2000 7.4 MPI fuel pressure question

[Mischief Managed]

Does the fuel injection system monitor fuel pressure and adjust injector opening cycles as needed or does it have a regulator and assume the fuel pressure is fixed?

 

[veeru]

Re: 2000 7.4 MPI fuel pressure question

from experience with autos most injected systems have
i) a fixed pressure valve that regulates a pressure from the electric pump
ii) an injector with one volume only and
iii) a return pipe.

its the frequency of opening the injector and duration that changes the volume of fuel going in based on throttle input.

 

[Mischief Managed]

Re: 2000 7.4 MPI fuel pressure question

from experience with autos most injected systems have
i) a fixed pressure valve that regulates a pressure from the electric pump
ii) an injector with one volume only and
iii) a return pipe.

its the frequency of opening the injector and duration that changes the volume of fuel going in based on throttle input.

I am fairly certain that my engine does not have a fuel return line. The injectors are rated for lbs per hour at a given pressure, but if you increase the pressure, they flow more for a given period of time. The difference is the square root of the quotient of (new pressure divided by the old pressure) times the old fuel flow rate. A 10% increase in pressure will give a 4.88% increase in flow.

I ask because I am concerned about E15 being approved and how much it will lean out the air to fuel ratio in my engine. Gasoline requires a 14.7:1 air to fuel ratio. Ethanol contains oxygen and requires less air so it has a 9:1 air to fuel ratio. E15 will be running 9% too lean if the injection system is setup for plain gasoline. That could be troublesome.

To offset the mixture problem, I'm thinking one could install bigger injectors, re-program the ECU, or simply increase the fuel pressure at the injectors. Increaseing the presssure would be the simplest and cheapest way to go, but it may not work if the systm monitors fuel pressure and adjusts the pulse width of the injector signals to compensate. I'm told cars do that.

 

[pnwboat]

Re: 2000 7.4 MPI fuel pressure question

I'm no expert on this, but some of the newer systems that do not have a return line actually control the fuel pump itself to adjust pressure. Here is a short video that explains how this is done. Not saying that this is how yours does it, but just in general.

http://www.youtube.com/watch?v=XNAhBHxX1S8

If you do have this type of system, then I think re-programming the ECM or slightly larger injectors may be your answer.

The other thing I would ask, is it really necessary, and what is this going to cost?

 

[Mischief Managed]

Re: 2000 7.4 MPI fuel pressure question

I'm no expert on this, but some of the newer systems that do not have a return line actually control the fuel pump itself to adjust pressure. Here is a short video that explains how this is done. Not saying that this is how yours does it, but just in general.

http://www.youtube.com/watch?v=XNAhBHxX1S8

If you do have this type of system, then I think re-programming the ECM or slightly larger injectors may be your answer.

The other thing I would ask, is it really necessary, and what is this going to cost?

Not sure if it's necessary or not. It's been running 6% lean on E10 for a few years now and does not seems to be suffering any problems. Perhaps 9% lean will still be OK. I'm just being proactive now while it's still Winter here in NH

If it does need to be adjusted, I can get the ECU re-programmed for $395, I'm pretty sure that there's also bigger (higher flow) fuel injectors available (not sure of the cost, but I assume they are expensive), and I found a kit for $70 that converts the Mercury fuel pressure regulator from a fixed unit to an adjustable one (not sure if this is applicable to a 7.4 MPI or not though). Clearly, I'd prefer to use the cheapest and simplest method, assuming all are equally effective.

 

[Tail_Gunner]

Re: 2000 7.4 MPI fuel pressure question

Well you should know that most if not all marine efi injections systems run very rich..With that said you can always do a simple plug read basic but without measuring your AFR with a wide band sensor its hard to know what is lean or rich.

Yes a fuel pressure reg is a quick and dirty solution...Injector pulses are timed in duration and any time you increase the pressure you are going to enrich the mixture. Personally on a expesnsive marine engine...maybe not the thing to do...

But if you want here is a source for reliable help. http://www.obd2allinone.com/sc/details.asp?item=mefitune

They specailize in MEFI systems which you might have...Or a Merc 555 which is different. If you wish to pursue this by yourself get your hands on a wide band analyzer and turn up the pressure..But understand that the tables preprogrammed (injection pulse rates) about every 200-400 rpm so whats a good AFR @ idle maynot be at 1500...3000 or 5000 and turning up the rail pressure will enrich the whole rpm band....aka you might flood it badly @ idle.

Using a wide band sensor creates a whole set of problems...Just how are you going to get oxygen sensors in the exhasut loop...It can be done for about 400-600... If you do have a mefi call obd...they really do know marine efi.

 

[E4ODnut]

Re: 2000 7.4 MPI fuel pressure question

Well you should know that most if not all marine efi injections systems run very rich..[/I]

Where do you get that information and why would you run an EFI system any richer than a carburetted system in the same application?

 

[Tail_Gunner]

Re: 2000 7.4 MPI fuel pressure question

Fair question...I am in the middle of of building a engine and tuning a tbi system. But lets think about some basics.....Marine engines are basically a heavy duty cycle engine.. very much like a nascar engine.... prolonged midrange rpm at heavy load's. So do you tune the burn for a lean mixture or a rich mixture...which one is the safer bet....rich burn.....Ask any engine builder...Ever wonder why the thermostat is set at 120-140....nice cold engine....no heat in the bilge...Its the only engine i know of that cools the exhaust stream with water.

Google marine engines AFR

 

[E4ODnut]

Re: 2000 7.4 MPI fuel pressure question

I ask because I am concerned about E15 being approved and how much it will lean out the air to fuel ratio in my engine. Gasoline requires a 14.7:1 air to fuel ratio. Ethanol contains oxygen and requires less air so it has a 9:1 air to fuel ratio. E15 will be running 9% too lean if the injection system is setup for plain gasoline. That could be troublesome.
.

You bring up a very interesting point, and one which I've never considered before, but before we get into that, I would be very surprised if Mercruiser uses a variable rate fuel pump and a fuel pressure input to the engine control unit. I have no direct experience with the Mercruiser system, but I suspect that it is a typical speed density control that uses inputs from engine RPM and manifold pressure as the prime inputs to calculate the injector pulse widths depending on engine speed and load.

As for the air fuel ratios, stoichiometric ratio ( the ratio where all fuel and all oxygen in the mixture is consumed ) is indeed 14.7:1 for gasoline and 9:1 for ethanol. This is assuming a standard specific gravity, so the ratios will vary a bit, but not much for practical purposes. Stoich ratio is not what is required to burn the fuel, it is just the ratio where least emissions are produced. What is important from a performance stand point is what ratios produce the best power, and what ratios produce the best economy? For best power on gasoline it is generally agreed that between 13 and 12.5:1 works best, any richer will produce less power and the excess fuel is just used for cooling. Best economy is as lean as the engine will run without a "lean stumble". That is usually between about 16 and 17:1. I have engines that will run quite happily lightly loaded at 17.5:1.

Now as far as ethanol is concerned, I have no idea where the best power ratio would be. Stoich is at 9:1, but the emissions from burning pure ethanol are completely different than what happens with gasoline. I would think that best economy would be determined the same way, by how the engine runs, but I really don't know.

So, getting back to your point that E15 would result in a 9% leaner mixture. First of all, I'm not sure how you came up with that figure. According to my math, and admittedly, math is not my strong suit, if you have a mixture of 85% gasoline, and 15% ethanol, at stoich, you end up with a fuel air ratio of 13.84:1. What does that mean in terms of energy available to burn or emissions? I have no idea.

So, How will this affect how your engine will run? I am also assuming that Mercruiser does not use an O2 sensor to provide feedback for a closed loop system. This means that the fuel maps were prepared by data from lots of dyno time using standard gasoline under typical conditions with a bit of safety factor thrown in. Now you will be burning a mixture that has not been used in preparing these maps. Will this mixture have leaning effect, or a richening effect? How much of a safety factor did they build into the maps, and will it make any real difference at all?

Not an easy question to answer from the layman's point of view.

 

[Tail_Gunner]

Re: 2000 7.4 MPI fuel pressure question

Heres a thread that wil articualte a bit

View attachment 86342
http://forum.efilive.com/showthread.php?6328-Tuning-Info

In the E38 PCM, stoichiometric fuel ratio is set in B3671 which is a table representing AFR vs Ethanol %. If your car is set up for "Flex Fuel" then this table is used to alter the Stoichiometric figure depending on the amount of ethanol in the fuel. If your car is not set up for "Flex Fuel", then I believe only the 0% Ethanol setting is used which in stock form is set to 14.68:1.

Some figures from the table are as follows.....
Ethanol AFR
00.0% 14.68
06.2% 14.32
12.5% 13.97
18.7% 13.61

and skipping to the end....

100% 9.00 ( which is the correct figure for 100% Ethanol )

Given that the B3671 graph is a straight line between 100% petrol and 100% Ethanol, we can probably assume it's correct.

So the stoichiometric ratio for 10% Ethanol is more like 14.09:1. I run 10% ethanol all the time and my car does not have the "Flex Fuel" option, so it's always using the 0% setting for stoichiometric.
​

Everything you would like to know...

http://www.michaelstractors.com/forum/topic.asp?TOPIC_ID=199

I think you will find most volvo or merc engines run somewhere around 10-11/1....Marine motors are not setup around emission regs or fuel econ concerns....just long term survivabilty and avoiding predetonation...Thats just a opinion..

Both merc and vovlo use a std GM fuel injection...just marinized...with one expection marine mpi manifolds are built again for heavy duty cycles..

http://www.fullthrottlev6.com/forum...ortec-Marine-Intake-Conversion-Helpfull-Hints

 

[wifisher]

Re: 2000 7.4 MPI fuel pressure question

I don't know of any engine anywhere that has a closed loop fuel pressure system. They just maintain a specific pressure to measure fuel flow.

The O2 sensor's purpose is to correct for differences in fuel volume or energy. It is there to detect a lean or rich condition, and adjust the pulse width accordingly. If the ethanol content gets too high for the pre-programmed corrections, it will set a code.

 

[Tail_Gunner]

Re: 2000 7.4 MPI fuel pressure question

I don't know of any engine anywhere that has a closed loop fuel pressure system. They just maintain a specific pressure to measure fuel flow.

The O2 sensor's purpose is to correct for differences in fuel volume or energy. It is there to detect a lean or rich condition, and adjust the pulse width accordingly. If the ethanol content gets too high for the pre-programmed corrections, it will set a code.

Marine motors 98 % use a open loop speed density

 

[wifisher]

Re: 2000 7.4 MPI fuel pressure question

Marine motors 98 % use a open loop speed density

So are you saying that there is no O2 sensor?

 

[Tail_Gunner]

Re: 2000 7.4 MPI fuel pressure question

No 02 sensors....Speed density tuning

http://www.ls1tech.com/forums/pcm-d...8574-please-expalin-speed-density-tuning.html

MAF = PCM calculates the cylinder fill airmass using MAF and RPM.

SD = PCM calculates the cylinder fill airmass using VE, MAP, IAT, and RPM.

VE is the table telling the PCM how much mass/volume the cylinder fill is at any RPM vs MAP.

When running with MAF, the PCM uses both MAF and VE at various times.

When running SD, the PCM obviously ignores MAF.

With your non-stock intake manifold, either way (MAF or SD) requires correcting both MAF and VE tables (the intake manifold change will alter airflow thru the MAF sensor, the MAF sensor was previously calibrated for it's immediate surroundings).

Either way you have to pay for a tune; SD eliminates the [cost of] the MAF sensor.

Eliminating the MAF sensor protects the PCM's airmass calculation from incorrect intake plumbing (bends too close to MAF) and from reversion effects.

Running SD is a fail-over mode and so it has side-effects, for example: spark timing adapt now defaults to low octane spark table (you lose the sliding between the HO and LO tables), and transmission pressure adapt defaults to fault mode (max pressure, no shift feel adapts), and other things not fully known.

 

[wifisher]

Re: 2000 7.4 MPI fuel pressure question

Thanks Tail gunner. didn't know that

 

[E4ODnut]

Re: 2000 7.4 MPI fuel pressure question

Heres a thread that wil articualte a bit

View attachment 86342
http://forum.efilive.com/showthread.php?6328-Tuning-Info



I think you will find most volvo or merc engines run somewhere around 10-11/1....Marine motors are not setup around emission regs or fuel econ concerns....just long term survivabilty and avoiding predetonation...Thats just a opinion..

Interesting thread, thanks.

I noticed that they state that at about 11:1 exhaust is black, and smells of gasoline. At about 10:1 the plugs foul and the engine barely runs.

That has been my experience as well. I'm afraid I don't buy into your rich mixture scenario. Mid to low 12s at wide open throttle, perhaps, but certainly no richer than that.

 

[E4ODnut]

Re: 2000 7.4 MPI fuel pressure question

http://www.michaelstractors.com/forum/topic.asp?TOPIC_ID=199

Another good link. Thanks again.

From the diagrams shown I see no mass air flow sensor. I also see no O2 sensor. This tells me that this particular system is open loop speed density. The fuel and (presumably) ignition maps were probably developed using hours of testing, probably on a dyno. There is no provision for closed loop feed back so if the composition of the fuel differs greatly than the fuel used during development of the tuning maps then it's a whole different batch of cats.

That gets us back to just what difference would E15 make? Haven't got a clue.

 

[Tail_Gunner]

Re: 2000 7.4 MPI fuel pressure question

Have a close look @ #10

 

[E4ODnut]

Re: 2000 7.4 MPI fuel pressure question

Have a close look @ #10

The number 10 I see is " Electronic Control Module (ECM)"

What am I missing here?

 

[Tail_Gunner]

Re: 2000 7.4 MPI fuel pressure question

Some figures from the table are as follows.....
Ethanol AFR
00.0% 14.68
06.2% 14.32
12.5% 13.97
18.7% 13.61

rounded off 20% ethanol lowers ARF 1%

Ok heres some more...literally

http://www.mastertune.net/faqsMasterTune-HD.html

Q: What is the best AFR for power?
A: Keep in mind that every motor is a bit different and that gas composition can vary substantially. In general, we find the best AFR for WOT power is between 13.0 to13.5:1 AFR. However, these settings can build a lot of heat so it is best to go a bit to the rich side. For street use, using a AFR setting of 13.0 is a good compromise between power and economy. Keep in mind that alcohol fuel blends require a lower AFR for maximum power than straight gasoline. Also, if heat is a problem it will be better to run a lower AFR


"Also, if heat is a problem it will be better to run a lower AFR"....................