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Fuel pressure

Diesel Tuning Fundamentals

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Discussion and questions related to the course Diesel Tuning Fundamentals

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How do one gets to know the maximum fuel pressure the pump can operate or withhold so that its safe for us to make changes(increase pressure) in the fuel pressure 2D table??

I think a load of information is out there on smaller CR fuel on what they can handle. Most professional diesel tuning shops can help you on that part.

Depending on the fuel system your running there is a safely valve eider on the fuel rail or fuelpump preventing overpressure. Some companies sell upgrades for these as well able to run more fuel pressure.

Just to bit to add to Rudd's post. Usually running within 10% of the max setting in the OEM application of the pump is considered fairly safe in the performance world. The electronic relief systems are much more robust and capable of consistently relieving pressure in the system. The mechanical reliefs in my opinion tend to degrade with use and should be size to only be used in emergencies.


Most engines I work on have these safely valves and yes if you open them it can be game over very fast as the keep leaking not shutting any more but most will give way slowly opening @ a lower fuel pressure over time so eider the valve/seat degrades or due to a lot of heat build-up on the valve the spring inside the valve loses force. Every brand has different problems and symptoms if it comes to that so if you want to run more pressure find you self a valve upgrade that’s within what the pump can handle.

We often get project with pre production or early production engines latest generation. For some engines we get full documentation on sensor and injector and others we do not ask but start generating data and compare them the older generation engines of the same engine family.

In this example we use a fuel rail valve that hold pressure up to 3000Bar but as you can see it opens @ a lower pressure so chance we apply to low current to the valve or the fuel press sensor scaling is incorrect.

Next question is how dos this effect engine performance. Well looking @ the RPM line it dos not make a difference and this is why. The fuel pressure error is proportional to the total injection angle chance. But if the total injection angle is very small the chance in total injection angle by fuel pressure is also very small so in all variations is less than 2 crank degrees in this example. Accurate start of injection is key to good performance.


I have problem with high injection timing under full boost on a 1KD with Motec. I can see flames from exhaust pipe (exhaust is 3" pipe, just 45" long) and making 180whp, fuel pressure is 190mpa @ 1.00 Lambda, and i read that max fuel pressure for that is 200mpa. Sense i need less injection timing, i need more fuel pressure. I also will like to know if i can work with 200mpa all the time, sense that engine have an electric solenoid, or can i go to 210.... or 215mpa

We know these engines as we mainly run them on long distance rally cars and these injector are not small making decent power easy so what injector duration or injection crank angle as for start en end of injection are you running. Also what is your EGT. Also presume your rail pressure sensor is connected correct and has the correct calibration on it???

What about your fuel pressure. Is it stabile? Or is it all over the place and as the ECU attempt to inject the same fuel quantity and injector duration will increase on lower fuel pressure reading.

What about the flames coming out of your downpipe. Is it a cone shape and what collar? Could be oxygen starvation as well as for hi EMP and EGR working. What is the position of your turbo activator in relation to boost pressure.

Tuning a diesel involved understanding the fuel model for your engine and yes for most this is not a easy thing. If the fuel model is correct and your not getting the power out of your engine some external factor will limit things.

Also take note getting from 2000Bar to 2100Bar fuel pressure you will gain 9Hp max on 190Hp. that’s marginal and will not solve your problem. So get these numbers out on INJ duration, start of injection and EGT and take it from there.

I dont have all numbers now, remember using like 25 or 27° BTDC for start injection timing and ending like 5 or 7° ATDC. Wiring its stock and a Motec M1 kit is on it, fuel pressure looks fine stable at 180 or 185mpa (this is the Motec Start Map tarjet, dont touch it yet) and injection timing is stable too. I dont have EGT yet, was installing that now. I delete EGR and instake throttle. Have a 47mm/49mm turbo with a 38mm external wastegate. Let me see if i can show you the exhaust picture so you can look at

On back you can see exhaust, and flames have same shape that dump pipe. This was testing, then take all off to ad EGT

Attached Files

Looks like your down on power but hard to say as we do not know drive train losses. If I calculate with 20% drive train losses I would expect between 190 and 200Hp on the wheels with the fuel injected as you mention it but do think you run your motec off the chard already as your fuel flow map only goes to 119?? and your way over this with max 124 micro litter with the data you posted.

Fuel consumption will be about 42L/hour making 0,1cc/stroke @ 3500Rpm for 228Hp on the crank.

We run more advanced timing due not using a pilot as we know it’s not needed and only taking power from the crank as return fuel from the injector pull more flow from the fuelpump that is essential a parasite if it come to power drain. But OK. Easy trick to pull some more Hp out of your engine.

As you can see we try so stay under this magic 30 degrees injection angle and end of injection close to TDC and it’s possible even with lower fuel pressure making decent power. Data is from a 3L CR Toyota rally pick-up. It runs a 3L V6 TDI VGT turbo @ 250Kpa making bit over 200 wheel Hp. We engineer for max efficiency end let the engine talk on numbers from the data log. So get your data log data out.

Looking @ your picture there is plenty of fuel but you need to start looking for more efficiency eider in your wastegate/turbo set-up or ECU system or the dyno show low numbers???



As soon i can go back to work after all this, i will try out more things. The dyno i use is a Mustang, and truck is AWD so i know its not nice with numbers. Just also will start moving fuel pressure to see if something change and will test with no pilot pulse also! Thanks for that info i never try that to see changes.

Well basic diesel tuning is boring for some but please start making data logs even if its low sample rate as some ECU manufactures charge load of money for a hi sample rate but even do you can still read between the lines getting an idea what is going on as to may things happen @ the same time in a CR diesel engine. I often make 2 or 3 runs and spend hours making changes to the control strategy even to optimise things but your still @ the basic level of making decent power and ones that is done your able to optimise on pilot to only main injection and shift start of injection toward that point as well as it all needs to work together as a team so aiming for these points and knowing here you are can only be done by creating data.

Yes i love to work with data, its just that this is the first CR engine i make, and then all this covid start and cant continue testing all, and want to have more teorical ideas to continue as soon i can

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