Calculating Air/ Fuel
01-05-2019, 11:51 PM
#1
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Car: 1982 Trans Am
Engine: LU5 CFI
Transmission: TH-200C
Calculating Air/ Fuel
So I am trying to find out the appropriate CFM capacity on this engine I would like to build. How do I go about setting the proper amount of fuel pressure, injector size, and idk if duty cycle is able to be change but it not wouldn't be a bad thing to know. So for 300Hp and wanted a gain of another 100+ I was thinking around 700CFM respective to my plenum. Dual throttle bodies and am going to run a parallel fuel system. I need to upgrade the fuel pump as well but how much is too much pressure? "Not really a question as much as a statement corresponding to my lack there of knowledge."
01-06-2019, 01:00 AM
#2
Re: Calculating Air/ Fuel
3 to 400 isn't that much, so don't feel you need to go nuts with eccentric systems. Many have gone before and worked out what works best. I'd start by discussing your goals with whichever EFI system you're using, that's assuming it doesn't already come with the proper injectors.
You'll pick the injectors size based on the power goals, same as fuel pump.
The duty cycle is just the percentage of time the injector is open and closed during one, 2 rotation cycle of the engine. During low rpm cruising, you'll have a low duty cycle, maybe 10% as it's closed more than open....as RPMs increase, the percentage goes up as the ECM commands the injectors to remain open longer to add fuel. At wide open throttle, you don't want more than 80% duty cycle. If you choose the right injector, and the engine is tuned properly, you'll be fine. This isn't something you program in, it's a diagnostic value you or your tuner will read after datalogging.
Fuel pressure is a fine tuning tool. As long as you're in the ballpark when you start tuning it's not something you need to worry about right away. You DO need to know what it is though, as you'll have to program it into your base tune. The way the pressure affects the fuel atomization, along with how the air flows inside the manifold can have minute effects on how the engine runs, but a tad higher or lower won't matter getting the car running, and it won't matter when choosing your injectors.
I've purchased my last few sets of injectors from our board sponsor SouthBay Fuel Injectors. Give them a call, let 'em know your set up and power goals and they'll get your squared away.
You'll pick the injectors size based on the power goals, same as fuel pump.
The duty cycle is just the percentage of time the injector is open and closed during one, 2 rotation cycle of the engine. During low rpm cruising, you'll have a low duty cycle, maybe 10% as it's closed more than open....as RPMs increase, the percentage goes up as the ECM commands the injectors to remain open longer to add fuel. At wide open throttle, you don't want more than 80% duty cycle. If you choose the right injector, and the engine is tuned properly, you'll be fine. This isn't something you program in, it's a diagnostic value you or your tuner will read after datalogging.
Fuel pressure is a fine tuning tool. As long as you're in the ballpark when you start tuning it's not something you need to worry about right away. You DO need to know what it is though, as you'll have to program it into your base tune. The way the pressure affects the fuel atomization, along with how the air flows inside the manifold can have minute effects on how the engine runs, but a tad higher or lower won't matter getting the car running, and it won't matter when choosing your injectors.
I've purchased my last few sets of injectors from our board sponsor SouthBay Fuel Injectors. Give them a call, let 'em know your set up and power goals and they'll get your squared away.
01-06-2019, 01:04 AM
#3
Re: Calculating Air/ Fuel
Here's a calculator to play with. ...they have a little blurb at the bottom talking about not running over a 90% duty cycle. I've heard 80%. It's one of those things. It's not set in stone.
https://www.deatschwerks.com/fuel-ca...tor-calculator
https://www.deatschwerks.com/fuel-ca...tor-calculator
01-06-2019, 12:20 PM
#4
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Re: Calculating Air/ Fuel
3 to 400 isn't that much, so don't feel you need to go nuts with eccentric systems. Many have gone before and worked out what works best. I'd start by discussing your goals with whichever EFI system you're using, that's assuming it doesn't already come with the proper injectors.
You'll pick the injectors size based on the power goals, same as fuel pump.
The duty cycle is just the percentage of time the injector is open and closed during one, 2 rotation cycle of the engine. During low rpm cruising, you'll have a low duty cycle, maybe 10% as it's closed more than open....as RPMs increase, the percentage goes up as the ECM commands the injectors to remain open longer to add fuel. At wide open throttle, you don't want more than 80% duty cycle. If you choose the right injector, and the engine is tuned properly, you'll be fine. This isn't something you program in, it's a diagnostic value you or your tuner will read after datalogging.
Fuel pressure is a fine tuning tool. As long as you're in the ballpark when you start tuning it's not something you need to worry about right away. You DO need to know what it is though, as you'll have to program it into your base tune. The way the pressure affects the fuel atomization, along with how the air flows inside the manifold can have minute effects on how the engine runs, but a tad higher or lower won't matter getting the car running, and it won't matter when choosing your injectors.
I've purchased my last few sets of injectors from our board sponsor SouthBay Fuel Injectors. Give them a call, let 'em know your set up and power goals and they'll get your squared away.
You'll pick the injectors size based on the power goals, same as fuel pump.
The duty cycle is just the percentage of time the injector is open and closed during one, 2 rotation cycle of the engine. During low rpm cruising, you'll have a low duty cycle, maybe 10% as it's closed more than open....as RPMs increase, the percentage goes up as the ECM commands the injectors to remain open longer to add fuel. At wide open throttle, you don't want more than 80% duty cycle. If you choose the right injector, and the engine is tuned properly, you'll be fine. This isn't something you program in, it's a diagnostic value you or your tuner will read after datalogging.
Fuel pressure is a fine tuning tool. As long as you're in the ballpark when you start tuning it's not something you need to worry about right away. You DO need to know what it is though, as you'll have to program it into your base tune. The way the pressure affects the fuel atomization, along with how the air flows inside the manifold can have minute effects on how the engine runs, but a tad higher or lower won't matter getting the car running, and it won't matter when choosing your injectors.
I've purchased my last few sets of injectors from our board sponsor SouthBay Fuel Injectors. Give them a call, let 'em know your set up and power goals and they'll get your squared away.
01-06-2019, 02:44 PM
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Car: 1982 Trans Am
Engine: LU5 CFI
Transmission: TH-200C
Re: Calculating Air/ Fuel
Awesome thank you, well to start I am doing an engine swap on my CFI. Only thing is im keeping the CFI set up and putting it onto an LT1. I already know this isnt just an easy let do it in a day type of swap. Increase of the cylinder size and intake along with a better exhaust flow is already too much for the stock sytem. So what I gathered so far is the stock throttle bodies on the 94 LT1 have a cfm rating close to 750. The stock throttle bodies on the CFI are close to 500 i believe. The plenum is a huge restriction so porting will help, but im not sure how much good it will do unless i just do an upgrade on it. The current fuel pressure is between 12-13 psi and is said to have a lot of difficulty keeping up with fuel delivery as it is. To add the throttle bodies run in a series instead of parallel so to account for the difference from factory the one injector is different from the other. Still working out the kinks as i go along should have figured i would have to describe my build im trying to do so please be patient lol. So far this is what I gathered.
01-07-2019, 08:29 AM
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Car: 1982 Trans Am
Engine: LU5 CFI
Transmission: TH-200C
Re: Calculating Air/ Fuel
I actually have, the thread has awesome information in there but does not instruct me on how to properly set it all up. I guess what im asking for is the formula for the equation to what is said above.
Last edited by 82crossfire305; 01-07-2019 at 08:41 AM.
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