Performance Fuel Pressure

[Schmieder]

Stock fuel pressure for my 96 Grand Prix 3100 is 43.5 psig.

I'm going turbo with a tune now, larger injectors.

Now, can I reduce the fuel pressure to help increase fuel flow volume? Knowing when I reduce fuel pressure I'll need to rescale the injectors, which is fine. The injectors will be well strong enough. 36 lb/hr on a 3100/3400TOP 5psig Turbo.

I need to have 330 cc/min for 250HP, planning to hit around 230-240HP. 36 lb/hr injectors could, in theory, flow 330 cc/min with fuel pressures of 35psig.

Will this cause negative results or could this be an effective method of increasing fuel volume flow?

Or should I boost fuel pressure up past the 43.5 psig stock levels?

Considering factory just loves to tune for convenience when I clearly don't want that, I want POWER!

Advice please...

[robertisaar]

i wouldn't reduce pressure, or else you'll start to have spray pattern issues...

[Schmieder]

i wouldn't reduce pressure, or else you'll start to have spray pattern issues...

Would the opposite also be true?

If I increased fuel pressures would I get better atomization/vaporization of the fuel?

Basically wondering if factory set the fuel psi specific for what ever reasons. And if so, why?

Is there a better performance fuel pressure?

I read, but do not remember where, that NASCAR uses 39.5psig in their performance engines. Is that true? And why do they do that?

I know thats a lot of questions to shoot off.....just looking for info.

Thanks

[robertisaar]

more pressure CAN improve spray pattern/atomization... but too much pressure can hydraulically lock them shut, but thats should be more pressure than anyone should be running...

the factory sets everything based on comprimises... so i would say, IMO, more pressure would be better. i mean the later 3x00 motors run 52-55PSI from the factory...

as for NASCAR: they run carbs... i can't imagine them needing 39.5PSI... but they are considering moving to fuel injection, so yay???

[Schmieder]

more pressure CAN improve spray pattern/atomization... but too much pressure can hydraulically lock them shut, but thats should be more pressure than anyone should be running...

the factory sets everything based on comprimises... so i would say, IMO, more pressure would be better. i mean the later 3x00 motors run 52-55PSI from the factory...

as for NASCAR: they run carbs... i can't imagine them needing 39.5PSI... but they are considering moving to fuel injection, so yay???

Ok, makes sense. A lot of this stuff is generally common sense that I should deduce on my own but I tend to be cautious with items that can grenade the car if not done right.

I'll probably up the pressure to 50-60 psig, adjust the injectors and then data log to see if there is a performance difference. That is where the compare feature I've read about in HP Tuners seems like a nice tool.

Any advice on "2-Bar Speed Density Modes"? Is it highly recommended to go 2-bar speed density when running 5psi boost turbo?

[robertisaar]

you don't want OBD2 advice from me.... not my area of study/play...

[Schmieder]

you don't want OBD2 advice from me.... not my area of study/play...

Hahahaha, gotcha. i won't ask OBDII.

Any life lessons you can point out regarding custom turbos. You know, the type of things most people miss at first, the things that make the difference between a novice and a professional.

I bet your beginning to see just how detailed I like to be. My budget is fine tuned all the way down to the nuts and bolts needed. I even have raw steel bar of various sizes to cut/drill/grind/weld support structure for the turbo and trans cooler. I even included shipping costs. My budget is to the penny. Right now I'm at exactly $3,679.23 with $100 aside for forgotten small items, if any, lol.

I bet I could get it down much more when I tour the junkyards. But there is only a few items I would want to use from there. Namely manifolds, tb, ect.

Thanks for all your help man, real cool!

[robertisaar]

assuming you've read through "Maximum Boost" about ~10 times, you should have a fairly complete list mentally for what you need. as for specific things: do you have check valves for the items that should only see vacuum, such as the vacuum modulator for the tranny, the brake booster, HVAC, cruise? that seems to be something most people forget...

[AaronGTR]

I would definitely run a little more fuel pressure rather than less. How much though? Well that depends on how much boost you are running, whether your injectors start approaching their max duty cycle, and if you have the ability to tune the computer.

More pressure will improve the atomization of the fuel in the spray pattern slightly. If your injectors are running out under boost it will give you a bit more fuel to keep from running lean as well, as long as you aren't pushing way more air than the injectors can keep up with. The trade off is running slightly rich at idle and part throttle. The pcm's closed loop mode using the oxygen sensor can adjust for this though, as long as you don't go too far with the fuel pressure. It's not optimum, but it works. That's basically all magnacharger did for their kit to accommodate 5-6psi of boost.

As far as running 2bar map and speed density mode.... you can't do it with the OBD2 pcm. It uses the MAF table to calculate too many things. It will run in speed density mode if the MAF fails and your engine isn't too highly modified (or if your VE table is corrected accurately enough)... but it isn't designed to run that way permanently. It also isn't possible to run a 2bar MAP without problems. Some of us have tried. It has to do with the way the pcm measures outside barometric pressure using the MAP sensor, and some code in the pcm that isn't available to us to modify. Basically you are better off leaving the MAF in and tuning properly with that, if tuning is what you intend to do.

[Schmieder]

assuming you've read through "Maximum Boost" about ~10 times, you should have a fairly complete list mentally for what you need. as for specific things: do you have check valves for the items that should only see vacuum, such as the vacuum modulator for the tranny, the brake booster, HVAC, cruise? that seems to be something most people forget...

yup, I have listed 4 check valves. Rated at a max of 150psi. Not that I would need that high strength, it's good to have quality items.

I read you mentioned once in another thread about check valves and people killing their tranny through boost pressure and the modulator.

The vacuum/boost system is like this, the way I see it so correct me if I'm wrong anywhere.

(1) Just after the turbo compressor, before the throttle body is the wastegate line, for maximum control accuracy.

(2) check valve > Trans Modulator
(3) Map > fpr (no check valve) is this ok for stock?
(4) PCV dumped into the air
(5) Brake Booster (check valve but I heard BB has it's own check valve, true?)
(6) check valve > accessory

Isn't a PCV a basic check valve in its own right?

I have 4 check valves budgeted in. I could probably run 2 vacuum lines from one check valve and be ok, but I don't want to compramise a systems vacuum. I read adding a vacuum manifold looks cool, but it can reduce vacuum itself. Multiple lines going through on vacuum line to TB. Unless one was made to fit the Brake Booster line. Tap 3-4 smaller vacs fromthe brake booster. But then were messing with teh brake booster so, I scrapped any vacuum compiling.


To the other poster, yes, definately tuning. HP Tuners Pro. Pricey but it nearly pays for itself when I don't have to buy HP Fuel Pump, Adj FPR, 12:1 FMU, Fuel Line, -6 AN adapters and they are expensive bugger, $4-$6 each for straight.

I have been doing a LOT of reading about HP Tuners and I really like the features. Smoothing functions, custom abilities, ect.

And it's a darn good thing Robertisaar helped me out big time w/ the 97 PCM swap. Now I can get rid of the RPM Limter. I read the block could grenade itself if I hit the rev limit under boost.

BTW, I read about a spark rev limit feature in HP Tuners and was hoping GM 97 PCM's incorporated the ability to rev limit with spark and/or fuel. A park/neutral safety rev limit at 6.5K would be nice if I could cut spark just before fuel was cut. Preventing detonation and still having the safety feature.

[Schmieder]

I would definitely run a little more fuel pressure rather than less. How much though? Well that depends on how much boost you are running, whether your injectors start approaching their max duty cycle, and if you have the ability to tune the computer.

More pressure will improve the atomization of the fuel in the spray pattern slightly. If your injectors are running out under boost it will give you a bit more fuel to keep from running lean as well, as long as you aren't pushing way more air than the injectors can keep up with.

Thanks, I will tune the turbo first and get it in a good ballpark figure before I adjust the fuel pressures. As you said, I need to know where my injectors stand first under 4-7psig boost. Running 36 lb/hr though so I know I'll have plenty of room to modify fuel pressure. But it is better safe then sorry.

Has there been any comprehensive study regarding fuel pressures? We can all assume higher pressures help atomization and it is most likely true, but at what point does the effect begin to reduce, if at all. Some one out there had to figure it out.

Thanks guys

[AaronGTR]

Is your pcm tuned for 36lb/hr injectors? You know those aren't the stock size and you can't just run them without tuning the pcm right?

I'm sure there have been plenty of studies on fuel pressure. I can't think of any place off the top of my head with all the info in one place, but it's generally accepted that too low a pressure hurts the spray pattern and atomization, and higher pressure improves it along with injecting more fuel. It's also pretty general knowledge that there is a limit to how high you can go as well though. As someone mentioned if the pressure is too high the injectors can lock up, but that would be way higher than you'll probably ever get it. It's pretty general knowledge that by increasing pressure you decrease flow through from the pump. A good pump can handle a certain amount of that, but too much pressure will kill the pump eventually. Usually they say to try and keep it under 70psi. I set my pressure to 48psi at idle with a max of about 60psi at 9psi of boost.

[Schmieder]

Is your pcm tuned for 36lb/hr injectors? You know those aren't the stock size and you can't just run them without tuning the pcm right?

I'm sure there have been plenty of studies on fuel pressure. I can't think of any place off the top of my head with all the info in one place, but it's generally accepted that too low a pressure hurts the spray pattern and atomization, and higher pressure improves it along with injecting more fuel. It's also pretty general knowledge that there is a limit to how high you can go as well though. As someone mentioned if the pressure is too high the injectors can lock up, but that would be way higher than you'll probably ever get it. It's pretty general knowledge that by increasing pressure you decrease flow through from the pump. A good pump can handle a certain amount of that, but too much pressure will kill the pump eventually. Usually they say to try and keep it under 70psi. I set my pressure to 48psi at idle with a max of about 60psi at 9psi of boost.

48psi idle
60psi boost

Sounds good.

Yeah, I will be tuning the larger injectors. Stock being 19#'s and new ones 36#'s that means I need to scale pulse width down 52.77%, multiplied by 0.52777. Also knowing if I bump fuel pressure to 48 psi at idle, the injectors new flow rate would put them at a rating of 37.8#'s. Thus reducing the pulse width by 50.26% from stock 19# injectors.

Anyone know if there is a tuning map for a turbocharged 3100/3400 hybrid I can base a starting point from?

[robertisaar]

the PCV valve is a check valve, though it's not all that great at stopping boost from getting into the crankcase. take the 5 seconds and pull it out, look at the bottom: i wouldn't call that a great seal... fine for vacuum only, though.

MAP: i would say don't allow boost to touch the diaphragm, and it will give basically a barometric reading until there is vacuum again.

brake booster: it does have it's own check valve now that i think about it.... i imagine a second one couldn't hurt, but i wouldn't call it a necessity now... just make sure it's in good condition.

[Schmieder]

the PCV valve is a check valve, though it's not all that great at stopping boost from getting into the crankcase. take the 5 seconds and pull it out, look at the bottom: i wouldn't call that a great seal... fine for vacuum only, though.

MAP: i would say don't allow boost to touch the diaphragm, and it will give basically a barometric reading until there is vacuum again.

brake booster: it does have it's own check valve now that i think about it.... i imagine a second one couldn't hurt, but i wouldn't call it a necessity now... just make sure it's in good condition.

hmm, I'm going to have to rig the vacuum line to the MAP.

Good info, thanks.


The more ground I cover now, the less headaches I'll have on my 1st start up.