Sizing Injectors - Cherokee SRT8 Forum
WK1 General Performance Discussion If you are planning on modifying your Jeep Grand Cherokee SRT8 with a performance enhancing modification, this is the section to post about it.

 
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post #1 of 9 (permalink) Old 06-08-2008, 09:34 PM Thread Starter
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Lightbulb Sizing Injectors

While trying to final tune my 426 Jeep, I have been to more injector sizing sites than what should be allowed. It seems that all injectors are rated using a 43.5psi fuel pump rating. That makes it difficult to figure out what is needed or the formula for our 58psi fuel system. Once you cross over into bigger cubic inches or forced induction you need to size a larger injector. The downside is loss of quality idle, drivability and long and short term fuel trim. Well until a tuner can SIZE the injectors into the tune....and even then I havent seen one that idled perfect!

Based on wheel horsepower and roughly a 20% driveline loss for a Jeep I had to swag the crank horsepower...so,

For a crank horsepower of;
580hp.... a 45.3 lb/hr or 475cc/min injector is required.
600hp.... a 46.88 lb/hr or 492.19 cc/min injector is required.
680hp.... a 53.13 lb/hr or 557.81 cc/min injector is required

Rated @43.5psi
38lb injector @58psi = 43.8 lbs/hr
40lb injector @58psi = 46 lb/hr
42lb injector @58psi = 48.4 lb/hr
50lb injector @58psi = 57.7 lb/hr

Out of curiosity I looked up the smallest SRT4 injector which is the
2003 SRT4 injector, rated at (43.5psi) 47lb/hr @ 58psi = 53lb/hr or 557cc/min.

2004 SRT4 injector rated at (43.5psi) 52lb/hr @ 58psi = 59lbs/hr or 577cc/min.

I wont even touch the Stage 2&3 injector rating.

Clearly the 2003 SRT4 injector is a bit overkill unless you are making well over 680 crank horsepower.....and at that point good luck with idle and streetability.


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post #2 of 9 (permalink) Old 06-08-2008, 10:06 PM
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Im glade we have a MOTOR HEAD in here to supply us with all this valuable info.

THANKS A LOT STEVE

2015 JEEP CHEROKEE LIMITED
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post #3 of 9 (permalink) Old 06-08-2008, 10:45 PM
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Steve, I will dont have access to my home computer but I have a excel spread sheet that shows injector needed by entering your injector rating at 40 psi and what fp you will be running under WOT. It will then calculate what size injector you will need for your specific application. If you dont get help, I will have somebody swing by the house and email me that speadsheet.

FYI- Im pretty sure that 20% drivetrain loss is on the very low side. The slush box and AWD have to make these rigs closer to 23-25%.
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post #4 of 9 (permalink) Old 06-08-2008, 10:54 PM
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http://www.rceng.com/technical.aspx

This calculator will tell you what size injector(based on 43.5 PSI rating) you will need to use with a FP of 59 psi.
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post #5 of 9 (permalink) Old 06-10-2008, 12:22 AM Thread Starter
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Quote:
Originally Posted by BTLFED View Post
http://www.rceng.com/technical.aspx

This calculator will tell you what size injector(based on 43.5 PSI rating) you will need to use with a FP of 59 psi.

I used this and it shows i need 46lb/hr.
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post #6 of 9 (permalink) Old 06-10-2008, 12:04 PM
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Quote:
Originally Posted by sarge View Post
I used this and it shows i need 46lb/hr.
You need to change the BSFC(Brake Specific Fuel Consumption) because the Hemi engine is pretty efficient and your 426 is probably even more efficient than stock. I played with the numbers a little and this is what I got.
#1

#2

#3

#4


I think the 3rd pic is the most accurate with a .40 BSFC and 85% IDC but if your engine is as efficient as the FRI 426 then the last pic shows what a .38 BSFC and 85% IDC can handle hp wise. That is why they got away with using the stock 6.1 injectors.
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post #7 of 9 (permalink) Old 06-10-2008, 12:08 PM
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I think most guys (supercharged lx's) have been switching to the srt4 injectors.....
hijack...I am thinking about upping the stall converter to the protorque 2800...do you think I will have the problems you did with the larger converter?
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post #8 of 9 (permalink) Old 06-10-2008, 12:16 PM
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Quote:
Originally Posted by sarge View Post
The downside is loss of quality idle, drivability and long and short term fuel trim. Well until a tuner can SIZE the injectors into the tune....and even then I havent seen one that idled perfect!


Clearly the 2003 SRT4 injector is a bit overkill unless you are making well over 680 crank horsepower.....and at that point good luck with idle and streetability.
Lots of false statements... you will be amazed what RDP can do for you. we are running 87lb injectors in BJBs with no issues. And the SRT4's are no problem to tune .

Bryan
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post #9 of 9 (permalink) Old 06-10-2008, 08:09 PM Thread Starter
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Lots of false statements...
These are not false statements!! This is fact at present with Chrysler and even Fords.

you will be amazed what RDP can do for you. I know you have teamed up and have your tour and we are scheduled for NEXT WEEK. Again, what I stated is the current condition and if you guys have fixed the situation you are ahead of the curve.


we are running 87lb injectors in BJBs with no issues. And the SRT4's are no problem to tune .
Again, I look forward to meeting you guys next week
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