Injector Sizing
07-15-2010, 10:40 PM
#1
Thread Starter
Join Date: Jul 2010
Location: Georgia
Posts: 7
Injector Sizing
I have a 2v GT with a forged stroker bottom end running 9.2 to 1 cr. Patriot Stage 2 heads. 75mm tb and plenum. Found a deal on some comp stage 2s. Does Any one now what size injectors will be needed. Still have stocks now and im sure they are about pegged.(had 85% duty on stock configuration).
I am thinking 24# should do it. But not sure.
I am thinking 24# should do it. But not sure.
07-16-2010, 07:31 AM
#4
3rd Gear Member
Join Date: Apr 2007
Location: Georgia
Posts: 931
No, the stock maf can support quite a bit of hp n/a iirc. Just get it tuned with the larger injectors, and change maf if you decide to put some boost to it. Although you might have to upgrade injectors again depending on which you go with the first time.
07-16-2010, 10:15 AM
#5
TECH SAVANT
Join Date: Dec 2006
Location: Saint Augustine, FL
Posts: 10,938
Here is a "rule of thumb" sizing guide..
These are "flywheel" HP numbers, and as stated above they are guidelines--so please, there's no need to post that "this chart is crap because I have a friend that is making 3000HP on 24lb/h injectors."
They are also based upon the 40psi pressure differential Ford chose for the returnless fuel system in our cars. If you alter the fuel pressure you will need to scale the values¹.
One other note, injectors that are too large can be difficult for the PCM to precisely control during idle and at low engine load/speed; leading to surging and other less than desirable operation. What constitutes "too large" depends on a number of factors, but generally when injector capacity gets to be 50% or more of what is needed is when problems crop up.
You will of course need to have the tune altered to accommodate the higher capacity injectors.
Your MAF will support up to 1150lb/h (at 4.8Vout), this correlates to 395fwHP or so...
-------------------------------------------------
¹ - This is the formula for scaling the injector values when you change the fuel pressure:
You can scale the HP values above by calculating the new flow rate and then multiplying the HP from above by the new flow/the old flow.
For example if you chose to run 60psi rail pressure with 42lb/h injectors (40.3 lb/h at 40 psi), then the new flow rate would be:
sqrt(60/40.3) * 40.3 = 49.2lb/h.
49.2/40.3 = 1.22; 1.22 * 483fwHP (from the chart @75%DC) = 589fwHP.
These are "flywheel" HP numbers, and as stated above they are guidelines--so please, there's no need to post that "this chart is crap because I have a friend that is making 3000HP on 24lb/h injectors."
They are also based upon the 40psi pressure differential Ford chose for the returnless fuel system in our cars. If you alter the fuel pressure you will need to scale the values¹.
One other note, injectors that are too large can be difficult for the PCM to precisely control during idle and at low engine load/speed; leading to surging and other less than desirable operation. What constitutes "too large" depends on a number of factors, but generally when injector capacity gets to be 50% or more of what is needed is when problems crop up.
You will of course need to have the tune altered to accommodate the higher capacity injectors.
Your MAF will support up to 1150lb/h (at 4.8Vout), this correlates to 395fwHP or so...
-------------------------------------------------
¹ - This is the formula for scaling the injector values when you change the fuel pressure:
You can scale the HP values above by calculating the new flow rate and then multiplying the HP from above by the new flow/the old flow.
For example if you chose to run 60psi rail pressure with 42lb/h injectors (40.3 lb/h at 40 psi), then the new flow rate would be:
sqrt(60/40.3) * 40.3 = 49.2lb/h.
49.2/40.3 = 1.22; 1.22 * 483fwHP (from the chart @75%DC) = 589fwHP.
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