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Ask Away with Jeff Smith: Fuel Volume vs. Fuel Pressure & How They Relate

I’m considering buying one of those self-learning TBI fuel injection systems. I’ve heard that the fuel delivery system is the most important part of the installation so I’ve been reading up on electric fuel pumps. I’ve noticed that companies rate their pumps at different power levels based on how they will be used. Why does it make a difference whether the engine is EFI or carbureted and why does a supercharger reduce the amount of horsepower the pump can deliver?

J.G.

Jeff Smith: This is a great question. Let’s say a given pump is rated to deliver 800 horsepower worth of fuel. This is a shorthand way of stating the pump’s capacity. The two most important values are the amount of fuel (often now given in liters per hour- lph) and the pressure. Basically, as the a fuel pump increases pressure, volume will decrease. This is just a basic design result of current fuel pumps. Let’s use that example of a pump rated at 800 horsepower. This is not an accurate evaluation because we don’t know the pressure. That’s why companies will state “800 horsepower for a normally aspirated engine with a carburetor.” What this means is the pump is capable of pumping 800 horsepower worth of fuel at probably around 6 to 8 psi. That same pump may drop its capacity to 600 horsepower when used with fuel injection. That’s because EFI requires anywhere from 43 to 60 psi of fuel pressure.

Remember, as pressure increases, capacity decreases.

These recommendations are still somewhat vague because they don’t generally list the specific pressures. The best thing to do to evaluate a fuel pump is to obtain the manufacturer’s official output rating. Sometimes this is in the form of a graph or specific output numbers on a chart. Another important variable for electric pumps is the operating voltage. The best information the manufacturer can give you is the operating pressure, the volume – usually stated in pounds per hour (lbs/hr), and the voltage the pump was operating on. Some companies will try to fool you with high output numbers at zero pressure. This is really just an advertising gimmick because all fuel delivery systems operate at some given pressure.

So perhaps now you can begin to see how pressure and operating voltage affect a pump’s output. Now let’s introduce yet another variable. Let’s say that you have a big block Chevy with a turbocharger and you are going to use a multi-point EFI system to feed the fuel to the engine. First off, this means the system will run at a higher fuel pressure of at least 43 to 45 psi. But there’s a further complication. As the turbo increases output, the boost in the manifold begins to climb. Let’s use a simple case of 10 psi of boost. With a 45 psi of line pressure, this means the injectors located in the manifold must supply the fuel but work against the boost pressure. This means that the effective fuel pressure drops from 45 psi to 35 because of the 10 psi head pressure in the manifold. The simple solution is to boost-reference the fuel pressure to increase 1:1 with boost pressure. This way the fuel pressure at the injectors remains the same. However, this also means that the fuel pump must now make 55 psi of pressure to deliver 45 psi at the injectors, which means the pump is working harder and its capacity at this higher pressure is reduced. So that’s why any supercharged or turbocharged EFI engine fuel pump application will have a lower horsepower capacity than just a normally aspirated EFI engine.

The important point is to match the pump’s capacity to the application and to actually look at the manufacturer’s flow chart rating to make sure that you are getting a pump that has the capacity you need.

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8 Comments

  1. “Remember as pressure increases, capacity decreases.” What is your definition of capacity? So top fuel dragsters set of pumps capable of huge pressures have less CAPACITY than a YUGO?

    • The issue is capacity vs pressure. Again, let’s go back to our classic garden hose as an example. In order to increase the pressure exiting the hose, we create a restriction by clamping our thumb over the end of the hose. The pressure increases, but now it takes longer to fill the bucket. This applies to all pump circuits. Generally, for a typical performance engine, we use a large pump and then install a regulator to manage the pressure. If we look inside a typical fuel pressure regulator, it uses a small steel ball on a seat that is controlled by an adjustable spring . Moving the ball closer to the seat increases pressure, but now the fuel must travel through a smaller orifice, restricting the flow. Our fuel pump is designed to run within a range of pressure and volume. To get to the exact question – “so a Top Fuel engine has less capacity than a Yugo?” Clearly, the answer is no. A Top Fuel requires somewhere in the neighborhood of 90 gallons of fuel per minute of pump capacity, this equal 765 lbs of 80/20 blend of nitro and alcohol per minute. Fuel pumps are most often rated in pounds per hour (lbs/hr) – so converting this equals almost 46,000 pounds of fuel per hour of pump capacity. Let’s assume a Yugo pump capacity is 100 liters per hour (it’s probably less than that, but we’ll use this as an example). This is approximately 264 gallons per hour and at 6.2 lbs per gallon is 1,637 lbs/hr – which is roughly 3.5 percent of the capacity of a Top Fuel engine pump.

      We might have gone overboard on this answer, but the bottom line is that capacity will be diminished in any pump when pressure demands increase. Either the pump must restrict the exit side of the pump to create the pressure – or a pressure regulator will do that downstream, but either way, if you increase the pressure – the volume decreases. So if high volume and high pressure are the requirements – that means you must use a larger capacity pump and delivery system – and a larger fuel pressure regulator – to supply both the high volume required and the pressure. I hope this answers your question.

    • Richard Dahmer says:

      Your Right Jim, I think Jeff has forgotten Basic Physics! The Harder you Blow something through a given size of say a 5/16 fuel line! The more you force through it! Look at Hilborn Constant Flow Fuel Injector! Small Lines! A lot of Pressure! 60 Psi! At Wide open Throttle and the barrel valve wide open I am still pushing 60 to 70 percent of the Fuel BACK to the tank! Yet I can change the pill a few steps and literally drowned the engine at Full Throttle! I understand Jeff’s Math! But its like any engineer! On paper Bumble Bees are aerodynamically impossible! Yet! They seem to Fly very Well! The Math is great! But Reality is Physics! I have been racing Professionally since I was 16. 1 of my cars run Hilborn full flow infection on the street! It is self starting with out a primer! Most hot rodders and the math says this is impossible! Well I’ll take you for a ride any time! I admit fuel mileage sucks! But, Than in Hi Performance we do not care about fuel mileage! Look at the fuel lines on Any New EFI Car! Tinny! Yet they can sell you a Mustang GT-500 with 1,050 HP to the rear tiers! And it can be adjusted to 1,300!!! On pump Gas! I just ordered one! form Shelby’s factory in Los Vegas! Check it out! A Good friend I race with spent 2,600 hundred dollars upgrading his fuel system! BBC 69 Camero! Hi volume everything! Went to the track and has NOT been able to gain 1/1000th of a second off the old time! With the old holly fuel system! I toled him: Thank you! For saving me! All the Money!!!! LOL! I always buy High Pressure Oil Pumps! Too! You can have all the High Volume you like but through the same size internal oil galleys! Your Not going to get any more oil to the bearings! Unless you increase Pressure!!! Fact! Buy your logic Or, Math If I take a pump that is capable of pumping 60 gallons per hour! @ 5 PSI (I am making this real simple to see OK?) And I pump fuel into a 1 gallon Bucket! I should fill the Bucket in about 1 minute Correct? Now, If I increases the PSI on the Pump! To 10 PSI (Not change anything else!) I should be able to fill that 1 gallon Bucket in about 30 Seconds! Yes or No? (By the way I have done this! And it is True!) for me anyway! I think you are 100% Correct about the boost in the intake system and the effect on the fuel pressure! 10 LBS of Boost and you must increase fuel pressure by 10 PSI Or, counter the Boost!, I have a 1968 Blown 302 cid Cougar! I have experimented on for many years! Bought the car New! Its been a lot of fun and taught me a lot, I worked with the engineers at old B&M done some time on the project when we first started on it! Street Car, Run on Regular Gas 7.3 :1 Compression, 8.5: 1 is the magic number! But, I wanted to see if I could run cheap gas and still make very good HP! 15 LBS Boost! And It worked! And N2O if I want to scare myself to get the blood flowing now and then! LOL! I guess we all have a way to get to the end of the finish line! That’s Racing! No Offense Jeff, Sounds good, Sells parts! Its all good!, My 2 & 1/2 cents worth! Rick Dahmer.

      • Think about it this way: if you take a garden hose and turn it on, it’s flowing some volume at some pressure. let’s say that’s 5 gallons a minute and 2 psi. Obviously, you can fill a 5-gallon pail in a minute flat. Now, put your thumb over the end as hard as you can and fill the bucket for a minute. You’re probably not even going to get to halfway.

        It’s the same deal with your fuel pump. Your thumb on the garden hose is your fuel pressure regulator. Depending on the pump design, some output the most flow at zero backpressure, some output the most at a low-moderate backpressure.

        For EFI, the size of the fuel lines makes very little difference over 1/4″ because of the high pressure. In this regard, you’re absolutely correct. On most carbed applications with a FPR near the carb, 1/4″ is also probably fine. If you’re trying to run enough fuel fuel for a performance engine at 6 psi from the tank, then you’ll need a much larger line to accommodate the flow.

  2. i got a 6.0 ls motor in a 1970 chevelle that has been changed over from fuel injection to carb,how much fuel pressure should i run with a number 10 braided fuel line.

  3. Sweet, info and knowledge.

  4. Alan summers says:

    I just installed a Paxton no I 1000 Paxton supercharger kit (used) on my 2002 mustang gt 4.6L 2v. Kit was made for this car. Every thing seems fine I installed a bigger fuel pump supplied bigger injectors and Paxton Ecuador chip. Having a weird problem occasionally after 10 minutes running in the morning once into normal temperature it will die 3 or 4 times in a row, doesn’t set any trouble codes, will refute every time after cranking 20 seconds or so, never seems to happen later in the day. I did notice 1 thing when it’s running fine when accelerating and as boost bypass opens when I let off to shift my obdii app shows fuel press going zero for a very brief moment! That seems odd to help. Any advice would be appreciated. I have never gotten an engine light. But app does seem to show a M$06. Which I cannot delve into. Thank you in advance

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