FAQ - Carbureted Fuel Pumps
1.) I’m looking at an Aeromotive EFI fuel pump for my new carbureted engine, but I need 7 PSI and your catalog (or your website) says it puts out 43 PSI, won’t this flood the engine?
It’s a common misconception for people to think that a particular fuel pump “puts out” a specific pressure. Though some pumps are pressure limited, which we’ll explain in a moment, the fact is no pump “puts out” a set pressure. What a pump does do is put out flow. And what it needs to do is put out the necessary flow, when regulated up to the required pressure needed for a particular application.
Actually, all electric pumps have a flow curve that changes with pressure. Not all companies provide flow curves for their pumps, which makes evaluating that fuel pump for a particular application virtually impossible. At Aeromotive we understand a pump’s flow curve, across a range of pressure, reveals crucial fuel pump performance characteristics, so when we quote flow, we always provide the test pressure and voltage. When you read; The A1000 flows 750 lb/hr at 45 PSI and 13.5 Volts, you’re being given vital flow data that is in the proper context. This doesn’t mean the pump “puts out” 45 PSI, rather it’s telling how much flow is available at 45 PSI. You’ll see 900 lb/hr at 8 PSI is also noted for carbureted engines.
There are basically two types of pumps used in automotive fuel systems, those that are pressure limited, for use with a static (non bypass) regulator, and those that are not pressure limited, and which must be used with a dynamic (bypass style) regulator. Pressure limited pumps are almost all intended for use with carbureted engines, and the static style carburetor regulators designed for 3-12 PSI. What happens with a pump like this is that when the flow is blocked by the regulator to prevent high pressure from flooding the carburetor, the pump bypass opens to prevent pressure from spiking and stalling the pump.
Some pressure limited pumps have an internal bypass (usually the lower flow street/strip type) that opens around 15 PSI and allows the flow from the outlet port to travel through an internal passage in the pump, back to the inlet port. The higher flow, racing specific pumps often feature an external bypass, set for 18-24 PSI. Here a return line is run from the fuel pump back to the top of the fuel tank so that when the maximum pressure is reached the excess flow returns to the tank. Either way, these pumps are not intended for use in high pressure, EFI systems, or for carbureted, boost referenced systems (blow through turbo or blower engines), even if the bypass is blocked to force pressure higher.
Many Aeromotive pumps are of the “non pressure limited” type, including the A1000 for example. This type of pump cannot be used with a static (non bypass) regulator, because to stop the flow coming from the pump completely would drive fuel pressure to 100-PSI or higher, creating excessive current draw and heat, and potentially damaging the pump permanently. Non pressure limited pumps can be operated in both low (carbureted) and high (EFI) pressure systems, as long as the proper bypass regulator is used. These types of pumps, in conjunction with bypass regulators and boost reference, are perfect choices for the high horsepower, blow through carbureted engines using centrifugal blowers or turbo chargers.
Aeromotive, adjustable bypass regulators are available to use with non pressure limited pumps that can handle flow from small to large pumps, and that can create and maintain pressure from carbureted to EFI levels. Most EFI regulators are adjustable from as low as 30 PSI to as high as 70 PSI, so those who want 43 PSI for the fuel rail and those who want 60 PSI will most probably be able to use the same pump and regulator combination. Just be sure the pump provides the necessary flow at the pressure you need.
2.) I’m building a new carbureted engine combination, what fuel pump do I need?
New pump development is itself an exhausting process that includes prototyping and testing, then more prototyping and testing, but once we know we can deliver a pump that will meet the objective and may be moved to durability and field testing, we begin a parallel effort to develop the supporting components required to create a complete fuel system around that pump. Everything from pre and post filters to port sizes and port fittings are considered. We engineer and develop a specific regulator that will maximize efficiency of that pump, enabling the buyer to extract every possible ounce of available flow while maintaining the desired pressure. The result is a complete fuel system with specific capabilities.
What does this mean to you? It takes the guess work out of choosing the right fuel delivery, and THAT makes your life easier in a meaningful way. All you have to do is determine what pump will meet your requirements. From there the system is defined and either available under one part number or outlined with respect to the individual components you need in our easy to use “Aeromotive Power Planner”. The “Power Planner” is available in our catalog and on our website at www.aeromotiveinc.com, at the top of any page, just click on the “Power Planner” link and choose the Carbureted Power Planner with one more click.
The “Power Planner” outlines fuel systems one at a time, starting with the lowest horsepower combinations and, as you scroll down, covering applications capable of increasing levels of horsepower. The two main questions you need to answer are simply “what will the engine’s peak horsepower be?”, and “What will the fuel system require for fuel pressure?”, including base pressure and boost reference if that is required. If you’re not sure of what your engine will make power-wise, there are numerous magazines and internet forums where you can research similar combinations to the one you’re building, that have already been dyno tested, to get you solidly in the ballpark.
It’s a good idea to be somewhat optimistic when estimating horsepower, or if you prefer, build in a little head room, just to make sure you cover the bases completely. Keep in mind, all ratings provided by Aeromotive are based on flywheel horsepower. Horsepower at the tire must be corrected up to flywheel horsepower. It’s safe to allow 15% drive line losses, so you can divide wheel horsepower numbers by 0.85 to get the flywheel estimate. For example, 500 WHP divided by 0.85 equals 588 FWHP.
Every Aeromotive fuel pump is rated for horsepower capability on its product page in our catalog, and on our website. You will find several horsepower ratings that apply to various engine combinations, naturally aspirated to forced induction, and allowances are made for carbureted and fuel injected engines where a given pump is capable of doing both.
For more detailed information on how to accurately calculate fuel delivery to support horsepower, see Aeromotive Tech Bulletin TB-501 at www.aeromotiveinc.com under the Tech Help, Tech Bulletin section.
3.) During hard stops or aggressive maneuvers, like after a burnout or during a road-course event, my fuel pressure falls and it takes a long time to come back up, why?
The first thing is to be certain the fuel cell or tank is full of fuel. Fuel surges forward in the tank or the fuel cell during hard braking, causing the pump to get a gulp of air. In drag racing the fuel cell should be topped off between each run. Fuel pump supply and/or pickup problems are another common cause of this dilemma. Frequently, the use of a restrictive filter before the pump, one that is too small or too fine or both, is a problem. This affects all types of electric pumps and will result in either cavitation or a loss of prime, after which the pump struggles to re-prime. Be certain the line connecting the fuel cell to the fuel pump inlet is big enough and that the inlet filter is course and free flowing. The tank or fuel cell must also have an adequate vent, otherwise vacuum builds in the tank, fighting the pump getting re-primed .
For more detailed information on potential causes of this and similar problems, see Aeromotive Tech Bulletin TB-101, TB-102 and TB-801 at www.aeromotiveinc.com under the Tech Help, Tech Bulletin section.
4.) With a newly installed fuel pump, or after disconnecting the fuel lines for maintenance, The pump does not prime or has run a long time before building pressure again.
These are symptoms of the same problems outlined in question number 3 above. Cracking the line connected to the pump inlet, then the line connected to the carburetor or fuel rail may help purge air. Briefly cycle the pump but be careful to avoid fuel leaks and prevent fire hazards when handling fuel!
5.) At the track, from the launch to mid-track, fuel pressure is steady, but then down track, it falls 2 PSI or more, what’s wrong?
The first thing to be blamed is usually the regulator, but in fact this is a strong indication of a fuel supply problem. If for some reason there is insufficient volume available from the fuel pump to feed the engine, fuel line pressure can drop to the carburetor. Even with a static regulator, a significant drop in line pressure affects flow through the regulator, causing regulator pressure to drop. Do not automatically assume the pump is bad or inadequate, inspect and resolve any supply line issues to the pump, ensure the tank is vented and the vent is functioning, and be sure to check the fuel pump wiring, along with the overall electrical system performance. Finally, if you still have problems, contact Aeromotive for a proper flow test you can perform in the field to verify if your fuel pump is performing properly.
6.) I’ve installed an A2000 (or A3000) Fuel pump, with the bypass from the pump connected to the fuel cell, along with a bypass regulator and return line at the carburetor. The pressure is low at the pump and adjusting the bypass regulator on the fuel pump has no affect on pressure, why?
When using a bypassing regulator at the carburetor, line pressure and regulated pressure are one-and-the-same. The bypass on the fuel pump should blocked when using a bypass regulator at the carburetor. The rule is, when two bypass regulators are connected to the same fuel pump, the regulator set for the lower pressure becomes the default regulator. A bypass regulator at the carburetor makes for an excellent fuel system, so be sure to use the recommended bypass regulator P/N’s 13202 or 13212 with fuel pump P/Ns 11202 and 11215, and install a –10 AN return line to properly control pressure.
7.) I just installed an A2000 (or A3000) fuel pump and can’t get the regulator on the pump to go below 11-12 PSI fuel pressure. The carburetor keeps flooding, what’s wrong?
The regulator on the A-2000 fuel pump, P/N 11202, is designed to control line pressure (inlet pressure) to a static, non-bypass, Aeromotive regulator, including P/N’s 13108, 13203, 13208 and 13205. The pump bypass is not capable of flowing sufficient volume to regulate pressure below 12 psi and cannot be used alone to regulate pressure directly to a carburetor. The regulator feature on these pumps is designed to present extremely consistent, adjustable fuel pressure to the inlet of a static regulator under the hood.
8.) I have the SS fuel pump P/N 11203 and a static regulator P/N 13205. My new engine will make more than 750 FWHP and your catalog says that’s more than the SS pump will support, unless I use a bypass regulator. Is this true, and how can a bypass regulator make that much difference?
Yes, it is true, although explaining why is somewhat more complicated. First, any electric pump flows more at a lower pressure, so more flow is available using a bypass regulator. But wait, if the carburetor runs at 7 PSI with either a static or bypass regulator, how is the pressure lower and the pump able to flow more?
At first glance, the SS pump doesn’t appear to have a bypass, like an A2000 pump for example, but in order to run a static regulator, it must, and it does have a bypass. However, the bypass in the SS pump is an internal bypass valve, meaning that when the static regulator closes to block flow to the carburetor the pressure between the pump and the regulator inlet rises until the internal bypass opens. This prevents stalling the pump and damaging the pumping mechanism. In the case of an SS pump, the internal pressure limit is 15 PSI. So, by changing to a bypass regulator that opens at 7 PSI, the flow from the pump is never stopped and the pressure to the regulator never goes above 7 PSI. The result is an increase in flow from the pump to the regulator and a higher HP limit from the same fuel pump.