MPGs run, Aspire SE

[FestYboy]

stock is ~36... yes duty cycle times will change to compensate for pressure. shorter bursts at higher pressures typicaly yield a finer spray

experiment time!!!: take a straw and a full glass of water. insert the straw into the water and seal the top off. remove the straw and lightly place a finger at the bottom of the straw (pintle valve). now uncap the top of the straw.... the water slowly dribbles out, large droplets fall straight down, a puddly mess.
now do the same steps again, but instead of lightly pressing on the bottom, press a little harder and then BLOW into the top of the straw (pressure). now you have a spray/mist of water that goes all over the place!
the volume of water hasn't changed, but the way it's delivered and dispersed does. same thing with the injector!

[TominMO]

1. what's the point of an adjustable FPR if you're going to run stock pressure?
2. you have to remember that the ECU can adjust the injector pulse length to maintain it's programed ideal AFR.

1. I was coming to that conclusion myself, but would like to play with fuel pressure just to see what happens.
2. Aha, that's what I wanted to know. If our Festy/Aspire computers are smart enough to compensate. So with lower fuel pressure, the computer will make the duty cycle longer? Will that be a problem? And with higher pressure, the duty cycle would be shorter? And BTW what is the stock pressure?

[rmoltis]

I guess I'm just uncertain how much the computer will adjust things.
Can't wait to see the results of the mpg cam either way.

[FestYboy]

well i'll ask this: what's the point of an adjustable FPR if you're going to run stock pressure?

in any case, reguardless of the cam profile, the idea is to get the MOST cylinder pressure for your power band (the MPG cam is designed to make the most UNDER 4000 rpm). given that, having higher pressure allows the injector to be closed longer (less lag) for a given volume of air. you have to remember that the ECU can adjust the injector pulse length to maintain it's programed ideal AFR.

[rmoltis]

I too have my pressure set at 42psi.
And it seems to do well.

But with the mpg cam designed to make the car gulp in less air.

Wouldn't tom need less fuel so it won't run rich.

Or will his header offset that.
And need a stock pressure or barely higher?

[FestYboy]

as mine sits, i'm at 42psi and have left it as such. 44mpg running arround Mass., and 34+ beating the snot out of it while short tripping. THAT is a 2mpg improvement over stock pressure AFTER the head rebuild. so yeah, more pressure GAINED mpg.

up to 5000 RPM, the injectors fire TWICE per combustion event per cylinder, so at low rpms, the injectors are open a VERY short time per pulse. I figure that the higher than normal rail pressure allows the pattern to actually happen and also atomize properly.

after (pay attention Zanzer!!!) 5000 rpm, the injectors fire ONCE per combustion event per cyl, because otherwise the injectors would start to go 100% and that makes for poor power.

[TominMO]

I think I get what your sayin.
If you reduce the pressure the fuel may not atomize fully through the injectors?

I think turning up the pressure would work good.
But turning it down may turn the spray into a stream
If turned down to low.

Only you will know for sure.

I bet matt would have some good advice on this matter.

Good luck on your endeavors.
Can't wait to see the results.

I will PM Matt on this question. Arty can chime in too BTW. I think what you or someone else was saying is that lower pressure might make the computer overtax the injectors by keeping them open longer, to get the same volume of fuel per cycle, something like that. But IDK if our cars' computers are even that smart. Nor am I sure that raising the pressure will do any good, or do any harm (or not). I figure there is a reason that the stock little FPR is set the way it is (whatever value that is).

[rmoltis]

Can't believe I said fuel pressure gauge. Meant regulator.

My concern is how to adjust it? rmoltis IIRC mentioned that I might actually overtax the injectors by dialing the pressure back on the FPR. My thought had been that by putting less pressure through, say 35 lbs, it would help MPGs, kinda like putting in a smaller primary main jet in a carb. But I don't know enough about fool injection to know whether this is correct thinking or not. I was planning on just leaving it at stock pressure, i.e. not a factor at all.

I think I get what your sayin.
If you reduce the pressure the fuel may not atomize fully through the injectors?

I think turning up the pressure would work good.
But turning it down may turn the spray into a stream
If turned down to low.

Only you will know for sure.

I bet matt would have some good advice on this matter.

Good luck on your endeavors.
Can't wait to see the results.

[TominMO]

running the FPR is going to smooth things out and should NOT be installed last (last should be the header). i would START with the FPR, that way it's there to support the cam and header as they are installed.

Can't believe I said fuel pressure gauge. Meant regulator.

My concern is how to adjust it? rmoltis IIRC mentioned that I might actually overtax the injectors by dialing the pressure back on the FPR. My thought had been that by putting less pressure through, say 35 lbs, it would help MPGs, kinda like putting in a smaller primary main jet in a carb. But I don't know enough about fool injection to know whether this is correct thinking or not. I was planning on just leaving it at stock pressure, i.e. not a factor at all.

[TominMO]

You gonna keep your stock exhaust with the header?

And are you gonna get new o2 sensors while your at it?

Sounds like fun.

Will probably redo the exhaust a bit; slightly larger pipe and muffler, no more than 2". O2 sensor was done pretty recently. Nippondenso of course.

[rmoltis]

running the FPR is going to smooth things out and should NOT be installed last (last should be the header). i would START with the FPR, that way it's there to support the cam and header as they are installed.

I agree.

To get the full benefit of either.
Or all together it would me most beneficial to install it first.

Not to mention if he wanted to do a stock run.
he could just dial the fpr back to stock pressure.

[FestYboy]

running the FPR is going to smooth things out and should NOT be installed last (last should be the header). i would START with the FPR, that way it's there to support the cam and header as they are installed.

[zoom zoom]

uh oh stuffs getting interesting now! I hope you do things individually with a couple tanks between changes so I can see whats worth buying and what's not! haha...do the fpr last!

[rmoltis]

You gonna keep your stock exhaust with the header?

And are you gonna get new o2 sensors while your at it?

Sounds like fun.

[TominMO]

Just got a PM today from Matt that my parts will be completed shortly; i.e., Aspire roller MPG cam, header and fuel pressure gauge. So when the parts box shows up on my doorstep, I will get them installed. The header will be installed later, so I can run a test on the cam alone.