The whole "X cc injectors = Y max hp" thing!

[Guest bastaad525]

You know, one thing I don't get and don't like about the X cc size injectors are good for Y amount of power math equations I see used a lot on here, are some of the variables that seem to be left out.

 

For instance, most people agree the stock injectors are good for in the area of 250-270 crank hp. But, at what air/fuel ratio is that assumed to be? 12:1? Stoich? The hp will vary with the fuel mixture... and should peak at a slightly lean (and dangerous) A/F ratio... so is there some ideal, assumed fuel mixture included in that mathmatical equation that you guys are figuring when you quote 250-270hp?

 

And what about fuel pressure? It's well known that you can squeeze more fuel thru a given injector by raising the fuel pressure. So when you say stock injectors are good for 250-270 crank hp, at what pressure am I to assume the injectors are running at? At what pressure do 270cc injectors actually flow exactly 270cc's (at 100% duty cycle)? And please don't say "stock fuel pressure" I'm sure there are other cars in the world that run 270cc injectors but run them at a different pressure than a stock Z does... there must be some set, known number to be used in this equation, right?

 

And, how much difference does fuel pressure really make? Let's just say that those stock 270cc injectors, running an ECU limited max 90% duty cycle (making them about 240cc's per minute), on a stock ZXT running at stock boost, which should have the fuel pressure at about 40-45psi, at full boost WOT, is good for how many actual cc's of flow per minute? What happens if you up the PSI in set increments? Say 5psi, or 10psi? How much more fuel will they flow (and thus, how much more hp would they be able to support) then??

 

I think it's a lot more complex than just saying "X cc injectors will support Y hp"... I really hope some of you more mathmatically inclined guys can answer these questions and shed some light on just how much stock injectors are REALLY good for.

[Sean73]

You know' date=' one thing I don't get and don't like about the X cc size injectors are good for Y amount of power math equations I see used a lot on here, are some of the variables that seem to be left out.

 

For instance, most people agree the stock injectors are good for in the area of 250-270 crank hp. But, at what air/fuel ratio is that assumed to be? 12:1? Stoich? The hp will vary with the fuel mixture... and should peak at a slightly lean (and dangerous) A/F ratio... so is there some ideal, assumed fuel mixture included in that mathmatical equation that you guys are figuring when you quote 250-270hp?[/quote']

 

The generally accepted equation is:

 

lbs fuel per hour per injector = (expected bhp X .55) / number of injectors

 

-->where .55 is the "brake specific fuel consumption of a typical turbocharged engine (bcfc)" The bcfc is a generally accepted value. HP is at the crank.

 

and ccs per minute per injector = lbs per hour X 10.5

 

280ZX turbo injectors flow 259 cc / min = 24.7 lb @ 37 psi

 

Solving equation 1 for bhp = 269.5 @ 100% duty cycle

or 242 HP @ 90% duty cycle.

 

And what about fuel pressure? It's well known that you can squeeze more fuel thru a given injector by raising the fuel pressure. So when you say stock injectors are good for 250-270 crank hp, at what pressure am I to assume the injectors are running at? At what pressure do 270cc injectors actually flow exactly 270cc's (at 100% duty cycle)? And please don't say "stock fuel pressure" I'm sure there are other cars in the world that run 270cc injectors but run them at a different pressure than a stock Z does... there must be some set, known number to be used in this equation, right?

 

Equation 1 is independant of fuel pressure, it only considers fuel quantity, which is the quantity that matters.

 

And, how much difference does fuel pressure really make? Let's just say that those stock 270cc injectors, running an ECU limited max 90% duty cycle (making them about 240cc's per minute), on a stock ZXT running at stock boost, which should have the fuel pressure at about 40-45psi, at full boost WOT, is good for how many actual cc's of flow per minute? What happens if you up the PSI in set increments? Say 5psi, or 10psi? How much more fuel will they flow (and thus, how much more hp would they be able to support) then??

 

Yes, if you increase fuel pressure, you increase flow capacity of the injectors, and you can re-solve equation 1 for the enhanced flow rate , but the gains are not 1:1. For example:

 

The square root of flow is proportional to pressure.

 

Increase fuel pressure 50%: 37 psi x 1.5 = 55 psi

 

Square root of pressure increase = (1.50)^1/2 = 1.225

 

So, you gain 22.5 % more flow, for a 50% pressure increase.

 

Your 24.7 lb injectors become essentially 30.26 lb injectors, that support 1.225 X 267 = 330 HP

 

That 55 psi is ON TOP OF the boost pressure. Anyone can immediately see that increasing fuel pressure substantially is a compromise with safety.

 

I think it's a lot more complex than just saying "X cc injectors will support Y hp"... I really hope some of you more mathmatically inclined guys can answer these questions and shed some light on just how much stock injectors are REALLY good for.

 

Maybe so, but these are the basic equations for sizing injectors that come straight out of Maximum Boost and other sources, and are known to be credible. Most turbocharging experts do not advocate radically increasing fuel pressure from stock to gain flow. So the statement, "X cc injectors support Y HP", is valid at an assumed stock fuel pressure.

 

If someone is pulling 250+ RWHP out of stock injectors, they are doing one of two things:

 

1) Increased fuel pressure.

2) BSFC is slightly lower, within an unknown margin of safety that's engine and fuel dependant. With high octane fuel, you can probably get away with a lower BSFC.

 

I suggest again and again and again, to anyone serious about turbocharging, get a copy of Maximum Boost, and you'll have the tools required to solve many problems in turbocharging.

[Guest Cronic]

Best freaking post EVAR!!!!

 

I have nothing more to add.

[yo2001]

 

If someone is pulling 250+ RWHP out of stock injectors' date=' they are doing one of two things:

 

1) Increased fuel pressure.

2) BSFC is slightly lower, within an unknown margin of safety that's engine and fuel dependant. With high octane fuel, you can probably get away with a lower BSFC.

[/quote']

 

I did run more fuel pressure with an aftermarket fuel pump.

[Guest bastaad525]

heheh I bit off a bit more than I could chew with that one... all this turbo math drowning me, trying to figure stuff out. No, I have not yet purchased Maximum Boost

 

 

Thanks for putting some of the equations up here for me though... this will help me figure some things out, like if I'm going to use an FPR for some extra flow and hp, I can have a baseline fuel pressure to aim for, to get my target hp (aiming for 240-250 hp to the wheels).

 

 

Best freaking post EVAR!!!!

 

I have nothing more to add.

 

 

Not to get big headed or anything but I HAVE been starting some good ones lately

 

 

lot's of good detailed info coming out that I haven't heard before.

[Guest bastaad525]

okay... maybe I"m missing some numbers here, or my algebra is getting rusty (most likely the latter) but I can't seem to figure out how to figure out the number that i"m looking for.

 

 

Trying to figure out a good, loose baseline fuel pressure number to shoot for to get my 240-250hp at the wheels.

 

My stock FPR sits at exactly 40psi at 0 vacuum (reference disconnected, not sure why it runs a big higher than stock)

 

I'm aiming to run 12 or 13 psi of boost (not really necessary info, I know)

 

Assuming the 90% duty cycle, on stock injectors, what would my fuel pressure need to be raised to to support about 300 crank hp even? I'm talking the actual psi setting w/o the boost or vacuum affecting it (so whatever my final pressure would be as seen under load and full boost, minus the 13 for the psi of boost I"m running)?

[Guest bastaad525]

EDIT: nevermind I figured it out

 

 

to get my 240-250hp to the wheels I'd need around 52-55psi of fuel pressure on top of boost (say 13 psi) so about 65-68 psi of actual fuel pressure as read on the gauge while on the dyno.

 

 

52 psi = 1.4 x 37psi

square root of 1.4 = 1.183

1.183 x 269.5 (stock hp limit at 100% flow) = 319 hp (rounded up, new hp limit)

319 less 10% (maximum duty cycle) = 287

287 less 16% (estimated driveline losses) = 241rwhp

 

Your equation already accounted for 250rwhp, as you got 330 crank hp at 55 psi and 100% duty cycle, so

 

330 less 10% = 297 crank hp at 90% duty cycle

297 less 16% driveline losses = 249rwhp

 

So did I get all this right? 52-55psi of fuel pressure (set at idle with no vacuum signal) should be good for 240-250rwhp? You mention that it's not generally considered safe to "substantially increase fuel pressure"... what is considered an unsafe substantial increase? Is 15-20psi over stock really bad?

 

Now my question is, what are the equations to use to figure out how much boost I need to run at to get that target hp?

 

 

Big thanks again Sean for typing all that out for me I will pick that book up sooner or later, now that I know more of exactly what kind of info is in it. I thought it was all about JUST the turbos themselves, like compressor maps and how to figure out what size turbo and what trim compressor and yadda yadda... didn't know it also covered fuel and such. It's good to know there are numbers you can use to know EXACTLY what you need to reach a certain amount of HP, instead of just shooting in the dark. It's disturbing to me when.... well like today I was calling around looking into different FPR's, and happened to call Top End performance, as I knew they carried the Cartech FMU's, which I'm interested in. And I asked, you know... do they recommend some baseline setting to set them at for given amounts of boost or hp targets or whatever. And the reply was that "we just set it to it's highest rate of gain and then back it off as we need to" ... you know, not to diss, I guess that is a safe method to do it, but, you know if you can just sit with a calculator and figure out more or less accurately exactly why you need, why not just do that instead?

 

Sorry for the rant, I just get bothered by the seemingly illogical way people do things sometimes.

 

Thanks again Sean for the great writeup.

[Guest bastaad525]

also, how do I know how much fuel my MSD pump can support? Here is the chart from their website showing flow vs. pressure... I have my pump running right off the alternator, so should be the 13.5 volt line, and if I am going to run 55psi + 13 psi of boost = 68psi... that's about 40gph of flow... is that enough for what I'm doing?

[Guest bastaad525]

also, how do I know how much fuel my MSD pump can support? Here is the chart from their website showing flow vs. pressure...

 

 

I have my pump running right off the alternator, so should be the 13.5 volt line, and if I am going to run 55psi + 13 psi of boost = 68psi... that's about 40gph of flow... is that enough for what I'm doing?

[Sean73]

So did I get all this right? 52-55psi of fuel pressure (set at idle with no vacuum signal) should be good for 240-250rwhp? You mention that it's not generally considered safe to "substantially increase fuel pressure"... what is considered an unsafe substantial increase? Is 15-20psi over stock really bad?

 

Looks good to me. How much faith do you have in your fuel rail? Has the rubber & clamps and o-rings ever been replaced?

 

I am in the same boat, but here's my take: If I am going to go to all of the trouble of tuning or fooling the stock ECU into handling the excess fuel at low boost/off boost, then I might as well just "punt" and go for the bigger injectors that can do the job right. Even though the stock fuel rail would probably take the extra pressure, it would certainly exploit a pre-existing weakness sooner rather than later. I don't like the idea of fuel spraying on a glowing hot turbo.

 

 

Sean

 

73 240Z; L28ET, T3/T04E, NPR IC, 8 psi, Z31 ECCS

300 miles on rebuild, down already with fried stock clutch.2400 lb RPS clutch going in.

[SleeperZ]

It's so much easier to put in the bigger injectors. You increase fuel pressure absurd amounts just to spray through factory squirters. What I did was not that hard (once I converted to a MAF). Bigger injectors + bigger intake pipe = more power at stock fuel pressure and factory driveability.

[Clifton]

With out a wide band O2 how do you know you need to increase the fuel pressure? Your a/f's might not be far off, or maybe they are. It's impossible to get them were you want them with out a dyno or wide band. Tunning on a dyno gets expensive.

[Guest bastaad525]

Sean - Well... I don't really know enough to say "yeah my fuel rail will or will not put up with the pressure increase" but we're not talking about this huge increase in pressure either... I wouldn't be the first guy to run this much pressure thru stock stuff, so I'm just gonna say, I'm not worried about it. By the way yeah a lot of the rubber HAS been replaced, I wanted to take care of a lot of that when I switched over to the 75 manifold.

 

About "going to all of the trouble of tuning or fooling the stock ECU into handling the excess fuel at low boost/off boost", this is EXACTLY why I want the BEGI FMU, the one with adjustable onset of gain, so that I wont have to futz with the AFM or anything like that... it wont affect fuel pressure at idle or off boost, and I just found out today it can be tuned to do nothing until about 5psi of boost is hit. I don't know why no one seems to take my word on this... I can get the little bit of extra fuel I need w/o affecting driveability or at least not affecting it that much if at all, w/o messing with new injectors.

 

SleeperZ - you and I have gone back and forth on the FMU and just the overall pressure thing a lot... I've gotten a lot of good info from you and you've made a lot of good points.

 

I have a few reasons for why I DONT want to mess with bigger injectors:

 

1. My target hp was, still is, and will be, 250hp to the wheels... nothing more.

 

2. I don't want to or plan on upgrading from the T3, at least, not for a VERY long time (years if at all).

 

3. I dont' want to deal with yet more used parts that may or may not work properly. Yes they can be aquired cheap, but will I have to send them to be cleaned? Would installing them w/o flowtesting them be a good idea? Before you answer that, keep in mind just how bad of luck I've had with some of this stuff... I'll go thru 20 injectors before I get a good working set of six, costing me how much $$$$?

 

4. I'm not going to be upgrading from the stock EFI any time soon either.

 

5. Bigger injectors on stock EFI is not perfect either. There are tuning issues, yeah it can be made to work, and work reasonably well, but you're still just 'tricking' the ECU into supplying more fuel, and we all know tuning via the AFM is not perfect either... there are gonna be inconsistencies there.

 

6. I think 370's are overkill if you're not upgrading from the stock turbo... Unless you're one of those guys who's severly overtaxing the stock turbo, running it way out of it's efficiency range and sending it to an early grave, the stock injectors CAN be made to compensate for whatever power you could reasonably extract from the stock turbo.

 

I really don't see how a mere 10-15psi increase in fuel pressure (FPR currently runs 40psi at 0 vacuum) is considered 'absurd amounts' or is gonna break anything. Seans equations show this is just about all the extra fuel I would need, and I'm probably going to need even less because I really dont want to exceed 13psi in the manifold (about 15psi at the turbo) and that just may not be enough to get my target 250hp to the wheels... if I can't then I can cut the fuel pressure even less. Anyways... either way I doubt I'll be breaking anything. It's just like when I kept reading that to run more than 10psi, I would absolutely NEED T-bar clamps, and all my piping would need the little lip around the edge, or I'd be blowing off pipes left and right. Well, I was running 14 psi for a couple of days, now running 12, getting on it all the time, using all worm gear clamps, and NONE of my piping has that little lip on the edge, and none of my clamps have loosened (I check them almost every day) and no pipes have worked loose. And to think I almost spent how much money on those T bar clamps and having lips welded on all my pipes....

 

The point is, you never know until you try, or until someone tries before you. I haven't seen anyone say they've had problems running higher than stock fuel pressure, even though there are quite a few guys doing so, no one has blown up their car yet Not to say it would have lasted a long time, but when I first installed the MSD pump and was still running the stock 240 return line, fuel pressure was up to 80psi!!! Nothing leaked. If something does go wrong, then I've learned something, and can pass that on to the next guy who comes in debating whether to upgrade injectors or just squeez more fuel thru the stockers. And if the BEGI FMU can be made to function as I've described or how it was described to me, then it wont affect driveability, rather it will only give me the extra fuel where I need it.

 

Clifton - I just dynoed two days ago. Unfortunately, the O2 readings were so badly lean, so low in the rev range, and even at just above stock boost (8psi) that I'm left wondering if there wasn't either some kind of equipment malfunction or user error. BUT, I will say, that when I was running 10 psi unintercooled, and dynoed, the A/F ratio had just started to run lean at 4500rpm or so, and now I've not only upped the boost but increased oxygen content per volume by intercooling the air... I can only assume it's leaned out a bit. However, even at 14psi on that same dyno, with a/f ratios off the chart (leaner than 18:1) there was not the slightest sign of detonation or really any clue at all in the power curve that something was up... again leading me to believe his wideband was on the fritz. And yes, I have spent a lot of money on dyno time with this car, and yes, whatever method I do finally choose to increase fuel supply, I will get back to the dyno ASAP to get it tuned, no doubt. Actually, having spent enough on dyno time by now to have fully covered the cost of my own wideband, this may just be my next purchase after fuel.

[Clifton]

1. My target hp was' date=' still is, and will be, 250hp to the wheels... nothing more.

 

 

6. I think 370's are overkill if you're not upgrading from the stock turbo... Unless you're one of those guys who's severly overtaxing the stock turbo, running it way out of it's efficiency range and sending it to an early grave, the stock injectors CAN be made to compensate for whatever power you could reasonably extract from the stock turbo.

 

 

 

Clifton - I just dynoed two days ago. Unfortunately, the O2 readings were so badly lean, so low in the rev range, and even at just above stock boost (8psi) that I'm left wondering if there wasn't either some kind of equipment malfunction or user error. BUT, I will say, that when I was running 10 psi unintercooled, and dynoed, the A/F ratio had just started to run lean at 4500rpm or so, and now I've not only upped the boost but increased oxygen content per volume by intercooling the air... I can only assume it's leaned out a bit. However, even at 14psi on that same dyno, with a/f ratios off the chart (leaner than 18:1) there was not the slightest sign of detonation or really any clue at all in the power curve that something was up... again leading me to believe his wideband was on the fritz. And yes, I have spent a lot of money on dyno time with this car, and yes, whatever method I do finally choose to increase fuel supply, I will get back to the dyno ASAP to get it tuned, no doubt. Actually, having spent enough on dyno time by now to have fully covered the cost of my own wideband, this may just be my next purchase after fuel.[/quote']

 

250HP to the wheels on the stock T3 is at the end of the map. Low effeciency and alot of extra heat. If you are using the dyno just to tune I would seriously look at a wideband. The intial cost is more than an hour on the dyno(in Az) but you can use it everyday and sell it for almost what you paid for it. I would have to have lived at a dyno to get what I have got out of mine. I feel like a spokes person for them . Once you get it right make a pull and get your hp results, but tunning on a dyno is to expensive unless it's not streetable.

http://www.innovatemotorsports.com/

http://www.dsmtuners.com/parts/product_info.php?cPath=3_126&products_id=779

[Guest bastaad525]

okay new hp goal then.. I figured 250rwhp was pushing it. How about 230 to the wheels? Or hell... whatever hp I can get w/o pushing past 13psi in the manifold and w/o running crazy amounts of fuel pressure will be good by me. The car feels a lot quicker now with the I/C and some 30+ more measured ft lbs of torque to the wheels, that's for sure.

[yo2001]

253RWHP @12psi with base FR @43psi.

 

D/C is overrated, injector pulse is where is at.

[mtcookson]

I thought the generally accepted bsfc for a turbocharged engine was .60-.65...?

[Sean73]

525- keep in mind, and I am sure you're aware of this already, that predictions of increased fuel flow from the equations are only valid if you know the baseline flow rate of the injector. Since you're injectors are untested, you have to base your HP prediction on an assumption, which may or may not be valid. The 20 or so junkyard injectors that I have tested, are mostly close to 260cc +/- 10 cc.

 

I think you can attain your goal of 230 RWHP with fuel pressure, but tuning is key. It would be a nice feature if you can get the FMU to kick on at 8-9 psi.

 

I love my in-car narrow-band A/F meter. Even though it's accuracy is not like a wideband, it's senstive enough that I can notice immediate changes in the A/F at WOT, after adjusting fuel or boost. If the lights don't go to the middle of green at WOT, then I know I need a little more fuel at that boost level.

 

My local dyno shops tap the wideband sensor off a secondary port on the downpipe. They send their customers to a muffler shop to have an auxillary O2 port welded on, so they can get an accurate wideband O2. Wideband at the tailpipe is sketchy, from what they tell me.

[Guest bastaad525]

Sean - yeah the injectors are an unknown commodity, but, they were in VERY good shape when I saw them before installing them, they are aftermarket, Python injectors, and looked hardly used at all... I know this really doesn't mean much either way, and of course I"m going to actually try to overshoot the fuel a bit for a nice safety margin... as w/o having a way to see mixture in EACH CYLINDER, even a w/b in the downpipe wont tell me if one of my injectors isn't flowing enough. All I can go by is plugs, and my plugs consistently look even across all six whenever I pull them, so if they are all running low, they're at least doing it evenly.

 

About the FMU, I will definately solve the issue of getting it to only come on at somewhere in the 7-8psi range, if working directly with Trey at BEGI doesn't work out I'll run the grainger valve inline with the boost signal to the FMU, and set it to not allow signal to reach the FMU until 8psi. Problem solved. I'm going to set the FMU to raise pressure at about 2:1, should give me just over the target psi I was shooting for and a little extra safety margin.

 

I dont trust narrow band meters, I've talked to a couple guys now who have used them while dyno testing on a wideband, and had the narrowband say they were running rich when they were actually running lean, or vice versa. I think they just react too slow. I haven't run into a dyno shop that runs the O2 in the downpipe yet and I've been to five different ones so far.

[Guest Aaron]

Sean's point about an O2 in the down pipe is a good one. If you have any kind of exhaust leak before the sensor (lots of places if it is in the tailpipe), it can throw the reading lean. I saw this on my pickup when the manifold cracked. I kept getting trouble codes indicating O2 sensor problems until I replaced the manifold. I guess as the pulse of exhaust passes the crack it realeases some of the fumes, and as it is moving away from the crack, it sucks in some air. The O2 senses the extra O2, and thinks the engine is lean. It could explain your odd readings at the dyno.