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Joined: Aug 2001
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The only way to know if you are maxing out your injectors is to do some fricking math! Just because you run lean doesn't mean that is what is happening. All you people out there are driving me nuts with this. FWIW I have WELL over 200 HP at the wheels with the stock 19# injectors and I was in no way running lean on the dyno.
You have to take a reading of the pulse width at WOT and max rpm.
If you factor on 7000 rpm, then you have a starting point. Your injectors fire once per two revolutions which means that at 7000 rpm each injector fires 3500 times per minute.
Something like 58 times per second which translates to 0.017 seconds between each injector pulse which is 17.1 milliseconds. Now if you assume that your injectors are "maxed" at 80% duty cycle, then you would have to see them on for 80% of the time between each pulse.
That would be 13.7 milliseconds at 7000rpm, longer at the 6750 rpm cutoff.
The injector pulse-width at 6750 rpm cutoff and 80% duty cycle is 17.8 milliseconds.
There, take that figure and use a scan tool to see what the injector pulsewidth is right at the top of your RPM range when you shift. If it is less than 17 mS then you are NOT maxing your injectors.
Yes it is that simple.
Tom
You can call me anything you like as long as it's nice.(all lies accepted)
99 Silver Frost SVT. #226 of 2760
Engine: 3.0 power!
Unique Stuff: Sunroof control module (#1 of 9)
Car Audio: Loaded and loud!
Check them out at
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Joined: Mar 2002
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A few things need to be known before we start.Injector 'duty cycle' time,this is the time ,in % ,the injector is open,sometimes called the pluse width.For race motors safe cycle is around 0.85.Rembember that an injector can only open & close so fast(min dynamic flow range).With too big an injector the ECU will try to lean out the fuel and can select a pulse width shorter than the min dynamic flow range of the injector.Bigger is not always better.To establish the fuel reqired can use the following:
Fuel in lbs/hr=(max HP x BSFC /(Number of injectors x duty cycle)
BSFC is the brake specific fuel consumption,the amount of fuel needed per unit of power produce.Only ENGINE dyno data can give you this butwe can estimate:-
High Compression 0.45>0.55, Low Compression 0.50>0.60, T/c & s/c 0.55>0.65.
An s/c set up has to take into account the parasitic losses to the s/c,an engine makes say 250hp with a s/c,it takes 30hp to drive the s/c so the net hp is 220hp.
Example on a 215hp motor.
Injector size =(215hp x 0.55) /(8 x 0.80)= 18.5lbs hr, thats on an 8cyl.The max safe duty cycle on OEM injectors is around 0.80duty cycle.Soc Ford pressure regulators operate at around 39-40PSI,with adjustable regulators you can increase the flow rate ,but dont go above 55 psi,the lines and connections wont take it!
To calc a new flow rating use the following:
New flow rate ={sq root of (new press/ old press)} x old flow rate.The two rules of thumb are ,dont go too big with injectors,dont go too lean with mixture.Hope some of this help...Regards Terry Haines, Haines Motor Sports Inc (PS TRUE engine dyno,not vechicle,data is best for these calcs) by Terry Haines
So with that equation, 240hp * .45 / 6 cylinders *.8 duty cycle = 22.5 lb injectors. or should use 24 lb injectors. So 6 cyl * 19 inject * .8 duty cycle / .45 = 200 max HP w/o increasing Fuel pressure. My interpretation....
98 T-Red Contour LX V6 Mtx 106k miles
9" K&N, Aussie Pipe, Dual Mode Damper, 19lb. injectors, SHM MAFterburner, Ford 9mm Wires, Mystery Mod, ShoShop Off-road Y pipe, Dynomax Resonator and Muffler, 2.5" custom cat-back.
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Joined: Jun 2001
Posts: 3,419
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Originally posted by HoosierContour:
So with that equation, 240hp * .45 / 6 cylinders *.8 duty cycle = 22.5 lb injectors. or should use 24 lb injectors.
So 6 cyl * 19 inject * .8 duty cycle / .45 = 200 max HP w/o increasing Fuel pressure.
My interpretation.... Yep. That exact math is what started my investigation into whether or not I was maxing my injectors, and that investigation ended with my dyno experience and installing 24# injectors. FWIW, last time I dyno'ed (4 months and quite a few problems solved ago) I peaked at 188 fwhp (~215 - 220 give or take at the crank) and that peak occurred exactly where my fuel curve fell off. So the math isn't exact (because of the BSFC estimate, mine must be slightly lower than 0.45, which makes sense since I run increased compression) but it is a theoretical start. When you back that up with hard data, it is tough to ignore the correlation. Along the same lines, if you find a way to increase your fuel pressure (which a few people have tried and failed, including myself) then you increase the "effective" flow rate of any set of injectors (from the same topic by Terry that Hoosier posted). It's exactly how Vortech gets the additional fuel into the motor on their S/C application, and why returnless can't work with a Vortech (different fuel pressure control system: returnable - mechanical vs. returnless - PCM controlled). However, you CAN use a chip to boost fuel pressure on a returnless system.
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Joined: Aug 2001
Posts: 1,050
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Yeah, all that stuff is correct, but the 19# injectors in our car are rated at 39psi, not the 55psi that our svt pressure regulators run at WOT.
Therefore they are acting like larger injectors due to the higher pressure and are therefore sufficient for light modifications over the stock applications.
Either way, you can't just assume that 'maxed injectors' is what is happening because you 'think' your engine is lean, or even if you have A/F data showing you are lean. That is why you need calculations and measurements. All reasons need to be explored...christ, you could have a plugged fuel filter and get the same effect.
warmonger
You can call me anything you like as long as it's nice.(all lies accepted)
99 Silver Frost SVT. #226 of 2760
Engine: 3.0 power!
Unique Stuff: Sunroof control module (#1 of 9)
Car Audio: Loaded and loud!
Check them out at
http://home.earthlink.net/~twilson1726
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