Fuel Pressure Revisited

[S2central.net]

Here's food for thought. This info comes from factory manuals on 3B and ABY engines regarding QUANTITY of fuel delivered by each injector.

With control hose disconnected on FPR in each case -
3bar nominal pressure on 3B
4bar nominal pressure on ABY
The fuel pressure under vacuum conditions on the control hose is 0.5bar lower in each case - confirming my view that the regulator is a stepped device.

In BOTH the 3B and ABY engines, the factory spec is for 160ml (+/- 10ml) fuel delivery per injector over a 30 second period. That gives a rating of 320cc per min - at 3bar on the 3B and 4bar on the ABY.

That means the 3B injector actually flows more fuel than the ABY item at the same pressure, assuming linear behaviour of course, but that is fair enough.

I can only presume that the ABY injectors work better as the four jet pattern is more effective than the single jet of the 3B injectors.


Paul

[audiman]

Mmmmmmm, that is food for thought. Dont know the answers to that lot, all I do know is that Steveh is having no problems running his car using the std 3B injectors.

Do you have info for the ADU fuel system Paul??

[Floddy]

Did it say anything about the fuel pressure when adding boost to the control hose?

And as i always said, the 3B is a better engine! (yeah right...)

[S2central.net]

@Audiman - I shall have to dig out the RS2 info over the weekend. I'm having some 'storage issues' at the moment and I couldn't find it last night. Its a vertical filing system From memory, the RS2 FPR is also 4bar max - same device as ABY engine innit, but of course the green injectors will flow a higher quantity of fuel over the same time.

@Floddy - I'm 99% certain that the fuel pressure stays at the higher level whenever there is positive pressure on the control hose. I'm not saying the 3B injectors are better than ABY, but they are certainly very capable. I couldn't possibly quantify the benefit in the 4-jet nature of the ABY injectors, but I strongly suspect that this produces a more efficient spray pattern and cleaner burn than the single jet 3B device.

What it does mean is that those 3B owners planning an RS2 conversion may not need worry themselves about the cost and hassle of upgrading to expensive green injectors. The only change required on the 3B fuel system is by fitting the FPR found on the 7A engine. This has a nominal max pressure of 4bar et voila you have the same fuelling capacity as an ABY engine - which can be readily remapped for RS2 applications - up to about 330bhp and 450Nm.


I've sketched out a little rig that I plan to build for testing this stuff - so that I can measure fuel quantity on each injector without expensive pressure test equipment. It will bolt onto my ECU breakout box of course, but I could develop a standalone tester that could be cloned on the cheap for a way to verify that our cars have a healthy fuel system. However, just thinking out loud here I think the check valve in the fuel pump can only be tested with a proper fuel pressure tester.


Cheers,

Paul

[s2driveruk]

Good news indeed Paul......One less cost to factor in when trying to convince my other half that actually, it doesn't quite go fast enough or deliver enough punch in its current form!!!!

David

[Markus]

Paul,

stepped device? You mean that the fuel pressure reg. doesnt follow manifold pressure linearily? Interesting.

Until now I've assumed that the reg (ABY in my case), will keep the fuel pressure 4 bar above manifold pressure, which would mean 5,1 bar fuel pressure on a stock ABY under full (over)boost.

The manifold pressure when idling is about -0,5 bar, so that would be the same as the -0,5 bar you mentioned. On the other hand, it will drop to about -0,8 bar when lifting off the throttle while in gear. It might, however, be that the reg never goes below 3,5 bar on an ABY.

I guess the general consensus is that, when both are run at stock fuel pressures, the 3B injectors will run out of steam earlier than the ABY ones. But if the 3B fuel pressure is raised, the injectors will flow more than the ABY ones.

BTW. all pressures mentioned are gauge pressures.

[s2driveruk]

I can confirm those vacuum figs of -0.5 and -0.8 on my guage too.

[Floddy]

I'm not saying the 3B injectors are better than ABY, but they are certainly very capable. I couldn't possibly quantify the benefit in the 4-jet nature of the ABY injectors, but I strongly suspect that this produces a more efficient spray pattern and cleaner burn than the single jet 3B device.

Hehe, just use to mess around with the ABY owners that 3B is better just because of the bigger injectors and the hard looking cam cover, but in other things, the 3B really sucks compared to the ABY. Tightening the cam belt with the water pump, for example...

stepped device? You mean that the fuel pressure reg. doesnt follow manifold pressure linearily? Interesting.

Until now I've assumed that the reg (ABY in my case), will keep the fuel pressure 4 bar above manifold pressure, which would mean 5,1 bar fuel pressure on a stock ABY under full (over)boost.

That's what I also always thought.

[S2central.net]

RS2 fuel pressure and quantity info as promised - direct from OFRM

System pressure (guage pressure in bar) with engine idling -
Without vaccum on control hose = 3.8-4.0
With vaccum on control hose = 3.2-3.7

Holding pressure for 10 minutes -
When engine hot = 3.6
When engine cold = 3.3

Injector QUANTITY, without vacuum = 130-140ml over 20s

Thats a nominal flow rate of 405cc/min at 4bar pressure. Quite an increase on the 3B and ABY injectors.

Cheers,

Paul

[Doug_RS2]

I'm not sure if this is a stupid questtion or not

When you have say 2.1 bar of air pressure in the cylinder and you are using a 4 bar regulator - does that mean on-boost flow rates through the injectors are going to be reduced significantly?

An injector is just a valve that opens - is that right? So 4 bar(fuel pressure) on one side and 2.1 bar(air pressure) in the cylinder means you have 1.9bar of pressure difference through the injector?

I'm just wanting to get this clear in my head

[Greg_S]

I always thought the fuel pressure varied depending on boost/vacuum by adjusting the regulator with the pressure.

[S2central.net]

Fuel is injected during the INTAKE cycle of our wonderful 4-stroke engines - in a sequential fashion per cylinder - so you needn't worry about manifold pressure in this regard.

4bar fuel pressure is 4bar fuel pressure


Paul

[Doug_RS2]

Paul - Thanks for clearing that up.

[Mihnea]

Paul,

The RS2 FPR is 3.8 Bar, not 4 Bar. Don't ask why they did it like that but it's not the same as the ABY.

Then, manifold pressure has to be added on top of the fuel pressure, so this means that when there's 2 Bar boost in the intake manifold, the fuel pressure at the injector is 6 Bar absolute with an ABY/AAN FPR. The FPR's rating is 4 Bar relative to whatever pressure it's being fed with, so if it's being fed with 0.5 Bar, you get 3.5 Bar fuel pressure (idle conditions).

The FPR is a linear device that always gives it's rating on top of the pressure it's being fed with, no matter if it's pressure or vacuum.
There's also rising rate FPRs out there that give even more fuel as boost pressure increases.

About 3B injectors, I knew they flow more than the ABYs, however, fuel flow rate increase isn't linear with fuel pressure increase, the actual formula is:

sqrt (newpressure/oldpressure)*old fuel flow= new fuel flow.

The fact is that the single hole of the 3B injectors means a worse fuel spray pattern and fuel particulate atomisation, which means that when an engine is running the same AFRs at WOT with 3B or RS2 injectors, the EGTs on the 3B injectors will be about 40 degree C higher than with the RS2 injectors, BTDT. 40 degree is pretty considerable, when one thinks that between 810 and 850 degs C there's about 20 HP gain/loss, so if one tries to achieve AFR=12.6-12.7:1 in order to get the most power out of the engine, the 3B injectors might be just the slight difference that makes one will have to lose power by running richer mixtures in order to keep the combustion chambers cool.

HTH,

Mihnea

[S2central.net]

See this is why I want a fuel pressure guage !

It seems the confusion comes from crossing terms with regards relative and absolute fuel pressure.

Like I said - 4bar pressure is 4bar pressure - RELATIVE to manifold (guage) pressure which is what had me screwed up.

I wasn't suggesting that 3B injectors would be as good as RS2 items - even at 4bar (nominal) pressure. I know the new_flow calcs you show here, so I just figured that the 3B injectors could be used on an RS2 conversion - albeit with a little less maximum power - because of the poorer spray pattern that you quite correctly talk about.

If I did the maths right though, a 3B injector at 4bar flows ~370cc/min versus the ABY injector which flows ~320cc/min at 4bar. This is derived from 3B injectors flowing 320cc/min at 3bar.

Looking at 6bar fuel pressure (running 2bar boost) we have the 3B injector at 453cc/min whilst the ABY injector does 392cc/min. Assuming a nominal 4bar FPR in both cases.

I don't have ETKA in front of me, but I'm almost certain (from memory) the FPR is identical on the RS2 and ABY/AAN engines and that the OFRM calls up 4bar nominal spec at idle.

PS - Have fun in Bristol by the way !

Cheers,

Paul