injector duty cycle calculations by FAST

[Noviblowngt]

I am running a 351 with a Novi 2000 and am having some issues with my fueling. I am using a FAST system to control a set of 83 lbs injectors. I am showing 120% DC at 19 pounds of boost. I increased base fuel pressure up to 55 and the DC came down to 105%. I am wondering if I should change the injector size in the setup on the FAST as the injectors are now acting like a larger injector. I also think my aeromotive pump may not be up to par for the job. I don't think I am near the true max of the injectors but I may be past the pumps capabilities. Would a lacking pump cause the injectors to try to compensate? My VE numbers are up around 135 to 140 at max boost.

 

[Craig Smith]

The injector DC calculation itself doesn't actually use the flow rating of the injector, but you should adjust the injector flow rate as you suspected. If you increased the pressure 10 psi, you increased the injector flow capability by about 10%. So if this value was at 55, 55 x 1.1 = 60.5 psi. The speed/density algorithm needs this value to be accurate in order to properly calculate how much fuel to inject to meet the a/f ratio you tell it you want.

Injector duty cycle is the pulsewidth time divided by the available time to inject fuel. That time is about 20 mS at 6000 RPM. You can look at data logs, do some math, and run the numbers yourself if you are curious to see how it works.

A pump running out of steam could cause this problem. Sounds like you are in closed loop and running too lean. The closed loop routine is attempting to increase the pulsewidth to richen the engine, but once you are at or above 100% this has no impact. If the pump can't deliver the fuel that the injectors require under a given set of parameters it's definitely time to upgrade.

A set of 55 lb/hr injectors can support a little over 900 hp on a naturally aspirated motor. I don't know how much power you are making, but whatever it is, you need to add in an extra amount of fuel to supply the power that the blower itself requires to run but will never be measured on the dyno. One of the other posts here goes into detail on this.

Hope this helps.

 

[Noviblowngt]

Thanks Craig I appreciate your help with this. The pump I am running is the A1000 from aeromotive and it is supposed to be good for 700 HP on blown motors. I think that with the parasitic loss of the blower and actaul HP I am beyond the limits of this pump. I am prolly going to step up to the larger Pro series pump with a voltage controller and a 10an feed line to replace the 8an I am running now. I hate having to buy more parts but at least my project seems to be making some power.

 

[bruce]

Originally posted by Noviblowngt
Thanks Craig I appreciate your help with this. The pump I am running is the A1000 from aeromotive and it is supposed to be good for 700 HP on blown motors. I think that with the parasitic loss of the blower and actaul HP I am beyond the limits of this pump. I am prolly going to step up to the larger Pro series pump with a voltage controller and a 10an feed line to replace the 8an I am running now. I hate having to buy more parts but at least my project seems to be making some power.

Before tearing into things you might want to investigate things further.

ie, install a pressure guage on the RETURN line. If at full load you show less then 5 PSI of backpressure take a close and accurate look. At less then 3 your out of pump. By looking at the return line you can verify that you have adequite fuel volume. If you can run a guage off of the high side that would also be an aid in diagnostics.

Pump volumes can change alot as a function of operating pressures, and voltage.

 

[Noviblowngt]

I'm not sure if I am following you on the return line theory.....I am running a 6 an return line so I am not sure how resistant it is to the return flow. Do you mean that if there is less than 3 psi in the return line than the pump is not keeping up with demand and all the fuel is being shot out the injectors and not against the regulator? I have a fp guage in the car but unfortunately I can't seem to get a look at it when the situation occurs. Things are happening real fast when the boost comes up in 3rd. Maybe thats a good idea for Craig and the new FAST box....an input for a fuel pressure sender so it can be datalogged along with everything else.

 

[AEkrot]

I think you are out of fuel pump.

I had 87 #/hr injectors with an FMU and Vortech XX mondo and 2 SX pumps with DFI - car was boarder line with a 77/32 pulley combo (26 psi) on a 347CID. Upgraded to FAST with 155 #/hr FMS injectors, no FMU, XX mondo with 30/77 pulley combo (28.9 psi) on 349 CID and same 2 SX pumps - ran out of pump running 164 mph at 3250 # race weight.

Upgraded again to a PROCHARGER F3 and one Weldon 2035 - been told that I will run out of fuel pump again by reputable sources but we'll see. I plan to go the belt driven aeromotive pump to overkill it.

I think the idea of looking at the return line pressure is that the lower it is, the less extra fuel pressure/fuel pump capacity you have left...

Alex

 

[TurboJim]

Yes, I believe bruces theory, which makes perfect sense, is the more bypass you have means more pressure in the return line, meaning the more excess you have, which means the more volume you HAVE vs what you NEED. A simply way to verify enough pump.

I like his way, and heres why. Months ago I had a S10 blazer in my shop. Had no power, but idled normal. Fuel pressure was 13psi, which 11-15 is spec, so I moved on. Couldnt find anything. I dont know what made me look, but I pulled the return line off the TB, and low and behold..nothing coming out with the engine running. Put a pump in it, problem solved. Moral of the story? Despite having acceptable PRESSURE, there wasnt enough VOLUME.....

The aeromotive 1000 pump is rated for 45psi if I remember right. As you go up from that (as in forced induction) the VOLUME falls off dramatically. If you look at the flow chart on their site, you'll see how pressure affects flow of that pump

 

[TwinTurboIroc]

one thing you can do to check on the volume of your pump is to just take off the feed from the rail, hook up your regulator there and set the regulator for you max psi setting under boost

turn the pump on and let the return end of the regulator pump into a gas container for 30 seconds and figure out what it is flowing after it has gone through your fuel line......its a good way to really see what the pump is doing versus ratings etc

 

[HighPSI]

I don't really see how reading pressure on the return line would work. I am always willing to learn something new, can you explain it a little better? To me, that would only measure a restriction in the return line. Are you saying that if you are out of pump the return line pressure would drop, regardless of weather you run a -6 or a -12?
I just watch the fuel pressure with the car at full load to see if it increases proportionally to boost. If you are running a 1:1 regulator, I look for the pressure to be "close" to base+boost. It is a simple method and may not be exact, but it has always worked for me.

 

[JDEstill]

Cal, I'll give a stab at what Bruce and Jim are saying.

If you put a pressure gauge at the regulator outlet, you will be measuring the pressure drop in the return line. If you had a lot of flow and a small return line, maybe you'll see 20 psi there, if it took 20 psi to push that amount of fuel to the tank.

So measuring the pressure at the regulator outlet would be indicating how much flow you have going back to the tank. More flow = higher pressure.

In Jim's example, at idle there should have been the maximum flow back to the tank and therefore the highest possible pressure at the regulator outlet. Since there was no flow period, obviously the pump wasn't doing a whole lot and needed replacement.

As you give more throttle and use more fuel, the pressure at the regulator outlet should drop as the return flow decreases. At WOT it should go to a minimum, but still positive pressure. If the pressure went to zero then that would indicate that there was zero return flow, and the engine was using every bit of fuel the pump could put out.

I think that measuring the rail pressure is still a better way of checking things though. We know what the pressure at the rail is supposed to be, and if it is off by 5 psi then we know something is wrong.

At the regulator outlet we can only see if there is pressure or not, and the pressure that is there depends on multiple factors, such as fuel pump size, return line size, etc and will change if any of those items changes. There isn't a spec that we can check against to see if it's good or bad (unless it's zero, then it is definitely bad).

I would think that if Jim had checked his rail pressure with a load on the engine that he would have seen the pressure drop below the 11-15 psi spec and would have therefore known that the pump wasn't keeping up? Similar to driving around with a fuel pressure gauge taped to the windshield to see if a pump is keeping up at WOT when it might be okay at idle and part throttle.

John

 

[JDEstill]

Oh yeah, as for the original question, Novi - I am presuming your fuel pressure regulator is referenced to manifold pressure, right? So your fuel pressure increases 1:1 with manifold pressure? If it doesn't, that's a big red flag right there. If you set your base pressure (line off) at 55 psi, at 19 psi boost you'd better be seeing 55+19 = 74 psi. If it's less, the pump isn't keeping up.

I just went and looked at the pump curve on their website. If set your base pressure at a typical 45 psi and are running 20 psi boost you need a pump that can supply enough flow at 65 psi.
Let's suppose you needed an injector duty cycle of 80% with those 83 lb/hr injectors. 0.8 x 83 x 8 = 531 lb/hr minimum from the fuel pump.

Looking at the pump curve on the aeromotive website, at 65 psi that A1000 pump can supply a little over 400 lb/hr at 12V. At 13.5V it can supply 525 lb/hr.

That tells me that even if you have 13.5V at the pump, and only needed an 80% injector duty cycle, you would still be right at the limit of the pump. More duty cycle, lower voltage, and that pump is too small. And that's if your base fuel pressure was at 45 psi, with it at 55 psi the pump puts out even less.

So far all signs point to the pump?

John

 

[Noviblowngt]

Yeah thats the what I am feeling is the problem . The regulator is refferenced to manifold at 1:1. I haven't been able to confirm that the pressure is falling off up top cause of a certain control issues as the boost comes up. Hard to look at a FP guage when the back end is trying to pass the front end. But doing all the math points to a lack of fuel pump so I guess I'll have to step up to the silver Aeromotive pump. That and a 10an feed to the y block should get the volume I need. Well at least I know the setup is gonna make some power. It has been a 3 year project putting all this thing together. Just to let everyone know what my car is:
93 GT convertible
351 wit vic jr heads and super victor intake converted for efi
51012 lunati cam with 1.6 rockers(for now,considering 1.7's)
novi 2000 blower 74-28 cog belt setup
6 core liquid to air intercooler
Fast bank to bank with 83lbs injectors
car is setup up for road racing
cobra r rims 255's up front 315's in the rear
3.27 gears
custom rollcage
corbeau seats and RJS harnesses
hopefuly I will be able to iron out all the bugs this winter and have some times and numbers in the spring, shootin to break the 200 mph mark. Thanks for all the good info, this board is the best one I have found for knowledgable users.

 

[JDEstill]

Looking at their pump curves, that silver one looks like it will do the job for you, even at 12V. The one that is a step below it looks like it would do the job at 13.5V, but might be marginal still at 12V.

If you want to save some money, your existing -8 feed line (downstream of the pump only) could be reused. Yes, the -8 will have about triple the restriction of the -10, so it will reduce your ultimate capacity a little bit, but I bet it won't be enough to matter. If the pressure drop in the feed line is 6 psi instead of 2 psi, I don't think you'll notice it, unless you are right on the edge of that pumps capability. I think it's worth a shot to see, then go back and replumb it with -10 if it really is a problem. Suction side of the pump though, definitely go big! I wouldn't chintz there... Catalog says it has -12 inlet and outlet, I'd go ahead and run -12 between the tank and the pump.

John

 

[Darren]

The only way the return line thoery could work is if you had a fixed orfice in the return line that was qualified under some testing. Now, it is possable to determine if you are out of pump by using this method. It may even be use by the JDEstill, using the stock fuel sender or his own worked out set up. If I returned my fuel from my regulator with say a fire hose, it would probable register 0 return pressure under all conditions. What would be the altimate way to test the pump is with a return flow meter. Ok Here I go again! Now put a flow meter on the inlet side, hook a data logger to both of them and subtrack the difference now you have fuel usage.

Darren

 

[Chuck Leeper]

-8 feed?

Here's someone at Aeromotive you can talk to:
Brett Clow. He's at 913-647-7300 ext 3.
You will have to use the -8 feed you now are using to function as the return if you go to the PRO pump.[pt# 11102]
A new -10,[minimum] will have to be used to feed the engine.
Also, there has to be a regulator change to the pt # 13110.
HTH,

 

[Noviblowngt]

Yep that was my plan.... use the current feed for the return and plumb in the new 10 an feed.....woohoooo like I don't have enough money in fittings and braid in this project. Thanks again for all the advice..

 

[Butch]

I didn't see it mentioned but this might be a good case for a 16 volt setup!

Just a thought.

Butch

 

[Noviblowngt]

I had been considering that also as I do have a ton of stuff working off my electrical system. Fast, MSD 7AL2, water pump, fan, intercooler pump, fuel pump, and a fan on my intercooler heat exchanger. Will prolly see how it works with the new pump and a 200 amp alternator. If I have voltage problems I may convert. So far the FAST has been showing 13.2 -13.7 driving around with my 130 amp alternator now. Thanks for the reply , it is definetely a good idea as I know FAST hates less than about 13 volts.

 

[TwinTurboIroc]

Originally posted by Noviblowngt
Yep that was my plan.... use the current feed for the return and plumb in the new 10 an feed.....woohoooo like I don't have enough money in fittings and braid in this project. Thanks again for all the advice..

use aluminum line and tube nuts......

 

[Craig Smith]

Originally posted by Noviblowngt
...as I know FAST hates less than about 13 volts.

Nope. Injectors, pumps, and that kind of stuff hate low voltage. The ECU works fine no matter what kind of voltage you see, for the most part. I don't mean to sound critical of your statement, but I want you to understand that the ECU itself does not operate differently under varying voltages. It's the parts that we run that are affected and it can sometimes be a handful.