Are 30 lb injectors too much for heads/cam combo?
Administrator
Joined: Nov 1998
Posts: 71,094
From: Hell was full so they sent me to NJ
To run 113.78 w/ 3680# race weight, you are well over 400flywheelHP (I'd say 430HP)... and that puts you near the upper limit of the stock 24's, at the stock fuel pressure. (Too close to the limit, in my opinion). Yes, you can go to 30's with the proper programming... there is no downside at all. Or you could raise the fuel pressure You'd have to go to 63psi to get 30# out of the 24's.
The popular sizing formula for injectors is:
INJ SIZE = (crankHP x BSFC) / (8cyls X duty cycle)
BSFC is Brake Specific Fuel Consumption.... a stock engine might see 0.50#/HR/HP. A well tuned race engine can be 0.45#/HR/HP (even less in narrow RPM applications). A power adder motor can easily run 0.50-0.60#/HR/HP.
Duty cycle is the amount of the time that the injector is open (pulse width), divided the time it takes the crank to make 2 revolutions (4-cycle). At 6,000rpm, there is only 0.020-seconds available in 2 crank revs... generally expressed as 20mS.
Anyone who says you can run 98% duty cycle on an injector is DEAD wrong. Well... .you can run them at 100%, but anyone who has ever tested an injector has seen flow and spray patterns start to become unstable at 80-85% duty cycle. Yes, you might not see that instability in the 1-second or so that you are at max fuel flow in a dyno pull, but I can assure you... run the injectors at 100% DC, or 98%DC for any period of time, and you will lose power. Eventually they will just fail.
Getting back to the formula, if you solve for HP instead of injector size, it becomes.....
crankHP = ( INJ SIZE X duty cycle X 8cyl) / BSFC
so for a 24# injector:
crankHP = (24 X 0.80 X 8) / 0.50 = 307.2 crankHP
as pointed out in the earlier post.
But this has some conservatism in it....
I found that with tuning, my BSFC was under 0.45.
You can stretch the DC a little... lets try 0.85
When you do this, and reflect that the factory uses 24.9#/HR as the flow constant in the PCM:
crankHP = (24.9 X 0.85 X 8) / 0.45 = 375 crankHP
Obvisouly, if you use 100% DC, this number becomes 442 crankHP.
Whiuch explains why people can make 400rwHP on the stock injectors... at least for less than 1-second max flow during a dyno pull.
I strongly believe in running the injectors at less than 85%. My own setup uses Bosch 64#, running at 58psi (=78#), and I run 70%DC or less at 800HP with a dry nitrous setup (all the fuel through the injectors).
I wouldn't even call this a "debate".... its simple facts.
The popular sizing formula for injectors is:
INJ SIZE = (crankHP x BSFC) / (8cyls X duty cycle)
BSFC is Brake Specific Fuel Consumption.... a stock engine might see 0.50#/HR/HP. A well tuned race engine can be 0.45#/HR/HP (even less in narrow RPM applications). A power adder motor can easily run 0.50-0.60#/HR/HP.
Duty cycle is the amount of the time that the injector is open (pulse width), divided the time it takes the crank to make 2 revolutions (4-cycle). At 6,000rpm, there is only 0.020-seconds available in 2 crank revs... generally expressed as 20mS.
Anyone who says you can run 98% duty cycle on an injector is DEAD wrong. Well... .you can run them at 100%, but anyone who has ever tested an injector has seen flow and spray patterns start to become unstable at 80-85% duty cycle. Yes, you might not see that instability in the 1-second or so that you are at max fuel flow in a dyno pull, but I can assure you... run the injectors at 100% DC, or 98%DC for any period of time, and you will lose power. Eventually they will just fail.
Getting back to the formula, if you solve for HP instead of injector size, it becomes.....
crankHP = ( INJ SIZE X duty cycle X 8cyl) / BSFC
so for a 24# injector:
crankHP = (24 X 0.80 X 8) / 0.50 = 307.2 crankHP
as pointed out in the earlier post.
But this has some conservatism in it....
I found that with tuning, my BSFC was under 0.45.
You can stretch the DC a little... lets try 0.85
When you do this, and reflect that the factory uses 24.9#/HR as the flow constant in the PCM:
crankHP = (24.9 X 0.85 X 8) / 0.45 = 375 crankHP
Obvisouly, if you use 100% DC, this number becomes 442 crankHP.
Whiuch explains why people can make 400rwHP on the stock injectors... at least for less than 1-second max flow during a dyno pull.
I strongly believe in running the injectors at less than 85%. My own setup uses Bosch 64#, running at 58psi (=78#), and I run 70%DC or less at 800HP with a dry nitrous setup (all the fuel through the injectors).
I wouldn't even call this a "debate".... its simple facts.
Administrator
Joined: Nov 1998
Posts: 71,094
From: Hell was full so they sent me to NJ
Check this link.
An excerpt from it:
[quote]
Next we test each injector at varying pulse widths, over the entire rpm range, from idle to 12,000 RPM. Starting the test at minimum Duty Cycle and increasing to 80% Duty Cycle, including the fast idle test at 2400 RPM. (80% is considered maximum usable Duty Cycle and is commonly used to calculate Brake Specific Fuel Consumption). We use the strobe light and the laser beam through out the testing cycle. It's a wonderful way to visually freeze the patterns, in mid air, and actually measure the results with the laser. No one else has this capability, and it's a real interesting light show. The last test that we run is the Tell All Test. We run each unit up in duty cycle, at 12,000 RPM, until it fails electronically. (Seen on the oscilloscope) This is prominently displayed with the laser and strobe. Most pintle type injectors fail at 86%/88% and the Disc will usually go to 92% +, depending on system pressure. All must cycle smoothly up to 85% to Pass. Bad injectors will sometimes go "static" at 70% or so and are discarded. NOTE: Most pintle injectors will increase flow rates up to 88%, 92% with Disc injectors, and then go "semi-static", half open-half closed, just before going full static. This time-out event occurs at different time/pulse width durations, in different injectors, but always produces a 50% or so Duty Cycle flow rate. This extremely dangerous situation will usually occur at the worst of times, full throttle-max boost-high RPM- just when you need 100% fuel delivery you get 50% and go dead lean. BANG!!! This problem seems to amplify a bit at higher pressures.
An excerpt from it:
[quote]
Next we test each injector at varying pulse widths, over the entire rpm range, from idle to 12,000 RPM. Starting the test at minimum Duty Cycle and increasing to 80% Duty Cycle, including the fast idle test at 2400 RPM. (80% is considered maximum usable Duty Cycle and is commonly used to calculate Brake Specific Fuel Consumption). We use the strobe light and the laser beam through out the testing cycle. It's a wonderful way to visually freeze the patterns, in mid air, and actually measure the results with the laser. No one else has this capability, and it's a real interesting light show. The last test that we run is the Tell All Test. We run each unit up in duty cycle, at 12,000 RPM, until it fails electronically. (Seen on the oscilloscope) This is prominently displayed with the laser and strobe. Most pintle type injectors fail at 86%/88% and the Disc will usually go to 92% +, depending on system pressure. All must cycle smoothly up to 85% to Pass. Bad injectors will sometimes go "static" at 70% or so and are discarded. NOTE: Most pintle injectors will increase flow rates up to 88%, 92% with Disc injectors, and then go "semi-static", half open-half closed, just before going full static. This time-out event occurs at different time/pulse width durations, in different injectors, but always produces a 50% or so Duty Cycle flow rate. This extremely dangerous situation will usually occur at the worst of times, full throttle-max boost-high RPM- just when you need 100% fuel delivery you get 50% and go dead lean. BANG!!! This problem seems to amplify a bit at higher pressures.
Registered User
Joined: Jan 2003
Posts: 408
From: West Coast
The factory put 28 lb injectors in the LT4 Vette. While they are conservative in their calculations, they don't just put in too big an injector for nothing. The LT4 was rated at 330 hp from the factory.
The injectors are bigger than they need to be for a reason: SAFETY. Ideally you don't want to max out the injector, because engine damage can result from going really lean at high RPM. When an injector is maxed out (i.e. open full time), it flows dramatically less than when it opens and closes normally.
The reason that you can push 400 hp from the stock 24 lb injectors is that you are eating into the safety margin the factory built in.
So, people spend thousands on heads and cam and all the other mods, but hesitate to spend $219 for injectors (FMS 30# @ Summit)? Sounds like fools gold to me.
The injectors are bigger than they need to be for a reason: SAFETY. Ideally you don't want to max out the injector, because engine damage can result from going really lean at high RPM. When an injector is maxed out (i.e. open full time), it flows dramatically less than when it opens and closes normally.
The reason that you can push 400 hp from the stock 24 lb injectors is that you are eating into the safety margin the factory built in.
So, people spend thousands on heads and cam and all the other mods, but hesitate to spend $219 for injectors (FMS 30# @ Summit)? Sounds like fools gold to me.
Administrator
Joined: Nov 1998
Posts: 71,094
From: Hell was full so they sent me to NJ
Originally posted by llafro
.....So, people spend thousands on heads and cam and all the other mods, but hesitate to spend $219 for injectors (FMS 30# @ Summit)? Sounds like fools gold to me.
.....So, people spend thousands on heads and cam and all the other mods, but hesitate to spend $219 for injectors (FMS 30# @ Summit)? Sounds like fools gold to me.
Registered User
Joined: Nov 2001
Posts: 646
From: West Palm Beach, FL
Originally posted by Injuneer
Duty cycle is the amount of the time that the injector is open (pulse width), divided the time it takes the crank to make 2 revolutions (4-cycle). At 6,000rpm, there is only 0.020-seconds available in 2 crank revs... generally expressed as 20mS.
Anyone who says you can run 98% duty cycle on an injector is DEAD wrong. Well... .you can run them at 100%, but anyone who has ever tested an injector has seen flow and spray patterns start to become unstable at 80-85% duty cycle. Yes, you might not see that instability in the 1-second or so that you are at max fuel flow in a dyno pull, but I can assure you... run the injectors at 100% DC, or 98%DC for any period of time, and you will lose power. Eventually they will just fail.
Duty cycle is the amount of the time that the injector is open (pulse width), divided the time it takes the crank to make 2 revolutions (4-cycle). At 6,000rpm, there is only 0.020-seconds available in 2 crank revs... generally expressed as 20mS.
Anyone who says you can run 98% duty cycle on an injector is DEAD wrong. Well... .you can run them at 100%, but anyone who has ever tested an injector has seen flow and spray patterns start to become unstable at 80-85% duty cycle. Yes, you might not see that instability in the 1-second or so that you are at max fuel flow in a dyno pull, but I can assure you... run the injectors at 100% DC, or 98%DC for any period of time, and you will lose power. Eventually they will just fail.
) What am I missing here?
Registered User
Joined: Aug 2000
Posts: 586
From: Oklahoma City, OK
Originally posted by llafro
The factory put 28 lb injectors in the LT4 Vette. While they are conservative in their calculations, they don't just put in too big an injector for nothing. The LT4 was rated at 330 hp from the factory.
The injectors are bigger than they need to be for a reason: SAFETY. Ideally you don't want to max out the injector, because engine damage can result from going really lean at high RPM. When an injector is maxed out (i.e. open full time), it flows dramatically less than when it opens and closes normally.
The reason that you can push 400 hp from the stock 24 lb injectors is that you are eating into the safety margin the factory built in.
So, people spend thousands on heads and cam and all the other mods, but hesitate to spend $219 for injectors (FMS 30# @ Summit)? Sounds like fools gold to me.
The factory put 28 lb injectors in the LT4 Vette. While they are conservative in their calculations, they don't just put in too big an injector for nothing. The LT4 was rated at 330 hp from the factory.
The injectors are bigger than they need to be for a reason: SAFETY. Ideally you don't want to max out the injector, because engine damage can result from going really lean at high RPM. When an injector is maxed out (i.e. open full time), it flows dramatically less than when it opens and closes normally.
The reason that you can push 400 hp from the stock 24 lb injectors is that you are eating into the safety margin the factory built in.
So, people spend thousands on heads and cam and all the other mods, but hesitate to spend $219 for injectors (FMS 30# @ Summit)? Sounds like fools gold to me.
Registered User
Joined: Mar 2002
Posts: 4,827
From: Waynesboro, PA
Originally posted by llafro
The factory put 28 lb injectors in the LT4 Vette. While they are conservative in their calculations, they don't just put in too big an injector for nothing. The LT4 was rated at 330 hp from the factory.
The injectors are bigger than they need to be for a reason: SAFETY. Ideally you don't want to max out the injector, because engine damage can result from going really lean at high RPM. When an injector is maxed out (i.e. open full time), it flows dramatically less than when it opens and closes normally.
The reason that you can push 400 hp from the stock 24 lb injectors is that you are eating into the safety margin the factory built in.
So, people spend thousands on heads and cam and all the other mods, but hesitate to spend $219 for injectors (FMS 30# @ Summit)? Sounds like fools gold to me.
The factory put 28 lb injectors in the LT4 Vette. While they are conservative in their calculations, they don't just put in too big an injector for nothing. The LT4 was rated at 330 hp from the factory.
The injectors are bigger than they need to be for a reason: SAFETY. Ideally you don't want to max out the injector, because engine damage can result from going really lean at high RPM. When an injector is maxed out (i.e. open full time), it flows dramatically less than when it opens and closes normally.
The reason that you can push 400 hp from the stock 24 lb injectors is that you are eating into the safety margin the factory built in.
So, people spend thousands on heads and cam and all the other mods, but hesitate to spend $219 for injectors (FMS 30# @ Summit)? Sounds like fools gold to me.
Administrator
Joined: Nov 1998
Posts: 71,094
From: Hell was full so they sent me to NJ
Originally posted by Mikey97Z
Can you please explain how it's harder on the injectors to run at high duty cycles? I don't understand why. It seems to me that the duty cycle percentage isn't how hard the injector is working, but how much of the 20ms window to spray for. So 100% means spray for 20ms. Doesn't seem like it would damage the injector. What would cause the flow and spray patterns to become unstable when the injector sprays for longer time? (i.e. spray for 10ms is stable but 20ms is unstable ) What am I missing here?
Can you please explain how it's harder on the injectors to run at high duty cycles? I don't understand why. It seems to me that the duty cycle percentage isn't how hard the injector is working, but how much of the 20ms window to spray for. So 100% means spray for 20ms. Doesn't seem like it would damage the injector. What would cause the flow and spray patterns to become unstable when the injector sprays for longer time? (i.e. spray for 10ms is stable but 20ms is unstable
) What am I missing here?
OPEN - SPRAY - CLOSE - OPEN - SPRAY - CLOSE
You're jerking that pintle (or disc or ball) up and down like a yo-yo. Small as it is, it still takes power to open it... .the faster it moves and the less time it rests, the harder it is to control the motion. As explained in the quote above, try to move that injector too fast, and the motion becomes unstable... it never fully closes, and worse, never fully opens.... it "flutters". The tests demonstrate the impacts of this instability on flow quantity and spray pattern.
Low impedance injectors help solve some of the problem... they use the huge inrush of current (PEAK) to snap the injector open faster, then reduce the current to the lower level required to keep the injector open (HOLD).
The issue of opening and closing the injector is what can produce the "downside" with oversized injectors. If the pulse width required for idle is too low, it can become less than the time required to open + close the injector, and the same thing happens... fuel delivery quantity becomes unstable, because thre is really no "open" time in the cylce. Combine the peak-and-hold low impedance injector with a high resolution, high quality driver circuit in the computer, and you can overcome some of these problems, and idle comfortably at 800rpm with 78# injectors. But it is harder to do with a stock PCM and its drivers, and a high impedance injector.
Registered User
Joined: Jul 2000
Posts: 1,428
From: Woodstock,Georgia
When I had hotcam/ported stock heads my DC was 95% with stockers at WOT. I really dont think a DC over 85% is a huge issue unless your road coarse racing or running a power adder as long as its under 100%.
Registered User
Joined: Feb 2002
Posts: 5,645
From: Bridgewater, MA
If I run 54-56psi, is there a conversion I can use to see what the "similated" flowrate for the injector would be compared to stock fuel pressure. Example, 24lb/hr is at 45psi stock right. If I upped the psi to 56psi, then is it like running 30lb/.hr on 45psi? Is there a conversion so I can tweek my fuel pressure?
Administrator
Joined: Nov 1998
Posts: 71,094
From: Hell was full so they sent me to NJ
The stock LT1 24# injector flows 24.9#/HR at 43.5psi (= 3 bar).
To calculate the new flow rate (there is nothing "simulated" about it),
[( Pnew / 43.5 ) ^ 0.5] X 24.9 = #/HRnew
In other words, divide the new operating pressure by the rated operating pressure, then take the square root of the result, and multiply it by the rated flow.
Example:
[( 56 / 43.5 ) ^ 0.5] X 24.9 =
[1.287 ^ 0.5] X 24.9 =
1.135 X 24.9 = 28.3 #/HR
If you are using a Bosch/Ford/SVO/etc pintle style injector, many of them (but not all) are rated at 39.15psi (= 2.7 bar), so you would have to replace 43.5 with 39.15 in the equation above. The general form of the equation is:
[( Pnew / Prated) ^ 0.5] X INJrated = INJnew
To calculate the new flow rate (there is nothing "simulated" about it),
[( Pnew / 43.5 ) ^ 0.5] X 24.9 = #/HRnew
In other words, divide the new operating pressure by the rated operating pressure, then take the square root of the result, and multiply it by the rated flow.
Example:
[( 56 / 43.5 ) ^ 0.5] X 24.9 =
[1.287 ^ 0.5] X 24.9 =
1.135 X 24.9 = 28.3 #/HR
If you are using a Bosch/Ford/SVO/etc pintle style injector, many of them (but not all) are rated at 39.15psi (= 2.7 bar), so you would have to replace 43.5 with 39.15 in the equation above. The general form of the equation is:
[( Pnew / Prated) ^ 0.5] X INJrated = INJnew