Hello Hot Rod fuel hose fans, today we will be talking about pressure drop & flow rates and what it means for hose size selection.
Hot Rod fuel size sells the two most common sizes of PTFE lined and stainless or nylon braided hose for multiple uses Including transmission cooler lines, fuel delivery, vacuum lines and many other uses on your rod. AN6 and AN8 - but which one do you need for your build? We will explain the decision process below.
When planning your plumbing project for fuel usage Horsepower plays a big role in what size you are going to need, as well if you are running a carbed application or fuel injected. We will look at both examples.
First we need to figure out the flow rate required for the HP of our engine (ugghhh more math) and depending on if we are using an electric or manual fuel pump the calculation is a bit different. (Electric is measured in LPH and Mechanical is measured in GPH)
Optimal LPH = (Max. HP x BSFC) / 1.585
Optimal GPH = (Max. HP x BSFC) / 6
BSFC = Brake specific fuel consumption
Ok. so what the heck does all of this mean?
First we need to figure out what BSFC is. Well luckily we can get pretty darn close for most engines as Naturally aspirated uses approx. 0.5 Lbs per hour at full throttle, Nitrous 0.6 Lbs, and boosted applications .6 to 0.75, obviously individual engine combinations can vary.
So let's say we had a 400HP small block chevy with no boost or nitrous and we were to compare both carburetor and EFI, don't worry I promise this will lead to PTFE hose size selection.
First let's figure out GPH for a mechanical pump used on a carbed engine.
(400 X 0.5) / 6 = 33.3 GPH
Next let's figure out LPH for an Electric fuel pump used on an EFI setup.
(400 x 0.5) / 1.585 = 126 LPH
Wow too much thinking for today, but we can now use these figures to get us to hose size. *Now a word of warning that many factors come into play that can effect the calculations such as if you add fitting size adapters, use extra fittings, too tight on the bend radius (see yesterdays blog) and anything else you put in line of the system.
How much pressure drop can we expect - for 10ft of smooth PTFE lined hose at this flow rate we can expect ~2.7 PSI of drop for AN6 size ptfe hose, and ~1.0 PSI of drop for AN8 size PTFE lined hose. Where did these numbers come from? For this example we used basic assumed values that can be found on google by searching for "pressure drop smooth hose".
Hopefully you can now see why we are doing two examples.
In order to overcome the needle float on a carb you need some pressure usually above 3psi. If we have a weak pump and run 10ft of hose to the inlet we may run out of oomph to fill the bowls. A pump running 6 Psi at the head of a length of 10ft hose may only see 2.5 PSI at the carb by the time all the variables are calculated in and preventing fuel flow.
Our second example on an EFI system running at 58PSI and a basic Electric OEM fuel pump capable of 155L of flow at 60PSI can much more easily handle the pressure drop delivered by the components in between the pump and the EFI system.
In this example our EFI system would function just fine on AN6 sized hose but our carbed engine may need to step it up to AN8.
Ok Mr. Hot Rod fuel hose what friggin size do I need? Well as you know every build is different but as general rule a carbed setup can run AN6 up to about 450HP with a strong mechanical pump. AN8 can take you up to approximately 600HP. An EFI setup you can run up to about 600HP using AN6 over that we recomend going to AN8 size. If you are running boosted applications and going over 1000HP well you are spending big bucks and can afford to get someone to do that math and build the proper system for you.
* The big warning - we used many estimates and assumptions in doing the calculations for this post. Be smart if you're not sure please ask an expert. Hot Rod fuel hose is always available to answer any questions around line sizing for your build.
* Second we did not show examples for uses like transmission cooler lines, usually flow rates are higher in these applications and we recommend AN8 size for cooler lines.
Here’s a simple online calculator for pressure drop from Gates Corp. It’s designed for hydraulic hoses, but if you enter the correct values for gasoline, it gives pretty good results. Just thought I’d pass this along.
Try it with the following values: hose ID: 0.320 in, flow rate: your GPH/60, hose length: 10 ft, viscosity: 0.480 cp (65F), specific gravity: 0.74 (60F)
As V8Nate pointed out, the fittings add a significant pressure drop. Enjoy!
Is it ok to run half inch fuel lines from the tank and Holley blue fuel pump at the back of the car up to the fuel pressure regulator then go down to 3/8 from regulator to carburetor.
I am getting ready to put a holley sniper on my fairly stock big block chevy and plan on boosting it in the future. The question I had to you is I already purchased some -8 ptfe fuel line for the efi I’m prep for being turboed and was wondering if that was going to cause any issues with a stock motor? The car is a 71 monte carlo and the fuel pump is a aem 50-1005 400 lph in line pump
Hi guys, I like your approach to selecting fuel size, however I think a huge factor was left out of your analysis.
AN fittings can vary greatly, and if you take measurements of the bore size through AN fittings I think you’re in for a horrible surprise. I have taken measurements from Aeroquip, Earl’s, Russell, and generic (Summit/Jegs) fittings and noticed that AN-6 fittings (3/8" hose) often have less than 1/4" through bore- especially fittings incorporating swept bends (45 degree through 180 degree). Even straight fittings represent a very significant orifice but are not usually as egregious as the swept bend fittings.
These dramatically affect flow capacity and pressure drop through the system and must be included in any valid analysis. Often, the pressure drop through the hose is completely negligible compared to the flow resistance in the fittings.