Ok, I have been pondering this for a while:

I have been running on a 12:1 FMU w/ stock pump/injectors for a couple of weeks. I am getting 450's and a custom circuit today/Tuesday. The FMU requires over 120psi in order to dump enough fuel w/ the stock injectors to keep the car running stoich/little rich.

My question though, how can I still use the stock pump w/ the new injectos and still be able to keep enough fuel in. I know the injectors are larger, and flow much more, but why wouldn't I need more psi now in order to keep them shooting that much fuel? Like, why is it taking the same amount of pressure to push 450cc's through as it is to push 240cc's through. I would think I would need more psi coming from the pump in order to keep a good volume of fuel going through the 450's in order to still run stoich/rich under boost.

Also. If I take off the FMU, 1 of 2 things should happen right:

1. I will not be adding more fuel in w/ the compressed air of the turbo...BOOM.
2. I will not be straining my fuel pump and would be able to keep up with the demand.

I'm going w/ 1, which I am sure is going to happen. Back to the first question now though, why can the pump handle the 450's to run rich and still not be strained?

jB

 

go to the toyota dealer and buy a 93.5+ Supra TT fuel pump. theyre great.

 

I would think it's because the 450cc is a volume rate, allowing much more fuel to be injected in the pulse. The 240cc injectors are being forced to pulse much more often (to achieve your desired a/f), therefore much more pressure in the fuel system is needed. The 450cc injectors provide the fuel with enough ease that high pressure isnt necessary. I'm just trying to think logically. It makes sense to me.

 

Fuel injectors are very much like the valves in your cylinder head. So I'm going to make an analogy, bear with me.

Under normal NA conditions the valves in your cylinder head can only flow in so much air/fuel and flow out so much exhaust. You can do two things to increase the flow into and out of the combustion chamber. One is you can increase the size of the valves. This is like increasing the size of your injectors. Or you can pressurize the intake charge with a turbo/super charger. This is like running an FMU.

So to summarize, flow depends on both pressure and valve size. A fuel injector is a valve (an electromechanical valve). You can flow more fuel through it by using a bigger valve or putting more pressure on the fuel.

The stock fuel pump is limited in the amount that it can pressurize the fuel. But it can flow relatively a lot of fuel at lower pressures. THis is why you need an inline pump to supplement the pressures required for an FMU. But larger injectors don't need more pressure to flow a lot of fuel though because they have a bigger outlet. So the stock pump will work fine with the 450cc injectors.

 

Hey, are you getting that circuit that allows you to lean out for the 450's? IM xDSMguy on here. He's an RNK guy and is turboing his GSR(it might already be on). He's been struggling with the EXACT same issues as you so maybe between the two of you, you can figure something out. Oh, I got the price on the MSD Inline and Rob still has the 450s. IM me, Ill hook you up .

 

Well, here's the answer to the question I think you're asking.

What you are comparing is this...

220 CC injectors rated at 40 PSI
450 CC injectors rated at 40 PSI
I know they are probably not rated at the same exact PSI, but close enough.

So...the 220s are intended to be open for X amount of time at X PSI, and they will deliver 220cc/min. With 12 times the stock fuel pressure, obviously more fuel will flow through the injector. Basically you're forcing more fuel than was intended thru that injector.

The 450s are intended to be open for the same amount of time and the same PSI as the 220s, and they will deliver 450cc/min. Basically they are bigger, or open wider, allowing more fuel to pass thru them.

Hopefully that's what you were looking for. BUT, you will need a better than stock pump for the 450s although you no longer need a high pressure pump as you should have had with a 12:1 FMU -but I know you didn't have one .

 

Ok, thanks guys. I think I have a grasp on it now

Jeff. I am getting some injectors tomorrow, and getting the circuit made for me. (Found injectors for $40 picked up in Orlando).

Would he do the pump for like a TT or for part of a BOV? If not, I'll see what I can do on here, or might use it for my setup after I get the 450cc's in.

jB

 

Hey, where can I get some info on this circuit? If it's cheap enough, I'd like to get one so I can sell my AFC to help fund a Hondata.

 

IM me, I sell the circuit for $20. ($21 if you use Paypal)

See also my webpage:

http://www.angelfire.com/ab7/diymaplimiter