David Buschur

iTrader: (53)

Most of this is way over my head, always has been, more than likely always will be. I am one guy that didn't go to college.

I am simple, if it works then run it. We made 390 WHP and 390 ft lbs of torque to the wheels in our EVO with the stock injectors. The calculations that people have used over the years have always fallen short of the horsepower we have been able to make with a particular injector.

We just made 541 whp with the EVO with a 660 cc injector in the car. They are maxed out and at 100% duty cycle right now at 30 psi.

David Buschur
www.buschurracing.com

TURBODAWG

iTrader: (54)

Hey David,

What was the air fuel ratio on those runs? Do you have any type of reading on the MAF flow rate? I am sure it was using the aem, I don't know if it has any type of readings on Mass air flow or not? That was done on your 2wd dyno, do you have any idea on the drivetrain efficiency, that would be needed to calculate approximate bhp. It appears at first look that you are defying the laws of physics, just like my Supra. It must be something in the air!!!

Brian

TURBODAWG

iTrader: (54)

If you assume only a 10% drivetrain loss to the front wheels and apply that to those numbers, you come up with about 600 bhp. The calculations look like this:

600 bhp * .50 lb/hr. bhp = 300
4 injectors * 100% duty cycle = 4
300 lb/hr / 4 = 75 lb/hr injectors

Converting to cc/min using average gasoline density 75 lb/hr x 10.5 = 787.5 cc/min

It appears that your numbers don't add up either!!!

What is missing here is the base fuel pressure (with the reference vac hose disconnected)?


Brian

propellerhead

iTrader: (3)

I guess I'm looking at the problem too simplistically and forgive me if this has already been stated.

For example, if an injector is rated to flow 660cc/min at 43.5psi of fuel pressure at 0psi of boost. For simplicity's sake, let's ignore what the fuel pressure should be when there is there is a vacuum in the manifold. When running 20psi of boost the absolute fuel pressure must be raised to 63.5 psi to maintain the same differential pressure (43.5 psi) across the injector. This will maintain the rated fuel flow rate. The fuel pressure regulator is used to increase the absolute fuel pressure.

Obviously when the differential fuel pressure is raised to greater than 43.5psi then the 660cc fuel injector now will flow more than 660cc/min. Raising the absolute fuel pressure, unless it is raises differential pressure greater than 43.5 psi, will not increase effective flow rate.

TURBODAWG, when you measured your fuel pressure you mentioned that you had 41psi at 0 psi of boost. Was that truely at a neutral manifold pressure (no boost, no vacuum) or at idle or partial throttle with a vacuum in the manifold?

TURBODAWG

iTrader: (54)

I am starting to somewhat agree with the differential pressure thinking. I am not sure about the fluid dynamics of the situation exactly. I am having a hard time believing that the pressures are directly additive. It seems to me like the two pressures are different systems. I would agree 100% if you were injecting fuel at 63.5 psi into a fuel cell that was at 20 psi. I would agree that the net differential pressure was 43.5 psi. Being that compressed air is a compressible fluid and gasoline is incompressible, I believe they may not follow a completely additive pressure differential equation. Just not sure on this yet.

I measured the fuel pressure, with the vac/boost reference line unhooked and it shows ~41 psi of fuel pressure. Also while idling, if you give it gas to get the boost gauge to show no vaccum, ie. 0 psi, the fuel pressure reads ~41 psi. At idle with the vac/boost line hooked up to the FPR, I read ~34 or 35 psi.

I think the biggest problem and sources of error in the calculations come from the assumed BSFC. We are assumning .6 or .55 lbs/hr bhp. When running race fuel and running leaner air fuel ratios, the BSFC will probably go down. Also the conversion to cc/min assumes an average density of gasoline. Race gas probably has a different density. We are also assuming drivetrain losses, and also assuming that they are a linear percentage no matter how high the hp level. This may not be a good assumption either.

All I know is, with my upgraded fuel pump, HKS 264 cams, Greddy FMIC, intake, 3" turboback, and 19.5 psi of boost. I can get 9.9:1 air fuel ratio with the Dynoflash upgrade that I have (mine was tuned before the Walbro tune was done). I am pulling back about 10% fuel from 5000 rpm's up to maintain a 11.7 to 11.4:1 air/fuel ratio. I have plenty of fuel available with my stock injectors, but I don't know how high a duty cycle I am running. That is what I was trying to determine when I started working with these equations.

I have seen 37.7 lbs/min on my datalogs and have run 118.7 mph.(before the FMIC). Don't know how much whp I am making because I haven't been to the dyno.

Brian

propellerhead

iTrader: (3)

I'll be frank, until I had scanned this thread I really hadn't given much thought to fuel pressure regulation and injector output. After I glanced over this thread I pulled out my AEM EFI Basics handbook. It has some pretty clear and succinct overviews of the various concepts used in tuning. Injector sizing, injector pulse width and fuel pressure regulation are fairly well covered. They basically cover the whole fuel pressure regulation topic in about 3 paragraphs.

So what your saying is if you were to better regulate your fuel pressure you could eek out some more fuel from the stock injectors... if you had to.

Absolutely. BSFC is a function of engine design and the type of tune. Obviously running lower AFRs (richer) will result in a higher BSFC. Plus there's too much voodoo involved to even remotely correlate BSFC and wheel dyno readings.

I wonder why this is. So why does the upgraded fuel pump result in higher fuel pressures? Based on very little reading I suspect the stock FPR or fuel return line is being overflowed and it can't bleed off the excess fuel fast enough. This results in a pressure build up and higher injector flow rate.

Ignore this if you already know how to calculate duty cycle. Can you measure injector pulse width? If you can then you can calculate duty cycle very easily. Figure out the available pulse width at a given RPM. For example: 6000rpm/60 = 100 revs per second or 10msec per a revolution. An injector can spray for one complete cycle or two revolutions so the available pulse time is 20msecs. If at that given RPM your injector pulse width is 15msecs then your using 75% of the available pulse width or duty cycle.

TURBODAWG

iTrader: (54)

I have no way to measure injectors pulse width. The only thing I really can datalog with my pocketlogger is MAF and O2 voltages that would be relevant to fuel delivery.

Brian

DystopiaMagnate

By "better regulate" do you mean raise the base fuel pressure, or are you thinking he's referring to accuracy?

Either way, if one wanted to go with some kind of RRFPR, that would of course give some more headroom with the stock injectors. You'd have to have the pump for it as well...


A pump is analogous to a compressor. As the pump pushes more fuel mass, the peak pressure it can reach drops. Put another way, as the FPR restricts the line to increase pressure, the volume the stock fuel pump can deliver falls off. If the combined open time of the injectors and the "desired" fuel pressure puts the pump at a point beyond where it can operate, it will fail to reach the fuel pressure the FPR wants, or it will oscillate, never reaching the time-averaged pressure the FPR is seeking. Net result: insufficient fuel delivery.

The "bigger" Walbro, at the target delivery pressure, can supply more fuel mass -- in this case, the correct and adequate amount of fuel that the ECU is expecting. It's not so much about the Walbro being "too strong" as it is the stock pump being insufficient. I don't think it has to do with inadequacies on the part of the FPR.


TURBODAWG, if it would be any more convincing, I can give you the relevant page numbers from the "Applied Fluid Dynamics Handbook" by Blevins (Krieger Publishing, 2003) which cover the governing equations for incompressible fluid flow through a discharge nozzle. It is the pressure at the discharge that is important, not the compressibility of the medium being discharged into.

Your suspicions regarding the accuracy of the BSFC, driveline losses, and fuel density all make sense. Another parameter is fluid viscosity as it changes the injector's "K factor" which I mentioned earlier. I tried to get some specs for C16 vs. 93/91 pump unleaded, but while I got a "specific gravity" (density) for C16 from VP's site, I wasn't able to get viscosity for C16 or either parameter for pump unleaded (didn't look super-hard though). The point of such an exercise would be to get a rough idea of the magnitude of the difference. For example, 1% difference in both parameters from fuel-to-fuel probably means the fuel isn't the culprit here.

I think ShapeGSX is someone on this forum that should have a crapload of logs with useful data for this discussion, including something for a street gas high-HP run.

propellerhead

iTrader: (3)

I mean raise the base fuel pressure to be "standard" at 43.5 psi.

I fully understand the analogy between the pump and a compressor. It's a simple concept.

I guess part of my problem in undestanding why TURBODAWG needs to trim fuel is that I have no idea what FP is when running stock tuning maps and what it is when running the DynoFlash remap. I'm guessing the reflash maps are overcompensating for the lack of fuel flow from the pump so when the proper fuel is delivered then a rich condition is generated. Do people running otherwise stock set ups aside from a upgraded fuel pump require fuel trim? I suspect they do, but why? If the fuel pressure on the rail is the same then the injector should deliver the right amount of fuel unless even the stock fuel maps are compensating for poor volume.

timzcat

iTrader: (10)

These results would mean the Brake Specific Fuel Consumption is around 45%.
The thing is, the equation has been around for along time. The overall efficiency of the cylinder head plays a big part in the BSFC.
The standard for the equation is 50 for NA, but it goes lower and lower the better the cylinder head. Well ported heads would use a 40 for BSCF. If I am not mistaken the Buschur car's head is not completely stock is it?
I think a good approach to the equation is to figure out a good stock head standard BSCF. The best way to do this is examples that show the injector duty cylcle for the given results. It looks like the real BSFC is around 48%.

David Buschur

iTrader: (53)

The head on our car is ported.

Like I say, I don't anything when it comes to all the "school" part of this. I just know what we can make on a particular injector with no fears of hurting our cars or a customers.

With that we have made 446 whp on a 550 cc fuel injector in a 4g63. That car weighed in at 2900 pounds and ran 10.72 at 132 mph.

We just made 548 at 30 psi in the EVO, this was on the 660 cc injector I had mentioned. We usually figure about 15 percent loss for the drivetrain, use the same equation on everything but the automatics which loose a ton more.

The fuel pressure is whatever it is stock in our car. All the fuel lines, fuel rail, filter and regulator are bone stock on the car. Just our fuel pump upgrade in it.

AFR's on the dyno we no leaner than 12.5:1. We are keeping the car on the richer side so it is safe and we can share these tunes with our customers.

The car has not MAF on it so I have no MAF flow rates. It is speed density.

David Buschur
www.buschurracing.com

propellerhead

iTrader: (3)

Hmm, wow. So what your saying is that my 11.5:1 AFR is very safe and it's possible to eek out some more power by leaning it out a tad with a custom flash instead of the "mail" flash? I guess stage 4 is next.

SuperHatch

iTrader: (23)

Can I make a general comment here...

We all know that rated injector flow rates are at 43.5psi (3bar), but does anyone know if that is gauge pressure or absolute pressure? I can't seem to find an answer and I've been searching all morning. If in fact that is at absolute pressure it could be an answer to all of your conflicts here. Since essentially it would mean that injectors are flow rated at ~30psig, and since the fuel systems run at ~43 psig it would account for the added flow capacity of the injectors.

I'm just fishing here, but hey, who knows?

- Steve

fre

iTrader: (4)

Keep in mind he is referring to running race fuel. On pump fuel 11.5:1 is about as far as you want to go. I run 12.0-12.5 on race fuel as well and I have 660cc injectors on a gt35r running 28psi and I am at 92-94% with 2640hz on the MAF. I think I am going to switch from 100 unleaded to 110 if I decide to try 30psi because 100 seems a bit low of an octane for that much boost. My fuel system is also stock except the injectors and the fuel pump. I have a FPR but I don't know that it would help my current situation. I will probably be close to 100% if I try 30psi. BTW I agree with Dave I prefer reality to theory, this is why I am an engineer and not a scientist. There are always variables you forget to take into account.

JustDSM

iTrader: (28)

I never did understand why people are so quick to jump into HUGE injectors. My Denso 660's are only running at ~80 duty cycle @29psi w/ 43.5psi base fuel pressure (Race Gas).

It's flowing 51lbs @6000ft ASL too It's gone 119mph with me blowing the spark out the last 2 seconds from the traps as well. You need to be making some SERIOUS power before you "need" 750+ cc/min.