injector experts please enter-injectors sizing for HP goals and fuel pressure and how they are relat
12-09-2005, 06:23 AM
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injector experts please enter-injectors sizing for HP goals and fuel pressure and how they are relat
Hey guys,
I was responding to a fellow H-T members post about whether 780cc injectors would be good for 450 HP. Now usually when I help somebody out with a question like this I use the formula on rceng.com to find out what HP a set of injectors is good for. But this time a lightbulb came on in my head. Since we are talking about boosted setups, I thought about how boost pressure affects fuel pressure. For non FMU setups we run either a stock or aftermarket 1:1 ratio FPR. So this means the fuel pressure increases for every pound of boost. That's when I realized that all this time I've been doing HP limits for injectors at only 45 psi of fuel pressure the pressure at which companies rate their injectors. I never took into account the fuel pressure increase from boost, and how this fuel pressure increase allows you to run more fuel out of a given injector.
Now that I have to account for the amount of boost being added to the fuel pressure, I want to put it into practice. I wanted to get your guys expert opinions on this to see if it all looks right.
EXAMPLE:
Usually when I find HP limits for a injector I do it for 85% injector duty cycle (IDC) and for 0.50 BSFC (naturally aspirated) and 0.60 (turbocharged). The formulas I'm using are from the tech portion of rwww.rceng.com
In this example we'll say the HP goal is going to be 450HP on a turbo which will be running at 20 psi. We want to see if the 780cc injectors we have will be enough for this power goal. Since we are running 20 psi of boost pressure and large injectors, we'll want to lower the base fuel pressure. This is so the injectors can idle better and also to lower overall fuel pressure so as to not limit the fuel pumps flow. So we are going from a base fuel pressure of 45 psi to 35 psi.
FIRST find out the injector flow changes from the rated flow @45 psi to our actual fuel pressure.
The first thing I would probably have to do is calculate the new injector's flow requirement or the most it will be able to flow at full load. So at full load/full boost the fuel pressure is going to be base+boost pressure=overall pressure, or 35+20=55 psi of fuel pressure. Here is the formula to calculate the new injector flow rate based on pressure change:
(new pressure/old pressure) SQUARE ROOT x old injector flow rate = new injector flow rate
Now lets do our example:
(55/45) SQUARE ROOT x 780 = 860cc's new injector flow rate
So what we did is took the new fuel pressure (base+boost) and divide it by the old pressure (injector flow rate at 45 psi as tested by manufacturer). 55 divded by 45 equals 1.22, hit the square root button on your calculator and 1.2222 becomes 1.105. Now multiply 1.105 by the old injector flow rate of 780 to get our new injector flow rate of 860.
NEXT finding HP limits of injectors:
The formula for this is also on http://www.rceng.com and it's pretty simple:
First convert CC per minute to LB per minute by dividing CC by 10.5
so 860/10.5 = 81.90 lb/min
From there it's:
(lb/min x duty cycle) / BSFC = HP per injector
For our example:
(81.90 x 0.85) / 0.60 = 116 HP per injector (4 injectors equals 460 HP)
I usually use 85% duty cycle for my "safe limit". I usually calculate it for 0.60 BSFC, then again for 0.50 BSFC to give me a good high and low HP limit. So we'll do it again for 0.60 BSFC:
(81.90 x 0.85) / 0.50 = 139 HP per injector (4 injectors equals 550 HP)
So that means at 35 base fuel pressure, and 20 lbs of boost pressure, our 780cc injectors are able to flow 860cc/min. Which would be good for 460-550HP, and since our goal was 450 HP @ 20 psi that means the injectors would be good. For you experts would you say that's a good way to find HP limits for injectors?
what do you guys think?
TIA
I was responding to a fellow H-T members post about whether 780cc injectors would be good for 450 HP. Now usually when I help somebody out with a question like this I use the formula on rceng.com to find out what HP a set of injectors is good for. But this time a lightbulb came on in my head. Since we are talking about boosted setups, I thought about how boost pressure affects fuel pressure. For non FMU setups we run either a stock or aftermarket 1:1 ratio FPR. So this means the fuel pressure increases for every pound of boost. That's when I realized that all this time I've been doing HP limits for injectors at only 45 psi of fuel pressure the pressure at which companies rate their injectors. I never took into account the fuel pressure increase from boost, and how this fuel pressure increase allows you to run more fuel out of a given injector.
Now that I have to account for the amount of boost being added to the fuel pressure, I want to put it into practice. I wanted to get your guys expert opinions on this to see if it all looks right.
EXAMPLE:
Usually when I find HP limits for a injector I do it for 85% injector duty cycle (IDC) and for 0.50 BSFC (naturally aspirated) and 0.60 (turbocharged). The formulas I'm using are from the tech portion of rwww.rceng.com
In this example we'll say the HP goal is going to be 450HP on a turbo which will be running at 20 psi. We want to see if the 780cc injectors we have will be enough for this power goal. Since we are running 20 psi of boost pressure and large injectors, we'll want to lower the base fuel pressure. This is so the injectors can idle better and also to lower overall fuel pressure so as to not limit the fuel pumps flow. So we are going from a base fuel pressure of 45 psi to 35 psi.
FIRST find out the injector flow changes from the rated flow @45 psi to our actual fuel pressure.
The first thing I would probably have to do is calculate the new injector's flow requirement or the most it will be able to flow at full load. So at full load/full boost the fuel pressure is going to be base+boost pressure=overall pressure, or 35+20=55 psi of fuel pressure. Here is the formula to calculate the new injector flow rate based on pressure change:
(new pressure/old pressure) SQUARE ROOT x old injector flow rate = new injector flow rate
Now lets do our example:
(55/45) SQUARE ROOT x 780 = 860cc's new injector flow rate
So what we did is took the new fuel pressure (base+boost) and divide it by the old pressure (injector flow rate at 45 psi as tested by manufacturer). 55 divded by 45 equals 1.22, hit the square root button on your calculator and 1.2222 becomes 1.105. Now multiply 1.105 by the old injector flow rate of 780 to get our new injector flow rate of 860.
NEXT finding HP limits of injectors:
The formula for this is also on http://www.rceng.com and it's pretty simple:
First convert CC per minute to LB per minute by dividing CC by 10.5
so 860/10.5 = 81.90 lb/min
From there it's:
(lb/min x duty cycle) / BSFC = HP per injector
For our example:
(81.90 x 0.85) / 0.60 = 116 HP per injector (4 injectors equals 460 HP)
I usually use 85% duty cycle for my "safe limit". I usually calculate it for 0.60 BSFC, then again for 0.50 BSFC to give me a good high and low HP limit. So we'll do it again for 0.60 BSFC:
(81.90 x 0.85) / 0.50 = 139 HP per injector (4 injectors equals 550 HP)
So that means at 35 base fuel pressure, and 20 lbs of boost pressure, our 780cc injectors are able to flow 860cc/min. Which would be good for 460-550HP, and since our goal was 450 HP @ 20 psi that means the injectors would be good. For you experts would you say that's a good way to find HP limits for injectors?
what do you guys think?
TIA
12-09-2005, 07:26 AM
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Re: injector experts please enter-injectors sizing for HP goals and fuel pressure and how they are r
I've never thought of that. I'm not any kind of "expert" on injectors, but it looks right to me. 
12-09-2005, 07:29 AM
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Re: injector experts please enter-injectors sizing for HP goals and fuel pressure and how they are r
Nice information, - want to see this thread develope into something really technical.
You might want to consider doing the same thing for fuel pumps and fuel line diameter. Seeing in how these are all directly related. These 780cc injectors may flow up to 860cc. But that doesnt do any good if you starve the injector from fuel.
I would like to see all this discussed, and i think this could turn into a REAL GOOD STICKY!!
You might want to consider doing the same thing for fuel pumps and fuel line diameter. Seeing in how these are all directly related. These 780cc injectors may flow up to 860cc. But that doesnt do any good if you starve the injector from fuel.
I would like to see all this discussed, and i think this could turn into a REAL GOOD STICKY!!
12-09-2005, 08:39 AM
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Re: injector experts please enter-injectors sizing for HP goals and fuel pressure and how they are r
when i always calculated, i figured in the 1:1 rise in the fuel pressure. i hope i wasnt doing this in error.
12-09-2005, 08:53 AM
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Re: injector experts please enter-injectors sizing for HP goals and fuel pressure and how they are r
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Importordomestic »</TD></TR><TR><TD CLASS="quote">Nice information, - want to see this thread develope into something really technical.
You might want to consider doing the same thing for fuel pumps and fuel line diameter. Seeing in how these are all directly related. These 780cc injectors may flow up to 860cc. But that doesnt do any good if you starve the injector from fuel.
I would like to see all this discussed, and i think this could turn into a REAL GOOD STICKY!!
</TD></TR></TABLE>
Yup, so far I've only been able to find fuel pressure to fuel flow charts for Aeromotives and Walbros. I cant find any for other brands like Holley, or whatever other brand fuel pumps people use. A few weeks ago I did some work to find limitations of Walbro and A1000 pumps. I could do a little something on that, but I need help figuring out how to convert gallons per hour to liters per hour. I use http://www.onlineconversions.com but they have liters per hour (UK) and liters per hour (US) conversions. From the looks of it I would need to use liters per hour (UK).
One other thing too is I'm trying to figure out how much extra fuel flow from a pump would be required to support a given injector size. For example in order to support a set of 1000c injectors maxed out you'd need a pump that can flow 4000+ cc's a minute. But this is where I'm unsure...would you be ok with a fuel pump that can flow a max of 4000cc/min 100% maxed out? or would you need a pump that can flow a max of 4500cc's and is only actually providing 4000cc's to the injectors...which would mean the pump is operating at about 88-90% capacity. That's what's keeping me from doing something on fuel pumps. You know how some people say you shouldn't operate an injector above 90% duty cycle for safety reasons? Do fuel pumps have a similar type of "duty cycle" that we dont want to exceed?
You might want to consider doing the same thing for fuel pumps and fuel line diameter. Seeing in how these are all directly related. These 780cc injectors may flow up to 860cc. But that doesnt do any good if you starve the injector from fuel.
I would like to see all this discussed, and i think this could turn into a REAL GOOD STICKY!!
</TD></TR></TABLE>Yup, so far I've only been able to find fuel pressure to fuel flow charts for Aeromotives and Walbros. I cant find any for other brands like Holley, or whatever other brand fuel pumps people use. A few weeks ago I did some work to find limitations of Walbro and A1000 pumps. I could do a little something on that, but I need help figuring out how to convert gallons per hour to liters per hour. I use http://www.onlineconversions.com but they have liters per hour (UK) and liters per hour (US) conversions. From the looks of it I would need to use liters per hour (UK).
One other thing too is I'm trying to figure out how much extra fuel flow from a pump would be required to support a given injector size. For example in order to support a set of 1000c injectors maxed out you'd need a pump that can flow 4000+ cc's a minute. But this is where I'm unsure...would you be ok with a fuel pump that can flow a max of 4000cc/min 100% maxed out? or would you need a pump that can flow a max of 4500cc's and is only actually providing 4000cc's to the injectors...which would mean the pump is operating at about 88-90% capacity. That's what's keeping me from doing something on fuel pumps. You know how some people say you shouldn't operate an injector above 90% duty cycle for safety reasons? Do fuel pumps have a similar type of "duty cycle" that we dont want to exceed?
12-09-2005, 10:36 AM
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Re: injector experts please enter-injectors sizing for HP goals and fuel pressure and how they are r
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by BlueShadow »</TD></TR><TR><TD CLASS="quote">
You know how some people say you shouldn't operate an injector above 90% duty cycle for safety reasons? Do fuel pumps have a similar type of "duty cycle" that we dont want to exceed?</TD></TR></TABLE>
you would think, but being submerged should help keep it cool. i dont think there would be a duty cycle since it is running 100% of the time. running at 90% capacity you would start thinking. the school of thought on all the buicks we built was to put the fuel pump just uinder the max, and because we always used new pumps, this was ok. example. the modern muscle pump ( close to our walbro ) we used to use was 40 gal per hour at 80 psi. we would set the line off pressure at 50 psi, and i used to run 25 psi boost. 75 psi rail pressure. under the pumps max. anything more than that we would install a bosch 286 outboard pump.
You know how some people say you shouldn't operate an injector above 90% duty cycle for safety reasons? Do fuel pumps have a similar type of "duty cycle" that we dont want to exceed?</TD></TR></TABLE>
you would think, but being submerged should help keep it cool. i dont think there would be a duty cycle since it is running 100% of the time. running at 90% capacity you would start thinking. the school of thought on all the buicks we built was to put the fuel pump just uinder the max, and because we always used new pumps, this was ok. example. the modern muscle pump ( close to our walbro ) we used to use was 40 gal per hour at 80 psi. we would set the line off pressure at 50 psi, and i used to run 25 psi boost. 75 psi rail pressure. under the pumps max. anything more than that we would install a bosch 286 outboard pump.
12-09-2005, 11:11 AM
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Re: injector experts please enter-injectors sizing for HP goals and fuel pressure and how they are r
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by boostedcivicsir »</TD></TR><TR><TD CLASS="quote">
you would think, but being submerged should help keep it cool. i dont think there would be a duty cycle since it is running 100% of the time. running at 90% capacity you would start thinking. the school of thought on all the buicks we built was to put the fuel pump just uinder the max, and because we always used new pumps, this was ok. example. the modern muscle pump ( close to our walbro ) we used to use was 40 gal per hour at 80 psi. we would set the line off pressure at 50 psi, and i used to run 25 psi boost. 75 psi rail pressure. under the pumps max. anything more than that we would install a bosch 286 outboard pump.</TD></TR></TABLE>
My concern wasn't with temperature, but with running out of flow. Like in my example, if the max the injectors can ever flow is 4000cc's for all 4, what size pump would be adequate to support that? would a 4000cc pump be ok?, or would you want a pump that can flow slightly more then the needed 4000cc? I'm guessing the answer is probably the second one, but how much more flow do we need?
you would think, but being submerged should help keep it cool. i dont think there would be a duty cycle since it is running 100% of the time. running at 90% capacity you would start thinking. the school of thought on all the buicks we built was to put the fuel pump just uinder the max, and because we always used new pumps, this was ok. example. the modern muscle pump ( close to our walbro ) we used to use was 40 gal per hour at 80 psi. we would set the line off pressure at 50 psi, and i used to run 25 psi boost. 75 psi rail pressure. under the pumps max. anything more than that we would install a bosch 286 outboard pump.</TD></TR></TABLE>
My concern wasn't with temperature, but with running out of flow. Like in my example, if the max the injectors can ever flow is 4000cc's for all 4, what size pump would be adequate to support that? would a 4000cc pump be ok?, or would you want a pump that can flow slightly more then the needed 4000cc? I'm guessing the answer is probably the second one, but how much more flow do we need?
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