Can somebody please explain (or point me to a resource that explains) the relationship between injector flow rate and fuel pressure?

I'm working with the disreputable factory TBI system on my '89 D100 w/5.2L (318) engine. Its fuel pressure is regulated at a nominal 14.5 psi. I want to try installing the smaller injectors from the 3.9L (238) V6, and installing a custom-made fuel pressure regulator that'll give 21 or 22 psi (I have one of each). My goals here are improved fuel atomisation and increased injector pulsewidth at small throttle openings. I was finally able to track down the specs on the two injectors, and they are as follows:

5.2 (318) injector, figures at 14.5 psi

Static flow in grams/minute: 478.6
2.5 mS at 100Hz flow in mg/pulse: 16.38
Dynamic flow at 100Hz, 2.5mS in grams/minute: 98.28
Static flow in Lb/hr: 63.3
Dynamic flow (2.5 mS @ 100Hz) in Lb/Hr: 13.0


3.9 (238) injector, figures at 14.5 psi

Static flow in grams/minute: 404.0
2.5 mS at 100Hz flow in mg/pulse: 14.10
Dynamic flow at 100Hz, 2.5mS in grams/minute: 84.60
Static flow in Lb/hr: 53.4
Dynamic flow (2.5 mS @ 100Hz) in Lb/Hr: 11.2

By any of these measures, the 3.9 injectors look to be about 15% smaller than the 5.2 injectors. I'm proposing to raise fuel pressure by about 50%. Aside from "Try it out and see what happens" (probably going to do that anyhow), does anyone see any potential problems here?

Thanks.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Flow rate is proportional to the square root of the pressure. So if you bumped it up to 22 psi, you'd have a static flow rate of around 65.8 lb/hr (53.4 * SQRT(22/14.5)). This assumes that the injector can still open at about the same rate, which is not a bad assumption with such a small pressure change.

If you want to change the pulse width, however, you'd need to reprogram (which most likely is spelled "replace" here) the ECU.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

Dan, the only thing I can think of is one of the calculators in the fuel section of the Megamanual http://www.megamanual.com/v22manual/minj.htm#size

There is a section that allows you to enter your injector size and then enter the proposed fuel preasure.

Hope this helps!

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Chris'
Autobody
Restoration
Service

Excellent link; thanks for sending me there. This tells me that if I use the 3.9 injectors with my 22.2 psi regulator, static flow will be 4% greater than stock overall. If I use the 3.9 injectors with my 21.4 psi regulator, static flow will be 2.5% greater than stock overall. That, in turn, tells me either move would be safe from the standpoint of avoiding leanout at high engine speeds and loads. Now I just need to figure out if the 3.9 injectors with either regulator would create an overly-rich situation at idle; the only way that could happen would be if the system is running its absolute minimum pulsewidth and frequency right now with the stock 5.2 injectors and 14.5 psi pressure regulator.

This pressure regulator has a vacuum hose on its backside, connected to manifold vacuum. Presumably this trims the fuel pressure to coincide with manifold vacuum. High vacuum = lower fuel pressure, low vacuum = higher fuel pressure. So my 21.4 and 22.2 (and the stock 14.5) should be maximum pressures attained at 0 manifold vacuum.

Getting close to time to try this out and see what happens. Truck is due for a smog check, so it'll be "trial by ordeal".

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I was actually thinking in terms of the ECU saying "Hey, whoah, I'm detecting a little too much fuel is going in" and reducing the pulsewidth accordingly. That will definitely happen via the O2S at anything above idle. The only open question is what'll happen at idle, since this (expletives deleted) system disregards the O2S at idle.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

As a long time TBI user.....

I would never run more than about 15 lbs....If you need more pressure yer injectors are too small.....

The smaller injectors will "duty cycle"more easily .

They can only open an close so fast.... then they hang open,,,,,or stammer...the thing leans ....and you melt 2 plugs before the top of the pass.

The throttle body and injectors are not the problem with that setup...(I live with the Dakota version.)

Mega squirt the thing ....

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Yeah....Im the one who destroyed this rare, vintage automobile.....

The idle / WOT issue with an injector change is interesting.

I deal with it by tuning without the o2 sensor.

Once the injectors have been changed un plug the o2 sensior and tune fuel pressure till you get the idle you want. Replug in o2 sensor and drive a few cycles.....check WOT for lean surge or ping. Richen as necessary.

For max towing power my work truck ends up a little rich at idle and will lean out/duty cycle on WOT/towing/8% grade.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Because...why?
If I parse you properly, you're thinking along my same lines: At idle and low speed, the duty cycle is so short that the injector may be open longer than the computer wants simply by dint of physical limitations (reaction time, inertia). To spray a given amount of fuel, a smaller injector needs to be open longer, which will slow down (space out) the open/close events over more time.
Well..it's halfåssed hardware controlled by halfåssed software!
H'mmm. More info on applying MS to this system, please. Send me a PM?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

The biggest problem is that, if the injectors are able to work correctly at the higher pressure, they'll behave almost exactly the same way as stock ones except maybe with better atomization and slightly larger flow rate. From a tuning standpoint, it doesn't matter very much if you get your high flow rate from turning up the injector size or turning up the pressure; flow rate is flow rate. A small bump up in pressure on the stock injectors could accomplish similar things with the duty cycle (minus the possible changes in atomization, and the possibility that the injectors may have a harder time opening under high pressure).

If you want big changes in duty cycle, Megasquirt or another standalone mighta better way to get your results. Particularly since you can pair an aftermarket ECU with a mechanical advance distributor like you were talking about earlier.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

That's really all I'm after. I have no indication that the engine is getting grossly too much or too little fuel. I'm primarily trying for better part-throttle driveability and cold-weather behaviour, and I think better atomisation may help there.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Dan, can you post the 21.4 and 22.2 fuel pressure regulator source(s), PNs, and approx prices?

I run my TBI injectors at about 19 psi in the Dart and wouldn't mind going higher.

Sandy, does pressure above 15 psi damage the injectors in some way?

Thanks,

Lou

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Home of Slant6-powered fun machines since 1988

I spent an afternoon on the phone until I found the company actually making FPRs (many companies distribute them), then another couple weeks playing phone tag with their engineering department, then about half a year waiting for a production run of this particular FPR (@ stock specs), then another several weeks while they custom-built and tested these two regulators for me.
What kind of FPR are you running?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I'm not sure if the Chrysler TB is the same as the GM one... is the regulator built in to the TB?

If it is, a common trick to increase fuel pressure is to open up the TB and regulator assembly and then add a quarter under (may be over?) the regulator diaphram. by "tightning up" the diaphram pressure is increased.

I can't find a link to that article but I will keep looking for it.

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Chris'
Autobody
Restoration
Service

I have had negative results trying higher than 15 PSI on my work truck.

I lose idle quality and get a huge part throttle/off idle bog

This setup is at it limits for a TBI factory setup. It makes about the same torque as Lous 64 and regularly runs on the ragged edge of the dreaded Duty Cycle meltdown.

A TBI injector will only operate within a certain range of flow rate.(idle to WOT) On my work truck I have to run as rich an idle as I dare in order to get the flow rate I need at even 75% throttle if Im at maximum torque RPM pulling a pass. I always pull a pass closed cycle.

The puter bases WOT on what it learns during closed cycle.

Both WOT and Idle are open cycle. It s hard to tune with fuel pressure and get both.

I have an over bored throttle body that makes both the idle end and top end iffy. The Throttle body will flow more than the injectors can supply until the pintles in the injectors are either open all the time....stuttering....or remain closed.

There are some things you can do with the IAC to make idle quality better.

Dan : check all your TPS circuit

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Yeah....Im the one who destroyed this rare, vintage automobile.....

I use a stock modified FPR. You can adjust fuel pressure while the thing is running.

It sounds like Dan may be trying the vacumn regulated FPR. I never had any luck with mine doing anything other than bouncing my fuel pressure around and causing puter fits.

Keep in mind these factory puters are very slow learners.

I always "kill the puter"by disconnecting the battery before doing any tuning. What you are after is a base tune the puter can easily learn from. This means the chip must be close. You fine tune fuel pressure and advance from there.

Keep in mind I use Rochester 220 with GM injectors injectors....your holley may differ.

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Yeah....Im the one who destroyed this rare, vintage automobile.....