I have heard that sizing the injectors at 45% max duty cycle is best. Any thoughts?

 

No. I don't know were that number is from, but some people think biggest is best when that's not true. When you aim for a Horsepower you also select injectors for that power. If you want only 400HP then 650 or 750cc injector will work great. This will give you the best idle and best spray pattern for mid to higher power. Around 80% is when you wanna stop. Others and I run 90% just fine though and is very doable.

 

100%

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by adseguy &raquo;</TD></TR><TR><TD CLASS="quote">No. I don't know were that number is from, but some people think biggest is best when that's not true. When you aim for a Horsepower you also select injectors for that power. If you want only 400HP then 650 or 750cc injector will work great. This will give you the best idle and best spray pattern for mid to higher power. Around 80% is when you wanna stop. Others and I run 90% just fine though and is very doable.</TD></TR></TABLE>

makes sense. I have heard of the 80% industry standard for duty cycle but I think that applies to N/A cars. The only reason I say this is I read something that I will post below. Let me know what you think of the logic.

"If you're turbo charged, you're better off staying on the low side of the duty cycle range (45%). Here's why: Under Turbo boost the fuel has to overcome the extra pressure in the intake manifold. Injectors are rated by using 45 pounds of fuel pressure (PSI), so this means that to maintain the same rated flow while under boost, the fuel pressure would have to be 45 PSI higher than the boost pressure. A fuel pressure that high would overburden the pump so what you want to do is use a larger injector that can flow more fuel at a lower fuel pressure. An injector operating at 45% of it's capacity should be a good starting point. In this case a 45 pound/hour injector will work and you could lower your fuel pressure to 25 PSI to get enough flow."

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by flood &raquo;</TD></TR><TR><TD CLASS="quote">makes sense. I have heard of the 80% industry standard for duty cycle but I think that applies to N/A cars. The only reason I say this is I read something that I will post below. Let me know what you think of the logic.

"If you're turbo charged, you're better off staying on the low side of the duty cycle range (45%). Here's why: Under Turbo boost the fuel has to overcome the extra pressure in the intake manifold. Injectors are rated by using 45 pounds of fuel pressure (PSI), so this means that to maintain the same rated flow while under boost, the fuel pressure would have to be 45 PSI higher than the boost pressure. A fuel pressure that high would overburden the pump so what you want to do is use a larger injector that can flow more fuel at a lower fuel pressure. An injector operating at 45% of it's capacity should be a good starting point. In this case a 45 pound/hour injector will work and you could lower your fuel pressure to 25 PSI to get enough flow."</TD></TR></TABLE>

that is the same theory that we all go with. Just our reasoning for sizing the injectors smaller to get a higher duty cycle is to maintain a nice idle.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by StolenTeg &raquo;</TD></TR><TR><TD CLASS="quote">100% </TD></TR></TABLE>

+ some fuel pressure =

+ 1. injector duty cycles

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by flood &raquo;</TD></TR><TR><TD CLASS="quote">"If you're turbo charged, you're better off staying on the low side of the duty cycle range (45%). Here's why: Under Turbo boost the fuel has to overcome the extra pressure in the intake manifold. Injectors are rated by using 45 pounds of fuel pressure (PSI), so this means that to maintain the same rated flow while under boost, the fuel pressure would have to be 45 PSI higher than the boost pressure. A fuel pressure that high would overburden the pump so what you want to do is use a larger injector that can flow more fuel at a lower fuel pressure. An injector operating at 45% of it's capacity should be a good starting point. In this case a 45 pound/hour injector will work and you could lower your fuel pressure to 25 PSI to get enough flow."</TD></TR></TABLE>

Unless I'm reading wrong - THAT is why a fuel pressure regulator has a vacuum/boost reference. This serves to increase the fuel pressure by the same amount of pressure inside the intake manifold thus keeping the pressure across the injector constant.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by EE_Chris &raquo;</TD></TR><TR><TD CLASS="quote">

Unless I'm reading wrong - THAT is why a fuel pressure regulator has a vacuum/boost reference. This serves to increase the fuel pressure by the same amount of pressure inside the intake manifold thus keeping the pressure across the injector constant.</TD></TR></TABLE>

That solves the issue at hand.

thanks