I have an aftermarket TPI system on my 383. When I ordered the system they asked for all the engine and car specs. H.P., compression, bore, stroke etc. etc. The system came with 24lb. injectors and seems to run great, good fuel mileage and all. (23 MPG). It will easily smoke the tires. My question is--I have seem charts for calculating injector size and they all say I should have 30 lb. injectors for that motor! What differences should I expect from changing? I suppose I would see a power increase(not that I need it) but what about mileage? More or less? :confused2: Art
Fuel injector sizing
- Thread starter AJ77
- Start date
mrvette
Phantom of the Opera
24 maybe a shade on the shy side for absolute WOT power, but you would loose something with much bigger, IMO....that is most computers have a trouble firing with too narrow a pulse width, the loarger injector will not respond properly, about barely open before it's pulse is gone....that will hurt spray patterns and economy, you have roughly 10% more engine, so you need 10% more injector over a 350, which is used for 24 lbs.... 305's came with just 19 lbs.....went to 21 lbs fairly quickly....
what you maybe try to do is if you on a dyno, bump up the pressure a tad....
get more flow, but I hear anything over 48? lbs messes up the injector function as it's not able to turn off nice and quick.....
what you maybe try to do is if you on a dyno, bump up the pressure a tad....
get more flow, but I hear anything over 48? lbs messes up the injector function as it's not able to turn off nice and quick.....
I actually did just raise the pressure from 40 to 44 and noticed a measureable increase in mileage last week. In the spring when we went to Hot Springs we got 18 MPG with the lower pressure and last week we drove to Eureka Sorings and averaged 22 up and back. In the fuel economy run we got 23.6. Not too bad I think.
Bullshark
Well-known member
Changing fuel injectors can be risky especially if you don't have the capability to recalibrate the fuel map. Increasing the injector size ( or fuel pressure for that matter) needs to be accompanied by the appropriate amount of increased air flow to keep optimum A/F ratio. A few things to consider:
1) Best all around performance (i.e power across the rpm/load map) will require different calibration numbers than those needed for best fuel economy. The don't go hand in hand. That is why Accel, Fast, and other after market EFI systems provide for two or more selectable fuel maps. Raising fuel pressure should provide more fuel for a given pulse width. In your case, (I'm guessing here) you raised your fuel pressure from 40 to 44 psi. probably resulted in a better more concise fuel delivery pulse. 40 psi is on the low side. Injectors like 43 - 48 depending on the injector and engine load.
2 Gene is correct about the larger saturated type injectors. Because of their higher impedance, they have a harder time turning off and on precisely, especially at high rpm. Low impedance (peak and hold) injectors require a larger drive current and can operate much faster and more precise in all conditions. Although more expensive, they are a much better injector if your ECU can handle the drive current and BTW you can get away with running larger injectors than you need without the penalty that the saturated injectors have.
3 The pintle type injectors that provide a cone spray are much preferred over the older type that spray multiple streams like a shower head. They atomize the fuel better.
Got to go, just some thoughts for your consideration
Bullshark
1) Best all around performance (i.e power across the rpm/load map) will require different calibration numbers than those needed for best fuel economy. The don't go hand in hand. That is why Accel, Fast, and other after market EFI systems provide for two or more selectable fuel maps. Raising fuel pressure should provide more fuel for a given pulse width. In your case, (I'm guessing here) you raised your fuel pressure from 40 to 44 psi. probably resulted in a better more concise fuel delivery pulse. 40 psi is on the low side. Injectors like 43 - 48 depending on the injector and engine load.
2 Gene is correct about the larger saturated type injectors. Because of their higher impedance, they have a harder time turning off and on precisely, especially at high rpm. Low impedance (peak and hold) injectors require a larger drive current and can operate much faster and more precise in all conditions. Although more expensive, they are a much better injector if your ECU can handle the drive current and BTW you can get away with running larger injectors than you need without the penalty that the saturated injectors have.
3 The pintle type injectors that provide a cone spray are much preferred over the older type that spray multiple streams like a shower head. They atomize the fuel better.
Got to go, just some thoughts for your consideration
Bullshark
Z-man
Active member
How much HP do you want to make?
Here's a link to the formula for fuel injection sizing. To use it, pick the HP you want to have (be reasonable but shoot a little high - like 20%). Size the injectors to get that HP. If you don't reach that HP when you put them in your engine, it won't be the fault of the injectors - but at least they'll be big enough. Then you can focus on changing other things on the engine...
If you pick way too large of injector size, your idle might suffer because the injectors can't run that slowly. If you go too small, they will max out and you'll never reach full power.
Link to Selecting Fuel Injectors
Here's a link to the formula for fuel injection sizing. To use it, pick the HP you want to have (be reasonable but shoot a little high - like 20%). Size the injectors to get that HP. If you don't reach that HP when you put them in your engine, it won't be the fault of the injectors - but at least they'll be big enough. Then you can focus on changing other things on the engine...
If you pick way too large of injector size, your idle might suffer because the injectors can't run that slowly. If you go too small, they will max out and you'll never reach full power.
Link to Selecting Fuel Injectors
Jsup
Well-known member
- Joined
- Oct 31, 2008
- Messages
- 812
I'm with your sizing calculators. Based on your compression, heads, intake, and exhaust I'd go 30lbs
You can tune it at idle, after idle there really isn't a problem.
There's two kinds of injectors basically, Pintle and Disc. I think I'm safe saying all DISC injectors are made by Lucas and OEMed by many people. Included are Accel (new style), TFS, Holley, Racetronix, more all come off the same line and have different stamps.
I have had very good success with www.fuelinjectorconnection.com as for Jon, he'll work with you if you need help.. He'll give you information, and not push you to buy anything.
FWIW, the Accel Super Ram intake is packaged with 30LB injectors.
You can tune it at idle, after idle there really isn't a problem.
There's two kinds of injectors basically, Pintle and Disc. I think I'm safe saying all DISC injectors are made by Lucas and OEMed by many people. Included are Accel (new style), TFS, Holley, Racetronix, more all come off the same line and have different stamps.
I have had very good success with www.fuelinjectorconnection.com as for Jon, he'll work with you if you need help.. He'll give you information, and not push you to buy anything.
FWIW, the Accel Super Ram intake is packaged with 30LB injectors.