Let's talk fuel injectors!

[yellowstang1994]

Ok guys, something to me is not making sense. Say I upgrade to 24lb/hr injectors and I get the whole mass air meter to calibrate it for the upgrade. Now what I want to know is: The stock fuel pressure is I think around 30lbs give or take with the tube off the regulator. I'm not sure if this is correct when I say this, but... from switching 19lbs to 24lbs there will be more fuel pressure being "allowed" to inject, from the bigger increase. Would I need to back off the fuel pressure from the regulator to compensate from the bigger increase? Or do I completely have it backwards... Please help me understand what exactly is going on here.

Also, a little fuel theory. (Not sure if I understand this completely either, haha). Say you have the 30lbs/hr stock at the regulator and (8) 19lb/hr injectors. That would be about 4lb/hr going to each individual injector. If each injector can handle 19lb/hr why would you ever need to upgrade? And are there any real advantages to upgrade on a somewhat mildly modified car if I am not running a supercharger or anything else? And finally, would my stock fuel pump be strong enough to provide fuel for 24/lb injectors? Or should I just stick stockers on? Thanks guys

 

[urban96]

ive also been wondering the same thing, im getting ready to upgrade to 24lbers when i do my heads. am i gonna need an AFPR with the 24's?

 

[yellowstang1994]

Another big problem is, even if I do get an AFPR, how would I know if i'm running rich or lean if the only way to determine that is by a wideband o2 kit? I mean I guess you could say if you have detonation or pinging then back off or give it more, but what if you have a pre-existing pinging problem already? I wouldn't beable to distinguish all of this, lol.

 

[WhiteDevil]

Well when u recal your MAF to 24# injectors then you technically should be ok. Like with a 19# MAF when your read say 2 volts it means say 110kg/hr and then with a 24# MAF 2volts is 149kg/hr. So it should compensate for the changes in he fuel, and its this way through the whole curve.

The changing the fuel pressure changes the amount of fuel released per pulse width. So up the pressure the more it will add fuel, but thats only at wot. When you are in closed loop which is at idle and part throttle the car will adjust to achieve a 14.64 A/F ratio based on input from your O2s. When you hit WOT the comp wont compensate so the added pressure will work to add fuel.

Make any sense?

 

[GRGT1994]

Make any sense?

A little. My head hurts.

 

[final5-0]

Your focus seems to be on fuel pressure.

Fuel pressure is really a fine tune thing when you look at the big picture of whats really going on here.

Airflow is the main thing to focus on when you mod your car.

Bigger h/c/i, exhaust, yada, yada, yada, all make greater airflow through the motor. You gotta have enough fuel to go along with all that increased airflow.

Say your goal is 325rwhp. Before you get to that goal, three things will happen due to the increased airflow:
1) stock fuel pump will be too small
2) stock inj's will be too small
3) stock maf will be too small

1 & 2 will happen before 3

You don't wanna get your maf before your inj's or you will have to get it recaled.

The most efficient (best bang for the buck spent) way to ensure you got enough fuel and a maf that will accurately meter the increased airflow is to decide on inj size you are gonna use and upgrade maf & inj's together.

Unless you can custom tune, you can't just put larger inj's on the car and lower the fuel pressure if that is what your asking.

Later
Grady

 

[yellowstang1994]

Heres what i'm going for basically. My car is basically stock except for a few exhaust bolt ons and the sort. I plan on getting an upper/lower intake and a TB, quite possibly a cam not too far down the road from now. But I want to get my fuel system situated first, my injectors are starting to die on me and the fuel injector cleaner isn't helping as much as it used to. So I figure, hey I might as well throw 24/lb injectors in there while i'm going to be there. Maybee a new pump etc... just basically have the whole fuel system out of the way. Is it a good idea upgrading the injectors now? or should I just stick to a new set of 19s?

I'm slowly understanding the whole concept so I think I pretty much got the tuning issue. *I think* lol.

 

[final5-0]

Heres what i'm going for basically. My car is basically stock except for a few exhaust bolt ons and the sort. I plan on getting an upper/lower intake and a TB, quite possibly a cam not too far down the road from now. But I want to get my fuel system situated first, my injectors are starting to die on me and the fuel injector cleaner isn't helping as much as it used to. So I figure, hey I might as well throw 24/lb injectors in there while i'm going to be there. Maybee a new pump etc... just basically have the whole fuel system out of the way. Is it a good idea upgrading the injectors now? or should I just stick to a new set of 19s?

I'm slowly understanding the whole concept so I think I pretty much got the tuning issue. *I think* lol.

I think the 24's are a good idea for your plans.

I'd suggest a ProM 80 caled for those 24's. Quality maf, very accurate too.

When you upgrade the fuel pump I'd go for the 255lph in tank.

Later
Grady

 

[urban96]

hey grady do you think i will need an AFPR when i upgrade to 24's (car will have H/C/I)

 

[CManT1914]

I'll throw in my 0.02. Your stock 19lb injectors are good to about 285hp at 40psi fuel pressure. 24's are only good for 350 or so at 40psi, so if you plan on making more than that, get 30's. I'm gonna get 30's (better safe than sorry, and it leaves room for improvement) for my heads and cam I'm putting in in a couple weeks, but until then (cus I gotta recal my MAF too) I'm gonna bump up my fuel pressure to around 60psi with my AFPR, which should make my stock 19's work like 24's. This should be enough TEMPORARILY until I can get my MAF recaled.

 

[mackey]

Yes the 40psi is good for 285 at full load and 100% efficiency. The injectors shouldn't have to run any higher than 85% in my opinion.

 

[WhiteDevil]

Yes the 40psi is good for 285 at full load and 100% efficiency. The injectors shouldn't have to run any higher than 85% in my opinion.

yup

 

[final5-0]

urban96 & CManT1914

Here is a bit of info and you can make up your own mind about your fuel concerns.

I talk to a guy on a regular basis who like me is a self tuner and he told me the other day he has seen the inj duty cycle on his 24's go to a level that he is not comfortable with and he will be moving up to 30's.

btw...His combo is very similar to mine.

The issue between 24's & 30's is all about how your gonna tune the combo.

I'll just say it flat out........................
The typical stock block h/c/i combo of about 300 to 325rwhp is better off using 30's IMHO.

However

30's can give probs during small pluse width driving conditions like idle, light load cruise, cold/hot start, etc but 24's usually don't do that kind of stuff.

Here is where the tuning method plays a critical part in this whole fuel thing.

30's for custom tune because:

Your fuel pressure will be at stock levels so the 30's don't appear as large as most might think and you can use the tune to take care of small pulse width probs.

24's with no custom tune because:

You have no way to tune for the probs 30's can cause. You will have to up the fuel pressure to make the 24's act larger. I don't think this method is the best way to go but many have done it.

Never forget this one thing..............

If you find out your fuel system is not like it should be, the event that made you aware of that fact might not bring you a whole lot of joy.

Better SAFE than SORRY

Just my thoughts about fuel stuff so take it for what it might be worth, lol.

Later
Grady

 

[cavscout371]

ive also been wondering the same thing, im getting ready to upgrade to 24lbers when i do my heads. am i gonna need an AFPR with the 24's?

I'm still running the stock ones for now and the plugs look pretty damn good.

 

[scarless]

One of the biggest mistakes is to get unmatched parts, high duration cam, large injectors, big throttle body, high stall converter. Many people install 24# or 30#, as final5-0 mentioned, and have idle or other problems. A custom tune is important if you change from 19#'ers. Here is the math.

( H * B ) / ( C * D ) = estimated injector size.
H = horsepower
B = B.S.F.C. .45 to .50
C = number of cylinders
D = duty cycle 80% to 85%

FYI, an engine with a good intake and heads (BSFC .45) with 19#, 38-40# pressure, running at 85% is good to 280-290HP at the (flywheel). The thing that everyone seems to skip is that this peak HP happens around 5500+ RPM. If for the brief period that the engine is at 5500-6000 RPM if the injectors are pushed to 95%, the peak HP they can support is 320HP (flywheel). This is for an efficient engine, not a stocker. A mostly stock engine (BSFC .50) with slightly dirty injectors (75% duty cycle) will peak at about 220HP (flywheel). Just a note, I've talked to "reliable" people that are making 255-280HP (at the rear wheels) with 19# injectors without lean a condition at WOT. Occasionally pushing the injectors to 90%+ is not a terrible thing.

As you can see there are many variables to account for. In the real world I've seen people get 300HP (flywheel) using 19# injectors. Remember that peak HP is not always the best thing, it's the area under the curve from 1500-5000 RPM that makes the real difference in normal driving. An engine may make good peak HP, but may bog at every stop light.

Here comes the advice part, take it or not. Don't switch to 24# injectors until your engine needs them. So after you change out the heads, intake, cam, MAF, and get a custom tune, and you are running lean on the dyno, then switch to 24#.

 

[scarless]

Also, the 94 stock 70mm MAF is good to around 320HP (flywheel) as well, but the 60mm throttle body is not. The stock 94 fuel pump is 90lph, I think. You can swap it out for a 155lph any time, any more than that is just over kill.

 

[CManT1914]

Also, the 94 stock 70mm MAF is good to around 320HP (flywheel) as well, but the 60mm throttle body is not. The stock 94 fuel pump is 90lph, I think. You can swap it out for a 155lph any time, any more than that is just over kill.

Not neccesarily. Especially for guys that want to have room for improvement down the road. Since our cars have a return line to the pump, then it's not a bad idea to go ahead and go with a bigger pump. I'm gonna go with a 255, merely because I want to have enough fuel down the road for when either 1) I wear down the rings and rebuild the bottom end with a 347 stroker kit, or 2) I slap a SC on it, or 3) I do both! lol

 

[final5-0]

One of the biggest mistakes is to get unmatched parts, high duration cam, large injectors, big throttle body, high stall converter. Many people install 24# or 30#, as final5-0 mentioned, and have idle or other problems. A custom tune is important if you change from 19#'ers. Here is the math.

( H * B ) / ( C * D ) = estimated injector size.
H = horsepower
B = B.S.F.C. .45 to .50
C = number of cylinders
D = duty cycle 80% to 85%

FYI, an engine with a good intake and heads (BSFC .45) with 19#, 38-40# pressure, running at 85% is good to 280-290HP at the (flywheel). The thing that everyone seems to skip is that this peak HP happens around 5500+ RPM. If for the brief period that the engine is at 5500-6000 RPM if the injectors are pushed to 95%, the peak HP they can support is 320HP (flywheel). This is for an efficient engine, not a stocker. A mostly stock engine (BSFC .50) with slightly dirty injectors (75% duty cycle) will peak at about 220HP (flywheel). Just a note, I've talked to "reliable" people that are making 255-280HP (at the rear wheels) with 19# injectors without lean a condition at WOT. Occasionally pushing the injectors to 90%+ is not a terrible thing.

Very nice job of presenting the facts.

I keep telling peeps to use the various formulas for inj size but thats about the best I've been able to expalin it.

As you can see there are many variables to account for. In the real world I've seen people get 300HP (flywheel) using 19# injectors. Remember that peak HP is not always the best thing, it's the area under the curve from 1500-5000 RPM that makes the real difference in normal driving. An engine may make good peak HP, but may bog at every stop light.

I'm so glad you brought up the two issues of peak power and power under the curve. The focus on my combo was the latter but I also had some thoughts about higher rpm power in the future so I just could not be comfortable with 24's.

Here comes the advice part, take it or not. Don't switch to 24# injectors until your engine needs them. So after you change out the heads, intake, cam, MAF, and get a custom tune, and you are running lean on the dyno, then switch to 24#.

I've been right with you and agree with all you've said up until now, so maybe I'm not understanding what your saying but....................

Why would you want to change out the stock maf for a 19lb caled maf and then have to send it off for a recal when it is time for larger inj's? You should be able to use your stock inj's, and maf until you are not comfortable with the 19's and then just switch to the larger inj's and the matching caled maf.

The need for more fuel from larger pump and inj's will come up before the need for a maf that will meter more air.

As for a power gain from an upgrade of the stock tb & maf on a stock or slightly modded Stang, it is next to nothing. See my sig, I almost made it to 300 with the stock tb & maf. Please take note...that was using 30lb inj's.

You make some great info very easy to understand and I look forward to your reply

Later
Grady

 

[final5-0]

Also, the 94 stock 70mm MAF is good to around 320HP (flywheel) as well, but the 60mm throttle body is not. The stock 94 fuel pump is 90lph, I think. You can swap it out for a 155lph any time, any more than that is just over kill.

Yeah, I found some of the same kinds of things as well.

Here is some stuff I saw from my combo and a little data from others to consider.

I tune using a Tweecer and the airflow values I saw in datalogs makes me think you can go a little higher ...... like to maybe 360fwhp until you peg the stock maf.

I've talked to a few other peeps who have pegged the stocker or came very close to it, and like me, they are at the 300rwhp level, give or take a few.

I'm basing that thought from 360fwhp with a 15% drive line loss (stick trans) when on the dyno.

As for the stock tb, my datalogs showed I gained some airflow from replacing it to be sure but I noticed the most gain from the maf upgrade. I went from the stock tb & maf to a FRPP65 tb & a ProM80 maf btw.

I might have gotten a better gain from the tb upgrade if I went to a larger tb but all that is hindsight as they say, lol.

Later
Grady

 

[CManT1914]

urban96 & CManT1914

Here is a bit of info and you can make up your own mind about your fuel concerns.

I talk to a guy on a regular basis who like me is a self tuner and he told me the other day he has seen the inj duty cycle on his 24's go to a level that he is not comfortable with and he will be moving up to 30's.

btw...His combo is very similar to mine.

The issue between 24's & 30's is all about how your gonna tune the combo.

I'll just say it flat out........................
The typical stock block h/c/i combo of about 300 to 325rwhp is better off using 30's IMHO.

However

30's can give probs during small pluse width driving conditions like idle, light load cruise, cold/hot start, etc but 24's usually don't do that kind of stuff.

Here is where the tuning method plays a critical part in this whole fuel thing.

30's for custom tune because:

Your fuel pressure will be at stock levels so the 30's don't appear as large as most might think and you can use the tune to take care of small pulse width probs.

24's with no custom tune because:

You have no way to tune for the probs 30's can cause. You will have to up the fuel pressure to make the 24's act larger. I don't think this method is the best way to go but many have done it.

Never forget this one thing..............

If you find out your fuel system is not like it should be, the event that made you aware of that fact might not bring you a whole lot of joy.

Better SAFE than SORRY

Just my thoughts about fuel stuff so take it for what it might be worth, lol.

Later
Grady

Thanks a bunch Grady! I suspected 24's would not be up to par for my setup, so I decided to go with some 30's. I bought some FRPP 30's today on eBay for quit cheap too. I will DEFINITELY be getting a custom tune once I get my MAF recaled (can't use my injectors til that happens ). Until then, I'll run stock 19's and bump the fuel pressure to around 60. Hopefully I won't have to do that for long, but we'll see. Thanks again!