fuel pressure.pdf

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/ TECH / FUEL PRESSURE /

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BEFORE

STARTING…

UNDER PRESSURE

The ﬁ rst rule of working on

cars and using tools of any

kind is don’t ever skimp

on decent protection.

Goggles, gloves, ear

defenders, masks and a

set of overalls should be in

your garage. Use them.

When using power tools,

protective gear is essential

— grinders and welders can

make a real mess of your

soft skin and bone if you get

it wrong.

Never work under a car

without supporting it using

axle stands. A car falling on

you is not something you’ll

be laughing about down

the pub.

If the dyno operator tells you you’re engine’s running lean at full

boost, don’t just turn the fuel pressure up. Here’s why.

day-to-

day work as a tuner, one of the

things I often find myself doing is

adjusting people’s fuel pressure

back to standard.

Now, given the fact that all

standard fuel regulators that I can

think of are tamper-proof in one

way or another to avoid DIY

adjustment, why on earth do they

always seem to be untamper-

proofed when they get to me?

Worst of all they are always set

too high, very rarely too low.

It seems to me that the answer

can only be that people can’t help

but mess with things they don’t fully

understand. The term ‘A little

knowledge is dangerous’ can easily

be applied to fuel pressure.

All too often I hear the excuse, “I

just turned it up for a bit of extra

fuel”, not to mention the old

chestnut, “More fuel pressure gives

it more power doesn’t it?”. Then I

end up spending the next half-an-

hour explaining why that is absolute

garbage, and how you are almost as

likely to end up with a melted piston

by running too much pressure as

you are with too little — not to

mention you are more likely to have

a meltdown with excess pressure in

some systems than if you simply

ran the correct pressure (presuming

air-fuel ratio (AFR) is correct at

standard pressure, of course).

So, why is it possible to melt an

engine with excess fuel pressure?

Surely, too much fuel is far safer

than too little? Well, the additional

pressure brings about its own pitfalls,

so let’s go back to basics and see if I

can explain it using some easy to

understand terminology.

fuel rail that in turn houses the

fuel injectors that deliver the fuel to

the engine.

At the end of this fuel rail will

normally be a fuel pressure

regulator. The regulator, as its name

suggests, regulates the pressure

seen in the pipe and fuel rail.

The fuel pump ﬁ res out fuel at a

maximum rate of approx 9 bar into

the fuel line and the fuel regulator

— depending on its settings — will

drop this pressure down to the

desired running pressure by simply

controlling the amount of fuel

allowed to leave the pipe and ﬂ ow

back to the fuel tank via the fuel

return lines.

So, for a system such as the

Sierra Cosworth, we start at 9 bar

and then the regulator opens up as

it reaches 3.5 bar and leaks fuel

back to the tank in a controlled

manner, thus achieving a regulated

3.5 bar of fuel pressure at the

injectors. This is the correct

setting for this system with most

commonly-used fuel injectors.

Simple you think? Yes it is simple,

at least until the engine actually

starts running...

Fuel injectors are simply

valves controlled by the ECU

environment and operating very

quickly and reliably — typically as

fast as 0.5 milliseconds.

Operation is simple; the injector

takes the fuel in at the top (or

sometimes from the side) and then

under command from the ECU,

opens for as long as it is told to and

injects the fuel.

The fuel is required to come out

of the nozzle in a particular spray

pattern and be atomised to a

certain level so as to ensure correct

mixture into the air stream. This

atomisation level and spray pattern

is chosen by the engine designers,

and they then choose an injector to

suit their requirements. The injector

Electric fuel pumps can ﬁ re

out fuel at a massive 9 bar...

HIGH PRESSURE

Why do we actually need to be

under such high pressure? OK,

that’s a good question, and the

answer lies at the all important fuel

injectors. The fuel injectors on most

EFi systems are simply solenoid

valves that are capable of both

handling fuel in a harsh unforgiving

PLUMBING

How are fuel injection systems

plumbed in? The most basic fuel

injection systems will have an

electric fuel pump, connected via

high pressure fuel lines (pipes) to a

WHO IS STU?

A Level 5-trained fuel-injection

technician, in the past Ford nut Stu’s

worked for a Ford RS dealer, a well-

known fuel-injection specialist and

various tuning companies. Then six

years ago, he joined forces with Kenny

Walker and opened up Motorsport

Developments near Blackpool,

specialising in engine management

live remapping, as well as developing

a range of Evolution chips which are

now sold all over the world.

He’s also jointly responsible with

Webmaster, Petrucci for www.

passionford.com. Started in 2003, it’s

grown rapidly from a few friends

contributing, to one of the biggest

Ford communities on the web.

Stu’s enviable knowledge of the

workings of modern-day Ford

performance engines means

that he’s just the man to explain

how and why things work, and

most importantly, how they can

be improved!

Words: Stewart Sanderson

Having worked as a tuner for over

16 years, Stewart ‘Stu’ Sanderson

is one of the most respected

names in the business.

0110 AUGUST 2006 FAST FORD

FAST FORD AUGUST 2006

0111

DURING my

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Fuel pressure regulator is the pork pie hat-shaped canister halfway up the left-hand side of this pic. The top pipe connects to the plenum

chamber and relays a pressure signal back to the spring and diaphragm inside the regulator to determine the fuel pressure supplied

chosen dictates the fuel pressure

required and the system is

designed from here upwards.

Most commonly, these pressures

are in the 3-4 bar range.

pipe on the top that is connected to

the plenum (or somewhere similar)

where it can sense manifold

absolute pressure (MAP). This pipe

has the incredibly important job of

carrying a vacuum or pressure

signal to the spring and diaphragm

assembly that is assembled within

the regulator.

becomes more complex. It is

deemed necessary to run a

different pressure depending on the

depression or pressure within the

inlet tract itself. The reason for this

is to keep fuel ﬂ ow linear regardless

of vacuum or boost pressure.

Please allow me to elaborate in the

most simple of layman’s terms...

Let’s imagine that your lungs are

the fuel pump, the water in your

mouth is the fuel, and a straw in

your mouth is the fuel line and

injector. A glass on the table can be

our engine’s cylinder. You have a

mouth full of water, and you wish to

empty that water, into the glass,

using the straw in your mouth.

Now let’s imagine we have

measured your lung capacity and

know it to be exactly 2 psi, so you

blow the water out into the glass

and it takes you 10 seconds to do

so. With no resistance at the end of

the straw you blew the water out

very easily, as you would expect.

Now try and imagine that exercise

mimicked a fuel injector that was

injecting fuel into the cylinder of an

engine that was turned off. No real

REGULATE

So does the regulator just regulate

to a set pressure and that’s it? No.

And this is where people start to

fall over and make silly and often

costly mistakes.

You will notice that most fuel

pressure regulators have a small

READ THE SIGNAL

Why does the regulator need this

signal? What does it do with it?

This is where the regulator

Inside the Cosworth YB plenum (top pic) you’ll ﬁ nd these four inlets. It’s the air pressure going into these that the fuel regulator needs to

monitor before it can distribute fuel at high pressure into those four holes on the inlet manifold via the injectors

FAST FORD AUGUST 2006

0113

fast tech / TECH / FUEL PRESSURE /

difference between a cylinder

and your glass in that scenario

(other than the injector being at

a higher pressure).

Now, what would happen if you

tried the same exercise again, but

this time someone was sucking at

the end of your straw very hard?

You would be right in thinking you

would empty your mouth full of

water faster than 10 seconds. Why?

Because your 2 psi pressure now

has something at the other end

helping it to travel through the

pipe, effectively increasing the

transfer rate of the ﬂ uid through

the straw (injector).

Let’s imagine then, that scenario

was the fuel injector ﬁ ring fuel into

an engine that is idling. (We

presumably all know that petrol

engines with a closed or low

throttle generate a partial vacuum

in the plenum don’t we? That’s

how the air is sucked in to be

mixed with the fuel.)

Now for an interesting scenario.

Let’s imagine the same job is to be

done, with your straw and mouthful

of water, but this time we are going

to put an equal pressure at the

other end of your straw. So you are

blowing water down the straw with

2 psi of lung pressure, but you now

have 2 psi at the other end of your

straw pushing back at you, so what

happens now? Well, nothing at all,

it’s a stalemate. The differential

pressure is nil so we have no ﬂ ow.

What you have just learnt is the

effects of a pressure differential on

ﬂ uid ﬂ ow, and that’s where the little

pipe comes in...

had applied 8 psi to the other end

of your straw... oops.

So, in a nutshell, we need to keep

the fuel pressure a set amount

above whatever pressure is seen in

the plenum at the injector pintle. If

we have 1-2 bar of vacuum in the

plenum, our regulator works the

other way and drops the pressure a

bar, too.

slightly on full boost after 6000 rpm.

What do you do? The correct

solution is to look at why you don’t

have quite enough fuel delivery up

there. Is it too much boost, is the

chip wrong for the spec, do we

have restricted fuel lines or ﬁ lters?

Maybe the alternator is getting too

hot at high rpm and starving the

injectors and fuel pump of valuable

voltage? All are very common and

simple to ﬁ x culprits.

However, the common bodge is

to simply wind up the fuel pressure

— let’s say for sake of discussion

they add another 1 bar of fuel

pressure. This instantly has two

negative effects.

Firstly, our pump and fuel lines

are now being asked to supply 6.5

bar instead of 5.5, as well as the

physical strain on the fuel pumps

and lines we must be aware of the

simple fact that when fuel pressure

goes up, the pump’s delivery ability

goes down while its current

consumption goes up and places

further strain on the wiring system.

Secondly, our perfectly good AFR

everywhere up to 6000 rpm is now

rich. OK, so we managed to obtain

the extra fuel from 6000 rpm at full

boost that we required to ﬁ x the

slight leanness, but everything else

that was previously OK is now far

too rich. Terrible fuel consumption

and greater engine wear are the

only real results from this exercise.

The only way additional pressure

can be run without this compromise

in overall fuelling is if the engine

management software was written

with this speciﬁ c fuel pressure in

mind from the outset — although

that doesn’t in any way alleviate the

additional physical and electrical

strains this extra pressure creates

on the fuel and electrical system.

IN-TANK PUMPS

The ﬁ nal issue is to do with in-tank

pumps, and this is worth a mention

as many of you run Fords with

an in-tank pump, and the most

notable one with problems is the

Fiesta Turbo.

It is not at all unknown for a

Fiesta RS Turbo engine to meltdown

due to incorrect and excessive fuel

pressure at Stage 1 and above

when using the standard fuel pump.

Why is this? Well, the standard in-

tank fuel pump has a little rubber

sealing section on it that connects

the high pressure outlet to the fuel

line in the tank — and this ruptures

at around 5 bar.

The scenario is this: standard fuel

pressure is 3.5 bar; boost at Stage 1

is 1 bar. This gives a fuel pump-

regulated pressure of 4.5 bar —

that’s just below where they usually

rupture and it is normally ﬁ ne there.

But then one of two things happens:

either Joe decides he can run

another 0.5 bar of boost; or Joe

and his pal down the pub decide

a little extra fuel won’t do it any

harm, and then proceed to wind

up the fuel pressure regulator. Hey

presto, an engine that was perfectly

safe with 1 bar of boost is now a

time bomb just waiting for the fuel

pump seal to rupture and lean

off the engine to the point of

destruction. Well done Joe, great

safety modiﬁ cation that was...

WHY IS MORE

PRESSURE BAD?

Let’s look at what pressure we are

likely to run on a Sierra Cosworth

running Bosch 403 injectors (greys).

We have 3.5 bar line pressure as

standard and then we run a held 2

bar of boost. This equates to a real

world fuel pressure of 5.5 bar (3.5 +

2) so our fuel pump is being asked

to supply the fuel required at 5.5

bar now — this incidentally is

over double your tyre pressures.

Quite some pressure to have fuel

at under your car, I’m sure you

will agree?

So, why do people adjust this

and turn it even higher? Well, it

is quite a common occurrence that

a person will discover his car is

running lean somewhere and thus

increase the fuel pressure to

account for this leanness and this,

sadly, is a really bad thing to do

because it is a big compromise and

has almost always compromised

the rest of the system.

Imagine we have a perfectly good

AFR on a dyno test, but as we

approach top rpm and boost you

are told that it leans out ever so

THE LITTLE PIPE?

What effectively happens when the

engine is running is this:

The fuel pressure is set in

this example, to 3.5 bar at

atmosphere. That’s with the signal

pipe disconnected. When we

connect the pipe, it acts on a

diaphragm and spring and affects

the fuel pressure with a ratio of 1:1.

This means that for every 1 psi of

difference the plenum has to

atmosphere, the fuel pressure will

be adjusted accordingly by 1 psi.

For example, if we have 1 bar of

pressure at the pipe (1 bar of turbo

boost), we will run our 3.5 bar plus 1

bar of additional pressure.

This has the effect of ensuring

the fuel pressure at the nozzle is

always exactly 3.5 bar above

whatever pressure is measured

within the inlet tract, so the fuel

ﬂ ow is always constant and of a

known entity, unlike the ﬂ ow we

had through the straw once we

equalised the pressure. Just think

what would have happened if we

Whack up the fuel pressure on a Fiesta

Turbo and you risk rupturing the in-tank

fuel pump seal, then it’s bye bye CVH...

NEXT MONTH

If the air-fuel ratio starts to lean out a high rpm on a power run,

it could be down to a number of elements (too much boost,

restricted fuel lines etc) — not just insufﬁ cient fuel pressure

Boost controllers: from the

Amal valve to the Japanese

controller — the various

options and how they work

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