Random Injector Replacement Q

[Cosmo Dude]

Originally Posted by twilightprotege

you are passing more past the injectors every minute as the revs climb, but you're only passing the same amount of fuel past the injectors for each induction stroke at 1000rpm as you do at 10000rpm

If you have a fuel pressure regulator then you will only have the same fuel available if you have the same manifold pressure at both points.
Aaron's calculations are based on maximum injector flow at maximum RPM and is (QED) correct.

[twilightprotege]

Originally Posted by Cosmo Dude

If you have a fuel pressure regulator then you will only have the same fuel available if you have the same manifold pressure at both points.

naturally i am referring to all else being equal

Aaron's calculations are based on maximum injector flow at maximum RPM and is (QED) correct.

sorry to be blunt, but you're missing the point.

http://www.witchhunter.com/injectorcalc1.php4
http://www.iroczone.com/calcs/injectorsize.htm
http://www.smokemup.com/auto_math/fuel_injector.php
http://www.importperformanceparts.ne...nj-choose.html
http://www.injector.com/injectorselection.php
http://www.haltech.com.au/injformula.htm
http://www.megamanual.com/v22manual/minj.htm

notice how there is little if any talk of rpm as a factor for determining fuel injector sizing (and those were just the ones i could find quickly)??? that's because it doesnt matter. if i can get an NA engine to make 200hp at 6000rpm, it will require far more injector than an engine that makes 100hp at 6000rpm, but using aaron's forumla this isnt taken into account...and if anyone thinks that the VE is taking that into account you're correct to a point, but accurately working out VE is nigh impossible

[dave0r]

i should try and get a screen shot of my Link's fueling tables
these are based on TPS, MAP and RPM values , so they are NEVER the same at any giving point, so WOT dyno tuning only tunes 1 part of the whole setup
then a Road test using a wideband was used to finish off the lean burn and cruz tables


in my case the fuel injectors tables are listed in % of saturation/duration ..ie idle....1.5% of injectors (based of satuation points, Dead times that i programed in)
at WOT and 9psi the injectors are going a 51% from around 3500rpm onwards , leans out a bit more 5700 to 6300 get the power back again , it just drops off for no reason
(data logging is so cool)

but yeah...the injectorsare still at 2-5% when driving at 100km/h doing 3200rpm with almost no throttle...

[Bear]

Originally Posted by twilightprotege

you're right - you are passing more past the injectors every minute as the revs climb, but you're only passing the same amount of fuel past the injectors for each induction stroke at 1000rpm as you do at 10000rpm

yup ... more or less true, on the assumption there is no requirement adjust fuel at different RPM points ... your's requires 20% more injector pulse at 6500rpm ....

I see where you're coming from, but I also understand Aaron's point which is to calculate back your peak fueling requirement based on peak air flow ... this will mean that your injectors should admittedly be a bit over sized, but that's just playing it safe ...

by the nature of the calculations on that link, it is determining the injector size based on BSFC (which includes your efficiency) and your projected hp figure ... aaron is basing his calculations on a mechanical air flow potential ...

same book ... different page

[Cosmo Dude]

Are the maths wrong or just not what you use?

[twilightprotege]

The reason RPM is not used to calculate fueling requirements is that:

Every ounce of horsepower your car makes comes from the combustion of fuel. There is energy in the fuel, which is released when it is burnt. This energy is transfered to the piston which in turn rotates the crank shaft and so on. It does this irrespective of revolutions. This explains, why without increasing RPM, you can make more power - by improving the effeciency of the burn, allowing you to BURN MORE, and release more energy.

The only point that RPM comes into any injector equation is under high RPM operation, where running injectors at near their peak duty cycle will cause the injectors to "overhang", i.e. they will still be opening, or partly closing when the next pulse is required. By running bigger injectors, you can get the same amount of fuel whilst operating at a lower duty cycle.

Now, when tuning, you may use RPM adjustments in your fuel maps to smooth out fuel delivery and so forth, but, as Aaron more or less stated, injector size is calculated based on the maximum they are going to be operating at. This invariably is going to translate to "the amount of fuel that is required for the maximum horsepower your engine will generate", Which is going to be at WOT at peak power. If your injector is up to that, it will suffice for every amount of power below it, regardless of RPM. Aaron's calculations suggest that that at WOT at 4500rpm I'll be using half the fuel of at 9000rpm, which is true from a flow point of view, but as I have been stating, I still require the same amount of fuel PER intake stroke (again, assuming everything else remains a constant) at both rpm points. Aaron's calculations would state that I require 232.5cc of injector at 4500rpm, then 465cc of injector at 9000rpm. that is simply not true.

Heres a quick calculation.

Assume a 300hp 4cylinder engine that revs to 9000rpm. Where peak power occurs isn't really important, but for the sake of fairness, we'll assume peak power occurs at peak RPM.

So, using Aaron's injector size calculation we come to roughly 390CC injectors (assuming 110% VE on an NA engine etc, on a more realistic 90% VE on a mildly worked modern 4cyl engine, this amount would be less)

Using the calculation on the RCeng, we come up with 496CC's with all their "recommended" settings (obviously it is worth while determining your exact BSFC and keying in your precise WOT fuel pressure to get a more accurate reading).

That is over 100CC/min difference between the 2 calculations, which isn't insignificant.

Now, either the calculation based on BSFC is way wrong, or Aaron's calculations are incorrect. You can't have it both ways. I've provided many links which indicate the method that people use to calculate correct injector sizing....I can understand that Aaron's calculations "make sense" on face value, but in reality it isn't how it works.

[dave0r]

im still confused on HOW you need such huge injectors to make power, from either turbo or N/A

N/A honda K20a engine for example has 310-330cc injectors (who you talk too) and they make 178wkw N/A ..VE? have a guess

humble GTX setup converted to FWD makes 158wkw from 330cc injectors, stock management/fpr etc..now this VE would be well over 120% as its turbo

my point is how come you are all saying he needs massive injectors?

510cc injectors with 16psi of boost on BP making 211wkw

my BP has 254cc injectors, makes 145hp (bp22)
my other injectors are 290cc toyota 4age-20v 165hp
Mitsi GSR turbo injectors 290cc 200hp <-- an you can get heaps more outa them

too me this has answered all question relating to "how big do i need to go? "

[Aaron]

Just to reiterate part of my original post with the maths:

It is based on getting MAXIMUM efficiency from the engine, NA, basically a very higly strung race engine, with well tuned runners etc etc to get a slight ram-charge affect to bump up VE to 110% (which is about the best NA is ever going to get).

Also I used 10:1 as the A/F Ratio, realistically you'd be loosing power if running that rich...

Also I set the injectors to a 80% duty cycle at max rpm

All of which introduce an actual error into the numbers, becuase my post WAS NOT about the injectors Ryan should use but about the MATHS to derive injector sizing.

The Maths that Twiggy uses is not wrong, but just the same as the maths I use it relies upon a "efficiency" factor to trim the values.

I personally don't like calculating back from expected power output becuase thats prone to falling victim to "claims" of performance by parts makers.

I believe that if I re-did my maths with numbers closer to what Ryan would expect in his build and Twiggy did the same we'd end up very close. I certainly only based my maths on a theoretical NA engine of about 1.8L NOT any one persons.

Truth be known you should only be sizing your injectors once you've got a flow figure for the head or throttle body (whatever is the limiting factor), because that's the limitation of the amount of air you can get in. That's far more accurate than using the swept capacity...

dave0r - are you quoting crank power outputs as measured? or guesstimates based on Dyno readings at the tyres?

I think you'd find that both Twiggy and I base our maths around having plenty of fuel "headroom" so that the injectors don't get saturated or need to be run beyond a safe duty cycle at peak load. Oversizing also takes into account flow degradation over time, etc etc

For ****s and giggles I may as well re-do my maths closer to a tweaked BP vs a balls out real race engine.

A.

[Aaron]

OK redid the maths for a tameish street engine so we're comparing apples with apples and end up around the 230-240cc area. Noting these injectors are running at 98% duty cycle at the engines peak flow. I trimmed down the VE as it's unlikely anyone will spend money on real decent cams and intake design to get past 90% VE and we're running a more acceptable 12:1 A/F ratio for power. I also reduced the swept capacity slightly and dropped the rev limit to 8000rpm.

So there you go - small injectors now recommended so that everyone can argue about the same thing.

However Ryan the answer to the question up the top still is - Those injectors will be too big for a stock ECU to trim out at low RPM & low load....


A.

[Aaron]

Originally Posted by twilightprotege

Using the calculation on the RCeng, we come up with 496CC's with all their "recommended" settings (obviously it is worth while determining your exact BSFC and keying in your precise WOT fuel pressure to get a more accurate reading).

That is over 100CC/min difference between the 2 calculations, which isn't insignificant.

Now, either the calculation based on BSFC is way wrong, or Aaron's calculations are incorrect. You can't have it both ways. I've provided many links which indicate the method that people use to calculate correct injector sizing....I can understand that Aaron's calculations "make sense" on face value, but in reality it isn't how it works.

Originally Posted by Aaron

This is at 100% duty cycle - set that to 80% for safety = 465cc

So you would want to find injectors in the range of say 440cc to 480cc for this motor if everything worked to its best.

Realistically the 390cc would be find I reckon as we've over compensated EVERY step of the way and it's unlikley any of us would be able to build a motor that efficient.

Errr I missed this before - but you notice that I calculated 465cc, then down scaled due to the high number of "optimums and estimates" in my quick maths.

So we are only 31cc apart and that's not much

[dave0r]

Aaron;
dyno figures are posted as wheel killing watts <--roller dyno
GTX,K20a <-- ive seen the cars and the dyno sheets so i know what they have not had modified to them and that was the injectors :P

the HP figures, ie BP22, 4AGE-20v, GSR are all stock crank HP at least on paper

[Cosmo Dude]

Originally Posted by twilightprotege

The reason RPM is not used to calculate fueling requirements is that:

Every ounce of horsepower your car makes comes from the combustion of fuel. There is energy in the fuel, which is released when it is burnt. This energy is transfered to the piston which in turn rotates the crank shaft and so on. It does this irrespective of revolutions. This explains, why without increasing RPM, you can make more power - by improving the effeciency of the burn, allowing you to BURN MORE, and release more energy.

The only point that RPM comes into any injector equation is under high RPM operation, where running injectors at near their peak duty cycle will cause the injectors to "overhang", i.e. they will still be opening, or partly closing when the next pulse is required. By running bigger injectors, you can get the same amount of fuel whilst operating at a lower duty cycle.

Now, when tuning, you may use RPM adjustments in your fuel maps to smooth out fuel delivery and so forth, but, as Aaron more or less stated, injector size is calculated based on the maximum they are going to be operating at. This invariably is going to translate to "the amount of fuel that is required for the maximum horsepower your engine will generate", Which is going to be at WOT at peak power. If your injector is up to that, it will suffice for every amount of power below it, regardless of RPM. Aaron's calculations suggest that that at WOT at 4500rpm I'll be using half the fuel of at 9000rpm, which is true from a flow point of view, but as I have been stating, I still require the same amount of fuel PER intake stroke (again, assuming everything else remains a constant) at both rpm points. Aaron's calculations would state that I require 232.5cc of injector at 4500rpm, then 465cc of injector at 9000rpm. that is simply not true.

Heres a quick calculation.

Assume a 300hp 4cylinder engine that revs to 9000rpm. Where peak power occurs isn't really important, but for the sake of fairness, we'll assume peak power occurs at peak RPM.

So, using Aaron's injector size calculation we come to roughly 390CC injectors (assuming 110% VE on an NA engine etc, on a more realistic 90% VE on a mildly worked modern 4cyl engine, this amount would be less)

Using the calculation on the RCeng, we come up with 496CC's with all their "recommended" settings (obviously it is worth while determining your exact BSFC and keying in your precise WOT fuel pressure to get a more accurate reading).

That is over 100CC/min difference between the 2 calculations, which isn't insignificant.

Now, either the calculation based on BSFC is way wrong, or Aaron's calculations are incorrect. You can't have it both ways. I've provided many links which indicate the method that people use to calculate correct injector sizing....I can understand that Aaron's calculations "make sense" on face value, but in reality it isn't how it works.

On an FI engine where you can turn up the boost to increase VE that may fly but the original question is an NA engine.
You have agreed that an increase of RPM will require a higher fuel rate and then argued that an increase in VE is the deciding factor. IMO they combine, neither are exclusive and both are important.

NB: Changes in VE can be made in NA engines but generaly mechanical in nature.

[twilightprotege]

Originally Posted by Cosmo Dude

You have agreed that an increase of RPM will require a higher fuel rate

geez....a higher volume of fuel per minute, but no more fuel per intake stroke (again, all else being equal).

if you're so hung up on rpm, tell me the answers to this scenario. i have an NA engine that makes 100fwhp at 6000rpm. my engine can rev to 10000rpm, sadly however i make less than 100fwhp at every single rpm point about 6000rpm. I do not know my VE at any point in the rev range. what injectors should i use? now, let's say that at 6000rpm on the injectors you choose, my ems pumps out 6ms to the injectors to give my my desired air fuel ratio when i have 0"Hg (aka wide open throttle). assuming once again that for this particular part of the scenario that my unknown VE is the same at 6000rpm as it is at 2000rpm (and all else has remained the same). what ms amount should my injectors require at 2000rpm at 0"Hg???

[Cosmo Dude]

Originally Posted by twilightprotege

if you're so hung up on rpm,

No, it is part of the equitation. Not all of it but an equally important of part of it.

[LordWorm]

this link here explains what is happening (as well as repeating the formula for calculating injector requirements) http://www.grmotorsports.com/backiss...h-the-flow.php

I think the issue is that the assumption is that injectors are always "on"... when infact (under ideal circumstances) the injectors should only be opening and closing within a 180 degrees of a 720 degree engine cycle (aka, the intake cycle)...

Idealy, the injector will fire a pulse, and completely close, during the intake cycle. Therefore RPM is irrelevant for determining the injector requirements DIRECTLY. Reason for this, is that regardless of 1 rpm, or 10 000rpm, the injector should open, and close, releasing the exact same volume of fuel, PER REVOLUTION to achieve the same horsepower.

The flow rating of an injector, is the amount of fuel that the injector can flow if completely open for a minute, at a given fuel pressure.

Interestingly, the accepted formula for calculating injector sizing DOES take rpm into account, in a round about way.


BSFC (brake specific fuel consumption) is expressed as lbs/hour/hp... that is, pounds of fuel burnt per hour, per horsepower. Heres a good primer on BSFC http://www.land-and-sea.com/dyno-tec...using_bsfc.htm

a quote from the link about BSFC

The beauty of BSFC #'s are that they remain similar over a wide range of engine sizes (assuming both are of similar mechanical design and compression ratio). For example, a tiny one cylinder 50cc four-stroke and a 454ci V8 might both have a BSFC ratio of 0.45 lb/Hp-hr (when optimally tuned) at their respective peak torque points. Thus, if either of these engine's were overly richened, its BSFC might climb into the 0.55 to 0.65 range (because the fuel flow in our equation will be going up as the power is going down.

Going off that, displacement doesn't even enter the equation.

Thats not to say that the methods which are stated by EMS manufacturers, injector manufacturers, engineers and engine builders are the ONLY way to calculate the required injector sizes. A more accurate means of calculating injector requirements using Aaron's equation, would be to solve the RPM and CFM side of the equation by solving VE correctly.


VE is expressed as:

VE = (3546 * CFM) / (CID * RPM)

Herein lays a problem with Aaron's original equation. He's using both VE and RPM in his calculations. Since the VE calculation already takes into account RPM, it doesn't make sense (and is inaccurate) to correct for it again. Additionally, the VE equation takes CFM into account, so solving this twice is also incorrect.

In a around about way he is trying to solve BSFC, however this is a value which can only really be accurately determined by measuring the amount of fuel which is being burnt for a given horsepower over an hour (or over a minute multiplied by 60, or over a second multiplied by 60 multiplied by 60....however you want to do it).

However, given that RPM and VE are really irrelevant to determining injector sizing, and BSFC is the number that we really want to get (which will be for an NA engine somewhere between 0.45 and 0.55 as a rule of thumb), that is, how much fuel do we need to burn per hour to achieve a target HP, the accepted formula is:

Flow rate (lbs/hr) = Max Hp x BSFC / no. injectors x duty cycle (formula taken from the haltech source link provided below) A more complete solution will also correct for fuel pressure, using new flow rate = SQRT(new pressure/old pressure) * old flow rate

It is interesting to note, that on the haltech website, the following is stated:

After much research it has been found that there are several different formulae used to determine injector size. It appears that most of the various formulae in use are basically identical, with only 2 real variables, being whether or not an 80% duty cycle is accounted for and what the desired Brake Specific Fuel Consumption (BSFC) should be. This is the formula with the correction based on the duty cycle

source: http://www.haltech.com.au/injformula.htm

its not a case of "knowing" what your crank horsepower is going to be - its a case of taking an educated guess as to what the crank horsepower is going to be. Anyone who has been around engines can work out with a fair degree of accuracy what this number will be.

A bit of overhead in injector size isn't a bad thing either. Given that injectors open in a linear fashion (that is, 20% opening is exactly 2x the flow of 10% opening etc), a highly strung NA engine can benefit greatly from having oversized injectors. The reason for this is that the speed in which the injector opens and closes depends on the % of injector opening. At very high RPM, the amount of time within that 180 degree intake stroke is not very long. As such, if you need to open a small injector a long way to provide fuel at high RPM, you may run into issues where by the injectors "overhang" the intake stroke. That is, they continue to be open after the intake valve has closed. By selecting a marginally oversized injector, you do not have to open it as far at high RPM to achieve a complete injector pulse within this small window.


Aaron's position on injector calculation isn't "wrong" exactly - he expresses many of the requirements for calculating the injector requirements and is on the "right track" - but his formula is somewhat inaccurate. Given that the industry standard for calculating injector sizing is as stated above, and that try as I might I cannot find any source (credible or otherwise) which expresses a formula even remotely close to Aaron's, I would suggest that when solving injector sizing, it is best to use the formula's provided by those that manufacture injectors, design EMS, or build engines suggest you use....


note: The only point that matters for solving injector requirements is the requriement at peak horsepower. If an injector can handle that, it can handle all power requirements below it, until obviously the minimum opening amount provides more fuel than is required - this isn't an issue on street cars, but is one of the reasons why race engines with large injectors run very rich on idle and at the lower side of their power curves.

Also, before someone asks, the injector formula works perfectly for different types of fuel as well, because the BSFC required (that is, lbs of fuel per hour per horsepower) of fuels such as methanol, are greater.

Hope this answers some of the questions - sorry for the haphazard way in which i have posted this.......I didn't go through and proof read it.... i think i've linked all my sources......

[Shaken]

So I guess a simple way to put it would be to say that the total volume of fuel required at the fuel rail to feed the engine should not be mistaken for the amount of fuel one injector has to flow to feed one single cylinder and maintain a desired afr.

To elaborate, RPM is a crucial factor in determining the total fuel requirements of an engine at the fuel rail. This is because at higher rpms, the injectors have to fire much more often over the same time increment.

However, each injector only fires once per combustion cycle and must only inject enough fuel to achieve the maximum ideal afr for one cylinder each time. This required amount should not vary much whether the engine is turning at 750 revolutions/minute or at 7500 revolutions/minute (with the most likely variance being a change in ideal afr at different points throughout the rpm range).

Could the above statements be considered mostly accurate (albeit overly simplified)?

[LordWorm]

Originally Posted by Shaken

So I guess a simple way to put it would be to say that the total volume of fuel required at the fuel rail to feed the engine should not be mistaken for the amount of fuel one injector has to flow to feed one single cylinder and maintain a desired afr.

To elaborate, RPM is a crucial factor in determining the total fuel requirements of an engine at the fuel rail. This is because at higher rpms, the injectors have to fire much more often over the same time increment.

However, each injector only fires once per combustion cycle and must only inject enough fuel to achieve the maximum ideal afr for one cylinder each time. This required amount should not vary much whether the engine is turning at 750 revolutions/minute or at 7500 revolutions/minute (with the most likely variance being a change in ideal afr at different points throughout the rpm range).

Could the above statements be considered mostly accurate (albeit overly simplified)?

"desired afr" doesn't really come into it... its more a case of maintaining the amount of fuel to support the power you are out to make...but yes, at higher RPM, the injectors need more fuel SUPPLY to prevent the injectors over running the available amount of fuel...this is more a fuel pump and rail size requirement problem though. Solving this is relatively easy. If your BSFC is 0.5 lbs per hour per horsepower, and you are making 100 crank horsepower, you would require a pump that can supply 50 lbs of fuel per hour.... "overkill" on a fuel pump isn't an issue though, as unused fuel is returned to the tank after the regulator.

Each injector fires once per INTAKE stroke (not combustion). Most if not all ECU's will have an injector phasing curve to make the injectors fire earlier as the engine speed increases to insure that the "package" of fuel is successfully delivered in the required valve opening time - but for the sake of simplicity, its safe to suggest that the fuel is delivered within the 180 degrees in which the intake stroke is occuring.

You are quite right with the amount of required fuel bit. 2 engines both with the same BSFC, same fuel pressure, and same horsepower, differing only in the RPM that the power is delivered, will require the same fuel injectors, as illustrated by the accepted fuel injector calculation formula.

[project.r.racing]

How much power will I need to achieve before I go from the original 220cc to 330cc injectors? (BP-05)

[LordWorm]

Originally Posted by project.r.racing

How much power will I need to achieve before I go from the original 220cc to 330cc injectors? (BP-05)

Assuming a BSFC of 0.5, and a fuel pressure of 43.5 PSI and a max injector duty cycle of 80%, 200 crank horse power would require 330CC injectors.

[chicaboo]

When you get there Ryan, I have 300cc (B6T), 330cc (B6T JDM) and 385cc (BPT) injectors laying around here. The plug style is a little different on the B6T JDM injectors if you didn't know, but I think regular B6T or BPT injectors would work for you (impedance, ohms...).

Gav.