jmc007

Hi ! I have a returnless style fuel system with a OEM fuel pressure regulator before the fuel rail (in fact it's in the fuel tank). Stock pressure is constant @ 60 PSI whatever at idle, cruising or WOT. My car is now being modded roughly with 25% more power than stock, so I upgraded to bigger injectors. But my air fuel ratio is way too rich under hard acceleration at mid rpm's, long term fuel trims when cruising are -15 to -20%. Since the boost comes way sooner (because of my mods; smaller pulley on the supercharger) the PCM throws a lot of fuel. Can I use an AFPR between my OEM fuel pressure regulator and the fuel rail ? Why ? Do I need a return line ? Why ? Thanks ! I also purchased an Apexi S-AFC2 to tune the engine with the load (MAP) factor ... installation soon.

DeltaT

iTrader: (5)

How did you choose that size injector? They are really big for a 2L car. Still, you should be able to tune them. What were the stock injectors?

If you add an AFPR you need to add a return line, because that's how the bypass regulators works - by bleeding off extra fuel and sending it back to the tank. With the stock system pumping out a constant 60, you should use a boost/vacuum referenced regulator to lower your pressures at cruise (hi vacuum), maybe down to 45psi, and increase it proportionally to your boost levels, maybe back up to the stock 60 under max boost.

Your car is trying to compensate for the monster injectors but probably is not allowed enough of a range to adjust it back into anywhere near spec. I suspect your stock injectors were like 22 or 26#, right?

Are you following a tried and true recipe here, or just throwing stuff from the parts catalog together and hoping to make it work? Have you ever seen a car with the same mods as you perform correctly? What are your HP goals?

Jim

jmc007

Stock Cobalt SS Supercharged injectors are 34 lb/hr (rated at 43,5 PSI). Since I'm now modded for roughly 25% more WHP than stock (on the same Mustang dyno), I choose the 42lb injectors. Peak whp is roughly 270 whp on a Dynojet.

There are some other street driven Cobalt with mods like mine and my HP goal is not more than 300 whp for now.

DeltaT

iTrader: (5)

Cool. Will the tranny hold up to those power levels?

Jim

Nasty N8

iTrader: (33)

You will need the fuel tables in the computer recalibrated for the extra flow.

nate

MIGHTYMOUSE

iTrader: (59)

im not familar with cobalts but our cars regulator is in the tank, only one fuel line in the engine bay, and yet we still have a return system... so u should double check that yours is definitely returnless.

jmc007

There isn't any problem yet with the tranny, but the stock clutch isn't that good; it slips.

jmc007

That would ne nice, it will happen if HPtuners comes out soon with a compatible software for the Cobalt SS PCM.

jmc007

There is a pressure regulator in the tank set at 60 PSI. No return line on the fuel rail.

I was wondering if an adjustable fuel pressure regulator can be used after the OEM regulator ... not to raise the boost, but to lower it at low and mid rpm's.

DeltaT

iTrader: (5)

This GM Canada site calls out the returnless fuel system just for the low end Cobalt:

http://www.gmcanada.com/english_migr.../coba_opti.jsp

Doesn't mean they didn't do a typo (I see at least 2 on the page, where they swapped pounds for kilos of car weight), but it is suspicious. Having a deadhead system makes more sense on an econobox than a blown mini-racer...

You need to look at your fuel tank and see how many lines are attached to the pump mount. Should have at least 3 lines: Output, Vent and Return. It's possible they will have another for emissions stuff like a vent canister line or something.

The return line and regulator could be in the middle of the fuel rail, well hidden. Or the regulator could be mounted off the engine, but nearby.

FYI - the GM HP rating is engine HP, not whp.

UPDATE: They do mention the returnless fuel system under the Emissions category for all three models, so I guess you have it.

Jim

andereck

You may have only one return regulator in a system. If you install a second one the system pressure will default to the lower pressure. Now if by chance your system is like a "returnless" Fbody and you accidentally hooked the return from the new regulator up to the return bleed line off the fuel filter you'd be back up to 60 psi again because you've closed the loop back to the factory regulator and the return line would fill and pressurize to the intank regulator setting.

Clear as mud? Best you could do now is install an aftermarket regulator using a dedicated return line and reference it to manifold vacuum to lower pressure at cruise if you're not able to get into the pcm.

jmc007

No just watch 5 or 6 lines below, they mention besides the emission controls category : "Close-coupled catalytic converters; High Resolution 58X ignition system; returnless fuel delivery system; fast-response O2 sensor" for the Cobalt SS Supercharged.

PLus the fact that I do have the Cobalt service manual set, and it's cleary written how it works and it's a returnless. LOL there is no hidden other fuel line; thrust me i get the fuel rail removed twice :o).

About the HP rating, I kbow GM rate engine BHP, but I did get my car dynoed at least 10 runs on an accurate Mustang dyno.

jmc007

[QUOTE=andereck]You may have only one return regulator in a system. If you install a second one the system pressure will default to the lower pressure. Now if by chance your system is like a "returnless" Fbody and you accidentally hooked the return from the new regulator up to the return bleed line off the fuel filter you'd be back up to 60 psi again because you've closed the loop back to the factory regulator and the return line would fill and pressurize to the intank regulator setting.

Clear as mud? Best you could do now is install an aftermarket regulator using a dedicated return line and reference it to manifold vacuum to lower pressure at cruise if you're not able to get into the pcm.[/QUOTE]

OK that's what I plan to do ... Any thought guys for a good (and not too expensive) adjustable fuel pressure regulator ?

andereck

13105 Aeromotive is a choice. Its compact and will adjust up to from 30 to 70-75 psi.

kp

iTrader: (34)

Thats what I run, has the same size orifice as the bigger regulators just the inlet is a little smaller. Its small, light and even comes with -6 fittings

DeltaT

iTrader: (5)

That's what I told you in my first post.

Jim

jmc007

Thanks guy, another alternative good quality model to the Aeromotive ?

DeltaT

iTrader: (5)

I use an Aeromotive as well.

jmc007

Any idea on the Apexi S-AFC 2 ?

jmc007

About the Aeromotive, it's said : "Fuel pressure will rise on a 1:1 ratio when referencing boost."

Does it means for each PSI of boost, the fuel pressure goes up by one PSI ? (Example : A FPR set to base 40 PSI will run at 50 PSI when 10 PSI of manifold boost).

Or it means the fuel pressure raise at the same time than the boost RATIO ? (Example : A FPR set to base 40 PSI will runs at 67 PSI when 10 PSI of manifold boost).