Simple question. I'm running 80 lb injectors on my 383 Vortec TBI setup. Would 90 lb injectors provide any improvement? I'm running GMPP Vortec Intake, Holley 670 TBI w/OEM pod, 255 l/hr fuel pump, & custom tuning. I understand that I would have to re-tune.

 

Not in my opinion. I'm just running 55# ones on my heavily modified 350 and I have no shortage of fuel at any RPM's.

 

If you cannot get enough fuel with your present injectors is when you want to increase the size.

Sorry but I am going to give you more questions than answers to begin with.

Are you running lean? Do you have any data logging capibility and or a Wide Band? What kind of "custom " tuning do you have? Are you doing it yourself? What cam are you running?

DM

 

cant you just up your fuel pre.

 

tunerpro rt give you in datalog the injector base pulse width. knowing that and the rpms you are turning will tell you if injs are large enough. OR one can run a WB and that will confirm engine has suffcient fuel.

80 bls inj can support 400 hp.

 

As others have mentioned it depends upon the setup. Can patch the data stream and log the PW, along with WB data, this would tell you whether more fuel is required. Not knowing what cam/valve-train/exhaust the engine has, we can still run some numbers.

Ball parking the fuel flow requirement going by the 1HP/CI value. And a .45 BSFC value, both of which are reasonable by todays standards. We can calculate the fuel flow requirement:

383 HP @ .45 BSFC is: 383 * .45 = 172 #/hr of fuel

Staying within 85% duty cycle: 172 / .85 = 203 #/hr fuel flow

Then two injectors, so divide by 2: 203 / 2 = 101.5 #/hr per injector

To get that from 80 #/hr injectors need to raise the fuel pressure to: psi = 11.5 * sq(101.5 / 80) (11.5 being the fuel pressure for 80 #/hr)

19 psi = 11.5 * sq(101.5 / 80)

To ensure enough fuel to cover transients such as AE (pump shot) I'd just go with 21-22 psi. Stock GM TBI injectors and pod will operate in excess of 30 psi. So there is still room for growth.

This calculation ignores the RPM requirements. If staying below 6,000 RPM there won't be a problem.

RBob.

 

Thank you very much for the info RBob.

 

How did u get 19psi? I get 12.95 when I run the same formula

 

I would guess you are missing some step or have some operation out of order.

Injector flow with pressure change is

(SqRT(NewPsi/Factory PSI)) × Flow Rate

So if you factor that GM flow rates TBI injectors at 11-12 psi (guessing why he used 11.5 since nobody can seem to actually agree). At 22 psi those 80 lb/hr injectors are about 108 lb/hr each.

 

When I was setting up my new motor recently, I wanted to know exactly what PSI it took to flow my 90# injector at 90#. I took the injextors to a friend that does nothing but injectors and put them on the injector flow tester and they needed 14.8PSI to flow 90#, checked them several times to get an accurate reading. That is the number I used to calculate what to use to set my fuel pressure up for my engine. Based on that number (14.8), my 383 needed 24.5psi fuel pressure to support 438HP and flows 116#phr each and which is really close to the HP I got on the dyno. DC was hoovering around 85% - 88% which I'm OK with.

 

That means you have 80# injectors, not 90#. In fact if you calculate what an 80# flows at 14.8 psi, working from RBob's 11.5 PSI it is almost exactly 90. Like 89.987.....

 

psi = 11.5 * sq(101.5 / 80)

Divide 101.5 by 80 = 1.26875

Square it: 1.26875 * 1.26875 = 1.6097265625

And multiply by 11.5: 1.6097265625 * 11.5 = 18.5 psi

I used 11.5 psi as the fuel pressure as it is around the middle of the 9 - 13 psi GM based acceptable service pressure. Using 13 psi is likely a better choice.

RBob.

 

Right but am trying to figure out how to calculate new fuel pressure if I know what the new flow rate is. so if I already know that existing fuel pressure is 11.5 and I know that the new flow rate needs to be 108 lb/hr, how would I determine what the new fuel pressure has to be in psi?

 

Here is a psi calculator , if your 80 #hr at 11.5 your 110#hr at 22psi.
http://www.megamanual.com/v22manual/minj.htm

 

They are not 80# injectors. These are 90# from a 454 truck motor which if you look at those from injector dealers are rated at 15psi, not 9-13 like stock CF injectors. Mine flow 90# at 14.8, close enough and that's what I use.

 

That sounds just about right, since Jim at DCS runs his 383 with 80# injectors @22psi.

 

No such thing as true 90 lb/hr injectors from GM. What you have are 80#. Even the 454SS had 80# at stock fuel pressure. They do flow 90# at 15 psi, but that is not what the OEM regulator was set at. Highest factory pressure I have seen on the low pressure stuff was 14 psi on a 1992 G20 van 350. All the TBI engines had the same spec fuel pressure except the 30 psi regulated stuff.

 

I see what I did wrong now. I though "sq(101.5 / 80)" meant I had to take the square root of this number as there is no square root symbol on standard keyboard. I really appreciate the answers I got here. This has been a tremendous help. The mega manual is a really good source of information too. I thank everyone who responded.

 

Sorry to dig up an old thread but I didn't want to start a new one if I didn't have to. This is a question I have been trying to find an answer to. I'm building up the 454 in my '94 C2500 and my stock injectors are the 46# (or what have you) injectors and the 30psi regulator. I have a set of 5235231 injectors from another, low pressure, BBC TBI laying around and I have found conflicting stories as to what their flow rate is. This will obviously play a big role in how the new setup is tuned, but either way these injectors are big enough to fuel the motor. So you're saying these are 81#/hr injectors?