AFPR questions.

[tekdeus]

I'm a little confused about how my fuel pressure regulator is supposed to be setup. I have a SX model 15402 http://www.sx-performance.com/15402.PDF

It came with my car and the vac line appears to be connected to the "fuel pressure up" VSV under the intake manifold. What is the purpose of this VSV, and would I be better off having my AFPR connected directly to vacuum?

My fuel pressure is set high (by the previous owner) at 50psi at idle. However, it does not change when I unplug it's vac line, and it does not change under boost either (I have an in-car fuel pressure gauge).

How much extra fuel pressure SHOULD I be seeing with each psi of boost? This SX AFPR has a pressure regulation slope of 3 psi/gal of flow rate change.

I pulled 20hg of vac on the AFPR with my vacuum pump and the pressure dropped to 45. I get the impression that the VSV it is connected to is not working, and I'd be better off with a direct vac line connection?

After hitting fuel cut for the first time the other day when it was cold out and I had my test pipe installed, I think I need to look at adding fuel with the AFPR and pulling fuel with my SAFC. Especially now that I'm looking at getting a larger turbo.

Mods are Lex AFM, 550's, dual walbro pumps, SAFC, LC-1 wideband.

 

[jdub]

Most AFPR's like this one increase FP on a 1:1 ratio with boost. This one specs it in GPH per 3 psi increase. You'll have to contact a SX Tech to confirm.

That is the hot start FP up VSV...it works by opening to the atmosphere to increase FP when coolant temps are above a certain value. It is controlled by the ECU.

Since yours does not increase FP when the vac line is disconnected, either the VSV is bad or it is not connected to a vac source. Confirm vac line routing per this pic:

Here's how to test the VSC:
http://www.cygnusx1.net/supra/Library/TSRM/MK3/manual.aspx?S=FI&P=116

You can attach the AFPR directly to a vac source, but you will lose the hot start function. It will be a factor on hot summer days.

50 psi FP with the vac line disconnected is a lot...I have the Lex/550 set-up and am running 28-30 psi with the vac line on. FP as set at the AFPR is a "global" correction...personally, I would drop the FP and adjust the SAFC fuel curve. Too much FP will cause the engine to "stumble" under acceleration...remember, the ECU increases fuel injector volume under acceleration. Increasing FP will cause you to go even more rich . Also, increasing FP at the AFPR will not help fuel cut...that is based on the Ks signal from the AFM.

 

[tekdeus]

Thanks Jdub,

So a 1:1 ratio means 15psi of boost adds 15psi of fuel pressure?

Does the lex AFM still read airflow volumes above the fuel cut Hz of 4.99 (iirc)? I'm wondering if I can pull more air through the AFM, but have the signal lowered by my SAFC to lower the fuel cut point on the ECU, and compensate with the rising fuel pressure under boost. How much richer will I be running if I get my AFPR pressure rising properly under boost? Since day one, my car has been tuned to the fuel pressure never changing from 50psi.

I'm planning to run a GT35R turbo at 15-17psi on the street and 25+ on race fuel, and not have to change my current engine management just yet if it's possible.

 

[jdub]

That's the way I understand it...30 psi base FP + 15 psi boost = 45 psi FP.

It's the ECU that cuts fuel at at a predetermined Ks value. I'm not familiar enough with the way the SAFC works to tell you how much you can clamp the AFM signal. Pretty sure you can raise fuel cut though.

The ECU controls injector duration as a means of controlling total fuel volume sent to the cylinder. FP is "global" in the sense that it affects all the injectors...pressure will increase volume for a given duration. You can only increase duration so much...an increase in FP will allow you to increase fuel delivered before you run into injector duty cycle problems.

Keep in mind the TCCS increases/decreases injector duration for specific conditions. That's the downfall of using a piggyback fuel control like the SAFC...you will be fighting the TCCS corrections.