fuel pressure regulaters

[randy 77zt]

if a person was installing a programmable efi system like sds it seems like it would be better to run system at set pressure at all times instead of using a regulater that varies pressure by manifold vacuem pressure.like a constant 50 psi and make all adjustments with injector pulse width.i think the system would be easier to program this way.please post opinions

[Guest Anonymous]

Nope. By keeping fuel pressure at a set pressure above manifold pressure you make the fuel flow rate constant all the time.

Also, if fuel pressure were constant, at, say, 20psi of boost you would only have 30psi of fuel pressure. And at idle you would have 50psi of fuel pressure, very possibly giving too much fuel.

THere's a reason almost all cars have manifold referenced fuel pressure regulators....

 

 

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Morgan

http://z31.com/~morgan/s30

http://carfiche.com

[TimZ]

quote:

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Originally posted by randy 77zt:

if a person was installing a programmable efi system like sds it seems like it would be better to run system at set pressure at all times instead of using a regulater that varies pressure by manifold vacuem pressure.like a constant 50 psi and make all adjustments with injector pulse width.i think the system would be easier to program this way.please post opinions

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ABSOLUTELY NOT.

The reason that the fuel injection regulators vary the fuel pressure is to keep the pressure differential across the injector constant. The pressure differential is what matters, NOT the fuel rail pressure.

 

The constant pressure differential is what allows you to reliably infer fuel flow from injector pulse width. If you keep the differential constant, then the change in fuel flow is pretty much directly related to the injector pulsewidth (i.e., double the pulsewidth, double the flow).

 

If you were to hold the fuel rail pressure constant, then the pressure differential across the injector would change with manifold pressure, and NOT in the direction that you want. At idle, you would have the highest differential, say 35psi fuel rail pressure plus 9psi from the manifold vacuum (I'm assuming 18inHg vacuum, which is -9psi).

Since fuel flow varies with the square of pressure, your 400cc injectors would flow like 450cc injectors, requiring you to artificially shorten your pulsewidths to compensate, if you can.

 

Now let's say that you are running 15psi of boost. Your injector pressure differential is now 35psi minus the 15psi of manifold pressure, or 20psi. Your 400cc injectors now flow like 300cc injectors.

 

In other words, holding the rail pressure constant will decrease the dynamic range of your injectors, and make it very difficult to tune properly.

[Evan Purple240zt]

TIMZ, can i get that in english please? Are you saying that its better to use a rising rate fpr or not?

 

I am NOT using a rising rate fpr. Well i dont think the SX unit is a rising rate fpr anyway.

 

My observation is this, as long as you have enough pressure and fuel, you should be able to tune your injector pulse to make up for needed fuel. After all, thats what SDS is for isnt it? I tend to observe a rising rate FPR as a bandaid. If you need to up the fuel pressure much past what the injector is rated for, you should get larger injectors IMO. Once youve reached too high of a duty cycle i would think it would be risky to run an undersized injector with too much fuel pressure no?

 

Hey, if im ass backwards fill me in here. I wont change regulators but am definitely curious!

 

Evan

[TimZ]

quote:

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Originally posted by Evan Purple240zt:

TIMZ, can i get that in english please? Are you saying that its better to use a rising rate fpr or not?

 

I am NOT using a rising rate fpr. Well i dont think the SX unit is a rising rate fpr anyway.

 

 

Evan

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I said nothing about a rising rate FPR. I was referring to a pressure referenced FPR, which is what most EFI systems require, including the SDS.

 

This was mentioned in an earlier post, as I recall, but I'll go over it quickly again.

 

'Rising Rate' and 'Pressure Referenced' are two distinctly different types of regulators. This is important to understand.

 

A pressure referenced regulator simply maintains a constant pressure difference between it's reference port (i.e., manifold pressure) and the fuel rail pressure. Let's say you are using a 35psi pressure referenced regulator, as is used in the stock Z. If you have zero manifold vacuum, then the fuel rail pressure will be 35psi. If you have 20psi of boost, the the fuel rail pressure will be 55psi. Remember that the injector has to spray into the manifold, so the pressure that it sees is 55psi minus 20psi, or 35psi, which is the proper pressure for the injector.

 

A Rising Rate FPR changes the fuel rail pressure in a non-linear fashion. Usually, if you have manifold vacuum, everything works as before. But when you have manifold pressure, the fuel rail pressure is increased by an amount (usually 2-5 times) greater than that of the manifold. So, if you have zero manifold vacuum, you'll have 35psi. If you have 20psi boost, you might have anywhere from 75 to 135psi of fuel rail pressure, depending on how the FPR is adjusted, and how much pressure the pump can actually supply. The idea here is to increase the flow capacity of a fairly small injector to allow it to function in a higher horsepower application than it was designed for.

 

In general, a pressure referenced regulator is necessary to get a properly tuned EFI system. A rising rate is only necessary if your injectors are too small for your maximum horsepower requirement, or if your maximum horsepower requirement is so high that you can't get a properly sized injector to work at idle.

 

If you run constant fuel rail pressure, however, you will have the worst of both worlds. In order to be able to supply enough fuel for max power, you will need to run a fairly high rail pressure, like 55psi or higher. At idle, then, the pressure that your injectors will see will be more like 65 psi, and you may very well not be able to make the pulsewidth short enough to keep from running wildly rich. Because all injectors have a minimum pulsewidth that they will react to, you'll most likely end up at a setting where the minimum pulse is still too rich, and you'll oscillate between too much fuel and none at all.

 

To sum up - if you run constant fuel rail pressure, you'll either be too lean at max power, or you won't idle well. Sorry - I know this is wordy, but I don't know how to make it any clearer.

 

 

[This message has been edited by TimZ (edited January 27, 2001).]

[pparaska]

With a bit of polishing, that would be a good magazine article. Great Stuff!

 

SuperDan, I think we might have a tech article here.

[BLKMGK]

Heh, here it is in easy to understand English

 

Under manifold vacuum the engine will actually try to SUCK fuel from th erail as the injector fires, thus you'd liek lower pressure in proportion to the vacuum. Under boost you want fuel pressure to go up 1PSI for each lb of boost due to the fact that the engine will be trying to BLOW fuel backwards up the fuel line to the tank. Carbs running under pressure have real problems with this to include collapsing floats bu tI digress...

 

The SX regulator is not "rising rate". Rising rate generally means that for each PSI in the manifold you'll see MORE pressure - say 1lb boost 2PSI pressure. Rising rate isn't waht we're talking about needing here and IMO is a crutch anyway ala Vortech's crappy FMU's (and Paxton and and...)

 

The SX WILL allow th epressure at idle to be reduced as you want, it will also raise pressure under boost correctly. That is if you hookup the vacuum hose to it's barbed fitting someplace after the throttlebody - I've got one on my Mustang. Note that the allen adjustment bolt on these things isn't stainless and will oten rust (ahem).

 

Does that help?