How to calculate flow rate with higher fuel pressure?

[Samckitt]

I was thinking about getting some LSx injectors for my LT1 that flow more fuel, but the ones I was looking at say they flow 28lbs/hr. Factory LT1 is 24lbs/hr. They were fairley cheap & I almost bought them. Then I got to looking at all the specs on flow rate on the LSx injectors & it is 28lbs at 58psi. The LT1 injectors from 24lbs at 43.5 psi. How can I calculate the LT1 injectors flow rate at 58 psi? How much pressure will the factory LT1 injectors handle?

[camcojb]

I was thinking about getting some LSx injectors for my LT1 that flow more fuel, but the ones I was looking at say they flow 28lbs/hr. Factory LT1 is 24lbs/hr. They were fairley cheap & I almost bought them. Then I got to looking at all the specs on flow rate on the LSx injectors & it is 28lbs at 58psi. The LT1 injectors from 24lbs at 43.5 psi. How can I calculate the LT1 injectors flow rate at 58 psi? How much pressure will the factory LT1 injectors handle?

http://www.rceng.com/technical.aspx

you can start plugging numbers into this calculator. A 24# injector at 43.5 is virtually identical in flow to a 28# @ 58 psi injector, so you'll gain nothing with this swap.

The proper way to pick an injector is by horsepower of your engine and fuel psi, bsfc, and duty cycle. How much power is your engine making and are there any plans for mods in the future?

Jody

[68Formula]

http://www.rceng.com/technical.aspx

you can start plugging numbers into this calculator. A 24# injector at 43.5 is virtually identical in flow to a 28# @ 58 psi injector, so you'll gain nothing with this swap.

The proper way to pick an injector is by horsepower of your engine and fuel psi, bsfc, and duty cycle. How much power is your engine making and are there any plans for mods in the future?

Jody

I woudn't call 15% nothing. It's enough to support another 40hp. Plus the OEMs usually calibrate quite rich at WOT. BThe above formula is a good estimate to start with, but the best thing is to get a wide band O2 sensor and really look at the numbers.

[camcojb]

I woudn't call 15% nothing. It's enough to support another 40hp. Plus the OEMs usually calibrate quite rich at WOT. BThe above formula is a good estimate to start with, but the best thing is to get a wide band O2 sensor and really look at the numbers.

what 15%? You have to watch out for the psi they're rated at. The 24's he mentioned are 24# at 43.5 psi of fuel pressure. The 28's are 28# at 58 psi. At equal pressures of 43.5 or 58 psi the two different injectors will flow basically the same. He gains nothing as far as fuel flow with the "larger" injectors, as they're rated at a higher pressure. The 24's at the same pressure as the 28's were rated at would be virtually the same flow.

A wideband is great to dial in the tuning but won't help at all to pick an injector to support the power level expected. You have to buy something to stick in the engine to start tuning, so the above calculator will tell you what you need if you know how much power you'll make, the bsfc of the engine, fuel pressure, and how high of a duty cycle you're willing to live with.

Jody

[68Formula]

what 15%? You have to watch out for the psi they're rated at. The 24's he mentioned are 24# at 43.5 psi of fuel pressure. The 28's are 28# at 58 psi. At equal pressures of 43.5 or 58 psi the two different injectors will flow basically the same. He gains nothing as far as fuel flow with the "larger" injectors, as they're rated at a higher pressure. The 24's at the same pressure as the 28's were rated at would be virtually the same flow.

A wideband is great to dial in the tuning but won't help at all to pick an injector to support the power level expected. You have to buy something to stick in the engine to start tuning, so the above calculator will tell you what you need if you know how much power you'll make, the bsfc of the engine, fuel pressure, and how high of a duty cycle you're willing to live with.

Jody

First, if you look, he is not actually swapping the injectors. He is asking how much the LT1s 24lbs are at 58psi vs. the rating at 43.5 psi. In this case the difference between 43.5psi and 58psi is about 15%.

Second I didn't disagree with using the formula to start with, but that it is just to rough in the injector sizing and not a replacement for tuning. That formula for sizing requires you've already run the engine on a dyno. So it is still a rough guess.

[Samckitt]

Screw buying the 28# injectors then. I'll just crank up the pressure. I already have an adjustable regulator. Motor has a CC306 cam, 11:1 compression, long tube headers & very free flowing exhaust, cold air intake & 58 mm throttle body. So I'll just hold off buying injectors until I really need them, maybe after going to a 383?

[68Formula]

The LT1 injectors from 24lbs at 43.5 psi. How can I calculate the LT1 injectors flow rate at 58 psi? How much pressure will the factory LT1 injectors handle?

It just so happens the rating for 24lbs at the 58psi injector level are the same as the LSXs he was comparing.

[camcojb]

It just so happens the rating for 24lbs at the 58psi injector level are the same as the LSXs he was comparing.

that was the point I was trying to make; no gain in fuel flow for the larger injectors as they're only larger at a higher psi.

Every engine builder I know uses a similar calculation for injector size as that chart. I'm not sure where you're going on injector size and tuning, they are two completely different things. I can tune a 30# injector and then put a 38# injector on the same engine, re-tune it, and get identical a/f's throughout the rpm/load ranges. But if he's got 480 hp or so the 30's will be at 100% duty cycle with 43 psi of fuel pressure and a bsfc of .5 whereas the 38's will be near 80% duty cycle and completely safe.

So unless you want to buy more than one set of injectors you need that chart or similar to have a starting place. Yes, you need to tune and datalog (or watch) the duty cycle, fuel pressure, and a/f to make sure everythings good.

How will a wideband tell you if the injector size is correct if you don't know the fp, duty cycle, etc? It will if the injectors way small and no matter what pulsewidth or ? you use in the fuel mapping it goes lean, but otherwise the wideband is no help at all in regards to injector sizing. Knowing what power you should be at and monitoring the duty cycle and fp will tell you if the injectors are correct or not. I'm not against widebands at all, have one in every car I build plus a handheld unit to swap in friends cars, but I've never used it to tell me if the injectors are correct.

Jody

[68Formula]

Screw buying the 28# injectors then. I'll just crank up the pressure. I already have an adjustable regulator. Motor has a CC306 cam, 11:1 compression, long tube headers & very free flowing exhaust, cold air intake & 58 mm throttle body. So I'll just hold off buying injectors until I really need them, maybe after going to a 383?

Should be fine running them up to 60psi. But I wouldn't try more than that. Could end up with an idle stability problem.

[68Formula]

that was the point I was trying to make; no gain in fuel flow for the larger injectors as they're only larger at a higher psi.

Every engine builder I know uses a similar calculation for injector size as that chart. I'm not sure where you're going on injector size and tuning, they are two completely different things. I can tune a 30# injector and then put a 38# injector on the same engine, re-tune it, and get identical a/f's throughout the rpm/load ranges. But if he's got 480 hp or so the 30's will be at 100% duty cycle with 43 psi of fuel pressure and a bsfc of .5 whereas the 38's will be near 80% duty cycle and completely safe.

So unless you want to buy more than one set of injectors you need that chart or similar to have a starting place. Yes, you need to tune and datalog (or watch) the duty cycle, fuel pressure, and a/f to make sure everythings good.

How will a wideband tell you if the injector size is correct if you don't know the fp, duty cycle, etc? It will if the injectors way small and no matter what pulsewidth or ? you use in the fuel mapping it goes lean, but otherwise the wideband is no help at all in regards to injector sizing. Knowing what power you should be at and monitoring the duty cycle and fp will tell you if the injectors are correct or not. I'm not against widebands at all, have one in every car I build plus a handheld unit to swap in friends cars, but I've never used it to tell me if the injectors are correct.

Jody

Yes you need to know what the duty cycle is, but without either a fuel consumption curve or a wide-band how would you know if the car is lean at various load/speed points? Unless you already have a wide-band feedback the engine is going to be in open loop so it will not command longer injector periods. You need to know both the air/fuel and duty cycle to tell if what you calculated for an injector is sufficient. That is my point.

I think we are arguing for no actual reason. We aren't really disagreeing so much as making statements from different angles. How about a truce? :box:

I think between the two of us, he has the info. he needs now.

[camcojb]

Yes you need to know what the duty cycle is, but without either a fuel consumption curve or a wide-band how would you know if the car is lean at various load/speed points? Unless you already have a wide-band feedback the engine is going to be in open loop so it will not command longer injector periods. You need to know both the air/fuel and duty cycle to tell if what you calculated for an injector is sufficient. That is my point.

I think we are arguing for no actual reason. We aren't really disagreeing so much as making statements from different angles. How about a truce? :box:

I think between the two of us, he has the info. he needs now.

sounds good to me. I just thought you were saying that you needed a wideband to tell you if the injector is correctly sized, and I couldn't get my brain around that. :hammer:

Jody

[68Formula]

I should make one more comment in regards to your fuel pump. While increasing the pressure will increase injector flow rate, it will decrease potential pump capacity by the same amount. This means if you raise the pressure and horsepower of the engine both will increase the demand on the pump. Usually there is quite a lot of extra capacity built in, so it's not a problem.

One way to tell you are short on pump is to watch the fuel pressure while monitoring duty cycle and air/fuel ratio. If increase the duty cycle does not enrichen the mixture, and you see a fuel pressure drop at higher rpms, you are running out of pump flow.

One of the magazines had this problem. They installed a combo package (that did not include a pump) to increase power. Even though the package added higher flow injectors that would have been more than adequate for the power increase, the stock pump was not up to the task. They saw the injector duty cycle go to 100% trying to enrich the mixture, but were not watching the fuel pressure, so they incorrectly assumed the injectors were not big enough. New even larger injectors of course did not help. Then they finally changed the pump for one with more flow at stock pressure and problem solved. What peeved me is they never actually explained the situation so some readers may have the false impression that the injectors were still necessary.

[camcojb]

I should make one more comment in regards to your fuel pump. While increasing the pressure will increase injector flow rate, it will decrease potential pump capacity by the same amount. This means if you raise the pressure and horsepower of the engine both will increase the demand on the pump. Usually there is quite a lot of extra capacity built in, so it's not a problem.

One way to tell you are short on pump is to watch the fuel pressure while monitoring duty cycle and air/fuel ratio. If increase the duty cycle does not enrichen the mixture, and you see a fuel pressure drop at higher rpms, you are running out of pump flow.

One of the magazines had this problem. They installed a combo package (that did not include a pump) to increase power. Even though the package added higher flow injectors that would have been more than adequate for the power increase, the stock pump was not up to the task. They saw the injector duty cycle go to 100% trying to enrich the mixture, but were not watching the fuel pressure, so they incorrectly assumed the injectors were not big enough. New even larger injectors of course did not help. Then they finally changed the pump for one with more flow at stock pressure and problem solved. What peeved me is they never actually explained the situation so some readers may have the false impression that the injectors were still necessary.

absolutely. You have to monitor duty cycle and fuel pressure for the exact reason above. Good info.

Jody