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Tech info on MP - Fuel Pressure Myth

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Tuning Technical Information

We are increasingly getting inquiries from customers having problems with standard vehicles. They are being frustrated with New Car Dealers who are not listening to their complaints about their vehicles poor Economy or Performance. Dealers are finding themselves caught in the middle as manufacturers dumb down the test procedures which the dealers are only allowed to carry out. They are only permitted to replace what their Diagnostic Equipment determines, so faulty sensors sending incorrectly calibrated signals, result in the Diagnostic Equipment not seeing the real faults. These Pages are here to give you a better understanding of how your vehicle works & help you diagnose problems you are having with your vehicle.

There is a “little” bit of fact to the Myth, but this really only affects the customers who search the internet & buy the cheapest product they can find. These cheap products will turn out to be a simple fixed resistor or screw driver adjusted variable resistor which will continue to increase fuel by the same amount at every point in the Tuning Map right through until the end of the engine. We still fail to understand why some people will purchase the cheapest products they can find, with no local warranty protection, & then fit these products to expensive engines?

This “Pressure Myth” is mainly used by the industry to attack Tuning Systems which plug to the Fuel Rail Pressure Sensor but this is also an “Industry Propagated Myth” & most surprisingly, the members of the industry who most use this as a selling tool are also selling Tuning systems which are making the same high pressure pump supply more fuel by making the Injectors open for longer (Duty Cycle) or via the Pressure Sensor. Even Australia’s largest 4X4 magazine makes direct profits by making its readers purchase Fuel Rail Tuning systems from an Exhaust company in which they own shares.
But none of these companies actually quote any figures of what the “Massive” Pressure Increase actually is. On the graph above, at any pressure over 200MPa (4.9V), the vehicle would go into limp mode.
To get a “very” large power gain the pressure increase required is actually less than 10%. On a Sensor which operates between 0 to 5volts, a very high powered “Race/Rally” program would change this by 0.5volt = 10%. An Eco or Towing style program would only need 2-8%.
To get the same power gain with an Injector Control (Duty Cycle) the High Pressure Pump is again, also running 10% higher.
It is basic mathematics that to get a Diesel engine to produce more power, it will need more fuel to deliver the power. If the dyno graph shows a big power gain at high engine rpm, it can only be achieved with their products by increased fuel delivered to combustion by more than was there before.
We sell both styles as well but do not see the need to use this Myth as a selling tool. Both styles “do work if programmed correctly”, but some engines work better & safer with one style instead of the other. On some engines the weak point is the Injectors, on others it is the Fuel Rail Sensors. So instead of compromising the Tuning on some vehicles, using the wrong method, this is why we sell both Injector Duty Cycle & Fuel Rail Sensor Tuning boxes.

Buy a Morepower Tuning Box for my vehicle.

Air/Fuel Ratio

The people who use this “Pressure Myth” also talk in great length about Air/Fuel ratios but sell products which Only Increase Fuel. By making the same high pressure pump supply more fuel either by making the Injectors open for longer or via the Pressure Sensor. Both methods increase fuel delivery to combustion, but the airflow is still the same, so what does this do to the Air/Fuel ratio? How much Fuel can be added & still have a correct Air/Fuel Ratio? We use a system which also connects to the MAP (Boost) Sensor & by increasing Boost (Air) as well as Fuel we can keep Air/Fuel ratios in correct proportions.
We find that most Diesel engines over fuel at full throttle & high rpm; you can demonstrate this on your own vehicle. If you put your foot flat to the floor the vehicle will accelerate fast, but if you gradually lift your foot off the pedal slightly, the vehicle goes faster again. This shows how leaning the rich mixture actually gave more power. So we find more power can be achieved by adding Boost at these over rich fuelling stages instead of just pumping in more fuel. Manufacturers are now using Sequential Turbo Systems to change the Boost Pressure & Air Volume. BMW use the same engine with one, two or three Turbo’s, the power & torque bands are completely different because of the Boost & Air Volume variations. We find we can do a similar effect even on single Turbo vehicles, by manipulating the Boost Sensor signal by different amounts at different RPM or Load points. Our Merc R350CDI which is 195kw 620Nm, we have biased the tune on this car to Eco/Towing. We know that 7th gear @ 100kmh is 1500rpm & the standard torque band hits peak torque at 1600rpm, so we have set the torque to come in at 1500rpm. It now holds 7th even on steep hills. The base model BMW X5 in 8th gear @ 100kmh is also 1500rpm, but the standard torque band does not hit its peak torque until 1750rpm. At low rpm we find this engine very rough with lots of vibration on slight inclines while it tries to hold 8th gear, the Twin & Triple Turbo models do not have this problem.

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Water Proof?

Many companies still sell Tuning Boxes to fit in engine bays with PC style Computer Plugs which are NOT water proof & tell customers to wrap plastic bags around them!
They also sell Boxes which are NOT resin filled, so they rely on thin rubber seals on flimsy plastic boxes, with the circuit board suspended inside.
One customer while removing one of these box brands, joked that the window allows you to see the water level & empty the box before any damage is done?
We have only one similar product in our range which has a PC style plug on a metal box, but it is only fitted inside the vehicle cabin, never in engine bay.

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Find out more about:

Why does the Dyno graph show all the power gain at only high RPM?

Throttle Position Sensor & Accelerator Pedal Position Sensor

Fuel Delivery systems

MAP & MAF Sensors

VNT - Turbo Control


VSV - Variable Swirl Valve

Technical info on Morepower Tuning

Morepower Tuning Overview


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