Last word on injectors--promise.
10-22-2007, 12:25 PM
#1
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Join Date: Dec 1969
Location: PORTLAND, OR, MULTNOMAH
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Last word on injectors--promise.
After pondering the need for larger injectors---I believe my FWHP somewhere around 425---I would like someone to answer the following question:
If you have 425 FWHP and are using stock 24# injectors, but you rarely ever drive more than 2000 RPM, e.g. 70 mph in 6th gear---wouldn't you be ok to use the HP that you are using at 2000 RPM x .070 to get the injector size that would be recommended? Example: You are running around 200 FWHP at 2000 RPM (just a guess) so taking 200 x .070 = 14.
I.e, you would only be needing a 14# injector at that HP and so a 24# injector would be more than enough to support the HP that you are using 95% of the time?
And what would that translate into regarding the percentage of injector that you are using?
I ask this because it seems that if you use too big of an injector for your normal driving wouldn't it mean that you are using too little of your injector's ability and perhaps causing a too rich a mixture at a really low rpm, e.g. idle around 950 rpm which in turn would cause a lousy idle?
I can see using larger injectors if you are talking about maxing your RPM and peak hp most of the time, e.g. peak hp 425 at 5700 rpm would be 425 x .070= 29.75,or a 30# injector, but seems like the injector that would be recommended if you were actually using your car off-road most of the time and going full out.
I read somewhere that using your injector exceeding 90% would be pushing it, but also read that using it a less than 10% is also a no no.
Just wondering. Thanks much.
If you have 425 FWHP and are using stock 24# injectors, but you rarely ever drive more than 2000 RPM, e.g. 70 mph in 6th gear---wouldn't you be ok to use the HP that you are using at 2000 RPM x .070 to get the injector size that would be recommended? Example: You are running around 200 FWHP at 2000 RPM (just a guess) so taking 200 x .070 = 14.
I.e, you would only be needing a 14# injector at that HP and so a 24# injector would be more than enough to support the HP that you are using 95% of the time?
And what would that translate into regarding the percentage of injector that you are using?
I ask this because it seems that if you use too big of an injector for your normal driving wouldn't it mean that you are using too little of your injector's ability and perhaps causing a too rich a mixture at a really low rpm, e.g. idle around 950 rpm which in turn would cause a lousy idle?
I can see using larger injectors if you are talking about maxing your RPM and peak hp most of the time, e.g. peak hp 425 at 5700 rpm would be 425 x .070= 29.75,or a 30# injector, but seems like the injector that would be recommended if you were actually using your car off-road most of the time and going full out.
I read somewhere that using your injector exceeding 90% would be pushing it, but also read that using it a less than 10% is also a no no.
Just wondering. Thanks much.
10-22-2007, 03:59 PM
#4
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You routinely use your injectors at less than 10% of their capability... its call "idle".
You size the injectors as I explained in the earlier post. They have to supply the amount of fuel required at peak fuel demand, or your engine will run lean. They have to supply that amount of fuel while staying open less than 100% of the time.
My "0.07 x flywheel HP" factor is applicable to peak fuel usage, assuming a BSFC of 0.476 #/HR/HP, and a DC not to exceed 85%. You can't use that factor at other engine operating conditions.
As long as your PCM has the correct injector size programmed into it, and as long as the PCM injector drivers are capable of operating the injectors at the small pulse widths required for idle, you will not have a problem running a large injector. Many people here have run 42 #/HR injectors on the stock PCM (with the correct injector size programming, of course) with no problems.
You size the injectors as I explained in the earlier post. They have to supply the amount of fuel required at peak fuel demand, or your engine will run lean. They have to supply that amount of fuel while staying open less than 100% of the time.
My "0.07 x flywheel HP" factor is applicable to peak fuel usage, assuming a BSFC of 0.476 #/HR/HP, and a DC not to exceed 85%. You can't use that factor at other engine operating conditions.
As long as your PCM has the correct injector size programmed into it, and as long as the PCM injector drivers are capable of operating the injectors at the small pulse widths required for idle, you will not have a problem running a large injector. Many people here have run 42 #/HR injectors on the stock PCM (with the correct injector size programming, of course) with no problems.
10-22-2007, 11:04 PM
#5
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Join Date: Jul 2002
Location: Phoenix, AZ
Posts: 1,277
If you have 425 FWHP and are using stock 24# injectors, but you rarely ever drive more than 2000 RPM, e.g. 70 mph in 6th gear---wouldn't you be ok to use the HP that you are using at 2000 RPM x .070 to get the injector size that would be recommended? Example: You are running around 200 FWHP at 2000 RPM (just a guess) so taking 200 x .070 = 14.
I.e, you would only be needing a 14# injector at that HP and so a 24# injector would be more than enough to support the HP that you are using 95% of the time?
I.e, you would only be needing a 14# injector at that HP and so a 24# injector would be more than enough to support the HP that you are using 95% of the time?
I really don't get your question. Are you trying to save yourself from buying larger injectors?
10-23-2007, 09:25 AM
#6
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Join Date: Dec 1969
Location: PORTLAND, OR, MULTNOMAH
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Of course: If a good set of injectors cost $400.00+- and a tune between 75-125 bucks plus a couple of dyno pulls you are looking at $600.00 +-.
Now if you are gaining 0 power.......
My point was that if you hardly ever go full throttle, that when you do, and it's a light to light situation, that you are only going to hit high RPM for a couple of seconds and it didn't seem that you would suffer that much loss in that short of time and it wouldn't really take a toll on your injectors.
However, after all the responses to the contrary, I guess , that if I ever have a few hundered lying around, I willl probably get some Denso/Bosch 36# and go for it. But meanwhile, with my 97SS with 10K on the car and about 2K on the motor, I don't see the urgency. JMHO.
Now if you are gaining 0 power.......
My point was that if you hardly ever go full throttle, that when you do, and it's a light to light situation, that you are only going to hit high RPM for a couple of seconds and it didn't seem that you would suffer that much loss in that short of time and it wouldn't really take a toll on your injectors.
However, after all the responses to the contrary, I guess , that if I ever have a few hundered lying around, I willl probably get some Denso/Bosch 36# and go for it. But meanwhile, with my 97SS with 10K on the car and about 2K on the motor, I don't see the urgency. JMHO.
10-23-2007, 11:12 AM
#7
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good luck though
10-23-2007, 11:38 AM
#8
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Posts: 109
Maybe I am crazy but the injectors are not what you need to worry about hurting. It is the motor... If you run lean even for a second or two it could do enough damage to destroy a piston. If you never planned to go WOT then don't worry about it. If you do go WOT and allow the injector duty cycles to go over 100% that is an indication that the motor is going lean and the injectors are the reason (provided fuel pressure is is spec).
Good luck,
TOM B
Good luck,
TOM B
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