Low pressure... Moves little fuel, pushes lightly

Low Volume, High Pressure... Moves little fuel, Pushes very hard

High Volume, Low pressure... Moves lots of fuel, pushes lightly

High Volume, High pressure... Moves Lots of fuel, Pushes very hard.

You probably want High Volume, Low pressure.

 

passion, do you agree with Pele??

Pele, should I get a high volume fuel pump with no pressure reg?

 

I hope i explain this right:

the reason you need a RRFPR(FMU) is that if you look at the carb under boost say 10psi, and fuel pressure is set at 4psi, the fuel can not come out of the bowls due to the pressure difference. 10psi>4psi


so if you start with a base FP of 4psi and want to boost 10psi
you have to make the fuel system go
stock fuel pressure(4psi in this case) +=1
so with 10psi of manifold/carburator pressure(boost) you should see 14psi of fuel pressure to make sure the carb is still supplied with the 4psi it requires

see what im saying?




or is that only for a complete carb bonnet(box) and not a blow thru hat?

 

You're probably going to want a High Volume pump at around 10-20 PSI with a regulator... The Extra pressure will help you so that you don't starve the engine under boost, and the regulator will keep you from blowing out needle valves in your carb.