Mike1982

I know that when I got my Guru chip set, I got the 3-bar fuel pressure, but what if I got an adjustable one and turned it up more to say the 43.5 that I see some do, would I be running rich then? Is this how people tune there air/fuel ratio? Does it increase the HP/TQ? What can the stock injectors/fuel pump handle if all is in good condition? I plan on upgrading fuel injectors in the summer, so would turning up the pressure be good or what?

Perry 951

You don't want to fool with it much if your A/F is fine. Have you tossed the car on a dyno? Increasing or decreasing fuel pressure has a liner affect on your overall A/F. For now, stay close to what your chips are mapped for. When you change injectors, change the FQS switch to the appropriate postion to change maps in your Guru chips.

Remember, it is bad for your rings if you run too rich.

Joe Jackson

Like Perry said, more fuel doesn't mean more power. You need to make sure you have the correct amount of air going into the cylinders too if you actually want the extra fuel to help (and not hinder by washing all the oil off the cylinder walls). If the Guru chips are mapped to run with a 3 bar regulator, run with a 3 bar regulator. Adjustable regulators are good for when you upgrade injectors so that you can LOWER the fuel pressure to avoid going too rich now that the injectors flow more fuel.

Danno

"Adjustable regulators are good for when you upgrade injectors so that you can LOWER the fuel pressure to avoid going too rich now that the injectors flow more fuel."

What good would that do? If you get 25% larger injectors and lower fuel-pressure by 25%, you're back to exactly the same place as before with stock injectors...

Joe Jackson

Who said anything about lowering it by the equivalent percentage of fuel injector flow increase? I can't remember my exact numbers but when I was running 18 psi (w/ MAF and Garret turbo) on stock injectors my base fuel pressure was up around 48 psi (again, can't remember exact numbers) but when I went with 55 lb/hr injectors I was able to lower it to a much safer (for the DME at least) 35 psi (and don't quote me on that number either).

Gary Gaukler

Yes, fuel flow is a square root function of fuel pressure. If you double fuel pressure, you only get sqrt(2) times the flow. 25% larger injectors, coupled with 25% less pressure would still yield more fuel flow than before. So you can dial down pressure much more.

But the question still is, what good does changing fuel pressure do? Not much, IMHO, if it's not matched to a specific set of chips that is designed for exactly that fuel pressure. Because otherwise all you are doing is change fuel flow by the same percentage across the rpm and load range. That would be far from optimal.

Gary.

Danno

Thanks Gary, didn't want to get too technical. Here's the bottom line:

35 lb/hr stock injectors @ 43.5psi = 35 lb/hr @ 100% duty-cycle
55 lb/hr aftermarket inj. @ 35psi = 49 lb/hr @ 100% duty-cycle

So you've just decreased your injector flow-volume rating, however, you'll still have too much fuel... and your injectors will still max out at 100% duty-cycle at the same RPM as before:

100% duty-cycle on stock injectors @ 5000rpm/18psi = 35 lb/hr flow
100% duty-cycle on aftermarket inj. @ 5000rpm/18psi = 49 lb/hr flow

If you're 10% too lean in the upper RPMs with stock injectors, you're gonna be 25% too rich now @ 5000rpm, but may end up just right by 6000rpm because of the same fixed 100% duty-cycle.

Also the DME has no idea about fuel-pressure. It only sends out a duty-cycle signal to the injectors based upon RPM vs. air-flow voltage. So to take advantage of 35% larger injectors, you need to scale back the duty-cycle by 35% across the board while maintaining the same fuel-pressure. The result may look like this at 2.5-bar:

100% duty-cycle on stock injectors @ 5000rpm/18psi = 35 lb/hr flow
65% duty-cycle on aftermarket inj. @ 5000rpm/18psi = 35 lb/hr flow

You'll have the same fuel-flow volume as before, but at a lower duty-cycle. The trick is.. how do you cut back on the duty-cycle coming out of the DME evenly?

It actually better to increase fuel-pressure in most cases because you'll have much better vaporization & atomization. Testing on experimental 100psi fuel-systems shows fuel being dispensed as a fine mist, even at high-volumes of 75-100 lb/hr per injector, whereas, with our systems, the fuel comes out as a solid-stream of liquid like a garden hose at those volumes. Anyway, raising the fuel-pressure to 3-bar with 55# injectors and dialing back the duty-cycle commensurately yields:

100% duty-cycle on stock injectors@2.5bar @ 5000rpm/18psi = 35 lb/hr flow
56% duty-cycle on aftermarket inj.@3.0bar @ 5000rpm/18psi = 35 lb/hr flow

Now, you've got identical fuel-delivery to maintain proper air-fuel ratios, but a lot more overhead in duty-cycle to match further increases in air-volume (turn up the boost!).

EDIT: ran the spell-check..

Joe Jackson

Thank you for the thorough explanation of your comments. They make sense now.

I'm running an "old school" setup with a Huntley Stage 4 MAF (w/ ARC2) and one of their "stage 2" turbos which apparently means nothing according to other posts on the same turbo. Do you think I'd be able to achieve satisfactory results (or possibly better results) increasing my fuel pressure to 3 bar from wherever it is now and tuning from there? At the time I set my fuel pressure, I was basing it off Tom McGuin's car which had the same setup. This was the pressure he got to work so I went with it.

I know the ARC2 and Huntley's standard chips aren't optimal based on your research but I'm sticking with them so I'd like to get the most out of them that I can.

mwc951

Danno.........
OUTSTANDING explanation!! Thankyou

gstreev

I'm running a stock 89 Turbo with Autothority Chips and might add a boost controller, is there any need to go with a 3 bar F.P.R. ?

Danno

"OUTSTANDING explanation!! Thankyou"

Why thank you and you're most welcome!

"I'm running an "old school" setup with a Huntley Stage 4 MAF (w/ ARC2) and one of their "stage 2" turbos"

Heh, heh, I had that same set-up for years as well. Yes, you can get it to work. I had the following settings for 15psi:

- Adjustable FPR set to 3-bar
- 55#/hr Siemens injectors
- ARC2 set to: LOW=8 o'clock, MID=10 o'clock, HIGH=2 o'clock, ACCEL=12-noon

I noticed the LOW **** had the most impact on idle & emissions and that position let me pass smog. There was a lean-spot from 1500-2500rpm that I couldn't ever dial out, and it got too rich from 3000-4000rpm. If I turned up the mid-**** to get rid of the lean-spot, it made the mid-range way too rich and the lag was horrible. So I just lived with the lean off-idle range by just getting through it quickly in 1st gear and revving it up high enough so that when I shifted into 2nd, the revs wouldn't be lower than 2500rpm.

"I'm running a stock 89 Turbo with Autothority Chips and might add a boost controller, is there any need to go with a 3 bar F.P.R. ?"

Well, without knowing exactly what your air-fuel mixture is across the board, adding more fuel with a 3-bar FPR may be a shot in the dark. You'd want to dyno-test the both with stock 2.5-bar and a 3.0bar FPR to see what the results are. The APE chips are mapped for a boost-curve that goes from 15psi max-boost in the mid-range and drops to about 10-11psi by redline. If you install a boost-controller that can hold high-RPM boost better, you'll end up with an air-fuel ratio that looks like this:


No amount of adjusting with fuel-pressure can compensate for this kind of an air-fuel curve. It can scale the entire curve up or down, but it won't be able to make opposite adjustments like leaning out the mid-range and richening up the high-end.

toddk911

Yep, so get GURU chips

Joe Jackson

Danno, I know a lot of us were running the same setup at one time and some of us still are (until we catch up money-wise after buying a house and go stand alone). I'll give those settings a try once my car is back together and see how it goes. Thanks for the reply (and for keeping all of those old settings close at hand).