How do YOU size injector upgrades?

NYSpeed Automotive
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How do you size your fuel injector upgrades?

I thought that most people had abandoned the stupid Java BSFC based calculators, but lately (on here & other forums) I’ve seen alot of people referencing them.

To that end let me ask a couple of questions:

Is your FUEL INJECTOR affected by what oil you use?
Is your FUEL INJECTOR affected by what rotating assembly you run?
Is your FUEL INJECTOR affected by muffler or turbine choice?
Is your FUEL INJECTOR affected by cam choice?

In other words is your injector itself, and its operation / flow rate affected by the engine’s brake specific fuel consumption rate as measured on a dyno? No.
Furthermore, you should realize that BSFC varies from engine to engine? Even for “identical” (LS1 to LS1, D16 to D16, etc) engines … YOU don’t know YOUR engine’s BSFC unless you had it on an engine dyno. If you don’t know your BSFC, why are you GUESTIMATING it when choosing injectors? These calculators have been “proven” wrong 100s of times over. Engines running injectors deemed “too small to be anywhere near safe” provide the case study.

KISS. Stick with math, facts, and simplify this whole process to what the injector acutally needs to deal with.

The fuel injector provides one service: delivering fuel. You want it to deliver the appropriate amount of fuel that your tune requires at any given load point. So you choose the flow rate based on the max an injector has to provide. This calculation assumes sea level but I’m not going to get into the air mass density changes @ altitude, mainly because WNY is pretty steadily between 300 & 1000’ above sea level. If you really wanted / needed to figure it out, you could adjust the [air mass] required by your engine.

[Air mass] / [AFR] * [60 minutes] / [# of injectors] / [duty cycle]

So we need to know how much air the engine will swallow [Airmass], the richest desired air/fuel [AFR] you’ll run, how many injectors you’ll run, and the max duty cycle you’re willing to submit your injectors to.

NA engines are pretty simple, lets start there.

To get max air mass: (CID x RPM) / 3456 = CFM

Lets take a stock LS1. 346*6200/3456 = 620 CFM.

Convert it to lbs/min (CFM0.069), to work more easily. 6200.069 = 42.8#/min

This is the max that your engine will flow. Now - we can account for documented VE if you know it, but we’ll skip that for now. It would only lower the number anyhow.

Now we know that you’ll need to fuel 42.8#/min of air, you’ll do this with 8 injectors, and we’ll use the generally accepted max duty cycle of 80%. Lets assume that the richest that this NA car will run at any given point is 12.5:1 AFR.

42.8/12.5*60/8/.8 = 32.1 lb/hr of fuel, per injector. Maybe the Fbody clan can chime in with what power levels they move up to 32# injectors at - keeping in mind the rule of thumb that 1#/min air = ~10.7 flywheel horsepower (458 in this case).

Now if you used RC Eng’s calculator http://www.rceng.com/technical.aspx for the LS1 engine, making 458 BHP with a .475 (average what they recommend) BFSC - they’re going to sell you 35#/hr injector — ASSUMING that guessed your BFSC correctly.


Turbo engines are a bit more complex as you need to account for <b>Pressure Ratio </b>[(P1+P2)/P2], but the constants are the same. Lets use stock D16 (with a turbo setup obviously) with any turbo setup - remember PR is <i>absolute</i>.

(98 x 6500) / 3456 = 184 CFM
184 * 0.069 = 12.7 #/min

Again, we can account for VE if we have it, but even in boosted honda apps (with really efficient race worked 4v heads) the actual VE is rarely much above 100% - so I'll stick with 100% because I don't have the details for these engines. 

Now, we're talking 12.7#/m <i>N/A</i>... but we need to add boost. Lets say we're running 15 psi. 
(P1+P2)/P1  --or-- (14.7+15)/14.7 = 2.02 PR.

Our D16 will swallow 12.7#/m NA (which is 1.0 PR), so at 2.02PR we get: <b>12.7*2.02 = 25.7 #/min</b>.

Now we know that you'll need to fuel 25.7#/min of air, you'll do this with 4 injectors, and we'll use the generally accepted max duty cycle of 80%. Lets <i>assume</i> that the richest that this car will run at any given point is 11.0:1 AFR.

<b>25.7/11*60/4/.8 = 43.8 lb/hr of fuel, per injector</b>. Maybe the Honda clan can chime in with what power levels they move up to 44# injectors at - keeping in mind the rule of thumb that 1#/min air = ~10.7 flywheel horsepower (275 in this case).

Now if you used RC Eng's calculator http://www.rceng.com/technical.aspx for the Honda engine, making 275 BHP with a .625 (average what they recommend) BFSC - they're going to sell you 54#/hr injector --- ASSUMING that guessed your BFSC correctly.
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Its basically a rant about the idea that “bigger is better” is fucking stupid. As well as guessing at engine parameters… :picard:

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holy thorough

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While I agree to an extent, I don’t think it’s unreasonable to have a little headroom…

Unless you’re so big that the duty cycle is suuuuper low at idle, i don’t think it’s a HUGE deal.

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Really newman? So I should get the 114 mm turbo so that I have some headroom? Not the same, but similar…Head room is one thing, buying injectors that are 150% of what you need at full tilt is a fucking joke and nothing more then a new way to measure your dick.

Especially when you factor VE into air needs and the flow drops 15%… Meaning your 64# injectors are now 170% more then neccesary.

Pet peeve…

But the root of it is to NOT guess at your engine parameters (bfsc specifically) because prolly 70% of the people on this site don’t understand them.

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obviously not the bigger the better. The larger the injector, the harder the idle tuning process is for cranking and post startup and so on. Yes it bothers me personally when someone looking to make 250 whp on their small turbo setup buys 750cc or higher injectors making it a pain to tune and/or basemap even. Going a little on the high side is fine, but going way overboard is annoying and senseless.

Guessing engine perameters is not really all that stupid in my book. I can “guess” which injector size is needed and go from there without going way overboard making other shit difficult. Sure I could probably downsize 100cc from most all of my estimates with zero problems of maxing the injector out. But seriously, is it really that big of a deal? Typically there’s a million more things on a boosted or all-motor built car that I would be worrying about than getting the injector sizing perfect.

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Typically there’s a million more things on a boosted or all-motor built car that I would be worrying about than getting the injector sizing perfect.[/quote]

lol, slight difference between you making an educated guess and the remaining 70% of of the people who “rely” on these sorts of things.And no its not a huge deal, its not make or break (outside of incredibly small PW tuning) issue. But its ignorant (in the truest from, not the derogatory one) IMO. Akin to slapping a particular turbo on because “its what everyone runs” mentality

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And because more then a handful of people have PMed or IMed me about it… heres the back story behind the rant, as it were.

im so ON about the injector issue because Ive been trying to explain to a particular moron something that has been explained a dozen times and he argues the fact that the “calclators told me so”, not to mention that he justifies his argument by ADJUSTING the parameters in the calculator… specifically BSFC - SOLELY because there is not verifiable BFSC rating for his particular engine. its moronic and fucking dumb. then he wonders why his car is having trouble maintiaing an OEM like idle quality. he’s running 95# injectors with an EMS that has a 0.1 ms resolution (not small enough IMO) so he is skipping HUGE ranges by bumping PW 0.1 either way with that size of injectors… oh, and the turbo hes running puts out ~35lb min MAX, even slightly out of efficiency. its MORANIC.

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Stock fbodies rock 26-28.8 injectors. 32# are usually good on lid/exhaust/cam/intake cars. 34-36 on heads/cam/exhaust/intake/TB/… Then you get into power added and dry kits.

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i don’t think injector calculators are worth dick, at least with what i’m used to. i basically reference those who have experience and are reliable in the hp range im looking to.

the only injectors i’ve ever bought were 60#. i’ve had those at 450rwhp, 600rwhp, 750rwhp, and now while approaching 900rwhp. shit, i ran these when i was running a 365ci NA with low compression and big heads, and the car STILL ran rock solid. that was probably under 400rwhp. i’d say that is a very large range, but these particular injectors (which seem to be a standard in the LSX world) have been EASILY tunable for idle and part throttle driving conditions, and all around performance at ALL mentioned hp levels. and only now am i starting to reach the end of their limits.

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with honda’s anything under 300whp only needs around 440-450cc
up to 400whp needs 600cc
up to 550whp needs 800cc
up to 650whp needs 1000cc
after that it becomes a game with fuel pressure and your choice of injectors

i personally have injector dynamics 1000cc injectors, they come with all the dead times for certain voltages and pressures, and start up like a stock injector, i can run them into the 16’s af while cruising without any hickups and with 100psi base pressure can do 800whp on a honda B series

a good article about injector dead times
http://www.yawpower.com/injectordeadtimesarticle.html

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is this thread really going to die this quick?

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Too much technical info for people to try to sound smart at. lol

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ugh. so i just had this conversation with someone on h-t about injectors, hes looking for 275hp tune, and i told him my 440cc hih imp injectors would be PERFECT. he replies with this. (tryna sell him shit… lol)

Yeah like I said I’m going under his guidance for this stuff. I’m looking for an inline pro or blox manifold if I had the choice. If that’s the log style then I was just confused. He said not to go under 650cc. He has 1000cc in his and it’s only making 250 wheel. It idles perfect without a hitch. As for tuning I will be taking it to our friends shop. Shops name is hybrid dynamics. Dennis is a awesome tuner. I’ll be using a chipped ecu with chrome.

p.s: dont go to hybrid dynamics

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He has 1000cc injectors and he’s only putting out 250 whp?

If that car has an engine smaller than 3 liters I’m calling bullshit, 1000cc injectors are notoriously difficult to get perfectly tuned at idle for smaller engines.

Nizuk: Nice post, you should go on tangents more often :smiley:

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considering it was hondatech, i would assume it’s a honda lol.

i stopped trying to have intelligent conversation with him after he sent me that

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Ah, I didn’t know what h-t meant until now lol, my bad. And I would too…

Just did a little browsing on the matter, someone has a 76GTS setup on an MKIV, and at the smallest resolution on his 1000cc injectors, it would change his AFR .4 points in low/medium rpms. That’s disgusting, considering weather/temperature/altitude changes.

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a lot of it depends on the injector itself and its spray pattern etc, not necessarily just the size. you guys are looking too far into it. yes, some injectors are a bitch to tune for idle/driveability. but think about this… a 1000hp car is NOT consuming 1000hp worth of fuel at idle and is NOT consuming 1000hp worth of fuel at low loaded driving; just like any other injector/hp combo. if a person buys injectors with a future build in mind that would require said injectors, i find nothing wrong with that as long as their tuner knows what they are doing

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this… is… NYSPEED. :lol:

Yea, this same rant sparked a 4 page discussion involving ideal gas law, static flow rate & VE effects as well as a tangent on running different octanes et al… on a more app specific forum. meh.

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True, at idle it isn’t consuming anywhere’s near it’s max. It’s easy to tune in the upper rpms where most of the power is being made, whether it’s N/A or turboed, because there is a LOT of air being shoved into the engine. Every little change you make with the injectors won’t make a big difference with such a large amount of air being crammed in, making them easy to tune.

I’m going to step away from your Camaro for a second and go to a ~2 liter engine, which is pushing ~4 lbs per minute at 2000 rpms, if my head math is right. Off boost…no multiplier. That’s a little under 7 lb/hr compared to the 43.8 Nizuk got at 15 psi. The smallest change you can make to the injectors will result in a large change for the AFR’s.

Another perspective: you’re trying to shoot a can of soda with a rifle from 400 yards. You think you’ve almost got it in your sights…if you move the barrel even a millimeter to the left or right, you’re going to miss the can by a few inches. The smallest change can have large results given a large enough distance…or a large enough injector.

Of course, I’m not saying it’s impossible. Certainly easier with your Camaro :smiley:

LINK.