# Maximum Fuel Flow through CIS hard lines?



## Southcross (Jul 2, 2004)

I have reached the max fuel flow of my Stock CIS fuel pump... 

I'm upgrading to a high-flow (190 or 255lph) fuel pump, what is the flow limit through a stock set of CIS hard lines? Am I better off cutting out the old lines and going straight AN fittings between pump and body hardline?


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## WaterWheels (Aug 14, 2005)

OK, I'm sure this will draw the wrath of someone by asking these questions, but hey, if not me the who? 
1) Are you making 300hp or more? Using a bad fuel usage figure, 190gph is enough for a normal engine to produce over 300hp and 255gph is around 450hp. So just why do you need this large amount of flow? CIS relies on pressure, that's a given, but more flow will do nothing if the engine does not have the ability to use the extra fuel, so 300hp? 
2) The injectors come into play here too. I can't recall just where I read this or if I saved it somewhere, but I believe the CIS injectors can flow enough fuel to support 50hp or 60hp per injector. So added flow will not even go through them and about 200hp is their limit. That said, they will not flow enough to support using the higher flowing fuel pump anyway. 

Maybe you have a monster 400hp engine and need the flow you are looking at, don't know as you don't say. But if you do then you need to also be thinking of a different method of getting it into the engine as the injectors themselves will not support that kind of power. So maybe a little background is needed.


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## Southcross (Jul 2, 2004)

WaterWheels said:


> 1) Are you making 300hp or more?


 uhm.. ya, the car is more than _slightly_ modified  

pretend this isn't a "CIS" question, but a question about the main CIS fuel lines  

I just need to know if the hard lines can handle the flow, specifically what I question is the stock pump banjo fitting/Accumulator/etc, I know the lines themselves are big enough.


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## WaterWheels (Aug 14, 2005)

I'm not so much of a "lets pretend" kind of guy, also not into "lets assume", "what if" or hypothetical questions. I feel they mask the real issue at hand or just pose as a cover for untruth of kinds. After all, this is a CIS forum so one would have to assume the question is about a CIS system or why be here, right? 

Never having tried this on a VW I can only guess based on what I know and have done, but no, I doubt the lines could handle it well. Of course the pressure behind the flow also makes a difference. The main lines are about 7mm (a little less) inside or roughly 1/4". The banjo fittings have small holes in the bolts for the fuel to pass through and it has to make some sharp turns, both not good for flow. I would say about 9mm (3/8") would be better, maybe even 5/16" but why risk it. Better flowing fittings would also be good, straight or curved, last resort 90° bends. But I have to say up front, I don't believe you have close to 300hp and require that amount of flow, but it sounds good I guess.


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## Southcross (Jul 2, 2004)

this is such a typical vortex thread, 90% opinion and 10% answer...


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## noizze (Aug 19, 2003)

I would say measure the open volume. Theirs something about it in the Bentley. If that volume is not enough to support your HP needs then an upgrade might be in order. Then check for a pressure drop at the primary inlet. 
Free flow volume is mostly an issue with line size. Pressure at a given volume is a pump issue. 
Is a 255lph pump rated at a given pressure? Will it support CIS pressure levels?


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## WaterWheels (Aug 14, 2005)

Although I really don't understand, or care about for that matter, the reason behind or the meaning of this


> this is such a typical vortex thread, 90% opinion and 10% answer...


 I have to correct something I stated above. The horsepower figures I stated were off due to a mistake on my part. You can just about double what I said above for the lower and upper numbers.


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## Southcross (Jul 2, 2004)

still... you people are hung up on the CIS fuel distributor and CIS injectors. These are not things I'm asking asking about  CIS Fuel pump and their fuel lines/fittings... the only things I am asking about here 

From Bentley Spec: 
Free Flow for 30s Min: .9L 
Equates to a free flow of 108Lph


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## noizze (Aug 19, 2003)

Southcross said:


> From Bentley Spec:
> Free Flow for 30s Min: .9L
> Equates to a free flow of 108Lph


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## Southcross (Jul 2, 2004)

noizze said:


> I would hate to have a sweet motor f'd up by a fuel delivery problem.


Yes! thats why I'm asking these questions :laugh:


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## WaterWheels (Aug 14, 2005)

As I said, it really doesn't matter, but I can't see where the fuel distributor was mentioned and you have to admit, if you make a post like your original in a CIS forum, the injectors just might come into the conversation. If your first post maybe stated a little more about what you want to do rather then just leaving it to the immagination, it’s possible the injectors would not have been talked about at all. And besides, once you did clear that up, they were not addressed about again, so. . . 

The plastic lines are about the same size inside as the metal lines. I have some sections in the garage and they run about 8mm outside and 6mm inside (give or take). Yes, the fittings reduce things again all of them, the pressed in ones on the plastic reduce down to about 4mm. The male threaded ends down to about 4.7mm and the metal tubes with the banjo welded on the ends are about 6mm inside. The holes into the banjo vary from about 4mm to 6mm depending on which one and who made it. I do not know the hole size in the banjo bolts, but it also varies on the bolt size. Seeing as there are at least 4 holes the area should be no less (more I think) then the tubing. The sharp bends the fuel has to make will hurt flow for sure on the banjo fittings. How much all this, fittings and such, effect the flow is something you would have to ask a person who can calculate flow good, it’s not my forte. 

Please feel free to add Smilies when and where you like, I just never find the time to click on them when posting. 

Above I was just off the top of my head calculating the horsepower possibility vs. the flow. Lets use the data posted by noizze of 1.4L in thirty seconds as a base line, better yet lets use the .760L/30 sec. manual test figure as a base line (CIS pump w/12volts). Lets also assume you are very good at building an engine and rather then a BFSC of .5 - .55 which is standard for a piston engine, lets use say .48 (just a ball park figure). That would mean that the CIS pump is capable of producing 310hp (.760 liters per 30 seconds = 91.2 liters an hour = 24 gallons an hour times the average weight of a gallon of gasoline (6.2lb) = 148.8lbs of gasoline in an hour divided by the BFSC of .48 = 310hp). So if the CIS pump can really deliver closer to1.7L per 30 seconds you can about double the power figure. Now this is measured at the fuel distributor most times so any flow issues up until there are in the calculation. 

Now for the real life comments. Go with 3/8” fuel lines and fitting if you intend to try and build an engine that can produce that 300hp. You can NEVER go wrong with having the fuel line bigger, but if it is too small you may never notice that it is stopping you from reaching your goal. If you want exact figures on what flow how much and just how the fittings effect things you’ll have to find someone who is good at calculating flow, I never dabbled in that area.


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## Southcross (Jul 2, 2004)

Southcross said:


> I have reached the max fuel flow of my Stock CIS fuel pump...
> 
> I'm upgrading to a high-flow (190 or 255lph) fuel pump, what is the flow limit through a stock set of CIS hard lines? Am I better off cutting out the old lines and going straight AN fittings between pump and body hardline?


 this question wasn't specific enough for you? 


oh, and your estimates on HP for a 90lph pump, and BSFC numbers are way off


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## noizze (Aug 19, 2003)

The original question unanswerable, flow is related to pressure. As flow increases, pressure drop increases at a (IIRC) logarithmic rate. So you don't get twice the volume with twice the pressure. 

So here's the answer: At CIS pressure levels there may be enough flow to support 300+hp. I would question reliability. 
If you plan on using EFI, as injectors don't like CIS level pressures, you would be advised to up grade the fuel lines. I would personally upgrade, piece of mind is worth a lot to me.


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## WaterWheels (Aug 14, 2005)

Southcross said:


> oh, and your estimates on HP for a 90lph pump, and BSFC numbers are way off


 Please point out just where I am “way” off on the above calculation (example if you like). 
~ The .760L per 30 seconds? That is straight out of many repair manuals for a CIS fuel pump with the measurment taken at the return line. You can even find it in the all knowing Bentley repair manual. 
~ If you have .760 liters in 30 seconds that would be 1.52 liters in one minute, right? And then seeing as there are 60 minutes in an hour that would be 91.2 liters an hour by my math, right? 
~ Then when you use the number 3.785, which is the number of liters in a gallon, and divide that into the 91.2 liters it comes out to 24 gallons (rounded down). Good so far? 
~ OK, so we have 24 gallons of gasoline feeding the engine in one hour. As we all know we burn gasoline by weight not air fuel ratios so we need to know just how much gasoline that is in pounds. Gasoline weight differs with temperature but a common figure of 6.2 lbs/gallon is used in many places, like air craft calculations. So, 24 gallons times the 6.2 pounds per gallon comes to 148.8 pounds of gasoline, correct? 
~ We know now that our fuel system, system not the pump as we are taking the measurment on the return line, is flowing 148lbs of gasoline an hour. So our next step is to divide that weight by the amount of gasoline, by weight, that a piston engine uses to produce 1 horsepower in one hour. That is what the Brake Specific Fuel Consumption (BSFC) is, right? The normal figure agreeded upon by most builders, tunners and designers is .5 - .55 but a really well built racing engine could be .4. I doubt you are on the same level as Formula 1 teams or other expensive racing platforms, so I gave you a .48 BSFC, which is really good I might add. 
To end our example we take our 148.8 pounds of gasoline an hour and divide that by the weight that is required to produce one horsepowe in an hour. 148.8 divided by 4.8 gives us, Ops, 310, which is the horsepower “potential” of our fuel system. Potential does not mean you will make that much but that you can. 

So exactly where is the problem here? Show me how you calculate things. 



Southcross said:


> this question wasn't specific enough for you?


 Actually no. “I have reached the max flow of my Stock CIS fuel pump…” Indicates we are talking about a CIS system and it’s fuel pump in stock form, nothing more and nothing less (doubt is thrown in with the reached the max flow part). The “question” states you are upgrading the pump, fine. The question asks what the flow limit for the CIS hard lines are, now it becomes an interpretation thing as what is happening once you reach the end of the fuel line? Is it going to be stuck into the intake manifold, is it going to be used with an EFI system, carbs? All we know is it is a stock CIS system so any response has to be addressed from that information. Sure, you know what you’re asking or talking about, but how about the rest of us? To be honest I was going to respond that upgrading the line was a waste of money and somewhat dumb really for a stock CIS system. But I had the feeling there was more behind the original post so I provided some information while also asking some questions. Guess just like your comment about typical Vortex responses, I should have remembered there are still some typical Old School Vortex members also.


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## bimmerboy02 (Oct 2, 2005)

If you're thinking of only upgrading to a 190lph I don't think you've reached the limit of the stock pump. CIS pumps are huge performers, and easily out do a walbro 255 if in an FI application. Depending on the model of the pump sometimes the only upgrade is an 044. Either your's is dying or its just the lines, which I'd bet reach their limit before the pump.


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## chois (May 12, 2000)

Southcross. Stop being a tool. 

You asked a really vague question and people started trying to fill in the blanks based on where you posted it. 

Sounds like you already have it all figured out, yet you are asking for an answer. 

Based on what you typed here, go get a bigger fuel pump and some larger diameter fuel line. You can find what you need at Summit, JEGS, HRP and Pegasus. 

Good luck.


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