Fuel Rail Pressure's relationship to fuel economy.

[killerbee]

In my personal experience, contrasting conventional wisdom, I found lower pressure for lower load to be the way to go, with do real downside in efficiency.

It realistically comes down to keeping pulsewidth to a reasonable CADeg. At higher loads, you would really need the pressure to lower CADeg, which can get as high as 30-40 degrees is some of the wilder power requirements.

[LBZoom]

I'm going to experiment with my fuel pressure some more in the next couple weeks, started a new job with about 30 miles of interstate driving every day so I should get some good testing in. I'm going to attempt to lower my pressure slightly, and I'll have to open my pulse some to still allow the fuel in I suppose... correct?

Or should I maybe lower my crusing pulse time and map the actual FRP vs. my current base FRP table and go that route? I guess only experimentation will tell.

[killerbee]

This is by no means the definitive technique, but I will say what worked for me on a long road, where I made plenty of learning mistakes before I settled on this. Find a pressure table that you like the feel of (easier said than done). Then create a pulse table that keeps around 800-900us as you climb the power ladder. As you approach your pressure cap, pulse should begin to grow past 1000us into the real power range. This was very important to my end product refinement, and this one process can take weeks or months if you are a perfectionist.

No load highway cruise uses 25-35HP. For me, I find no use for more than 60Mpa. That should give a good place to start from. Eventually you will draw your own conclusions, but trial and error is definately involved. Develop the buttmeter to tell you what you need, trust the force. Eventually, you will develop keen ideas of what changes the feel of things, and make correlations that the Dash will provide. I use dashB almost exclusively.

[DmaxHawk]

In my personal experience, contrasting conventional wisdom, I found lower pressure for lower load to be the way to go, with do real downside in efficiency.

It realistically comes down to keeping pulsewidth to a reasonable CADeg. At higher loads, you would really need the pressure to lower CADeg, which can get as high as 30-40 degrees is some of the wilder power requirements.

So lower pressure will lower CADegs? I'm a little confused

[killerbee]

No. As you probably know, lowering pressure, changing nothing else will increase CADeg.

I know some are wondering what CADeg is. It is the pulswidth measured as degrees of crank rotation.

[DmaxHawk]

No. As you probably know, lowering pressure, changing nothing else will increase CADeg.

I know some are wondering what CADeg is. It is the pulswidth measured as degrees of crank rotation.

Ok so lowering pressure will increase the pulse because it is still trying to get the same mm3s into the cylinder? The increase in pulse is compensation for lower pressure?
Is this right?

[killerbee]

Yes. More or less, a given HP requirement (or drag total) requires the same fuel volume.

The actual fuel volume is best defined by fuel pulse multiplied by the pressure square root. Don't get too hung up on "mm3" as illustrated in the tables. It would be best considered a unitless index.

[bballer182]

I'm going to experiment with my fuel pressure some more in the next couple weeks, started a new job with about 30 miles of interstate driving every day so I should get some good testing in. I'm going to attempt to lower my pressure slightly, and I'll have to open my pulse some to still allow the fuel in I suppose... correct?

Or should I maybe lower my crusing pulse time and map the actual FRP vs. my current base FRP table and go that route? I guess only experimentation will tell.

As posted by Michael earlier there is no need to alter the pulse width table when lowering or raising the pressure you are automatically increasing or decreasing the PW. you cant expect to run @ 65 mph and have a pressure of 60Mpa and a PW or 800 and then change the pressure to 70Mpa and have the same PW. you would be creating more power than is required to move the truck down the road @65. so just leave the PW alone.

[LBZoom]

I've already noticed a slight difference in my economy with the lower pressure.... Why exactly does lower pressure improve MPG's if the fuel volume essentially remains static? Just curious...

[bballer182]

you are playing with how the fuel burns. either short and quick with a fast pressure spike or a slow burn with a slow rolling pressure curve.