Injector Flow Mathematics

[undefined0000]

Alright, so I know the basics of the fuel injection system, but it is unclear to me how exactly fuel pressure plays a role in it, I'll just give my situation.

I have some vac leaks, and to help the car run better, I turned up the fuel pressure to a whopping 51psi. Anything near the stock pressure and the car runs like poopoo. So Heres the question.

The injector flow rate,for example, on my auto is 390cc, is with the suggested fuel pressure (43.5), correct?

Does this essentially increase the "cc" with pressure?

So at 51psi, am I running like 410cc?

If so, is there any basic math that can be used to determine flow rate based on PSI?

Math tells me that with the 43.5 psi on 390, it is 9cc -> 1psi

So 51psi -> 459cc?

Theoretically it works, but only in a linear application, since we're dealing with fluids, I would feel that its more of a curved pattern.

Please tell me if I'm crazy, I'm just trying to get some numbers instead of just guessing. Wideband is in the mail, and having these formulas down will help me a ton.

Also, is it too harsh on the injectors to run a higher PSI or not until its overboard pressure.

 

[PrimerPLus]

1G MT = 37psi
1G A/T = 43.5psi
1G NT = 48psi
All 2G turbo cars are 43.5psi

This is for stock. Most aftermarket fuel injectors are flow tested at 43.5psi I believe.
I would not turn up your fuel pressure. You need to fix your vacuum leaks.

 

[steve]

If so, is there any basic math that can be used to determine flow rate based on PSI?

I'm sure the formula is in several of my old posts, which I learned from RC Engineering RC Fuel Injection

New_Flow_Rate = SQRT (New_pressure / Old_Pressure) * Old_Flow_Rate

SQRT(51 psi / 43.5 psi) * 390 cc = 422 cc effective flow rate.

 

[1990AWD]

I have some vac leaks, and to help the car run better, I turned up the fuel pressure to a whopping 51psi. Anything near the stock pressure and the car runs like poopoo. So Heres the question.

Why not just fix the leaks?

 

[awdtalon4451]

Why not just fix the leaks?

Word!! Not trying to be a prick or anything but stop being lazy bro. Your just band aiding the problem and putting unnecessary strain where its not needed.

 

[undefined0000]

Because I am having a VERY hard time finding the leaks, I know a decent sized one is the TB shaft seals, so I need to drop like $100 for a new tb and whatever else is leaking after I fix that, rent and all the good stuff is due soon, and I still need to drive to work.

 

[awdtalon4451]

You can get the whole gasket rebuild kit offline for like $20. They have tutorials that are step by step on how to rebuild them yourself.

 

[miguelmcv]

Why spending that much of money on a new or rebuild throttle body?.
Just go to the hardware store, get the O rings and replace them, All what you need is 2 O rings and a can of carburator cleaner to clean the throttle body once is apart, the gaskets are like $2 ish.

The process is very simple and we are all here to help, there are a lot of videos and photos to guide you on all the process.

I dought you will spend more then $25 total.

 

[My1GLaser]

Throttle Body O-Ring

TB Shaft Seals VFAQ

I've done 4 TBs. It will take you about 1.5 hrs the first time. I've only ever used the seal in the second link. NAPA can order them and I have also gotten them from kamandirect.com. I've never used the orings...

vfaq.com is your friend!

 

[Groundation]

If you are going to rebuild it, I wouldn't use o-rings, get the proper seals. I used the ones from chicago rawhide or something that I found on a writeup here.

I would also get new screws for the ones that you have to remove that are holding the throttle plate in. make sure to peen them after you reinstall the new screws. I got new screws at Home Depot.

Don't forget 2 new gaskets, one for the tb elbow side, one for the intake manifold side

 

[undefined0000]

I get the basic idea of changing the seals, I took apart my old mangled 1g TB completely and I see what I'll have to change out, I just wanted something shinier. Thanks for the links though! I guess I will just do a rebuild for now. I'll post it in my journal next week when I get some more money. I'm hoping to the god that is my only leak because I've replaced everything else.

 

[ThunderChild]

you could also contact snowborder714 on here and get a complete rebuild kit from him with seals, hardware etc. my kit with mil.spec seals was very reasonable. there are 3 types of seals he offers, just pm him and hell set you up.

and yes, fixing the real problem is the best idea. far better than covering up the problem by cranking fuel pressure.

 

[undefined0000]

Thanks for that! I'm bandaiding the problem for a reason. Poor .

 

[undefined0000]

Back with more!

Alright, so I plan to get link around tax time and as I get into actual tuning, I'd like to understand what's going on. I'm starting understanding injector duty cycle and AFRs and stuff.

I had a question about our MAFs. What does the computer see in the end.

Does it see inHg/PSI or lbs/min?

What does that turn into at the ecu pin?

eg. 1psi = 1 volt or some kind of resistance.

With the stock fuel mappings, how does the ecu calculate how long for the injector to be open

so

going to lbs/min just for example

4lbs/min = (4)/somenumber to the power of anotherone something something = 20ms.

so in my case, I'll use idle

My idle is about 900rpms, which is actually 450 for one cycle.

Just a guess here, but my idle intake pressure is about 14inHg which is ?lbs/min?

injectors are 390s at 43.5psi during WOT, about 25 at idle, so they're flowing (296cc at idle?) excuse my math, been a long day and this has just kinda been on my mind.

Mind you, my ecu is expecting 450cc injectors, because the previous owner of the car was a derp, however, it should be able to adjust the trims. Its doing a decent job for me having a massive leak at the TB 15.5 on the wideband.

I'd just like to know exactly what the computer is thinking in this situation. I'm a software developer, so I like to know what computers are thinking.

I hope this makes some kind of sense, I am just trying to educate myself. Thanks!

 

[WES_393]

Our MAF is known as a Karman Vortex sensor. It disrupts the airflow to create oscillating waves of pressure (much like sound waves) which are measured in frequency, specifically Hertz (Hz). As airflow increases so does the frequency of the waves. So the ECU uses the Hz signal to help determine how much fuel needs to be delivered. As for how exactly the ECU receives or uses the signal, I can't help you there. But I can tell you that it's not really necessary info. As long as the MAF reading is accurate, the ECU knows exactly what to do with it. How to calibrate the MAF using a wideband can be found here.

As for fuel, you have 2 different "modes" of fuel control/delivery: Open loop and closed loop. Closed loop is typically during idle/cruise. The ECU checks it's air/fuel calculations against the front O2 sensor. The front O2 gives off a .2-.8v signal with .5 being stoich (14.7). As the voltage falls above or below .5 the ECU will constantly adjust (and overshoot) the amount of fuel in order to reach 14.7:1 A/F ratio. Open loop is when the ECU isn't using any feedback to check it's calculations. It uses the airflow reading (Hz) along with the programmed fuel tables that are load and RPM based. The ECU simply assumes these readings are correct, which is why boost leaks, fuel pressure, injector size, etc are a big issue on our cars. You can read more about it here.

Now, when it comes to injectors, the ECU is programmed for your stock injector size and fuel pressure. It can control fuel delivery by increasing or decreasing the pulse width/frequency of the injectors. Since Link uses the stock ECU it takes advantage of it's fuel control abilities, which is shown as dead time and global adjustment percentage. This is how you are able to run 1000cc injectors without the ECU dumping twice as much fuel, yet still being able to deliver twice as much fuel. More info found here.

So maybe somebody can explain the exact hardware/software for your interests, but otherwise that's the basics behind fuel control. Just understanding those concepts will give a much better grasp on tuning since you'll mostly be editing fuel/timing tables, adjusting injector settings, and dialing in MAF frequency.

 

[Diamond Star Werks]

Back with more!

Alright, so I plan to get link around tax time and as I get into actual tuning, I'd like to understand what's going on. I'm starting understanding injector duty cycle and AFRs and stuff.

I had a question about our MAFs. What does the computer see in the end.

Does it see inHg/PSI or lbs/min?

What does that turn into at the ecu pin?

eg. 1psi = 1 volt or some kind of resistance.

With the stock fuel mappings, how does the ecu calculate how long for the injector to be open

so

going to lbs/min just for example

4lbs/min = (4)/somenumber to the power of anotherone something something = 20ms.

so in my case, I'll use idle

My idle is about 900rpms, which is actually 450 for one cycle.

Just a guess here, but my idle intake pressure is about 14inHg which is ?lbs/min?

injectors are 390s at 43.5psi during WOT, about 25 at idle, so they're flowing (296cc at idle?) excuse my math, been a long day and this has just kinda been on my mind.

Mind you, my ecu is expecting 450cc injectors, because the previous owner of the car was a derp, however, it should be able to adjust the trims. Its doing a decent job for me having a massive leak at the TB 15.5 on the wideband.

I'd just like to know exactly what the computer is thinking in this situation. I'm a software developer, so I like to know what computers are thinking.

I hope this makes some kind of sense, I am just trying to educate myself. Thanks!

Lbs/min is way to large of a number for the ecu to calc with and it wouldn't be accurate enough. Like using metric instead of standard. Metric is more precise.

There is no way it would be accurate using pressure to determine airflow on a dsm. Other turbo cars use pressure ratings for fuel calculations. Most turbo chryslers do. Srt4, lebaron turbo, glh, glhs, and others. Non-turbo vehicles also use map sensors for fuel calcs but dsm's use them only for egr purposes.

Ecu reads airflow in grams/sec. At idle you will only see 4/gsec +-1/gsec. Usually between 4/gsec and 5/gsec.

There are 453.592 grams in one pound.

4/grams/second is 240 grams per minute. 240÷453.592=.529. So at idle your seeing .529lbs/min.

You're in no way seeing 296cc's of fuel at idle. 296÷390=.75897=75.9% of your fuel capacity. At idle your more around 10%-20%. Around 58cc.

 

[steve]

Get yourself a copy of the Laser/Talon 1990 Technical Information Manual and read section 14 over and over. Google should find a copy if this doesn't work.
Chrysler Laser Talon 1990 Technical Information Manual.pdf download - 2shared

 

[undefined0000]

Thanks for all of the replies and helping me understand! I'm really interested in the sciences of how all of this works. Thanks for the clear explanation of open loop vs closed loop fuel calculations, explains why my idle afrs are better than my slight acceleration ones. So with that in mind. Does the ECU take into account what the afrs are during cruise and such. My average cruise is about 15.5. I have the TB shaft seals leaky, but I would think that the computer could have a little more correction, but at the same time, it is expecting bigger injectors. I'll see what happens when I put the 450cc in and new shaft seals. Getting a wideband really opened up a lot of thought for me .

Steve, I really do need to just grab a book . I keep putting it off, but I'll grab one this weds.