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Injector size calculations

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Old 02-27-2012, 06:30 PM   #1
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Injector size calculations

Okay, let's be scientific about this. I've seen guys say they're going to put such & such an injector in their build, but not how to best come up with that value.

I did an internet search, and came up with this site:
http://www.watkinsmotorsports.com/Fu..._required.aspx

Note the fuel pressure does not affect the recommended injector size. That makes sense, as the injector should be sized for the flow you need at the pressure you're going to run. A 22 lb injector rated at 48 psi will flow the same at 48 psi as a 22 lb injector rated at 58 psi will flow at 58 psi.

I'm assuming my LQ4/L92 build will make as much as 500 HP at the crank (at sea level), so using this calculator, it says use a 39 lb injector.

On the other hand, my 28 lb injectors in the LS1/4L60E car will only support 350 HP (at 80% duty cycle - up to 450 if you allow them to max out). Since various internet HP calculators estimate I'm putting out more than that at sea level based on ET & MPH, I'm probably running them at more than 80% at sea level (but should be fine at our 5800' elevation).

On the other hand (if there is another hand), the 36 lb injectors my son just put in his 5.3 punched out to 5.7l will support up to 460 HP according to this calculator (doubt he'll get them to 80% duty cycle, though). His 24 lb injectors (I think that's what they were - stock 5.3's as best we could tell) were maxing out on the dyno at a little over 300 RWHP. He should get it back on the dyno sometime in March, so we'll see how it does (also did a BBK intake to replace the LS1 intake that came with it).

So, back to the basic topic, what calculating tool do you use?
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Old 02-27-2012, 07:53 PM   #2
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Re: Injector size calculations

This was a very timely post! I'm trying to decide whether to stick with my stock ls3 injectors or upgrade. Everybody on 'tech insists that they are fine to well over 550rwhp, but based on the link you provided, 500rwhp(600fwhp) puts them at 90% duty cycle.

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Old 02-27-2012, 09:09 PM   #3
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Hmmm, LS3 injectors look like they'd fill the bill for me.
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Old 02-28-2012, 01:24 AM   #4
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Re: Injector size calculations

You can raise fuel pressure to make those injectors support more HP though. Lots of guys are running the LS3 injectors which are 42 pounders right?
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Old 02-28-2012, 01:49 AM   #5
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Re: Injector size calculations

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Originally Posted by xpndbl3 View Post
You can raise fuel pressure to make those injectors support more HP though. Lots of guys are running the LS3 injectors which are 42 pounders right?
About 42lb. At 58psi. Iirc. However they are ev6 connector plug. So need the ev1 to ev6 adapter harness to work in some cars.
The are also significantly shorter than the ev1 type injectors.
A local is around 520+rw with stock ls3 injectors on his 2010 ss. H/c/i .etc.
Idk his duty cycle. But no probs, and hes been running like that going on 4mo.
Correct tune,afr. Turns 7000+ rpm.
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Old 02-28-2012, 08:05 AM   #6
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Re: Injector size calculations

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Originally Posted by xpndbl3 View Post
You can raise fuel pressure to make those injectors support more HP though. Lots of guys are running the LS3 injectors which are 42 pounders right?
those injectors will support up to 90 psi without a problem. As far as the length is concerned we have the "hats" that fit on top...makes them the same length as the standard bosch
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Old 02-28-2012, 08:25 AM   #7
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Re: Injector size calculations

i used this site:

http://www.corvetteinjectors.com/calculator.html

figured 500fwhp, which put me at 39lb/hr and change. i picked up a set of refurbished buick supercharged injectors(white in color), which flow at 41.5 at ls1 pressure. this will give me room in the future when i add better heads and intake.
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Old 02-28-2012, 09:34 AM   #8
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Re: Injector size calculations

39's for 500 flywheel hp is overkill. You can do that with 30's.

The calculators are ok, but you need accurate BSFC's to make it function right. Good n/a motors typically are in the low mid .40's. Using .50 or so is conservative.

87_TA has 42lb injectors on his 406sbc making 521whp in the low 6000 rpm range. Says its going static on him at 6400 and forcing a rich condition. Calculators using a .48 bsfc say thats 90's % cycle duty so its pretty close. I had 42's on my 383 for 400whp. Way overkill but I wanted to be safe for future upgrades.

I"ve seen guys claim 400whp is possible with stock 28lb LS1 injectors.
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Old 02-28-2012, 12:03 PM   #9
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Obviously BSFC and duty cycle will have a significant effect on the results.

As I understand the physics: you don't want 100% duty cycle; higher fuel pressure decreases injector life; and the bigger the injector the less optimal the spray pattern for low power situations (idle, cruise). So, you don't want too small of an injector so that you don't max out the duty cycle or have to raise fuel pressure, and you don't want too big of an injector that will mess up low power driveability and fuel economy (and emissions if that is a factor for you).

I couldn't tell difference in performance with the LS1/4L60E car with 26 vs. 28 lb injectors at altitude (mind you, no tuning performed for either one), but I did have the 28's in when at lower altitude for the runs that produced the estimated power outputs. Simply changing the BSFC to .45 and duty cycle to 90% in either calculator changed the power level to 450 FWHP with 28's. So, I doubt they were holding me back.

Oh, about the LQ4/L92 build - SLP is selling LS3 take-off intake manifolds with rails and injectors, so I don't think injector height is going to be an issue for me. I will need to change the injector connectors on the harness, though. No biggie, common enough to be easy to find, and easy enough to do.
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Old 02-29-2012, 07:13 PM   #10
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Re: Injector size calculations

I am going through sizing a new set of injectors for my '85 IROC LS3 / TVS2300 configuration right now as i am well out of injector right now using the stock ZR1 injector

https://www.thirdgen.org/forums/ltx-...ne-swap-7.html (LS3 Engine Swap)

Fuel pressure plays an important aspect especially when you consider that on a force blown application that pressure that is forcing that additional air in the engine is also opposing the fuel pressure so your effective injector flow is dropping and is why on a zr1 they use a variable fuel pump pressure that increases as boost occurs

Flow rate is calculated for a given pressure by (square root (adjusted pressure/rated pressure)) X rated injector flow

On most of our mods (including mine) we are running a fixed fuel pump pressure, but you need to adjust your injector flow for your boost pressure when considering injector sizing, but that is for another discussion

However, here are some basics you can consider... To calculate basic fuel needed you can roughly estimate by

GPH = HP X Specific Fuel Consumption / Fuel Weight X Safety Buffer

Using SFC of 0.55 lbs / hp /hr, 6.35 lbs/gal fuel weight and then also a 15% buffer, for 600HP you get about 60 gallons per hour

Flow rate would then be 60 gallons per hour * 6.35 lbs / gal = 381 lbs / hr

Take 381 lbs / hr and divide by 8 cylinder = 48 lbs / hr

Remember that most injectors have their label rating at 3 bar vs our 4 bar system, you can use the formula above to adjust

attached is a screenshot of my present zr1 injector flow vs delta pressure... you can see how much difference the ultimate pressure delta change can make... also a critical reason why you need to make sure you have your injector table correct when changing injectors... there is more than just this table to work with as battery volts, IAT etc also affect injector flow performance and have there own tables... installing injectors with different characteristics and not adjusting these values will make for a very bad situation as remember you are running open loop on start-up with no oxygen sensor adjustment, and even in closed loop they can only trim so much
Attached Thumbnails
Injector size calculations-tvs2300-zr1-injectors.png  

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Old 02-29-2012, 07:35 PM   #11
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Re: Injector size calculations

i dont want to hi-jack or mess up the flow of this thread, but im turboing a 5.3 with ls6 intake and cam. what injectors should i use for this build.?
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Old 03-20-2012, 09:52 AM   #12
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Re: Injector size calculations

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So, back to the basic topic, what calculating tool do you use?
The basic Brake Specific Fuel Consumption calculus will get you the delivery volume you need, but for LS platform tuning you really want to select injectors that are well-documented in terms of their offset (latency) and flow rates across the range of manifold pressures.

I went through this last year when I upgraded to Ford/SVO 30# "skinny" injectors and it was a bear. The offset table in GM's software is 3D and huge, no way you'll ever calibrate it using observational data like exhaust O2 content.

I ended up buying Greg Banish's DVD and using his pre-fabricated offset and flow rate tables after weeks of frustrating tuning attempts...negative LTFTs in some cells, positive LTFTs in others, tip-in stumbling, etc.

My numbers were nowhere close to his, and his tables smoothed the engine right out.

If you can find GM injectors sized right for your application, and the factory calibration tables to go with them, this would be the cheapest approach. Other than that, I'd find injectors on Banish's DVD and make the investment.
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Old 03-20-2012, 07:42 PM   #13
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Re: Injector size calculations

so...injectors for ~900rwhp on E-85 with 17lbs boost...go lol
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Old 03-20-2012, 08:05 PM   #14
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Re: Injector size calculations

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so...injectors for ~900rwhp on E-85 with 17lbs boost...go lol
120lb or near that
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Old 03-21-2012, 12:31 AM   #15
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Re: Injector size calculations

imo.. you can NEVER have too much injector... i have ls3's in my ls2 and there just peachy... stock 34lb ls2 injectors will support like 540 flywheel hp
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Old 03-21-2012, 02:54 AM   #16
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Originally Posted by 1nasty86 View Post
imo.. you can NEVER have too much injector... i have ls3's in my ls2 and there just peachy... stock 34lb ls2 injectors will support like 540 flywheel hp
120 lb'ers would probably be too much for your LS2...
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Old 03-21-2012, 12:10 PM   #17
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Re: Injector size calculations

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120 lb'ers would probably be too much for your LS2...
Yeah well any idiot that puts 120lb injectors in a n/a ls2 should keep them in there for sure! You get my point, figures I throw in my .02 and get a remark like that.
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Old 03-21-2012, 12:23 PM   #18
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Re: Injector size calculations

I know a ford red top injector(30lb) will be almost 36 lbs at 58psi, same for any ford injector rated at 45 psi.
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Old 03-21-2012, 01:11 PM   #19
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Re: Injector size calculations

It just comes down to what was said above about offsets and tuneability at low engine speeds. A good tuner will need to play with injector offsets unless precise values are actually known/calculated and can be trusted for use.

Large injectors generally have issues have low pulsewidths and accurate fuel control is hard to come by at low speeds/low fuel demand. Not all are like this. That being said, I have no issues with my 80's on my mild 401 when off boost. Boosted setups need very large injectors but they need to work off boost when fuel demand is low.

I know my buddy has done some turbo LSx motors and used 72-118lb injectors on 4.8's/5.3's/5.7's, 6.0's and all idle just fine with cam/heads/turbo.

So its hard to say you can go too big as long as the injector can handle low pulsewidths, or atleast the software/hardware drivers in the ecm can handle it.

Say you find a deal on 60's when you need 36 lb injectors for your LS2 motor. If the price is too hard to pass up, 60's can be used. Just require extra attention to tuning detail
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Old 03-21-2012, 02:52 PM   #20
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Re: Injector size calculations

120lb injectors would be at 25% duty cycle at wot on your lq4/l92 setup so stick the 120's in it! Then you can support 1200hp easy
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Old 03-22-2012, 01:13 AM   #21
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Re: Injector size calculations

theres a lot to injectors and tuning them.

a few tips:
ALWAYS get a flow matched set.

get a big enough set that you have room for more but not so large that the car wont run right (ie: 120lb injectors on a 500hp engine)

use the correct O-Rings for the injectors.

tune them correctly. dont trick the computer. its recomended you base your ifr table(s) on actual logs. the calculations out of that will affect your ve directly. basically, if your tune on a mild motor is ~110% ve... your ifr is borked. fix it.

as always... USE A W/B
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Old 03-22-2012, 05:22 AM   #22
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Re: Injector size calculations

injector dynamics make a number of different injector sizes and styles that fit the bill... the great thing is they are flow matched and all the required programming information is posted on their web site.

several high end tuning shops are now exclusively using these due to the factors above.

hutter performance is one such shop in my area

i just got a set to put in my tvs2300 / ls3 swap as my zr1 type injectors were out of room... i went with the ID850's as they are direct physical size replacement for the zr1's

http://www.injectordynamics.com/
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Old 03-22-2012, 05:01 PM   #23
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Re: Injector size calculations

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Originally Posted by 1nasty86 View Post
120lb injectors would be at 25% duty cycle at wot on your lq4/l92 setup so stick the 120's in it! Then you can support 1200hp easy



oh, you were serious?

anything larger than a 60lb injector is very difficult to tune for as you have to trick the stock ecm to run them correctly. 60lb squirters are enough of a pita themselves on an engine that actually needs them..

then the insanely difficult task of getting the idle correct with injectors that large. its a great way to wash the cylinders down with fuel and destroy the engine. also, the issue of transitional and transient fueling along with injector timing.

this is not an area to "overbuild." it will cause you a hell of a lot of headaches and make the car run like dog crap.


alotquicka: 60lb'ers are a good bet for the mild/moderate boost builds.
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Old 03-22-2012, 05:05 PM   #24
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Re: Injector size calculations

Just strickly making a point, Efi live will scale 120lb injectors and a good tuner could make them work. I have 42lb injectors in my h/c/i ls2 which will be plenty for anything I want to do n/a
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Old 03-22-2012, 06:36 PM   #25
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Re: Injector size calculations

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Originally Posted by 1nasty86 View Post
Just strickly making a point, Efi live will scale 120lb injectors and a good tuner could make them work. I have 42lb injectors in my h/c/i ls2 which will be plenty for anything I want to do n/a
the point is, use the right set of injectors, not the largest you can afford. its about as nutty as throwing a 1050cfm carb on a motor that only needs a 650 at the most.
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Old 03-22-2012, 06:44 PM   #26
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Re: Injector size calculations

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the point is, use the right set of injectors, not the largest you can afford. its about as nutty as throwing a 1050cfm carb on a motor that only needs a 650 at the most.
I didnt say go out and buy the largest injector you can afford, I stated 120lb injectors could work.
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Old 03-23-2012, 01:45 AM   #27
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Re: Injector size calculations

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I didnt say go out and buy the largest injector you can afford, I stated 120lb injectors could work.
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Old 09-02-2014, 02:51 PM   #28
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Re: Injector size calculations

ok I have a 6.0 with a large cam ported 243 heads going to be on e85 making about 475 hp at crank I est.,what lb do I need will a ls3 injector fit my truck intake or what fits
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Old 09-02-2014, 03:04 PM   #29
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Re: Injector size calculations

Ls3 style wont fit i dont think, not the cathedral port intakes, correct me if i am wrong

Injector size assuming 475 crank on e85 a 42 lb at 58 psi should cover it.
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Old 04-04-2017, 08:48 AM   #30
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Re: Injector size calculations

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Ls3 style wont fit i dont think, not the cathedral port intakes, correct me if i am wrong

Injector size assuming 475 crank on e85 a 42 lb at 58 psi should cover it.

Bringing a thread back from the dead here for a moment.

So based on what I read...my swap will be in the 450-550 BHP range. 39lb injectors should be plenty for what I need but the stock 28lb LS6 injectors I have on my manifold now that came with it when I bought it will be holding me back?
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Old 04-04-2017, 09:59 AM   #31
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Re: Injector size calculations

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Bringing a thread back from the dead here for a moment.

So based on what I read...my swap will be in the 450-550 BHP range. 39lb injectors should be plenty for what I need but the stock 28lb LS6 injectors I have on my manifold now that came with it when I bought it will be holding me back?
28s at 58 psi would likely max out at WOT and may even be lean. So it would be best to get larger injectors.

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Old 04-06-2017, 05:41 PM   #32
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Re: Injector size calculations

larger is always the better way to go. With a good tune, you can pretty much use anything. By the way...we not only sell tpi injectors but we have every size performance injector you could possibly need. From 440cc up to 2200cc. If we can help any of you give us a call
516-442-4707
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Old 04-06-2017, 06:02 PM   #33
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Re: Injector size calculations

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larger is always the better way to go. With a good tune, you can pretty much use anything. By the way...we not only sell tpi injectors but we have every size performance injector you could possibly need. From 440cc up to 2200cc. If we can help any of you give us a call
516-442-4707
I will definitely take a look.
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Old 04-06-2017, 06:11 PM   #34
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Re: Injector size calculations

well our website is fairly new so not every product is listed. If you need something that you do not see please give us a call at the shop
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Old 04-06-2017, 08:52 PM   #35
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Re: Injector size calculations

Most people go too small because they are looking at the old 80% duty cycle rule of thumb. It is not unreasonable to aim for 50% or 60% max. You can buy injectors with modern internals that have excellent control at low pulse width so that's just not even a concern any more.

There is a preferred window of opportunity you want fuel to enter the cylinder, and you want all the fuel in the cylinder by the time the intake valve closes. Larger injectors can hit that optimized window of opportunity to higher engine speed. Smaller injectors can be open so long that the fuel is still flowing even after the intake valve closes.

I'm going on memory here, but I think I max out around 60% duty cycle. My buddy has a LS1 turning 8000 rpm and he maxes out at 50% duty cycle.
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Old 04-06-2017, 09:06 PM   #36
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Re: Injector size calculations

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Originally Posted by RaverRacerX View Post
.. then the insanely difficult task of getting the idle correct with injectors that large
All you do is send the fuel in early so it evaporates on the backside of the hot intake valve. GM does the same thing from the factory because LS engine does not atomize well at idle. Tremendous gain of torque at idle when you do that.

Last edited by QwkTrip; 04-06-2017 at 09:11 PM.
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Old 04-07-2017, 07:54 AM   #37
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Re: Injector size calculations

Lol i had 36 lb inj on my stock 305 tpi. Idled great. 127 lbs on my old 400" sbc. Idled great. You can do alot with todays injectors but you definately can go too big. 2200cc on a 5.3 turbo motor may get a little rich at idle
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Old 04-07-2017, 07:55 AM   #38
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Re: Injector size calculations

Well, my tuner suggested 36# injectors because i'm running NA, but would 42# work if, in the future I plan to add, in the immortal words of Jeremy Clarkson, " MORE POWER!!"? I just don't want to leave myself short and have to re-do it again later on since the price is basically the same initially.
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Old 04-07-2017, 08:03 AM   #39
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Re: Injector size calculations

I use 60 lb/hr injectors on my n/a 427. My buddy has 80 lb/hr injectors on his high revving n/a LS1. We both make the same Hp.
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Old 04-07-2017, 08:10 AM   #40
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Re: Injector size calculations

SO #42lb per hour wouldn't be too big to shoot for (6.0 LQ9) and will give me upgrade ability later on?
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Old 04-07-2017, 07:26 PM   #41
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Re: Injector size calculations

GM put them in every 6.2L LS3.
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Old 04-26-2017, 02:13 PM   #42
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Re: Injector size calculations

42lb it is. Leaves me room to upgrade later without changing injectors.
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Old 04-28-2017, 10:55 PM   #43
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Re: Injector size calculations

You can buy injectors that can be resized as your project grows. That's what I did. My injectors can be resized up to 110 lb/hr if I remember right.

Stock LS3 injectors are and are not 42 lb/hr. Basically they are rated by GM at 42 lb/hr but the test is at a different pressure than what is common in the aftermarket. If you adjust for apples to apples comparison to most the aftermarket then the LS3 injectors must be de-rated to something in the mid to high 30's.

Again, I would not hesitate to go bigger.
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Old 04-29-2017, 10:44 AM   #44
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Re: Injector size calculations

with a good tune you can actually use any size injector you want. It depends on what you want to accomplish. We modify/resize the bosch injectors anywhere from 750cc up to 1300cc for people. The rest are all unmodified up to 2200cc
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