How Do 550cc Injectors Make 300rwkw On An Rb25det?

[Harey]

Hi,

I have searched and read many threads on here about injector sizing. Many times people mention that 550cc injectors will supply enough fuel for 300rwkw. How is this possible on the standard fuel pressure of 3 bar (43.5psi)?

I have entered the following figures into many different fuel injector size calculators on the net and they all say I will only make 250rwkw on 3 bar of fuel pressure.

I am using the following data:

BSFC: 0.60 (for turbocharged cars)

No of Injectors: 6

Max Duty Cycle: 0.8

Fuel Pressure: 43.5

On the following site it only provides fuel for ~420hp at the engine, which is ~250rwkw.

Am I missing something? Are people upping the fuel pressure or running the duty cycle above 80?

Any help much appreciated!

[Sheriff Wan]

You would be missing duty cycle being higher, i.e most are happy to run at 90+%

Also fuel pressure regs could be used to increase pressure...

Just some thoughts...

[R31Nismoid]

550cc x6, as a rule, will support 550hp. It's a rough guide of course.

Also remember the fuel reg is rising rate. Base pressure is what, 38psi or so.

Add 20psi of boost, you are now seeing 58psi.

[Steve]

as stated, you run 1 bar boost, a base pressure of 3 bar is now 4 bar

It will easily support 300rwkw, I had 321rwkw at 1.2bar boost without hitting 90% duty cycle on RB25 (with 3kg base fuel pressure - nissan FPR is only 2.55kg btw)

[daisu]

550cc x6, as a rule, will support 550hp. It's a rough guide of course.

Also remember the fuel reg is rising rate. Base pressure is what, 38psi or so.

Add 20psi of boost, you are now seeing 58psi.

The boost pressure in the manifold restricts flow from the injectors, which is why we have rising rate regs.

If the manifold was at atmospheric pressure, and the fuel rail at 38psi (base pressure), there is a differential pressure of 38psi.

If the manifold pressure is 20psi and the rail pressure was 58psi you still have a differential of 38psi.

Therefore at any given duty cycle, the flow rate will be the same regardless of the amount of boost. (providing the fuel reg is sized correctly and operating within it's limits).

Edited February 11, 2010 by daisu

[Harey]

The boost pressure in the manifold restricts flow from the injectors, which is why we have rising rate regs.

If the manifold was at atmospheric pressure, and the fuel rail at 38psi (base pressure), there is a differential pressure of 38psi.

If the manifold pressure is 20psi and the rail pressure was 58psi you still have a differential of 38psi.

Therefore at any given duty cycle, the flow rate will be the same regardless of the amount of boost. (providing the fuel reg is sized correctly and operating within it's limits).

Exactly right. Its only ever effectively supplying the base fuel pressure. 1psi of air pressure cancels out 1psi of fuel pressure, hence the factory 1:1 rising rate fuel regulator for forced induction cars.

Where did you get 550cc ~ 550hp?

[Granthem]

My 550's are hiting 85% duty and making 340rwhp... however, the current tune is apparently somewhere around 10:1 af's

[Harey]

My 550's are hiting 85% duty and making 340rwhp... however, the current tune is apparently somewhere around 10:1 af's

Standard fuel reg I assume. See that sounds more like it. Sure theres more to be gained from 12:1 afrs but surely not 60rwhp. So people must be running them at over 85% duty cycle to make 300rwkw?

Edited February 11, 2010 by Harey

[Sean W]

1cc for 1hp is just an old rule of thumb, i have no idea where it comes from but ive heard heaps of ppl mention it. but yeah +1 for rising rate reg, high duty cycle and boost. 550s make 260rwkw in mine running nowhere near their duty cycle limits

EDIT: nismo fuel reg and 19psi, cant rmb afr or exact duty cycles as its only recently been retuned

[Granthem]

Standard fuel reg I assume. See that sounds more like it. Sure theres more to be gained from 12:1 afrs but surely not 60rwhp. So people must be running them at over 85% duty cycle to make 300rwkw?

Nismo reg. not sure what the base pressure is set at. But im pretty certain its not 3 bar. Is base fuel pressure measured at 0 manifold pressure?

Edited February 11, 2010 by Granthem

[Harey]

1cc for 1hp is just an old rule of thumb, i have no idea where it comes from but ive heard heaps of ppl mention it. but yeah +1 for rising rate reg, high duty cycle and boost. 550s make 260rwkw in mine running nowhere near their duty cycle limits

Do you mean an adjustable fuel pressure reg? Cause the factory fuel pressure reg is rising rate. As said previously boost does not help the amount of fuel getting through the injectors.

They must be high duty cycle if only the standard reg.

1cc for 1hp must be at 100% duty cycle.

Edited February 11, 2010 by Harey

[Sean W]

edited post above, nismo reg. and im only learning more about this recently myself but i was just posting my experience

[Harey]

Thanks fellers so the nismo reg is the missing piece! Cheers!

[Granthem]

To be honest I dont think my Nismo reg is being used to crank up the base pressure at all. I believe its simply there from when one of the previous owners was trying to get the most out of stock injectors

[lachlanw]

you all need to remember/state the 1cc=1hp is only true for a 6 cyl engine . for a 4 cylinder engine it only has 4 injectors so theyd have to be 30% bigger to get same power

[R31Nismoid]

Thanks fellers so the nismo reg is the missing piece! Cheers!

No it's not.

Factory reg is perfectly fine is the injector size is correctly given. Which 550's are for 300rwkw.

Here is the numbers @ base fuel pressure (6 cyc motor):

550 cc = 550 bhp

550 bhp / 1.34 = 410 rwkw

410 rwkw – 70kw loss = 340rwkw

The facts.

1. 550's will support 300rwkw.

2. Factory reg is all you need.

If these "sites" say you can only get 250rwkw from 550's... then how is it possible that with 370cc injectors people are making 200-220rwkw???

*think for a second*

Exactly, it wouldn't be possible. To back this up look @ the RB25 dyno results thread every single car with a factory turbo making 190-200rwkw is using factory injectors.

Whatever site you are getting your calc's from - are wrong. OR you are inputting information wrong.

We are the people here that have experience and knowledge of this specific application. Not some "cater for all" website calcuator without any idea.

Injector calcuators are the same as ET/Drag calculators, most of them are absolute rubbish.

[Ryno]

Using 555cc injectors, my car makes 250 rwkw at 72% inj duty.

[Adriano]

Guys just for clarification the stock reg is not rising rate, they are 1:1. Rising rate regs increase the fuel pressure more than the manifold pressure increases.

[32TAXI]

I was told 550cc make approx 100hp per injector, so six should make 600hp.

I've made 240rwkw using 4x 555cc on sr20!

[Granthem]

can anyone answer where base pressue is measured from... idle? of 0 manifold pressure

My car at idle is 2kg/cm2 fuel pressure... at 0 on the boost gauge... around 2.4kg/cm2

Seems abit off 3 bar?

Edited February 11, 2010 by Granthem