Turbos (obviously) make more power.  More power requires more fuel.

(Up-dated as I learn more)

I've run some tests with an automotive multimeter that has a duty cycle function clipped to one of the injectors.  At max boost of 10 psi the injectors hit 100% duty cycle at just under 5000 rpm.

Getting the extra fuel

1. Additional Injector(s)
2. Rising Rate Fuel Pressure Regulator 
3. Larger Injectors 

Manipulating the ECM

1. Chip Change 
2. MAF Signal Conditioning
   

How Much Fuel?

Wide Open Throttle

Getting the fuel

There's a few ways to get the extra fuel.  Some not so good and some good.   But that's not the end of the story.  You then have to allow the ECM to deal with the extra fuel you're dumping. 

The ECM pulses the injectors on and off at a regular interval (the duty cycle).   If an injector is on for say 60% of the time, it's at a 60% duty cycle.  The amount of fuel flowed through the injector in this amount of time is dependent on two major items.  The size of the injector (the orifice the fuel flows through) and the fuel pressure.  A larger orifice or more fuel pressure will flow more fuel in the same amount of time.

Regardless of how you get the extra fuel, see the following section Manipulating the ECM

Additional Injector(s)

These can be standard fuel injectors or an on-off solenoid like the cold start injectors found on VWs.  Controlling the injectors is another matter. 

On-off injectors just need battery voltage to operate.  You can use a pressure switch to turn the injector on at a certain boost level.  This is pretty simple and easy to hook up, but it's less than optimal.  It's a binary thing;  on or off.   Power levels don't change from one instant to the next to require this much fuel.   As power comes up, the injector turns on and suddenly dumps enough fuel for say 30 h.p., but the engine can't suddenly use that much fuel so the mixture goes way rich until the rpms, boost and power levels go up enough to need all of it.  You end up wasting a lot of gas.

A slightly better method would be to use say 3 smaller injectors, each with their own pressure switch that turns them on progressively at different boost levels.  This buffers the full fuel shot over 3 events instead of one and give a little bit better fuel distribution.

Traditional pulsed fuel injectors then are even better.  They gradually add more and more fuel as boost comes up so you don't get a rich spike at all.  Just the extra bit of fuel you need from one instant to the next.  These do require an AIC (Auxiliary Injector Controller) though to operate them.  There are many plans out there on the net that are pretty simple.  You can also buy them from some places for about the cost of a chip change, but you have full control over future mods as well.

Rising Rate Fuel Pressure Regulator 

Rising rate regulators operate on the principle that more fuel pressure will force more fuel through the same injector. 

For a non-boosted EFI engine, the stock regulator works to keep a constant fuel pressure across the injector.  On one end of the injector, you have manifold vacuum.   On the other is pressure from the fuel pump.  When you open the throttle, manifold vacuum drops which means fuel pressure has to rise to keep the same pressure differential. 

A regulator for a boosted EFI engine works the same way, except that it can continue to raise the fuel pressure when positive pressure exists in the manifold.  Every psi of boost in the manifold raises the fuel pressure 1 psi as well to keep the fuel pressure differential the same.  At 0 psi in the manifold, fuel pressure may be 45 psi.   At 10 psi of boost, fuel pressure will be 55 psi.  In both cases, the injector will flow the same amount of fuel because there's always a pressure differential of 45 psi across the injector.

A rising rate regulator increases the amount of fuel injected under boost by raising the fuel pressure faster than manifold pressure.  A 3:1 regulator will raise fuel pressure 3 psi for every pound of boost.  In the above example, 10 psi of boost would result in 75 psi of fuel pressure giving a pressure across the injector of 65 psi.  A 44% increase in pressure from a stock 1:1 regulator.  Following the rules of flow vs. pressure, this gives about a 20% increase in fuel flow through the same injectors at 10 psi of boost.

This has the advantage of being relatively easy to install and doesn't require wiring in pressure switches or an AIC that extra injectors does while still giving a fairly decent (not ideal, but decent) and smooth increase in fuel as boost pressure and power go up.

A down side is the elevated fuel pressures seen.  These can easily go to 100 psi if you're not careful.  Higher fuel pressure makes the injectors work harder and can even keep them from opening at all depending on how high you go and how long you stay there.  Also, you'll need to make sure your fuel hoses are in good condition and you'll want to replace the stock spring type hose clamps with stronger worm type clamps.   If you buy replacement hose, be absolutely sure it high pressure EFI rated hose.   Don't take the parts counter guy's word for it either.  Check the hose yourself.  It'll be printed on the outside of it.  Another thing is that the volume a fuel pump can flow goes down as the pressure goes up so you might run out of pump capacity at 75 psi where you would have enough at 55 psi with larger injectors.

70 - 80 psi of fuel pressure seems to be about the max to shoot for while keeping things reliable, but I've heard of others going to 100 psi with no problems.  The type of injectors you have will also be a player in this.  Pintle type injectors are less likely to stick closed under high fuel pressure than disc type injectors are.

Larger Injectors 

You can also replace your stock injectors with larger ones assuming you can find a set that are physically compatible with yours.  This means physical dimensions, wiring connector and impedance of the injector.  This is actually not a hard thing to do as there are a large number of injectors available from several sources, but with the exception of the occasional oddball, most use one of a few different body styles.

The biggest concern is matching the impedance of the injectors.  They come in high impedance (typically 12 - 16 ohms) saturated injectors and low impedance (around 2 ohm I think) pulse-and-hold injectors.  Don't mix them.  Saturated are the most common in stock engines.  If you have 12 ohm injectors now, make sure you upgrade them with some of the same impedance.

Manipulating the ECM

Regardless of how you actually get the extra fuel, you still need to make the ECM work with it.  If you switch to larger injectors or increase the fuel pressure, the injectors will flow more fuel in the same amount of time.  The ECM doesn't know this however.   It'll still open the injectors for the same amount of time as before and you'll end up with an overly rich mixture.  The same occurs with additional injectors.   There's more fuel flowing per unit time when the additional injectors are on than the ECM knows about.  The ECM needs to be re-calibrated (new chip and/or EPROM) for the larger fuel flows, or it needs to be fooled into thinking there's less air mass coming in than there actually is.

Chip Change 

For a lot of applications, this is done by modifying the look-up tables the ECM uses with a relatively expensive chip change.  That's not an option for me and I wouldn't use it if it were.  Because it's basically a new look-up table, it's pretty much a one shot change.  Any fueling changes in the future would require another chip.   If you have a programmable ECM, then you're in luck as it may be possible to update the table yourself (if you can find a commercial programmer) as you modify things. 

MAF Signal Conditioning

The MAF sensor measure directly the mass of the air entering the engine.  From that, the ECM calculates (or looks up) the required amount of fuel needed for that mass.   This calculation takes into account several factors.  The ones we're concerned with here are the size of the stock injectors and the stock fuel pressure.

If the MAF was linear (i.e., the voltage signal from the sensor was linear in relation to the amount of air flowing) you could use a simple resistor to drop the signal slightly.   This would cause the ECM to think there was less air mass coming in and would then command a shorter fuel pulse.  This would normally create a lean condition, but because the injectors are now flowing more fuel, the shorter pulse makes the mixture correct.

The MAF sensor output is not linear though.  It's logarithmic.  At low air flow rates (idle and off-idle), the signal changes a lot.  At higher flow rates (wide open throttle) the signal changes less and less.  The way to condition this logarithmic signal is with a couple pots.  Basically, the pots are wired such that pot 1 is sets the baseline (coarse) adjustment, or adjustment at idle.  Pot 2 then is the gain (or fine) control that creates the logarithmic effect as air flows (and sensor signal) go up

For details and a schematic, see this page of an article on AutoSpeed for details.   Read the whole article as well.  It has a lot of info about swapping larger injectors.

How Much Fuel?

How much extra fuel to dump is another matter.  Boost pressure alone is not an entirely accurate way of computing how much more fuel you need.  5 psi at 2000 rpm requires less fuel than 5 psi at 5000 rpm does.

If you switch to larger injectors and manipulate the MAF sensor output to compensate, the amount of extra fuel is handled automatically by the ECM regardless of rpm or boost pressure.  If you have additional injectors and/or use a rising rate FPR, there's more to the picture.

Additional injectors that use a boost sensitive pressure switch to activate them are limited in that they dump all the extra fuel or none.  In addition to being a very unprecise and binary sort of fueling, it's also additionally in-accurate in that boost pressure alone is not an accurate way of determining how much more fuel you need.

A better way if you're using pulsed injectors and an AIC, would be to use boost pressure to set the baseline and then modify the amount to inject by looking at the rpm signal. 

Other ways could be to look at the current pulse width for one of the stock injectors and/or the current signal level from the MAF.

One thing to keep in mind though, is that all of these methods require tuning.   You can't just plug in larger injectors, hook up the MAF signal modifier, twist the dials and go.  You need at the very least a good O2 sensor and an air-fuel ratio meter.  With these (and preferable a good multimeter and boost gauge) and some time on a deserted stretch of road you can get it tuned in pretty close.  Ideally, you'd use a chassis dyno with a wide range O2 sensor monitoring the fuel ratio.  These are *much* more accurate and will make tuning a lot easier.  Rent for use of these items is a spendy proposition though and will likely require the time of a technician / tuner that knows what they're doing.

Wide Open Throttle (WOT)

WOT causes other things to happen as well.  Most ECM controlled cars run in closed loop mode for the most part.  This is where the ECM monitors the O2 sensor and compares the actual mixture to what it thinks the mixture should be.  By doing this, it can continually compensate and alter the injector pulse duration to inject a little more or less fuel to keep the mixture just right.  This helps economy and emissions immensely.

When you stomp on it though, all that goes out the window.  The ECM reverts to open loop mode and dumps fuel solely by it's internal calculations.  It also tries to dump a somewhat rich mixture which gives best power and a margin of safety.  A lean mixture at WOT will overheat and start melting things like pistons.  Very bad.

What can be worse is if your favorite method of adding extra fuel isn't calibrated quite right and is off to the lean side.  During cruise and light throttle that doesn't trip the ECM into open loop mode, the O2 feedback registers this lean condition and the ECM compensates for it with the stock injectors.  Once you go to WOT though, it no longer does this compensation since it's ignoring the O2 sensor and you're at WOT going into a lean condition.  Not good. 

It's always *much* better to err on the rich side than the lean side.  Power and economy will suffer by going *too* rich, but it's better than a blown engine from going lean.

Created by: Dan Houlton
This page was last updated on 14 Apr 2004
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